DE102020115460B3 - Method for determining the time difference between the time at which an optical signal is transmitted by an object and the time at which the optical signal is received by another object - Google Patents

Abstract

In the method for determining the time difference between the transmission time and the reception time of an optical signal with a modulation code from a sequence of symbols, a correlation function is first maximized in the received object of the optical signal in a manner known per se. A reference signal with a symbol sequence identical to the modulation code is generated and, in order to maximize the correlation, the symbol sequence is shifted symbol-by-symbol and sub-symbol-wise. The time of reception is determined on the basis of the time stamp of the temporally first symbol of the shifted symbol sequence of the reference signal and the partial symbol duration, namely when the symbol sequence of the reference signal (18) is shifted by one or more symbol durations and times to maximize the degree of correlation the symbol part-time.

Description

The present invention relates to a method for transmitting time between at least two locations optically linked by a laser.

Frequency references used to generate a time have achieved great short term frequency stability on the order of 10 ^-15 [s / s] (Allan deviation) and more for sample times between 0.1 and a few hundred seconds. Time intervals can now be routinely measured at sub-fs levels with low noise. This presents a challenge when comparing the time generated by remote clocks using communication devices, since these introduce additive measurement noise and the channel delays must be taken into account at a low level below the target time transmission accuracy. Microwave-based timing techniques are inadequate for prior art optical clocks and transmission over optical links is required.

Time transmission methods are divided into two categories: one-way methods, in which only one-way measurements are used and the channel propagation delays are modeled (e.g. GNSS); and two-way methods, in which measurements are exchanged by both sides and channel delay is canceled (if symmetrical), mitigated or modeled (e.g. the network time protocol NTP or two-way time transmission via communication satellite).

A common one-way time transmission (OWTT) technique uses Global Navigation Satellite Systems (GNSSs). These are made up of tens of satellites that send navigation signals in an L-band. These globally and continuously available signals are processed by a GNSS receiver in order to determine the position in space. Furthermore, after removing channel delays related to signal propagation from the satellite to the receiver, disruptive atmospheric (ionospheric and tropospheric) delays and device-related components (receiver and satellite antenna delays and line biases) provide a GNSS pseudorange (i.e. code) and Carrier phase measurements the offset between a local time reference and the selected GNSS system time (e.g. GPS system time or Galileo system time). This enables a) transferring time from a GNSS timescale to a clock on the ground and b) retrieving the time offset between the clock on the ground at different locations by forwarding each clock offset to the selected GNSS system time and Distinguish between the two. This technique is often improved when operating in a so-called "common mode": two remote stations are observing the same (set of) satellites, and their time offset relative to the selected GNSS system time is determined only by using the satellite (s) that that they have in view together, thereby eliminating all satellite hardware delays and satellite clock errors.

In general, one-way methods are less precise due to the incorrect modeling of channel delays and device deviations.

Two-way time transmission (TWTT) methods are based on time stamping the time of transmission and time of reception of signals traveling between remote units in both directions. Each side calculates the measured difference between "data packets" traveling in opposite directions and extracts the time offset between the clocks at two stations. TWTT techniques allow for greater timing accuracy than one-way techniques due to the cancellation (if perfectly symmetrical) or mitigation of the channel delays and device delays on each side. A widespread TWTT method is the "Network Time Protocol" (NTP) and its further development "Precise Time Protocol" (PTP), in which a client sends a message containing the transmission time to a time server, which in turn sends a response indicating the time of receipt Contains the original message on the server and the time the response was sent. The client can then call up the difference between the server clock and the client clock if it is assumed that the transit time over the Internet is symmetrical for the two transmissions. TWTT over a satellite communication channel, with the satellite acting as a relay between two remote stations, is also commonly used to compare clocks over large areas, possibly intercontinental.

Both OWTT and TWTT technology can be applied over a physical layer (such as the cables or fibers that carry Internet traffic) or over open space communication. The former often enables better time transmission accuracy, for example the clock network that is implemented between SYRTE in Paris and PTB in Braunschweig on a dedicated glass fiber synchronized to a 10-as-level. However, they lack the flexibility offered by free space connections between distant clocks on dynamic platforms.

The possibilities of modern TWTT techniques over free space are nowadays limited by the carrier used, typically in the microwave region, whereby the time transmission accuracy is limited to 0.1-1 ns, which is only improved to 0.01 ns if long mean time intervals are possible are. Optical carriers allow much higher accuracy without long integration times. However, the techniques for transmitting time over an optical carrier with a target accuracy of ps or better at low latency are still under development. An example is Optical TWTT (OTWTT) in free space, which is being developed at NIST, where the fs-level accuracy is demonstrated in a laboratory setup (see [1]), but at the expense of the use of multiple frequency combs. The implementation of a compact device for a TWTT compared to optical connections with an accuracy of 1 ps is described here.

Time transmission over the Internet using the NTP protocol is limited by asymmetrical channel delays, and accuracies of up to 1 ms can generally only be achieved. GNSS only allows OWTT techniques and is therefore limited by the accuracy of the L-band signals, equipment noise and incorrectly modeled channel delays. An OWTT accuracy via GNSS is in the order of magnitude of 1 ns and is usually not available online with this level of precision (lengthy post-processing required). TWTT over a communications satellite in microwave bands typically achieves an accuracy of 0.1 ns (0.01 ns for longer integration times), but requires an available communications window to the satellite, which is expensive and time-limited.

The most precise time transmission techniques today are TWTT laboratory setups connected with dedicated fiber optics. While possible timing accuracies of 0.1 fs or better, they lack the flexibility required for widespread use in dynamic applications such as inter-satellite or ground-to-satellite connections where free space communications must be used. OWTT in free space with fs level accuracy is covered (see [1]), but the technique proposed there relies on multiple frequency combs on each side in order to work.

The idea underlying all time transmission methods is to time stamp the start of a sequence of data bits that are rigidly structured (i.e. known pattern) so that the receiver can identify the start of the sequence and record the time of arrival. Then, based on the knowledge of the difference between the recorded arrival time and the transmission time, the offset between the two clocks at different locations can be called up (time transmission) as soon as all propagation delays have been taken into account, such as transit time, atmospheric disturbances and so on.

A distance measuring method is known from [6] which is suitable for determining the distance between two satellites. A high-frequency signal, which is modulated with data, is additionally spread by a PN sequence. By correlation with the PN sequence and subsequent sub-chip path determination, the time difference between the transmission of the PN sequence and the reception of the PN sequence can be determined, and thus the distance between the satellites can also be determined.

The use of a code spreading method is also described in [7], specifically for the determination of a time difference between two locations by means of the sending and receiving of signals via a communication satellite. The use of optical carrier frequencies for the satellite paths is also shown.

the end US 2003/0048430 A1 an optical distance measuring method is known which is also based on the use of a PN sequence modulated signal and which can determine the correlation peak with sub-chip resolution.

Finally it's over DE 10 2014 111 589 A1 Another optical distance measuring method is known which is based on the two-way exchange of time stamps in order to determine and correct a time offset between two systems. A fine determination of a received edge position is then carried out in order to determine an exact distance value.

The present invention is based on the object of creating an improved time transmission between at least two optically connected objects.

2 represents exactly this concept for introducing the time transmission technology in a simplified way. The time transmission concept is not a new idea: GNSS, to name just one example of many, works by using precisely this technology. The basic time transfer concept shown in the two diagrams is part of the IEEE 1588 protocol (see e.g. https://endruntechnologies.com/pdf/PTP-1588.pdf) and is largely used in modern communication systems: the Internet Time synchronization protocol works in the same way.

The time transfer protocol itself is not the subject of the invention, but rather a method for its physical implementation.

• - ein erstes Objekt von einem zweiten Objekt ein mittels eines Codes moduliertes optisches Signal mit einer bekannten Trägerfrequenz als optisches erstes Empfangssignal empfängt, wobei der Modulationscode eine Sequenz von Symbolen aufweist, die jeweils gleich lange Intervalle einnehmen, und wobei das optische Signal einen ersten Absendezeitstempel aufweist, der angibt, zu welchem Zeitpunkt das zweite Objekt, basierend auf seiner Zeitbasis, das optische Signal an das erste Objekt gesendet hat,
• - in dem ersten Objekt
  • - ein optisches Referenzsignal erzeugt wird, das als ein optisches Trägersignal mit der gleichen Frequenz wie diejenige des ersten Empfangssignals und mit einer aufmodulierten, der Symbolsequenz des ersten Empfangssignals gleichenden Symbolsequenz ausgebildet ist,
  • - in einem Korrelator die Korrelation des ersten Empfangssignals mit dem Referenzsignal ermittelt wird,
  • - zur Maximierung des Grades der Korrelation in einem Bit-Verschieber eine Verschiebung der Symbolsequenz des Referenzsignals um eine Symbol-Zeitdauer, die gleich dem Intervall eines Symbols oder gleich einem ganzzahligen Vielfachen des Intervalls ist, erfolgt,
  • - zur weiteren Maximierung des Grades der Korrelation eine Verschiebung der Symbolsequenz des Referenzsignals um eine Symbol-Teilzeitdauer, die kleiner ist als das Intervall eines Symbols, durch Ansteuerung einer dem Korrelator vorgeschalteten und von diesem angesteuerten Analogverschiebung erfolgt und
  • - die Empfangszeit des von dem ersten Objekt empfangenen, von dem zweiten Objekt ausgesendeten optischen Signals anhand des basierend auf der lokalen Zeitbasis des ersten Objekts vergebenen Zeitstempels des zeitlich ersten Symbols der Symbolsequenz des Referenzsignals und der Symbol-Teilzeitdauer ermittelt wird, und zwar bei der durch die Maximierung des Korrelationsgrades erfolgten Verschiebung der Symbolsequenz des Referenzsignals um die ein oder mehreren Symbol-Zeitdauern und um die Symbol-Teilzeitdauer.

To solve this problem, the invention proposes a method for determining the time difference between the transmission time at which a modulated, optical frequency signal having a transmission time stamp is transmitted from a second object which has a local time base, in particular from a satellite, and the time of reception at which the optical frequency signal of the second object is received by a first object that has a local time base, in particular from a satellite, specifically for the synchronization of the time bases of both objects by means of a one-way or two-way known per se Time transmission method, with the method for determining the time difference

• a first object receives from a second object an optical signal modulated by means of a code with a known carrier frequency as the optical first received signal, the modulation code having a sequence of symbols which each occupy equally long intervals, and the optical signal having a first transmission time stamp , which indicates at what point in time the second object, based on its time base, sent the optical signal to the first object,
• - in the first object
  • - An optical reference signal is generated, which is designed as an optical carrier signal with the same frequency as that of the first received signal and with a symbol sequence modulated on that is the same as the symbol sequence of the first received signal,
  • - the correlation of the first received signal with the reference signal is determined in a correlator,
  • - To maximize the degree of correlation in a bit shifter, the symbol sequence of the reference signal is shifted by a symbol duration which is equal to the interval of a symbol or equal to an integral multiple of the interval,
  • - To further maximize the degree of correlation, the symbol sequence of the reference signal is shifted by a partial symbol duration which is smaller than the interval of a symbol by controlling an analog shift connected upstream of the correlator and controlled by it, and
  • - The time of receipt of the optical signal received by the first object and emitted by the second object is determined using the time stamp of the first symbol of the symbol sequence of the reference signal and the symbol part-time duration, which is assigned based on the local time base of the first object, namely during the the degree of correlation is maximized, the symbol sequence of the reference signal is shifted by the one or more symbol periods and by the partial symbol period.

