EP1299745A4 - Recepteur de systeme de positionnement par satellite pour un fonctionnement en signal faible - Google Patents

Recepteur de systeme de positionnement par satellite pour un fonctionnement en signal faible

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Publication number
EP1299745A4
EP1299745A4 EP01929105A EP01929105A EP1299745A4 EP 1299745 A4 EP1299745 A4 EP 1299745A4 EP 01929105 A EP01929105 A EP 01929105A EP 01929105 A EP01929105 A EP 01929105A EP 1299745 A4 EP1299745 A4 EP 1299745A4
Authority
EP
European Patent Office
Prior art keywords
receiver
satellite
signal
location
signals
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP01929105A
Other languages
German (de)
English (en)
Other versions
EP1299745A1 (fr
Inventor
Roderick C Bryant
Eamonn P Glennon
Stanley L Dougan
Andrew G Dempster
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Sigtec Navigation Pty Ltd
Original Assignee
Sigtec Navigation Pty Ltd
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Sigtec Navigation Pty Ltd filed Critical Sigtec Navigation Pty Ltd
Publication of EP1299745A1 publication Critical patent/EP1299745A1/fr
Publication of EP1299745A4 publication Critical patent/EP1299745A4/fr
Withdrawn legal-status Critical Current

Links

Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S19/00—Satellite radio beacon positioning systems; Determining position, velocity or attitude using signals transmitted by such systems
    • G01S19/01—Satellite radio beacon positioning systems transmitting time-stamped messages, e.g. GPS [Global Positioning System], GLONASS [Global Orbiting Navigation Satellite System] or GALILEO
    • G01S19/13—Receivers
    • G01S19/24—Acquisition or tracking or demodulation of signals transmitted by the system
    • G01S19/25—Acquisition or tracking or demodulation of signals transmitted by the system involving aiding data received from a cooperating element, e.g. assisted GPS
    • G01S19/258—Acquisition or tracking or demodulation of signals transmitted by the system involving aiding data received from a cooperating element, e.g. assisted GPS relating to the satellite constellation, e.g. almanac, ephemeris data, lists of satellites in view
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S19/00—Satellite radio beacon positioning systems; Determining position, velocity or attitude using signals transmitted by such systems
    • G01S19/01—Satellite radio beacon positioning systems transmitting time-stamped messages, e.g. GPS [Global Positioning System], GLONASS [Global Orbiting Navigation Satellite System] or GALILEO
    • G01S19/13—Receivers
    • G01S19/24—Acquisition or tracking or demodulation of signals transmitted by the system
    • G01S19/25—Acquisition or tracking or demodulation of signals transmitted by the system involving aiding data received from a cooperating element, e.g. assisted GPS
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S19/00—Satellite radio beacon positioning systems; Determining position, velocity or attitude using signals transmitted by such systems
    • G01S19/01—Satellite radio beacon positioning systems transmitting time-stamped messages, e.g. GPS [Global Positioning System], GLONASS [Global Orbiting Navigation Satellite System] or GALILEO
    • G01S19/13—Receivers
    • G01S19/24—Acquisition or tracking or demodulation of signals transmitted by the system
    • G01S19/25—Acquisition or tracking or demodulation of signals transmitted by the system involving aiding data received from a cooperating element, e.g. assisted GPS
    • G01S19/252—Employing an initial estimate of location in generating assistance data
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S19/00—Satellite radio beacon positioning systems; Determining position, velocity or attitude using signals transmitted by such systems
    • G01S19/01—Satellite radio beacon positioning systems transmitting time-stamped messages, e.g. GPS [Global Positioning System], GLONASS [Global Orbiting Navigation Satellite System] or GALILEO
    • G01S19/13—Receivers
    • G01S19/24—Acquisition or tracking or demodulation of signals transmitted by the system
    • G01S19/29—Acquisition or tracking or demodulation of signals transmitted by the system carrier including Doppler, related
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S19/00—Satellite radio beacon positioning systems; Determining position, velocity or attitude using signals transmitted by such systems
    • G01S19/01—Satellite radio beacon positioning systems transmitting time-stamped messages, e.g. GPS [Global Positioning System], GLONASS [Global Orbiting Navigation Satellite System] or GALILEO