To maximize the synchronization of a locally generated optical reference signal with a symbol sequence that is the same as the symbol sequence of the received signal, the inventive method uses a symbol-wise shift of the symbol sequence of the reference signal and a subsymbol-wise shift of the symbol sequence of the reference signal. This is controlled by an optical PLL, which was previously used for the symbol-by-symbol shift of the symbol frequency of the reference signal. According to the invention, the sub-symbol-wise shift is added here.

This procedure is now used according to the invention in order to determine extremely precisely the time of reception at which a satellite, for example, receives an optical signal from another satellite. Both satellites have a local time base. The symbol sequences of the reference signal are provided with time stamps that are assigned using the local time base. According to the invention, the time of reception of the optical signal received by the first object and transmitted by the second object is determined using the time stamp of the first symbol of the symbol sequence of the reference signal and the symbol part-time duration, which is assigned based on the local time base of the first object the degree of correlation is maximized, the symbol sequence of the reference signal is shifted by the one or more symbol periods and by the partial symbol period.

The said time difference can then be calculated by calculating the difference between the time stamp of the received optical signal and the reception time.

• - Bereitstellen optischer Frequenzreferenzen für die optisch verbundenen Standorte,
• - Bereitstellen von optischen Frequenzkämmen für die optisch verbundenen Standorte,
• - Übertragen einer Datenstruktur mittels eines Laserstrahls von einem der optisch verbundenen Standorte zum anderen,
• - Verrasten in die Phase eines ankommenden Laserstrahls und Justieren der lokal reproduzierten Datenstrukturen auf die empfangene Datenstruktur,
• - Ermöglichen des Justierens der zwei Datensequenzen innerhalb eines Bits mittels eines ersten „Grob“justageprozesses,
• - Ermöglichen des Justierens der ansteigenden Flanken von zwei Referenzbits (lokal versus empfangen) mit verringertem Rauschen mittels eines zweiten Justageprozesses (nachstehend als Analogverschiebung bezeichnet), und
• - durch Bestimmen der bei der lokalen Nachbildung verwendeten Verzögerung Abrufen der Ankunftszeit auf einem Pikosekunden-Level.

Accordingly, the present invention generally provides a method for time transmission between at least two locations that are optically connected to one another by means of a laser, comprising

• - Provision of optical frequency references for the optically connected locations,
• - Provision of optical frequency combs for the optically connected locations,
• - Transfer of a data structure by means of a laser beam from one of the optically connected locations to the other,
• - Locking into the phase of an incoming laser beam and adjusting the locally reproduced data structures to the received data structure,
• - Enabling the adjustment of the two data sequences within one bit by means of a first "coarse" adjustment process,
• - Enabling the adjustment of the rising edges of two reference bits (local versus received) with reduced noise by means of a second adjustment process (hereinafter referred to as analog shift), and
• - by determining the delay used in the local replication, retrieving the time of arrival on a picosecond level.

The invention relates to a method for realizing the above-described time transmission with very high accuracy (1 ps accurate) between two connected locations by means of a laser link, optical frequency references, optical frequency combs and FPGAs. The data structures are implemented with a very high bit rate; the receiver is able to lock into the phase of the incoming laser and adjust the locally reproduced data structures to the received one. A first “rough” adjustment enables the two sequences to be adjusted in one bit. A second adjustment process ("analog shift") enables the rising edges of the two reference bits (local versus received) to be adjusted with minimal noise. Knowing the delay applied in the local replication, the time of arrival can be obtained at a 1 ps level. The two-step adjustment process is the main feature of the invention.

In an advantageous development of the invention, a correlation value describing the degree of correlation is generated as an electrical signal which has a lower frequency than the first received signal.

It can expediently be provided that the symbol sequence for the reference signal is modulated onto the carrier signal with a time shift by means of an optical modulator, the time shift being determined on the basis of the magnitude of the correlation value so that the reference signal comes into phase with the first received signal.

Expediently, provision can also be made for the symbol sequence of the reference signal to be shifted by one or more symbol periods by corresponding time-delayed modulation of the carrier signal by means of an electrical digital signal having the symbol sequence of the reference signal.

In a further advantageous development of the invention it can be provided that the analog shift, which receives the symbol sequence of the reference signal, receives the electrical digital signal and passes this digital signal on to the modulator with a time delay by the delay time determined by an electrical time delay control signal for the analog shift.

It can expediently also be provided that the time delay control signal is an electrical digital signal that is converted in a digital-to-analog converter into an electrical analog signal, the electrical analog signal controlling the analog shift, or that the time delay control signal is an electrical digital signal and the analog shift is digital -Analog shift that is driven by the digital signal.

• 1 eine schematische Darstellung der Zeitsynchronisation zweier sich bewegender Objekte, wie beispielsweise zweier Satelliten,
• 2 eine schematische Darstellung eines Datenpakets eines optischen Signals, wobei die übertragenen Daten zu Beginn eines Datenframes und an dessen Ende Synchronisationsbits aufweisen,
• 3 eine Möglichkeit der Modulation eines optischen Trägersignals mit einer Frequenz im THz-Bereich durch eine an sich bekannte PRN-Sequenz (beispielsweise 20 bis 30 Gigabits pro Sekunde (Gbps)), und zwar jeweils moduliert durch Datenbits (10 bis 1000 Mbps),
• 4 eine schematische Darstellung der Hauptfunktionskomponenten eines Satelliten, ausgelegt für ein Zeitübertragungsverfahren,
• 5 ein Beispiel für eine Satelliten-Konstellation, bei der die Erfindung eingesetzt werden kann,
• 6 ein Blockdiagramm der Hauptkomponenten eines Systems zur Umsetzung des erfindungsgemäßen Verfahrens und
• 7 und 8 Signalverläufe mit den korrespondierenden Korrelationswerten für verschiedene Verschiebungen der Symbolsequenz im Sub-Symbolintervall-Bereich.

The invention is explained below with reference to the drawing. In detail, show:

• 1 a schematic representation of the time synchronization of two moving objects, such as two satellites,
• 2 a schematic representation of a data packet of an optical signal, the transmitted data having synchronization bits at the beginning of a data frame and at its end,
• 3 a possibility of modulating an optical carrier signal with a frequency in the THz range using a PRN sequence known per se (for example 20 to 30 gigabits per second (Gbps)), in each case modulated by data bits (10 to 1000 Mbps),
• 4th a schematic representation of the main functional components of a satellite, designed for a time transmission method,
• 5 an example of a satellite constellation in which the invention can be used,
• 6th a block diagram of the main components of a system for implementing the method according to the invention and
• 7th and 8th Signal curves with the corresponding correlation values for various shifts in the symbol sequence in the sub-symbol interval area.

In 5 a satellite constellation is shown in which several satellites are located 10 move on a MEO orbit. Neighboring MEO satellites 10 are in communication with each other through optical data communication. Further (LEO) satellites 12 are located on a LEO orbit closer to the earth, which likewise use optical data communication with the MEO satellites 10 communicate with ground stations on earth using down and up links.

Some peculiarities of such a satellite constellation, which are of subordinate importance, are shown in WO 2019/011919 A1 described.

For high-precision applications in geodesy, for example, not only time synchronization of the satellites is required, but also knowledge of the exact distance between neighboring MEO satellites 10 or between a MEO satellite 10 and a LEO satellite 12th and keeping track of their changes is of great importance. For this purpose, one can use the optical distance measurement method, for example.

• Plattform 1 unterzieht eine ausgehende Mitteilung (oder eine Unterseite von dieser) einer Zeitstempelung und sendet sie zu einer Zeit taulT zu Plattform 2. Die Mitteilung wird über eine PRN- (Pseudo-Random-Noise -pseudozufälliges Rauschen) Sequenz kodiert. Plattform 2 empfängt die Mitteilung durch Justieren einer lokalen Nachbildung der PRN-Sequenz und unterzieht den Empfangszeitraum der (Subseite der) Mitteilung einer Zeitstempelung bei tau2R. Nach einem kurzen Intervall antwortet Plattform 2 mit einer zeitgestempelten Mitteilung (mit ihrer eigenen PRN-Sequenz kodiert) und sendet sie zur Zeit tau2T. Plattform 1 empfängt die Antwort und stempelt sie zur Empfangszeit tau1R.

1 shows a simplified exchange of time-stamped messages (i.e. bit sequences) between two locations:

• Platform 1 subjects an outgoing message (or a subpage of this) to a time stamp and sends it to platform 2 at a time taulT. The message is encoded via a PRN (pseudo-random noise) sequence. Platform 2 receives the message by adjusting a local replica of the PRN sequence and subjects the reception period of the (sub-page of) the message to a time stamp at tau2R. After a short interval, platform 2 responds with a time stamped message (encoded with its own PRN sequence) and sends it at time tau2T. Platform 1 receives the response and stamps it at the time of reception tau1R.

$${\Delta }_{12}^{\text{'}}={\tau }_{1R}-\left({\tau }_{2T}+{\delta }_{2T}+d_{21}+{\delta }_{1R}\right)$$

Wobei Δ12 der Zeit-Offset (positiv, wenn die Uhr auf Plattform 1 schneller läuft als die Uhr auf Plattform 2) zwischen den zwei Uhren ist, tau die Zeitstempel bezeichnet, Delta Hardware-Verzögerungen bezeichnet und d die Ausbreitungsverzögerung ist. Die berechnete Ausbreitungsverzögerung muss möglicherweise relativistische Effekte (Lichtausbreitung im Gravitätsfeld) bis zur dritten Ordnung aufweisen beim Vergleichen der Zeitablesewerte zwischen sich schnell bewegenden Plattformen (Satelliten) und/oder Plattformen bei unterschiedlichem Gravitätspotenzial (z.B. Satellit-Boden-Link) mit der Soll-1-ps-Genauigkeit. Ferner beziehen sich der Zeitstempel auf die korrekte Zeit, d.h. die lokale Zeit in der unmittelbaren Raum-Zeit-Metrik des Satelliten. Die zwei Zeit-Offsets Δ12 und Δ12' werden aufgrund der Rigorosität unterschiedlich wahrgenommen: da sie zu unterschiedlichen Zeiten extrahiert werden, können die Taktraten während des Synchronisationsprozesses variieren. Dies ist proportional zu der Stabilität der zu vergleichenden Frequenzreferenzen. Die Geräteverzögerungen sind möglicherweise nicht vernachlässigbar bei Potenzialvariationen proportional zu z.B. der Temperatur der (zu bewertenden) Ausstattung. Schließlich können die Ausbreitungsverzögerungen über die optische Abstandsmessung mit einer Soll-Genauigkeit von 1 ps gemessen werden.All time stamps are relative to each platform's own time scale. The aim of the time transfer is to provide a means of extracting the time offset between the references for timing the sending / receiving. A simplified model for the passage of time in the creation, transmission, reception and decoding of the two exchanged messages as in FIG 1 (some terms are not visualized in the figure for clarity) can be specified as $${\Delta }_{\mathrm{12th}}=\left({\tau }_{1T}+{\delta }_{1T}+d_{\mathrm{12th}}+{\mathrm{<figure-callout\; id="2"\; label="\delta "\; filenames="DE102020115460B3\_0009.png,DE102020115460B3\_0012.png"\; state="\{\{state\}\}">\delta </figure-callout>}}_{2\mathrm{R.}}\right)-{\tau }_{2\mathrm{R.}}$$

$${\Delta }_{\mathrm{12th}}^{\text{'}}={\mathrm{<figure-callout\; id="1"\; label="\tau "\; filenames="DE102020115460B3\_0009.png,DE102020115460B3\_0013.png"\; state="\{\{state\}\}">\tau </figure-callout>}}_{1\mathrm{R.}}-\left({\tau }_{2T}+{\delta }_{2T}+d_{21}+{\mathrm{<figure-callout\; id="1"\; label="\delta "\; filenames="DE102020115460B3\_0009.png,DE102020115460B3\_0013.png"\; state="\{\{state\}\}">\delta </figure-callout>}}_{1\mathrm{R.}}\right)$$

Where Δ12 is the time offset (positive if the clock on platform 1 is running faster than the clock on platform 2) between the two clocks, tau denotes the time stamps, Delta denotes hardware delays, and d is the propagation delay. The calculated propagation delay must possibly have relativistic effects (light propagation in the gravitational field) up to the third order when comparing the time readings between fast-moving platforms (satellites) and / or platforms with different gravitational potential (e.g. satellite-ground link) with the target 1- ps accuracy. Also refer to the Timestamp to the correct time, ie the local time in the immediate space-time metric of the satellite. The two time offsets Δ12 and Δ12 'are perceived differently due to the rigor: since they are extracted at different times, the clock rates can vary during the synchronization process. This is proportional to the stability of the frequency references to be compared. The device delays are possibly not negligible in the case of potential variations proportional to, for example, the temperature of the equipment (to be assessed). Finally, the propagation delays can be measured using the optical distance measurement with a target accuracy of 1 ps.

Dies ist die Gleichung, die sämtlichen bestehenden Zweiweg-Zeitübertragungs-Algorithmen zugrundeliegt. Jede Plattform berechnet den Offset ihrer eigenen Zeit relativ zu der Zeit der anderen Plattform durch Ablesen sämtlicher Zeitstempel (taus) und Modellieren oder Messen der Aussendeverzögerungen (ds und Deltas). Sobald der Offset geschätzt ist, werden die zwei Uhren als synchronisiert betrachtet, da jede dann relativ zu der Zeitskala der anderen justiert werden kann.By averaging the two delays measured above on the broadcast platform, one obtains: $${\Delta }_{\mathrm{12th}}\approx \frac{\left({\tau }_{1T}+{\mathrm{<figure-callout\; id="1"\; label="\tau "\; filenames="DE102020115460B3\_0009.png,DE102020115460B3\_0013.png"\; state="\{\{state\}\}">\tau </figure-callout>}}_{1\mathrm{R.}}\right)-\left({\tau }_{2T}+{\mathrm{<figure-callout\; id="2"\; label="\tau "\; filenames="DE102020115460B3\_0009.png,DE102020115460B3\_0012.png"\; state="\{\{state\}\}">\tau </figure-callout>}}_{2\mathrm{R.}}\right)+\left(d_{\mathrm{12th}}-d_{21}\right)+\left({\delta }_{1T}-{\mathrm{<figure-callout\; id="1"\; label="\delta "\; filenames="DE102020115460B3\_0009.png,DE102020115460B3\_0013.png"\; state="\{\{state\}\}">\delta </figure-callout>}}_{1\mathrm{R.}}\right)-\left({\delta }_{2T}-{\mathrm{<figure-callout\; id="2"\; label="\delta "\; filenames="DE102020115460B3\_0009.png,DE102020115460B3\_0012.png"\; state="\{\{state\}\}">\delta </figure-callout>}}_{2\mathrm{R.}}\right)}{2}$$

This is the equation that underlies all existing two-way timing algorithms. Each platform calculates the offset of its own time relative to the time of the other platform by reading all the time stamps (ts) and modeling or measuring the transmission delays (ds and deltas). Once the offset is estimated, the two clocks are considered to be synchronized since each can then be adjusted relative to the other's time scale.

Dies erfordert vier Zeitstempel (erzeugt mittels des in dem vorhergehenden Abschnitt beschriebenen Verfahrens) und die Kenntnis der Wegverzögerung, die bei der ersten Ordnung mittels einer Laserabstandsmessung geschätzt wird und möglicherweise je nach der angesprochenen Anwendung über das Kanalverzögerungsmodellieren korrigiert wird (relativistische Korrekturen und durch Atmosphäre hervorgerufene Verzögerungen).A notable example of the two-way time transfer algorithm is the Network Time Protocol (NTP), which is used to distribute time over the Worldwide Web. Most algorithms (for example NTPv4) work assuming symmetrical delays; all clutter terms are canceled and the time offset is estimated simply by holding the first two terms on the right hand side of the equation above. This simplification is not possible in the case of fast-moving platforms (e.g. transmission between two satellites): if not taken into account, the line of sight relative to the speed can transmit time errors that are too large. However, it is not necessary to apply this simplification: the optical distance measurement provides an estimate for both d12 and d21. The differences between device delays make it possible to mitigate their impact to a great extent. If the hardware delays are nearly constant over a short period of time (e.g. the thermal response of the optical transceivers and terminals is sufficiently slow), the time can be translated using the relatively simple equation $${\Delta }_{\mathrm{12th}}\approx \frac{\left({\tau }_{1T}+{\mathrm{<figure-callout\; id="1"\; label="\tau "\; filenames="DE102020115460B3\_0009.png,DE102020115460B3\_0013.png"\; state="\{\{state\}\}">\tau </figure-callout>}}_{1\mathrm{R.}}\right)-\left({\tau }_{2T}+{\mathrm{<figure-callout\; id="2"\; label="\tau "\; filenames="DE102020115460B3\_0009.png,DE102020115460B3\_0012.png"\; state="\{\{state\}\}">\tau </figure-callout>}}_{2\mathrm{R.}}\right)+\left(d_{\mathrm{12th}}-d_{21}\right)}{2}$$

This requires four time stamps (generated using the method described in the previous section) and knowledge of the path delay, which is estimated for the first order using a laser distance measurement and may be corrected using channel delay modeling depending on the application in question (relativistic corrections and atmospheric corrections Delays).

1. a) Erzeugen der Zeitstempel mit dem gleichen Genauigkeitslevel, und
2. b) Entfernen der Wegverzögerung mit dem gleichen Genauigkeitslevel.

The challenge of synchronizing two clocks with an accuracy of 1 ps lies in

1. a) Generating the timestamps with the same level of accuracy, and
2. b) Removal of the path delay with the same level of accuracy.

Point b) requires the knowledge of the distance covered by the light between two clocks with an accuracy of 1 ps corresponding to 1 ps * c = 300 mm. This can be achieved with laser distance measurement functions (subject of another patent) and accurate propagation models (e.g. relativistic and / or atmospheric models).

The subject of the invention is point a): how the time offset is generated and, more importantly, recovered with 1 ps accuracy.

First, a clock is built from two components: a stable oscillator and a counter. The oscillator, also known as the “frequency reference” (examples: the rotation of the earth, a pendulum, microwave radiation, a laser) is a device that generates a stable cyclic signal (e.g. a sinusoidal wave). A counter registers the number of cycles that occur over time. The combination of the two results in a clock that measures the passage of time from an arbitrary starting moment by continuously tracking the number of cycles that occur. Nowadays the most stable time scales are made with atomic Generates frequency references that work by “tuning” to a fixed emission radiation of a given substance: an example is the hydrogen maser, in which the 21 cm line in the hydrogen spectrum, which corresponds to a frequency emission of 1.42 GHz, is the structure of very stable watches.

For some years now, lasers have become more and more interesting for measuring the passage of time, especially since the invention of marketing frequency combs (see https://www.nobelprize.org/prizes/physics/2005/summary/) in the early 2000s , a device capable of measuring optical frequencies (THz) and dividing optical frequencies down to the microwave (GHz) range. A stabilized laser can then be used as a frequency reference as one is now able to count cycles based on its frequency over the frequency comb, which acts as a kind of "gear". The principle of laser stabilization is beyond the scope of this brief explanation: suffice it to say that a laser can be stabilized to a level well above what microwave atomic clocks can achieve in the short and medium term, and a clock that based on a stabilized laser, its performance is superior to atomic clocks in terms of short and medium-term stability (typically between 0.01 and 1000 s).

Regardless of the frequency reference actually used, a frequency comb is used to I) generate an optical signal that serves as a "carrier" in the two-way laser communication system, and II) generate a microwave signal, typically at 10 MHz, to control the electronics, ie for example of the FPGA on which the electrical transceivers are implemented.

The counter implemented in the FPGA is able to operate at 400 MHz (via internal 40x multipliers applied to the 10 MHz input), resulting in a time resolution of 1 / (400 Hz) : the time scale generated in the FPGA is thus incremented every 2.5 ns.

In the FPGA, a pseudo random noise (PRN) sequence is generated at a high rate, e.g., 20 Gbps (bits-per-second, note: it can be higher). This sequence is used for both distance measurement and time transmission. The sequence is contained in an internal high-speed access memory: when it is sent, the sequence is read from the memory and used to modulate the optical carrier (the laser transmission between two units takes place at wavelengths of 1550 nm or 1064 nm, these being the current standards for optical transmission frequencies are).

An additional data layer is generated and injected into the transmitted signals: this data layer is made up of data packets that are initially limited to a number of synchronization bits (see 2 ).

The transmitter works as follows: the optical carrier (1550 nm or 1064 nm) is first modulated by means of a PRN sequence (e.g. at 20 Gbps), then modulated again by a data channel (assumed here at 50 Mbps), see 3 . The first rising edge of the first synchronization bit of the data packet is adjusted with the first rising edge of the PRN sequence, and it is marked with a time stamp: this information is sent in the data packet. Thus, each outgoing data packet is assigned a time stamp relative to its first synchronization bit: this information is used by the receiver to retrieve a comparison time stamp.

Assuming that the total length of the PRN sequence (generated with 16 bits) is 2 ¹⁶ = 65536 bits, the PRN sequence repeats every 3.28 ms and each bit is 50 ps long. Instead, each data bit is 1 / 50Mbps = 20ns long, so 400 consecutive PRN bits are modulated by each data bit.

The data channel carries the time stamp of the first synchronization bit, which can then be called up by the receiver when the signal is demodulated.

An optical phase-locked loop (OPLL) is used on the receiver side to initially track the incoming laser signals. It should be noted that it is not possible to directly time stamp the received PRN sequence at ps level, since the internal FPGA processing speed limits the time resolution to 2.5 ns. Therefore, the following process is used.

When the OPLL is locked to the received optical signal, the same known PRN that is used to send the signal (it must then be known to the receiver) is generated locally and time stamped according to the same principle as above: the first rising edge of the The first bit of the PRN sequence received is used to record the timestamp of the locally generated sequence at a resolution of 2.5 ns (assuming the FPGA is operating at 400 MHz).