    • G01S19/13—Receivers
    • G01S19/24—Acquisition or tracking or demodulation of signals transmitted by the system
    • G01S19/30—Acquisition or tracking or demodulation of signals transmitted by the system code related
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S19/00—Satellite radio beacon positioning systems; Determining position, velocity or attitude using signals transmitted by such systems
    • G01S19/01—Satellite radio beacon positioning systems transmitting time-stamped messages, e.g. GPS [Global Positioning System], GLONASS [Global Orbiting Navigation Satellite System] or GALILEO
    • G01S19/13—Receivers
    • G01S19/35—Constructional details or hardware or software details of the signal processing chain
    • G01S19/37—Hardware or software details of the signal processing chain

Definitions

  • This invention relates to the design of receivers employed in satellite- based positioning systems (SPS) such as the US Navstar Global Positioning System (GPS), the Russian Global Navigation Satellite System (GLONASS) and the European Galileo system. More specifically, the invention relates to methods, devices and systems for determining a receiver location using weak signal satellite transmissions.
  • SPS satellite- based positioning systems
  • GPS Global Positioning System
  • GLONASS Russian Global Navigation Satellite System
  • European Galileo system More specifically, the invention relates to methods, devices and systems for determining a receiver location using weak signal satellite transmissions.
  • Satellite based positioning systems operate by utilizing constellations of satellites which transmit to earth continuous direct sequence spread spectrum signals. Receivers within receiving range of these satellites intercept these signals which carry data (navigation messages) modulated onto a spread spectrum carrier. This data provides the precise time of transmission at certain instants in the signal along with orbital parameters (e.g., precise ephemeris data and less precise almanac data in the case of GPS) for the satellites themselves.
  • orbital parameters e.g., precise ephemeris data and less precise almanac data in the case of GPS
  • each pseudorange is computed as the time of flight from one satellite to the receiver multiplied by the speed of light and is thus an estimate of the distance or 'range' between the satellite and the receiver.
  • the time of flight is estimated as the difference between the time of transmission determined from the navigation message and the time of receipt as determined using a clock in the receiver. Since the receiver's clock will inevitably have a different present time when compared to the clock of the satellites, the four range computations will have a common error.
  • the common error is the error in the receiver's clock multiplied by the speed of light.
  • the signals from the satellites consist of a carrier signal which is biphase modulated by a pseudo-random binary spreading code at a relatively high "chipping" rate (e.g., 1.023 MHz) and then biphase modulated by the binary navigation message at a low data rate (e.g., 50 Hz).
  • the carrier to noise ratio is typically very low (e.g., 31dBHz to 51dBHz) at the earth's surface for a receiver with unobstructed line of sight to the satellite from its antenna. However, it is sufficient to permit the signals to be detected, acquired and tracked using conventional phase-locked loop and delay-locked loop techniques and for the data to be extracted.
  • the process of tracking the code of a signal in a conventional SPS receiver involves the use of a hardware code generator and signal mixer.
  • the output from the mixer contains no code modulation at all.
  • the bandwidth of the signal is much less and it can be filtered to greatly increase the signal to noise ratio.
  • This is usually done using a decimation filter such that the correlator output sampling rate is much lower than the input sampling rate (e.g., 1kHz at the output compared to 1.3MHz at the input) .
  • the precise time of transmission of this signal corresponding to any given instant at the receiver can be determined by latching the state of the code generator to get the code phase and by counting the code epochs within each bit of the data and by counting the bits within each word of the navigation message and by counting the words within each subframe of the message and by extracting and decoding the times of transmission corresponding to the subframe boundaries.