The local PRN simulation is then adjusted to the incoming PRN sequence by means of a maximum correlation process. The point of best adjustment is sought by bit-wise shifting (bit shifting) of the local PRN simulation until the maximum correlation is detected. Since each bit shift is equal to an applied delay of 50 ps (for a PRN sequence at 20 Gbps), it is possible to roughly determine the time of receipt of the rising edge of the first PRN bit to within 50 ps. Thus, the rough reception time is given by $$\begin{array}{c}\text{Time stamp of the locally generated PRN sequence}\\ +\\ \left(\text{Number of bit shifts applied}\times 50\text{ps}\right).\end{array}$$

The next step is to adjust two rising edges of the received and locally generated PRN sequences. This is implemented via what is known as an analog shift, which enables the PRN sequence to be shifted by a very small part of the PRN bit length. The analog shift is applied to find the maximum correlation within the one-bit coarse adjustment determined in the previous step, as described below (see also EP 3 751 308 A1 ). This process offers an adjustment of the two rising edges to an accuracy between 1/50 and 1/100 of the bit length and thus enables a 1 ps accurate time stamp of the received signal as $$\begin{array}{c}\text{Time stamp of the locally generated PRN sequence}\\ +\\ \text{Number of bit shifts applied}\times 50\text{ps}\\ +\\ \text{Sub-chip shift}.\end{array}$$

4th illustrates the functional architecture for implementing the process detailed above and below.

4th shows schematically a first object 60 , which has a local time base 62 with a counter 64. Furthermore, the first object 60 a PRN sequence generator 66 and an optical terminal 68 on.

The connection of these elements by signal paths is, if optical signals are involved, in 4th shown as dashed lines and, when electrical signals are concerned, as solid lines. Between the object 60 and another object 70 which is like the object 60 can be, for example, a satellite, there is an optical communication channel 72 . The object 70 again has an optical terminal 74 and a local time base 76 in the form of a reference frequency. The optical terminal is operated by a local time base 76 powered meter 78 and an internal PRN sequence generator 80 controlled. In a correlator 82 becomes the correlation of the received optical transmission signal 83 of the object 60 and the internal reference signal. According to the invention, the correlation function is maximized by shifting the symbols bit by bit by means of a bit shifter 84 , which is based on a further internal PRN sequence generator 86 acts that a reference signal 88 generated with a symbol sequence which is equal to the symbol sequence of the received transmission signal 83 is. The correlator 82 controls an analog shift as a function of the value of the correlation function 90 ), with which it is possible to generate the PRN sequence in the PRN sequence generator 86 generated reference signal 88 to be shifted temporally in the sub-symbol area. If the correlation function is then maximized, the time information about the symbol-wise shift and the sub-symbol-wise shift are a time stamp generator 92 supplied, in which the time stamp of the first symbol of the symbol-wise shifted symbol sequence is determined, plus the information about the subsymbol-wise shifting. Based on the counter value (output of the counter 78 ) the time of reception can then be determined at which the object 70 based on its local time base, the received transmission signal 83 of the other object 60 received. This signal also has the time stamp of the time it was sent, so that the time difference between the two can now be determined. This means that the essential information that is required is available in order to achieve a synchronization of the time bases of several objects with the help of the known one-way or two-way time transmission method.

• - Optische Sendeempfänger: sie werden zum Erzeugen und Empfangen der optischen Signale verwendet, wobei sie die elektrooptische Wandlung durchführen. Kohärente Kommunikationen, die auf einem Homodyn-Empfang basieren, ermöglichen eine Korrelation in der optischen Domäne, wodurch die Signalempfindlichkeit erhöht wird. Diese Technologie wird derzeit bei Satellitenkommunikationen in den EDRS-Systemen angewendet (siehe Airbus, 2019).
• - Hochleistungs-Verarbeitungssysteme, wie FPGAs: sie werden zum Aufnehmen, Auswerten und Erzeugen von elektrischen Signalen verwendet. Die derzeitige FPGA-Technologie umfasst Multi-Gigabit-Sendeempfänger (MGTs), die eine Hochgeschwindigkeits-Datenerfassung und -erzeugung im Bereich von mehreren zehn Gigabits pro Sekunde ermöglichen. Die interne Struktur ermöglicht z.B. das Implementieren von zeitdeterministischen Algorithmen und eine Parallelverarbeitung.

A number of technologies are involved in the process:

• Optical transceivers: they are used to generate and receive the optical signals, performing the electro-optical conversion. Coherent communications based on homodyne reception allow correlation in the optical domain, thereby increasing signal sensitivity. This technology is currently used in satellite communications in the EDRS systems (see Airbus, 2019).
• - High-performance processing systems such as FPGAs: they are used to record, evaluate and generate electrical signals. Current FPGA technology includes multi-gigabit transceivers (MGTs) that enable high-speed data acquisition and generation in the tens of gigabits per second range. The internal structure enables, for example, the implementation of time-deterministic algorithms and parallel processing.

The invention provides an advanced two-way timing method that is essentially the same as the NTP used in an optical link with optical transceivers at both ends of the channel.

Two partners, for example two satellites, have an optical terminal that provides bi-directional optical links. With the Laserlink, a PRN sequence and the communication data are modulated at a high data rate. The system can be described as follows

1. 1. Satellite 1 has a local counter which is synchronized with the time reference via a frequency comb, for example.
2. 2. Satellite 1 generates a PRN sequence, which is also synchronized with the time reference via the frequency comb.
3. 3. The beginning of the PRN sequence is synchronized with the first data bit transition.
4. 4. The PRN sequence and the data are multiplexed and modulated by BPSK in the laser link. The data contain the time stamp of the start of the sequence. Since both the sequence and the counter are synchronized by the frequency comb, the PRN sequence can be initiated when the counter is updated.
5. 5. The same applies to satellite 2.
6. 6. Both satellites exchange the range and data signal that has been modulated on the optical link.
7. 7. Satellite 1 receives the signal, locks OPLL and DLL. To lock the DLL, a bit shift of the sequence is first applied, then a coarse shift and an analog shift of the sequence are applied (see EP 3 751 308 A1 ). This operation is carried out by observing the electrical signal of the optical correlation between the reception signal and the modulated local oscillator.
8. 8. The received timestamp can be generated by concatenating three parts, namely the local time related to the start of the PRN sequence, the bit shift relative to the initial PRN position, and the analog shift.
9. 9. The subtraction of the received and the sent time stamp contains the propagation delay that has to be estimated, taking into account relativistic effects, in order to achieve a ps accuracy.

With the current FPGA technology, system clocks of almost 1 GHz and transceiver data rates of more than 30 Gbits / s can be achieved. On the basis of a cautious assumption that an FPGA counter runs at a system clock of 400 MHz, this means that the time resolution of the internal counter is 1 / (400 MHz) = 2.5 ns. With a PRN bit rate of 20 Gbit / s, the length of a single bit is 50 ps, which defines the time resolution of the PRN bit shift. Finally, the analog shift, which adjusts the received PRN sequence with a local replica to an expected accuracy of 1/50 of the bit length, enables a 1 ps resolution in the time stamp of the received data.

The main advantage: this technique can be used in one-way and two-way time transmission, and only optical transceivers are used and no dedicated fibers or a third comb (as in [1]). It also enables snapshot (no averaging over long periods of time) measurements.

An exemplary embodiment for the synchronization of the time bases of two objects is described below.

• - Sender erzeugt Daten- und Abstandssequenzen, die Chip-Phasen-justiert sind. Jede Datensequenz enthält einen Zeitstempel, der von dem Zähler erzeugt wird, welcher von einer lokalen Frequenzreferenz von Objekt 1 betrieben wird.
• - Das Signal wird optisch zu Objekt 2 übertragen.

For object 1 (e.g. satellite 1)

• - Transmitter generates data and distance sequences that are chip-phase-adjusted. Each data sequence contains a time stamp that is generated by the counter, which is operated by a local frequency reference of object 1.
• - The signal is transmitted optically to object 2.

1. 1. Der Empfänger verrastet sich auf die ankommende Abstandssequenz durch Maximieren der Korrelation zwischen der ankommenden Abstandssequenz und einer lokal erzeugten Abstandssequenz (wie z.B. in DE 10 2019 120 700 B4 beschrieben).
2. 2. Die lokal erzeugte Abstandssequenz wird mit einem lokalen Zähler zeitlich synchronisiert.
3. 3. Der erste Chip der lokalen Abstandssequenz wird einer Zeitstempelung unterzogen.
4. 4. Der Verrastungsprozess wird in zwei Stufen unterteilt, wobei zuerst die Sequenzen auf einem Chip-Level und zweitens auf einem Sub-Chip-Level justiert werden.
5. 5. Die erste Stufe justiert die empfangene und die lokal erzeugte Sequenz mit der Zeitgenauigkeit, die der Chipdauer entspricht.
6. 6. Die zweite Stufe justiert die empfangene und die lokal erzeugte Sequenz mit der Sub-Chip-Zeitgenauigkeit, die der Auflösung des Digital-Analog-Wandlers entspricht, der die analoge Delay-Line ansteuert, wie z.B. definiert in EP 3 751 308 A1 und DE 10 2019 120 700 B4 .
7. 7. Die Ankunftszeit des Signals wird durch Kombinieren der Verzögerungen der ersten und der zweiten Stufe und Verwenden derselben bei dem Zeitstempel relativ zu dem ersten Chip der lokalen Abstandssequenz bestimmt (siehe Schritt 3).
8. 8. Die Laufzeitinformation wird durch Subtrahieren der Ankunftszeit an Objekt 2 und der Aussendezeit an Objekt 1 berechnet.
9. 9. Die Laufzeitinformation wird über den Datenkanal optisch zu Objekt 1 zurückgesendet. Zu diesem Zweck erzeugt der Sender an Objekt 2 Daten- und Abstandssequenzen, die Chip-Phase-justiert sind. Jede Datensequenz enthält einen Zeitstempel, der von einem Zähler erzeugt wird, welcher von der lokalen Frequenzreferenz von Objekt 1 betrieben wird, und die gemessene Laufzeit (siehe Schritt 7).
10. 10. Das Signal wird optisch zu Objekt 1 übertragen.

For object 2 (e.g. satellite 2)

1. 1. The receiver locks onto the incoming distance sequence by maximizing the correlation between the incoming distance sequence and a locally generated distance sequence (such as in DE 10 2019 120 700 B4 described).
2. 2. The locally generated distance sequence is time-synchronized with a local counter.
3. 3. The first chip in the local distance sequence is time stamped.
4. 4. The locking process is divided into two stages, with the sequences being adjusted first on a chip level and second on a sub-chip level.
5. 5. The first stage adjusts the received and the locally generated sequence with the time accuracy that corresponds to the chip duration.
6. 6. The second stage adjusts the received and the locally generated sequence with the sub-chip timing accuracy that corresponds to the resolution of the digital-to-analog converter that controls the analog delay line, as defined in, for example EP 3 751 308 A1 and DE 10 2019 120 700 B4 .
7. 7. The time of arrival of the signal is determined by combining the first and second stage delays and using them in the time stamp relative to the first chip of the local distance sequence (see step 3).
8. 8. The transit time information is calculated by subtracting the arrival time at object 2 and the transmission time at object 1.
9. 9. The runtime information is optically sent back to object 1 via the data channel. For this purpose, the transmitter generates data and distance sequences on object 2, which are chip-phase-adjusted. Each data sequence contains a time stamp, which is generated by a counter, which is operated by the local frequency reference of object 1, and the measured transit time (see step 7).
10. 10. The signal is transmitted optically to object 1.

• - Wiederholen von Schritten 1 bis 8, die für Objekt 2 definiert sind.
• - Die Zeitübertragung wird durch Berechnen der halben Differenz zwischen den Informationen über die zwei Laufzeiten ermittelt, wobei die erste an Objekt 2 gemessen und über einen Datenkanal übertragen wird und die zweite an Objekt 1 gemessen wird (vorhergehender Schritt).
• - Sobald die erste Synchronisation erfolgt ist, ist der absolute Abstand zwischen den zwei Objekten bekannt und können nachfolgende Zeitübertragungen in einer Richtung mittels Schritten 1 bis 8 an einem der Objekte durchgeführt werden.

At object 1

• - Repeat steps 1 to 8 that are defined for object 2.
• The time transmission is determined by calculating half the difference between the information about the two transit times, the first being measured on object 2 and transmitted via a data channel and the second being measured on object 1 (previous step).
• As soon as the first synchronization has taken place, the absolute distance between the two objects is known and subsequent time transfers can be carried out in one direction using steps 1 to 8 on one of the objects.

Using a numerical example, it will be briefly explained below that it is possible to synchronize with an accuracy of less than 1 ps (picosecond) accuracy.

The running time is determined by converting the offset between the local PRN sequence and the received PRN sequence in a corresponding time interval and adding the current system time. In the case of a 20 Gcps (20 Gigachips per second) PRN sequence, the chip duration is 50 ps, which is also that of is the time resolution used by the mechanism to move symbol by symbol. To further increase the resolution, assuming a range of 70 ps, for example, an analog delay line that is controlled by a 10-bit DAC, for example, is introduced. This achieves a time resolution in the range of less than 1 ps (<1 ps range).

For the synchronization of the local time bases of two objects that are in communication with one another through an optical communication channel, it is important, as can be seen from the above, to know the distance between the two objects, among other things. This is because this calculates the time required for the transmission of an optical signal between the two objects.

A method that can advantageously be used within the scope of the invention for determining the distance between two objects, in particular two satellites connected by optical data communication, will be described below.

For various applications it may be necessary to measure the distance between two objects. One example is the distance measurement between two satellites in a satellite constellation with several satellites in a common orbit. A satellite constellation in which the distance between neighboring satellites can be important is, for example, in WO 2019/01191 A1 described. Experts also speak of “optical ranging” when determining the distance using an optical signal.

For geodesy applications in particular, it is sometimes necessary to determine the distance between two satellites with millimeter precision. Satellite signals are optical signals that are provided with a code. Satellite signals are typically so-called PRN codes, that is, sequences of symbols, where a symbol can have a duration of only 50 ps.

There is a requirement to be able to determine the distance from satellites, for example, with accuracies in the sub-millimeter range.

A possible laboratory setup for testing an optical inter-satellite link for bi-directional transmission of time and data and for determining the distance between the two satellites is described in [5]. First, the various possible scenarios between a LEO (Low Earth Orbit) and an MEO (Medium Earth Orbit) are described and the resulting optomechanical requirements are explained. An input laser and a reference laser are synchronized with an optical phase-locked loop for communication. A delay locked loop (DLL) is used to correlate the input signal with the reference signal. The results of a simplified system test for distance measurement are described. Finally, the opto-mechanical design is explained in more detail and hardware developments are briefly shown.

• - ein erstes Objekt von einem zweiten Objekt ein mittels eines Codes moduliertes optisches Signal mit einer bekannten Trägerfrequenz als Empfangssignal empfängt, wobei der Modulationscode eine Sequenz von Symbolen aufweist, die jeweils gleich lange Intervalle einnehmen, und
• - in dem ersten Objekt
  • - ein optisches Referenzsignal erzeugt wird, das als ein optisches Trägersignal mit der gleichen Frequenz wie diejenige des Empfangssignals und mit einer aufmodulierten, der Symbolsequenz des Empfangssignals gleichenden Symbolsequenz ausgebildet ist,
  • - die Korrelation des Empfangssignals mit dem Referenzsignal ermittelt wird,
  • - ein den Grad der Korrelation beschreibender Korrelationswert als ein elektrisches Signal erzeugt wird, das eine niedrigere Frequenz als das Empfangssignal aufweist,
  • - die Symbolsequenz für das Referenzsignal mittels eines optischen Modulators zeitlich verschoben auf das Trägersignal moduliert wird, wobei die zeitliche Verschiebung anhand der Größe des Korrelationswerts bestimmt wird, so dass das Referenzsignal mit dem Empfangssignal in Phase gelangt,
  • - eine Verschiebung der Symbolsequenz des Referenzsignals um eine Zeitdauer, die gleich dem Intervall eines Symbols oder gleich einem ganzzahligen Vielfachen des Intervalls ist, durch entsprechende zeitverzögerte Modulation des Trägersignals mittels eines die Symbolsequenz des Referenzsignals aufweisenden, elektrischen Digitalsignals erfolgt,
  • - eine Verschiebung der Symbolsequenz des Referenzsignals um eine Zeitdauer, die kleiner ist als das Intervall eines Symbols, durch Ansteuerung einer dem Modulator vorgeschalteten Analogverschiebung erfolgt, welche das die Symbolsequenz des Referenzsignals aufweisende, elektrische Digitalsignal empfängt und dieses Digitalsignal um die durch ein elektrisches Zeitverzögerungssteuersignal für die Analogverschiebung bestimmte Verzögerungszeit zeitverzögert an den Modulator weitergibt, und
  • - der Grad der zeitlichen Verschiebung, mit der die Symbolsequenz für das Referenzsignal auf das Trägersignal moduliert wird, den Abstand beider Objekte repräsentiert und/oder der Abstand beider Objekte anhand des Grads der zeitlichen Verschiebung ermittelt wird.

The exemplary method provides that

• a first object receives from a second object an optical signal modulated by means of a code with a known carrier frequency as a received signal, the modulation code having a sequence of symbols which each occupy equally long intervals, and
• - in the first object
  • - An optical reference signal is generated, which is designed as an optical carrier signal with the same frequency as that of the received signal and with a symbol sequence modulated on that is the same as the symbol sequence of the received signal,
  • - the correlation of the received signal with the reference signal is determined,
  • - A correlation value describing the degree of correlation is generated as an electrical signal which has a lower frequency than the received signal,
  • the symbol sequence for the reference signal is modulated onto the carrier signal with a time shift by means of an optical modulator, the time shift being determined on the basis of the magnitude of the correlation value, so that the reference signal comes into phase with the received signal,
  • - the symbol sequence of the reference signal is shifted by a period of time that is equal to the interval of a symbol or equal to an integral multiple of the interval by means of a corresponding time-delayed modulation of the carrier signal by means of an electrical digital signal having the symbol sequence of the reference signal,
  • - The symbol sequence of the reference signal is shifted by a period of time that is shorter than the interval of a symbol by controlling an analog shift connected upstream of the modulator, which receives the electrical digital signal having the symbol sequence of the reference signal and this digital signal by the amount determined by an electrical time delay control signal for the analog shift forwards a certain delay time to the modulator with a time delay, and
  • the degree of the time shift with which the symbol sequence for the reference signal is modulated onto the carrier signal, represents the distance between the two objects and / or the distance between the two objects is determined on the basis of the degree of the time shift.

This concept is based on initially known elements. For example, an optical signal which is sent from the second object to the first object and has a modulation code with a sequence of symbols is used to determine the distance. This modulation code is in particular a PRN code, that is to say a sequence of PRN symbols. In order to be able to extract the optical received signal from the noise of the receiver of the first object, an optical reference signal generated in the first object is correlated with the optical received signal. As an example of a correlator that can be used for this purpose, reference is made to an optical coherence receiver.

The correlation converts the extremely high-frequency optical signal, the frequency of which is in the tera-Hertz range, into an electrical signal that is shifted to a frequency range that is at least three orders of magnitude lower. This means that further signal processing can now be implemented with conventional hardware and software.

The optical reference signal, which is generated in the first object by means of a laser, for example, has an optical carrier signal which has the same frequency as the optical received signal. The same sequence of symbols as in the received signal, that is to say the modulation code of the received signal, is then modulated onto this carrier signal. The symbol sequence is shifted in phase relative to the carrier signal until one recognizes from the correlation value that the reference signal is in phase with the received signal or at least comes very close to this state.

The shift of the symbol sequence for the reference signal, with which the symbol sequence is modulated onto the carrier signal, can in principle only take place symbol-by-symbol. The minimum possible phase shift is thus limited to the duration of a symbol interval. However, this can be too imprecise for certain applications. For example, due to a phase shift in the order of magnitude of a symbol interval of the modulation code of the received signal, one could already be too large, with which one would no longer obtain a usable correlation signal or a usable correlation value.

In this method it is therefore proposed, in addition to the comparatively already large-resolution shift of the symbol sequence of the reference signal by an interval of a symbol or by a multiple of an interval of a symbol, also to enable a very fine shift of the symbol sequence. This very fine shift is in the sub-symbol area and its smallest shift can be, for example, less than 1/10, for example less than 1/20 of the duration of a symbol interval, i.e. less than, for example, 2.5 ps. The resolution is even in the femtosecond range and depends, among other things, on the resolution of the DAC, since the delay line works continuously. This means that with a 12-bit DAC you can achieve a resolution of up to 20 femtoseconds, which corresponds to a symbol component of 1/2500. Instead of an analog delay line, a digitally controllable analog shift can also be used, which would eliminate the DAC as a dedicated element. The very high resolution is achieved according to the invention by a controlled time delay of the symbol sequence of the reference signal in that it passes an analog shift as a digital signal, i.e. as a binary signal in the analog or digital domain, which is connected upstream of the modulator for the carrier signal. The analog shift, in turn, is controlled by means of a likewise digital electrical time delay control signal, which is optionally subjected to a digital-to-analog conversion in order to then control the analog shift as an electrical analog signal or as an electrical digital signal.

A digital signal processor, an FPGA or another type of signal processing unit is typically used for signal processing, the input variable of which is an electrical analog signal describing the correlation value and which outputs two electrical digital signals, one of which describes the symbol sequence for the reference signal and the other ensures the sub-symbol shifting of the symbol sequence for the reference signal, the electrical digital signal shifted in time in this way the symbol sequence is given to the modulator, in which the symbol sequence is then modulated onto the optical carrier signal.

With the concept, the optical received signal can be tracked much more precisely. In addition, by applying a known small "disturbance" (one also speaks of "dither" in English) to the electrical digital time delay control signal, ie to the fine displacement signal (the direction of the displacement - namely delaying or leading - the symbol sequence is known) Deliberately influencing the correlation, and because of the extremely high resolution to such a small extent that the tracking continues to work. On the basis of the change in the correlation value, i.e. if one evaluates the correlation and sets it in relation to the known interference signal, one can find out which branch (left or right of the correlation peak) of the correlation function one is on. This procedure is much more difficult and complicated to implement if one proceeds accordingly by shifting the symbol sequence for the reference signal symbol by symbol when modulating the symbol sequence onto the optical carrier signal. This advantageously makes it possible to additionally shift the symbol sequence for the reference signal in the time division multiplex method in order to emulate an early-middle-late algorithm. In this way one can react to frequency shifts of the optical received signal and to phase shifts of its code (symbol sequence) and thus guarantee the tracking of the received signal more reliably when the distance between the two objects changes.