  • a similar scheme can be used for any SPS.
  • traditional SPS receivers can suffer from troublesome lapses in position identification in the presence of weakened transmission signals. When the direct line of sight between the antenna and the satellites is obstructed, signals may be severely attenuated when they reach the antenna. Conventional techniques can not be used to detect, acquire and track these signals. Moreover, under these circumstances even if the signal could be detected, the carrier-to-noise ratio of a GPS signal, for example, may be as low as or lower than 24dBHz and as such it is not possible to extract the data from the signals.
  • aiding information requires additional transmission capabilities. For example, aiding information may be sent to the SPS receiver using additional satellite transmitters or wireless telephone systems.
  • aiding information supplied to limit the use of such additional resources.
  • the voice communication will be interrupted by the aiding message.
  • the aiding messages must therefore be as short as possible in order to limit the voice interruptions to tolerable durations and frequencies.
  • no matter how the aiding data is communicated its communication will delay the operation of the receiver. In many applications the location data is needed promptly and therefore any delay must be minimized.
  • An objective of the present invention is to provide a method and device for use in a satellite positioning system that has improved performance in the presence of obstructed ox weak satellite transmission signals while maintaining robust performance in the presence of strong signals.
  • a further objective is to improve performance of the system utilizing minimal external assistance while maintaining a graceful degradation in performance when this aiding fails.
  • a still further objective of the invention is to provide a device that achieves a minimal Time To First Fix (TTFF).
  • TTFF Time To First Fix
  • a device made in accordance with this invention utilizes a novel signal processing scheme for detecting, acquiring and tracking attenuated satellite signals, such as those that might be received at an indoor location, and computes location solutions.
  • the scheme makes novel use of attenuated satellite signals and minimal externally-supplied aiding information.
  • an aiding source supplies two types of information in an ordered sequence.
  • the aiding source provides an approximate location of the receiver preferably to within 20km and certainly in the GPS case to within 100km.
  • the aiding source provides precise satellite positions and velocities for the set of tracked satellites. These satellite positions and velocities are computed by the aiding source from ephemeris data for the satellites. No further aiding information is needed.
  • the device detects and acquires a set of satellites for tracking based upon information from internally stored almanac data and its approximate location received from the aiding source.
  • the device relies upon the code phases of the weak satellite signals rather than the transmission time data within the weakened signal.
  • the code phases of the signals are measured at the same instant so that there is a common time of receipt. Then, by determining the differences between the code phases, the resulting values or code phase differences, are taken as ambiguous measurements of the differences in the times of transmission of the satellite signals.
  • these code phase differences are then employed to generate pseudoranges with the assistance of the approximate location received from the aiding source.
  • the approximate location of the receiver and the precise satellite positions are combined to determine approximate ranges to the satellites.
  • precise pseudorange differences are derived.
  • the precise SPS receiver location may be resolved using the precise pseudoranges and the precise satellite positions.
  • FIG. 1 is a sequence diagram describing the interactions between an aiding source, a call taker and a handset with an integrated SPS receiver according to one embodiment of this invention
  • FIG. 2 is a flowchart describing the overall algorithm according to one embodiment of this invention for acquiring satellite signals, measuring code phases, carrier smoothing these measurements, computing pseudorange differences and computing handset location;
  • FIG. 3 is a block diagram of a typical SPS receiver according to this invention.
  • FIG. 4 is a block diagram describing the signal processing algorithm used to measure amplitude in each of an early and a late arm of each channel of the correlator according to one embodiment of this invention
  • FIG. 5 is a block diagram describing the carrier smoothing algorithm used to reduce the error in the code phase measurements according to one embodiment of this invention
  • FIG. 6 is a block diagram describing the algorithm used to compute the location and velocity from the code phase and carrier frequency differences according to one embodiment of this invention. Detailed Description of the Invention
  • the first element is the nature of the aiding information and the manner in which the SPS receiver and the aiding source interact to provide the aiding information.
  • the second relates to a procedure for detecting, acquiring and tracking weak signals while avoiding jamming by strong signals and ensuring graceful degradation under adverse conditions.
  • the third relates to the design of a device for tracking of multiple satellite signals to determine code phases at a common measuring instant.
  • the fourth element of the invention involves a set of algorithms used to process a weak satellite signal in order to compute a position solution from measured code phase differences.
  • the aiding data used in accordance with the present invention is limited to information that includes an approximate location for an SPS receiver and the positions and velocities of a specific set of satellites. This information is determined and provided through a request/response sequence.
  • a model of one embodiment of such an exchange in accordance with the present invention is depicted in FIG. 1.
  • a typical exchange might involve an SPS Receiver 1, an Aiding Source
  • the SPS Receiver 1 might be a GPS receiver embedded in or co-located with a wireless telephone or other handset.
  • the Aiding Source 2 may be located at a call center or cell site or elsewhere in the wireless network such that the aiding data is transmitted via a wireless communication link to the handset.
  • the Call Taker 3 may also be located at the call center or other location accessible from the wireless network. The ultimate user of the location data may be either the Call Taker
  • the SPS Receiver 1 sends a First Aiding Request 4 to the Aiding Source 2. This would typically occur upon activation of the SPS Receiver 1 but may occur at other times as well.
  • the Aiding Source 2 sends a First Aiding Response 5 which contains the approximate location of the SPS Receiver 1.
  • the approximate location of the SPS Receiver 1 is accurate to better than one half of a code epoch of a satellite signal multiplied by the speed of light or about 100km in the case of GPS.
  • the approximate location may also be sent to the Call Taker 3 in a First Aiding Report 6.
  • the SPS Receiver 1 performs its correlation search to acquire satellite signals.
  • the almanac data and the approximate location help to constrain the initial search once at least one satellite has been acquired.
  • the SPS Receiver 1 sends a Second Aiding Request 7 to the Aiding Source 2.
  • the Second Aiding Request 7 includes information for identifying the specific set of satellites used by the SPS Receiver 1 in determining pseudorange differences.
  • the Aiding Source 2 determines the precise positions and velocities of the identified set of satellites from ephemeris data for the satellites.
  • the determined positions and velocities are then sent to the SPS Receiver 1 in a Second Aiding Response 8. Since this elapsed time is known and assuming that the latency between transmission and reception of the request for aiding can be determined it is possible for the aiding source to determine the time of reception of the satellite signals to within a few tens of milliseconds. Therefore, under this scheme since the Aiding Source 2 provides precise satellite positions and velocities, the Aiding Source 2 rather than the SPS Receiver 1 needs to be able to determine specific time synchronization data from the satellite signals and needs to maintain or acquire ephemeris data.
  • the Latency period for the communication between the two must be within a few tens of milliseconds. This will ensure a limitation on the error in the computed satellite locations to a few meters.
  • the Second Aiding Request 7 occurs at a known elapsed time from the instant when the code phases of the satellite signals latch. Since this elapsed time is known and assuming that the latency between transmission and reception of the request for aiding can be determined it is possible for the Aiding Source 2 to determine the time of reception of the satellite signals to within a few tens of milliseconds.
  • the SPS Receiver 1 After receiving the satellite positions and velocities and using pseudorange and range rate differences, the SPS Receiver 1 then computes a Position and Velocity (PV) solution to determine its precise location, speed, heading, etc. After such determination, the SPS Receiver 1 then sends a Receiver Report 9 to the Aiding Source 2 which includes the raw location, speed, heading, height, satellite identifications and the solution mode used by it (i.e. 3D or 2D with altitude aiding).