• - dass die Symbolsequenz für das Referenzsignal zur Reduktion der Gefahr des Verlustes der Nachverfolgung des Empfangssignals durch das Trägersignal mit aufmoduliertem Referenzsignal bei einer Veränderung der Frequenz und/oder Phase des Empfangssignals zu höheren Werten in Folge einer Verringerung des Abstandes der beiden Objekte und zu niedrigeren Werten in Folge einer Vergrößerung des Abstandes der beiden Objekte durch Modulation mittels eines Zeitmultiplex-Signals um jeweils vorbestimmte Ausmaße d.h. Zeit-Offsets zwecks Emulation einer Frequenzänderung nacheilend oder vorauseilend verschoben wird, ohne dass der Korrelationswert einen vorbestimmten, einen Mindestgrad an Korrelation definierenden Schwellwert unterschreitet, wobei der Korrelationswert von einem ersten Wert, der ein Nacheilen der Symbolsequenz für das Referenzsignal gegenüber derjenigen des Empfangssignals repräsentiert (und damit sozusagen eine mit einer Verringerung des Abstandes der beiden Objekte einhergehenden Vergrößerung der Frequenz des Empfangssignals emuliert), auf einen Optimalwert, der im Wesentlichen die Synchronisation der Symbolsequenzen für das Referenzsignal und derjenigen des Empfangssignals repräsentiert (und damit sozusagen einen im Wesentlichen gleichbleibenden Abstand beider Objekte emuliert), und ferner von diesem Optimalwert auf einen zweiten Korrelationswert, der ein Vorauseilen der Symbolsequenz für das Referenzsignal gegenüber derjenigen des Empfangssignals repräsentiert (und mit anderen Worten damit eine mit einer Vergrößerung des Abstandes der beiden Objekte einhergehenden Verringerung der Frequenz des Empfangssignals emuliert) wechselt und zwischen diesen Werten umgekehrt oder zyklisch wechselt und
• - wobei im Falle einer Abstandsänderung der beiden Objekte die dann erfolgende Frequenz- und/oder Phasenverschiebung zu entweder einer höheren Frequenz oder einer niedrigeren Frequenz des Empfangssignals durch Wahl eines Nacheilens oder Vorauseilens der Symbolsequenz für das Referenzsignal gegenüber derjenigen des Empfangssignals kompensiert wird (und damit bei einer Vergrößerung oder Verringerung des Abstandes die Nachverfolgung des Empfangssignals vereinfacht ist und zuverlässiger erfolgt).

For this purpose, it is provided according to a further development of the invention,

• - That the symbol sequence for the reference signal to reduce the risk of losing tracking of the received signal by the carrier signal with modulated reference signal in the event of a change in the frequency and / or phase of the received signal to higher values as a result of a reduction in the distance between the two objects and to lower values as a result of an increase in the distance between the two objects by modulation by means of a time-division multiplex signal by respectively predetermined amounts, i.e. time offsets for the purpose of emulating a frequency change are shifted lagging or leading, without the correlation value falling below a predetermined threshold value defining a minimum degree of correlation, whereby the correlation value of a first value which represents a lag of the symbol sequence for the reference signal compared to that of the received signal (and thus, so to speak, a magnification associated with a reduction in the distance between the two objects emulation of the frequency of the received signal), to an optimum value, which essentially represents the synchronization of the symbol sequences for the reference signal and that of the received signal (and thus, so to speak, emulates an essentially constant distance between the two objects), and further from this optimum value to a second correlation value , which represents an advance of the symbol sequence for the reference signal compared to that of the received signal (and in other words emulates a reduction in the frequency of the received signal associated with an increase in the distance between the two objects) and alternates between these values reversely or cyclically and
• - In the case of a change in the distance between the two objects, the then occurring frequency and / or phase shift to either a higher frequency or a lower frequency of the received signal is compensated for by selecting a lagging or leading of the symbol sequence for the reference signal compared to that of the received signal (and thus at an increase or decrease in the distance, the tracking of the received signal is simplified and takes place more reliably).

This additional modulation of the finest displacement signal, i.e. the electrical time delay signal, takes place in the time division multiplex process, i.e. in the time domain and thus one after the other. In principle, the early-middle-late method, known per se, is used here for tracking navigation satellite carrier signals that shift in terms of frequency and phase of modulated codes due to changes in the distance between the satellite and the receiver. However, the invention advantageously only requires a single optical receiver (or, in other words, a single optical receiving channel), while the hardware implementations of the early-middle-late method are used in the RF range, as is the case with navigation satellite systems , requires three receivers or three receiving channels, which has a detrimental effect on the power that the received signal must have in order to be sufficiently large after division into the three receiving channels (with the electrical circuits assigned to them such as the correlators) . This restriction does not apply to the approach according to the invention of emulating the early-middle-late method, which is a significant advantage over the realizations of this method known in the prior art.

In an expedient development, it can be provided that the time delay control signal is an electrical digital signal that is converted into an electrical analog signal in a digital-to-analog converter, the electrical analog signal controlling the analog shift.

As already mentioned above, the received signal from the first object is received by a coherent receiver (optical coherence receiver), which carries out the correlation and a conversion of the correlation into an electrical analog signal, which is used for further processing and in particular for the formation of the correlation value and for Generation of the shift of the symbol sequence of the reference signal relative to the phase of the carrier signal is used in a digital signal.

A preferred field of application relates to the case that the two objects are two satellites and / or that the modulation code has a PRN sequence of symbols or a gold code. Basically, the modulation code can be chosen as desired.

As already described above, the two previously mentioned satellites can be part of a satellite constellation in which several satellites move on a common orbit, the orbit in particular being a MEO orbit located at a medium distance from Earth , and wherein the two satellites are adjacent to each other in orbit. Alternatively, the two satellites mentioned above can be part of a satellite constellation in which at least one first satellite is on a first orbit, for example a MEO orbit located at a medium distance from Earth, and at least one second satellite is on a different from the first orbit second orbit, e.g. a LEO orbit located a short distance from Earth.

• - Die Korrelation von Referenzsignal und Empfangssignal findet in der optischen Domäne statt, die anders als die elektrische Domäne im Wesentlichen nicht bandbreitenlimitiert ist.
• - Die optische Korrelation reduziert die Bandbreitenanforderungen an den Empfänger und sämtliche diesem nachgeschalteten Komponenten. Somit kann eine höhere Auflösung erreicht werden, ohne die Komplexität des Empfängers zu vergrößern. Es erfolgt eine zweistufige Referenzsignalerzeugung, und zwar
  • - eine symbolweise Verschiebung der digitalen Symbolsequenz (Grobverschiebung bzw. diskontinuierliche Verschiebung) und
  • - Erzeugung eines Signals zur quasi analogen (auf die Zeitdomäne bezogen), d. h. quasi kontinuierlichen Feinstverschiebung der Symbolsequenz vor deren Aufmodulation auf das optischen Trägersignal.
• - Die in der elektrischen Domäne vorgenommene Verschiebung verändert nicht die optische Phase des Signals, was andernfalls den optischen PLL beeinträchtigen würde.
• - Das elektrische Signal für die Sub-Symbolintervall-Verschiebung der Phase der Symbolsequenz erhöht die maximal mögliche Auflösung bei der Entfernungsmessung in Schritten von weniger als einer Pikosekunde.
• - Die phasenmodulierte Datenübertragung über den optischen Kanal lässt Datenraten von mindestens 50 Mbps zu.
• - Das elektrische Signal für die Feinstverschiebung erlaubt die Modulation eines Zeitmultiplex-Signals (Time Division Multiplexed Signal) zur Vermeidung der Verwendung von drei Empfängern (für early, middle und late bei der Korrelation), was von Hochfrequenzempfängern an sich bekannt ist, aber sehr komplex bei optischen Datenübertragungssystemen ist. Nach der Erfindung bedarf es lediglich eines einzigen in diesen Fall optischen Empfängers mit einem einzigen Empfangskanal, so dass die Leistung des Empfangssignals den wegen Aufteilung auf mehrere Empfangskanäle gegebenen Einschränkungen an eine bestimmte Mindestgröße nicht unterliegt.
• - Eine hochauflösende Drift-Interpolation des Referenzsignals ist mit dem elektrischen Signal für die Sub-Symbolintervall-Verschiebung möglich. Anders ausgedrückt wird also bei dem Verfahren zur Ermittlung des Abstands zweier Objekte, insbesondere zweier Satelliten, unter Verwendung eines optischen Signals, das von dem einen Objekt an das andere Objekt gesendet wird, ein Referenzlaser mittels einer optischen Phasenregelschleife auf den Takt des optischen Eingangssignals synchronisiert. Eine Verzögerungsregelschleife wird verwendet, um das optische Eingangssignal mit dem auf Basis des Referenzlasers erzeugten Referenzsignals zu korrelieren. Dabei wird eine Grobjustage der Symbolsequenz im Referenzsignal durch symbolweises Verschieben dieser Sequenz realisiert. Vorzugsweise wird nun zusätzlich zu dieser Grobverschiebung eine Feinstverschiebung vorgenommen, indem ein Subsymbol-Verschiebungssignal (oben mit Zeitverzögerungssteuersignal bezeichnet) generiert wird, das für eine Verschiebung der Symbolsequenz um einen Bruchteil des Intervalls eines Symbols sorgt.

The essential features for the exemplary method for determining distance presented here are as follows:

• The correlation of the reference signal and the received signal takes place in the optical domain, which, unlike the electrical domain, is essentially not bandwidth-limited.
• - The optical correlation reduces the bandwidth requirements on the receiver and all of the downstream components. Thus, a higher resolution can be achieved without increasing the complexity of the receiver. A two-stage reference signal generation takes place, namely
  • - a symbol-by-symbol shift of the digital symbol sequence (coarse shift or discontinuous shift) and
  • Generation of a signal for the quasi-analog (based on the time domain), ie quasi-continuous, very fine shifting of the symbol sequence before it is modulated onto the optical carrier signal.
• The shift made in the electrical domain does not change the optical phase of the signal, which would otherwise affect the optical PLL.
• The electrical signal for the sub-symbol interval shift of the phase of the symbol sequence increases the maximum possible resolution for the distance measurement in steps of less than one picosecond.
• - The phase-modulated data transmission over the optical channel allows data rates of at least 50 Mbps.
• - The electrical signal for the fine displacement allows the modulation of a time division multiplex signal (Time Division Multiplexed Signal) to avoid the use of three receivers (for early, middle and late in the correlation), which is known from high-frequency receivers, but is very complex in optical data transmission systems. According to the invention, only a single, in this case optical, receiver with a single receiving channel is required, so that the power of the received signal is not subject to the restrictions of a certain minimum size given due to the division into several receiving channels.
• - A high-resolution drift interpolation of the reference signal is possible with the electrical signal for the sub-symbol interval shift. In other words, in the method for determining the distance between two objects, in particular two satellites, using an optical signal that is sent from one object to the other object, a reference laser is synchronized to the clock of the optical input signal by means of an optical phase-locked loop. A delay locked loop is used to correlate the optical input signal with the reference signal generated on the basis of the reference laser. A rough adjustment of the symbol sequence in Reference signal realized by shifting this sequence symbol by symbol. In addition to this coarse shift, a very fine shift is preferably carried out by generating a sub-symbol shift signal (referred to above as time delay control signal), which shifts the symbol sequence by a fraction of the interval of a symbol.

The method for determining distance can basically be used in all areas in which distance measurement by means of lasers is used. In particular, the method can be used for satellite navigation (GNSS), for geodesy and in optical distance measurement (see for example in [2] and [3]).

For the distance measurement, the optical (received) signal received from a first satellite, which is sent to the first satellite by a second satellite (possibly at the instigation of an optical signal previously sent from the first satellite to the second satellite), is processed and evaluated .

First of all, the first satellite must receive the received signal from the second satellite 16 extract from the noise. According to the in 6th shown example of a processing unit 14th this takes place in a manner known per se by forming the correlation of an optical reference signal generated in the first satellite 18th with the received signal 16 . Both signals have the same carrier frequency (typically in the THz range) and are modulated in the same way. The modulation is (also typically) a sequence of symbols (eg PRN sequence). By phase shifting the symbol sequence, which is required for modulating the reference signal, one looks for the maximum of the correlation function (see also the 7th and 8th ). An essential feature of the invention is the particularly high-resolution time shift of the symbol sequence with which it is applied to the carrier signal 20th , which after modulation with the symbol sequence is the reference signal 18th forms, is modulated.

According to 6th , in which optical signals or signal paths are identified by means of dashed lines and electrical signals or signal paths are identified by solid lines, becomes the optical carrier signal 20th by means of a laser 22nd generated. The laser 22nd is electrical through an optical PLL 24 controlled by a coherence receiver 26th the I and Q output signals 28 , 30th receives, which are also electrical signals and have a much lower frequency (typically in the MHz range). The two output signals 28 , 30th are analog signals and are converted into an analog-to-digital converter (ADC) 32 into an electrical digital signal, ie into a binary signal 34 implemented in the continuous time domain (analog domain). Depending on the current value of the correlation (correlation value), the optical carrier signal 20th out of phase and in an optical modulator 36 modulated. The control of the modulator 36 is described below.

E.g. a digital signal processor 38 (Controller) receives the signal 34 and a detector 40 evaluates the correlation function. After filtering by, for example, a loop filter 42 takes place in the unit 44 the generation of two digital signals 46 , 48 that are used to control the phase shift with which the symbol sequence of the reference signal 18th on its optical carrier signal 20th is optically modulated offset in time. This time shift represents the distance between the two satellites. The unit 44 outputs a corresponding signal.

The electrical digital signal 46 depicts the symbol sequence. This digital signal 46 is delayed accordingly either by the block 44 output or the block 44 controls a symbol sequence generator, which then generates the digital signal 46 issues. The output of the digital signal 46 takes place with a corresponding time delay, but this can only take place with a comparatively coarse resolution, since here there can only be shifts with a smallest time unit, which is equal to the duration of an interval of a symbol.

If, however, as provided according to the invention, one also wants to be able to shift in the sub-symbol interval range, then according to the invention the second electrical digital signal is 48 generated with which the transmission of the first digital signal 46 on the optical modulator 36 can be delayed. The second digital signal is used for this 48 in a digital-to-analog converter 50 converted into an electrical analog signal with which an analog shift 52 which controls the first digital signal 46 delayed by the time specified by the aforementioned analog signal and to the modulator 36 passes on. This process can also take place in the digital domain by adding the analog shift time delay element 52 is controlled digitally, the digital-to-analog converter 50 so can be omitted.

With this approach it is now possible that the symbol sequence for the reference signal 18th representing first digital signal 46 in the sub icon area as it is in 7th is shown. In 8th the correlation curve shows that this fine adjustment in the sub-symbol interval range means that the correlation value remains in the range of the correlation peak, i.e. the tracking can be carried out reliably even with the smallest changes in distance between the two satellites, which is not possible with a phase shift by symbol intervals as the smallest time unit. The time shift of the symbol sequence for the reference signal 18th on the carrier signal 20th is modulated, can thus be selected and set in such a way that the correlation value of the correlation function is maximized and / or is and / or remains.

It can be assumed that the distance between two MEO satellites, for example, can be determined in the sub-millimeter range and a change in distance can be tracked with high accuracy.

• - Der Kohärenz-Empfänger empfängt eine PRN-Sequenz und Daten von weit entfernt und untersucht die Interferenz mit dem lokal erzeugten Referenzsignal in der optischen Domäne. Die Schleife ist eingerastet bei konstruktiver Interferenz durch Nachführung des Korrelationsmaximums.
• - Die Ausgänge des Kohärenz-Empfängers werden verwendet, um den Restphasenfehler zwischen dem Eingangssignal und dem Referenzsignal zu berechnen. Dieser Phasenfehler wird der optischen PLL zugeführt, um den Laser einzurasten.
• - Die Ausgangssignale des Kohärenz-Empfängers werden zur Wiedergewinnung der Daten abgetastet. Der ADC braucht lediglich so schnell zu sein, dass die Datengewinnung korrekt erfolgt. Er braucht nicht das gesamte Spektrum der PRN-Sequenz abzudecken.
• - Der Detektor demultiplext digital das Datensignal (Phase des Signals) und den Korrelationswert (Amplitude des Signals). Die Veränderungsrate des Korrelationssignals (ansteigend, abfallend oder 100%ige Korrelation) wird bestimmt entweder durch
  • - einen early-late-Algorithmus gemäß z.B. [4], der durch Hinzufügung eines Zeit-Offset zum Signalgenerator emuliert wird (time Division Multiplexed Time Offset) oder
  • - Schätzung der Dopplerzeitverschiebung aus der empfangenen Datenphase, da die Dopplerverschiebung (Phase) proportional ist zur Dopplerzeitverschiebung (siehe z.B. in [4]).
• - Zur Stabilisierung der „closed loop“-Antwort des DLL (Delay Locked Loop) dient ein Loop-Filter.
• - Der Signalgenerator erzeugt das die Referenz-PRN-Sequenz repräsentierende Signal. Die aktuelle Phase der erzeugten PRN-Sequenz (Maximierung der Korrelation durch die DLL) repräsentiert die Entfernungsinformation. Die PRN-Sequenz-Erzeugung wird durchgeführt in zwei Schritten:
  • - digitale Erzeugung einer binären PRN-Sequenz (Grobauflösung)
  • - Erzeugung eines analogen Signals zur Verschiebung der PRN-Sequenz im Subsymbolintervallbereich (Feinstverschiebung)
• - Das Signal für die Grobverschiebung (Digital Coarse PNN Sequence) passiert eine Analogverschiebung, dessen Zeitverzögerungsgröße gesteuert wird durch das analoge Signal für die Feinstverschiebung. Dies resultiert in einem elektrischen Signal, das die Auflösung im Zeitbereich wie das analoge Feinstverschiebungssignal hat. Der mit diesem Signal erzielbare Bereich der Zeitverschiebung ist größer als das Intervall eines Symbols, wodurch ein kontinuierliches Verschieben der Phase der PRN-Sequenz für das Referenzsignal erzielt werden kann. Drei verschiedene verschobene Signale sind in 7 gezeigt (siehe oben). Das Ergebnis der Korrelationsfunktion bei Verschiebung entsprechend den drei in 6 gezeigten Zuständen „zu früh“, „Mitte“ und „zu spät“ ist in 8 gezeigt.
• - Das elektrische Signal wird phasenmoduliert auf das optische Signal des Lasers (optisches Trägerfrequenzsignal) moduliert und bildet das optische Referenzsignal.

The exemplary embodiment shows that the method for determining the distance is characterized in particular by one or more of the following features:

• - The coherence receiver receives a PRN sequence and data from far away and examines the interference with the locally generated reference signal in the optical domain. The loop is locked in the event of constructive interference by tracking the correlation maximum.
• - The outputs of the coherence receiver are used to calculate the residual phase error between the input signal and the reference signal. This phase error is fed to the optical PLL to lock the laser into place.
• - The output signals of the coherence receiver are sampled to recover the data. The ADC only needs to be fast enough for the data to be obtained correctly. It does not need to cover the entire spectrum of the PRN sequence.
• - The detector digitally demultiplexes the data signal (phase of the signal) and the correlation value (amplitude of the signal). The rate of change of the correlation signal (rising, falling or 100% correlation) is determined by either
  • - an early-late algorithm according to, for example, [4], which is emulated by adding a time offset to the signal generator (time division multiplexed time offset) or
  • - Estimation of the Doppler time shift from the received data phase, since the Doppler shift (phase) is proportional to the Doppler time shift (see eg in [4]).
• - A loop filter is used to stabilize the "closed loop" response of the DLL (Delay Locked Loop).
• - The signal generator generates the signal representing the reference PRN sequence. The current phase of the generated PRN sequence (maximizing the correlation by the DLL) represents the distance information. The PRN sequence generation is carried out in two steps:
  • - digital generation of a binary PRN sequence (coarse resolution)
  • - Generation of an analog signal to shift the PRN sequence in the sub-symbol interval range (fine shift)
• - The signal for the coarse shift (Digital Coarse PNN Sequence) passes an analog shift, the time delay value of which is controlled by the analog signal for the fine shift. This results in an electrical signal that has the same resolution in the time domain as the analog fine displacement signal. The range of time shift that can be achieved with this signal is greater than the interval of a symbol, as a result of which a continuous shift of the phase of the PRN sequence for the reference signal can be achieved. Three different shifted signals are in 7th shown (see above). The result of the correlation function when shifted according to the three in 6th The states "too early", "middle" and "too late" shown are in 8th shown.
• - The electrical signal is phase-modulated on the optical signal of the laser (optical carrier frequency signal) and forms the optical reference signal.

List of reference symbols

10

MEO satellites

12th

LEO satellites

14th

Processing unit

16

optical received signal

18th

optical reference signal

20th

optical carrier signal

22nd

laser

24

optical PLL

26th

Coherence receiver

28

electrical output signal

30th

electrical output signal

32

Analog-to-digital converter

34

electrical digital signal

36

optical modulator

38

Digital signal processor

40

detector

42

Loop filter

44

Signal generation unit

46

electrical digital signal

48

electrical digital signal (time delay control signal or sub-symbol shift signal)

50

Digital-to-analog converter

52

Analog shift

60

other object

68

terminal

70

object

72

Communication channel

74

terminal

76

Time base

78

counter

80

PRN sequence generator

82

Correlator

83

Transmission signal

84

Bit shifter

86

PRN sequence generator

88

Reference signal

90

Analog shift

92

Timestamp generator

List of Abbreviations

GNSS Global Navigation Satellite System NTP network time protocol OTWTT optical two way time transfer OWTT one-way time transfer PTP precise time protocol TWTT two way time transfer

bibliography

• [1] Deschênes, J.D., Sinclair, L.C., Giorgetta, F.R., Swann, W.C., Baumann, E., Bergeron, H., Cermak, M., Coddington, I., Newbury, N.R., Synchronization of Distant Optical Clocks at the Femtosecond Level. Physical Review X, 6, 021016, 2016. DOI: 10.1103/PhysRevX.6.021016 ,
• [2] Sheard, B., Heinzel, G., Danzmann, K., Shaddock, D., Klipstein, W. and Folkner, W. 2012. Intersatellite laser ranging instrument for the GRACE follow-on mission. Journal ofGeodesy. 86, 12 (2012), 1083-1095 .
• [3] Sutton, A., McKenzie, K., Ware, B. and Shaddock, D.A. 2010. Laser ranging and communications for LISA. Optics express. 18, 20 (2010), 20759-20773 .
• [4] Kaplan, E. and Hegarty, C. 2005. Understanding GPS: principles and applications. Artech house .
• [5] Poliak, J., Calvo, R.M., Surof, J., Richerzhagen and Mathias, R. Wolf 2018. Laboratory Demonstrator of Optical Inter-satellite Links for the Kepler System .
• [6] MITCHELL, G.: High-accuracy ranging using spread-spectrum technology. In: Proceedings of the AIAA/USU Conference on Small Satellites. 2001, SSC01-VI-2, S. 1-11 .
• [7] KIRCHNER, D.: Two-way time transfer via communication satellites. In: Proceedings of the IEEE, Vol., 79, 1991, No. 7, S. 938-990. - ISSN 1558-2256 .

https://www.nobelprize.org/prizes/physics/2005/summary/
https://endruntechnologies.com/pdf/PTP-1588.pdf
WO 2019/011919 A1
US 2003/0048430 A1
DE 10 2014 111 589 A1
DE 10 2019 120 700 B4
EP 3 751 308 A1

• [1] Deschênes, JD, Sinclair, LC, Giorgetta, FR, Swann, WC, Baumann, E., Bergeron, H., Cermak, M., Coddington, I., Newbury, NR, Synchronization of Distant Optical Clocks at the Femtosecond Level. Physical Review X, 6, 021016, 2016. DOI: 10.1103 / PhysRevX.6.021016 ,
• [2] Sheard, B., Heinzel, G., Danzmann, K., Shaddock, D., Klipstein, W. and Folkner, W. 2012. Intersatellite laser ranging instrument for the GRACE follow-on mission. Journal of Geodesy. 86, 12 (2012), 1083-1095 .
• [3] Sutton, A., McKenzie, K., Ware, B. and Shaddock, DA 2010. Laser ranging and communications for LISA. Optics express. 18, 20 (2010), 20759-20773 .
• [4] Kaplan, E. and Hegarty, C. 2005. Understanding GPS: principles and applications. Artech house .
• [5] Poliak, J., Calvo, RM, Surof, J., Richerzhagen and Mathias, R. Wolf 2018. Laboratory Demonstrator of Optical Inter-satellite Links for the Kepler System .
• [6] MITCHELL, G .: High-accuracy ranging using spread-spectrum technology. In: Proceedings of the AIAA / USU Conference on Small Satellites. 2001, SSC01-VI-2, pp. 1-11 .
• [7] KIRCHNER, D .: Two-way time transfer via communication satellites. In: Proceedings of the IEEE, Vol., 79, 1991, No. 7, pp. 938-990. - ISSN 1558-2256 .

https://www.nobelprize.org/prizes/physics/2005/summary/
https://endruntechnologies.com/pdf/PTP-1588.pdf
WO 2019/011919 A1
US 2003/0048430 A1
DE 10 2014 111 589 A1
DE 10 2019 120 700 B4
EP 3 751 308 A1

Claims (21)

Method for determining the time difference between the transmission time at which a modulated, optical frequency signal having a transmission time stamp is transmitted from a second object which has a local time base, in particular from a satellite, and the reception time at which the optical frequency signal of the second Object is received by a first object, which has a local time base, in particular from a satellite, to synchronize the time bases of both objects by means of a one-way or two-way time transmission method known per se, with the method for determining the time difference - a first object receives from a second object an optical signal modulated by means of a code with a known carrier frequency as an optical first received signal (16), the modulation code having a sequence of symbols which each occupy equally long intervals, and the optical signal (16 ) e has a first sending time stamp, which indicates at what point in time the second object, based on its time base, sent the optical signal to the first object, - in the first object - an optical reference signal (18) is generated, which is an optical carrier signal (20) is formed with the same frequency as that of the first received signal (16) and with a modulated symbol sequence which is the same as the symbol sequence of the first received signal (16), - in a correlator (26) the correlation of the first received signal (16) with the Reference signal (18) is determined, - to maximize the degree of correlation in a bit shifter (24) a shift of the symbol sequence of the reference signal (18) by a symbol duration equal to the interval of a symbol or equal to an integral multiple of the interval is carried out, - to further maximize the degree of correlation, a shift of the symbol sequence of the reference signal (18) by one Partial symbol duration, which is smaller than the interval of a symbol, is carried out by controlling a time delay element (28) connected upstream of the correlator (26) and controlled by it, and - the time of receipt of the optical signal received by the first object and emitted by the second object is determined based on the time stamp of the temporally first symbol of the symbol sequence of the reference signal (18) and the symbol part-time duration assigned on the basis of the local time base of the first object, namely at the shifting of the symbol sequence of the reference signal (18) by the one or more symbol time periods and by the symbol part-time period which is effected by maximizing the degree of correlation. Procedure according to Claim 1 , characterized in that a correlation value describing the degree of correlation is generated as an electrical signal which has a lower frequency than the first received signal (16). Procedure according to Claim 1 or 2 , characterized in that the symbol sequence for the reference signal (18) is modulated on the carrier signal (20) with a time shift by means of an optical modulator (36), the time shift being determined on the basis of the magnitude of the correlation value, so that the reference signal (18) gets in phase with the first received signal (16). Procedure according to Claim 3 , characterized in that the symbol sequence of the reference signal (18) is shifted by one or more symbol periods by corresponding time-delayed modulation of the carrier signal (20) by means of an electrical digital signal (46) having the symbol sequence of the reference signal (18). Procedure according to Claim 4 , characterized in that the time delay element (28) which has the symbol sequence of the reference signal (18) receives electrical digital signal (46) and this digital signal (46) by the delay time determined by an electrical time delay control signal (48) for the time delay element (52) passes on with a time delay to the modulator (36). Procedure according to Claim 5 , characterized in that the time delay control signal (48) is an electrical digital signal which is converted in a digital-to-analog converter (50) into an electrical analog signal, wherein the electrical analog signal controls the time delay element (52), or that the time delay control signal (48 ) is an electrical digital signal and the time delay element (52) is a digital time delay element which is controlled by the digital signal. Method according to one of the Claims 1 until 6th , characterized in that the first received signal (16) is received from the first object by a coherent receiver (26) which forms the correlator (26) and carries out the correlation and a conversion of the correlation into an electrical analog signal which is used for further processing and in particular to form the correlation value and to generate the time shift of the symbol sequence for the reference signal (18) is converted into an electrical digital signal (34). Method according to one of the Claims 1 until 7th , characterized in that the time bases of the two objects are synchronized by means of a one-way or two-way time transmission method. Method according to one of the Claims 1 until 8th , characterized in that - the first object (70), based on its time base, sends the received signal provided with a second sending time stamp to the second object as a second received signal and - in the second object (60) - an optical reference signal is generated that is designed as an optical carrier signal with the same frequency as that of the second received signal and with a modulated symbol sequence identical to the symbol sequence of the second received signal, - the correlation of the second received signal with the reference signal is determined in a correlator, - to maximize the degree of correlation In a bit shifter, the symbol sequence of the reference signal is shifted by a symbol duration that is equal to the interval of a symbol or equal to an integral multiple of the interval, and to further maximize the degree of correlation, the symbol sequence of the reference signal is shifted by one symbol part time that is smaller than the interval of a symbol is carried out by controlling a time delay element connected upstream of the correlator and controlled by it and The time stamp assigned to the object of the temporally first symbol of the symbol sequence of the reference signal (18) and the symbol part-time duration is determined, namely with the displacement of the Symbol sequence of the reference signal (18) around the one or more symbol periods and around the symbol part period. Procedure according to Claim 9 characterized in that a correlation value describing the degree of correlation is generated in the second object (60) as an electrical signal which has a lower frequency than the second received signal. Procedure according to Claim 9 or 10 , characterized in that in the second object (60) the symbol sequence for the reference signal is modulated on the carrier signal with a time shift by means of an optical modulator, the time shift being determined on the basis of the magnitude of the correlation value, so that the reference signal with the second received signal in Phase reached. Procedure according to Claim 11 , characterized in that in the second object (60) the displacement of the symbol sequence of the reference signal by one or more symbol periods takes place by corresponding time-delayed modulation of the carrier signal by means of an electrical digital signal having the symbol sequence of the reference signal. Procedure according to Claim 12 , characterized in that in the second object (60) the time delay element which receives the symbol sequence of the reference signal, electrical digital signal and passes this digital signal on to the modulator with a time delay by the delay time determined by an electrical time delay control signal for the time delay element. Method according to one of the Claims 9 until 13th , characterized in that - the symbol sequence for the reference signal (18) to reduce the risk of losing tracking of the received signal (16) by the carrier signal (20) with modulated reference signal (18) when the frequency and / or the phase of the Received signal (16) is shifted to higher values as a result of a reduction in the distance between the two objects and to lower values as a result of an increase in the distance between the two objects by modulation by means of a time-division multiplex signal by respectively predetermined amounts, i.e. time offsets lagging or leading, without the correlation value falling below a predetermined threshold value defining a minimum degree of correlation, the correlation value from a first value, which represents a lag of the symbol sequence for the reference signal (18) compared to that of the received signal (16), to an optimum value, which is essentially the synchronization of the symbol seq uenzen for the reference signal (18) and that of the received signal (16), and also from this optimum value to a second correlation value, which represents an advance of the symbol sequence for the reference signal (18) compared to that of the received signal (16), and alternates between these values reversed or cyclically changes and - in the case of a change in the distance between the two objects, the frequency shift then taking place to either a higher frequency or a lower frequency of the received signal (16) and / or phase shift by selecting a lagging or leading of the symbol sequence for the reference signal (18) compared to that of the received signal (16) is compensated. Procedure according to Claim 14 , characterized in that the time delay control signal (48) is modulated by means of the time division multiplex signal. Method according to one of the Claims 13 until 15th , characterized in that in the second object (60) the time delay control signal is an electrical digital signal which is converted in a digital-to-analog converter into an electrical analog signal, wherein the electrical analog signal controls the time delay element, or that the time delay control signal (48) a is an electrical digital signal and that the time delay element (52) operates digitally and is controlled by the digital signal. Method according to one of the Claims 9 until 16 , characterized in that the second received signal from the second object (60) is received by a coherent receiver forming the correlator which carries out the correlation and a conversion of the correlation into an electrical analog signal that is used for further processing and in particular for forming the correlation value and to generate the time shift of the symbol sequence for the reference signal is converted into an electrical digital signal. Method according to one of the Claims 9 until 17th , characterized in that the two objects (70, 60) are two satellites. Procedure according to Claim 18 , characterized in that the two satellites are part of a satellite constellation in which several satellites move on a common orbit, the orbit being in particular a MEO orbit located at a medium distance from earth, and the two Satellites are adjacent to each other in orbit. Procedure according to Claim 18 , characterized in that the two satellites are part of a satellite constellation in which at least one first satellite is on a first orbit, for example a MEO orbit located in the middle distance from Earth, and at least one second satellite is on one of the first orbit different second orbit, e.g. a LEO orbit located a short distance from Earth. Method according to one of the Claims 9 until 20th , characterized in that the modulation code comprises a PRN sequence of symbols or a gold code.

Images