  • PV Position and Velocity
  • the Aiding Source 2 may take further action. For example, the Aiding Source 2 may use the known satellite set and times of transmission, select differential pseudorange corrections (obtained by any available means) and then compute a corresponding location correction consistent with the reported mode of solution. With this later computation, the Aiding Source 2 may then apply the correction to the location reported by the SPS Receiver to obtain a more current location. This precise location then may be sent to the Call Taker 3 in a Second Aiding Report 10. As an alternative embodiment of this aforementioned exchange, the
  • SPS Receiver 1 reports the code phase differences to the Aiding Source 2.
  • the Aiding Source 2 could compute a PV solution for the SPS Receiver 1 using a method like that of the SPS Receiver 1.
  • the scheme requires the SPS Receiver 1 to compute pseudoranges without the benefit of having actual time synchronization data from the satellite signals.
  • the use of this data is avoided because the code phases of the satellite signals are taken as ambiguous measurements of the differences in the times of transmissions of the satellite signals. This is accomplished by measuring the code phases at the same instant so that there is a common time of receipt.
  • the ambiguity resolution needed to convert code phase differences into pseudorange differences is achieved by utilizing the approximate location for the receiver obtained from the Aiding Source 2.
  • the SPS Receiver 1 estimates Doppler information using stored current almanac data.
  • the SPS Receiver 1 uses this current almanac data to estimate the Doppler frequencies of the satellites to an accuracy of better than about 250Hz for GPS given approximate locations of the satellites known to better than about one hundred kilometers. This is sufficient accuracy to achieve a rapid acquisition provided that the frequency offset of the reference oscillator of the SPS Receiver 1 is known to within a few Hz. To achieve this latter requirement, the reference frequency offset is estimated each time a PV solution is computed and thus it can be tracked. Moreover, to the extent that the reference frequency varies with temperature as well as aging and to the extent that a large change in temperature between PV solutions will lead to a degradation in acquisition performance, the SPS Receiver 1, as described below, utilizes a method that copes with the change and ensures graceful degradation.
  • the SPS Receiver 1 To ensure the presence of current almanac data without the use of an Aiding Source 1, the SPS Receiver 1 must be activated often enough and for long enough in the presence of strong signals to keep the data current. In the case of the GPS system, the almanac data must be less than 2 months old to remain current. To meet this goal under GPS, the SPS Receiver 1 must gather approximately 27 sets of orbital coefficients over a period of around 2 months. On average it would take around 20s plus signal acquisition time to gather one set and it would take around 60 such gatherings to acquire all of the sets. Hence, if the SPS Receiver 1 was activated in the presence of strong signals approximately once per day on average for about 30s then the stored almanac would remain current. B. Weak Signal Acquisition/Tracking in Presence of Jamming Signals
  • any SPS system has a limited dynamic range.
  • any signal that is weaker by more than about 20dB than another signal that is also present may be jammed by the stronger signal.
  • FIG. 2 outlines an example procedure for a GPS receiver that ensures that any strong signals are acquired first using a high threshold. A threshold for acquisition of any remaining needed signals will then be set, in the case of GPS, 20dB below the strongest signal acquired. While the FIG. 2 contains information pertinent to GPS system, its general application would be equally applicable to any other SPS system.
  • the device starts with the first request/response exchange in attempt to acquire the SPS Receiver's 1 approximate location from the Aiding Source 2. If the exchange is successful and the approximate location aiding data is received, the device sets its initial search parameters in step 11 with the goal of acquiring the strong signals.
  • the parameters for the search are initially set to a high threshold and with a short integration period adequate for acquisition at the selected threshold. There is no prior knowledge of the code phases of the satellites and therefore the search is unrestricted.
  • the reference frequency offset is assumed to be a prior value that was measured when the receiver was previously active (i.e. "old").
  • the SPS Receiver 1 uses a multi- channel device to perform a concurrent search for the strong signals of all visible satellites.
  • the assumed frequency offset of the receiver's reference oscillator may have changed by a larger amount than can be accommodated for by the sampling rate of a correlator's output samples.
  • the offset would be adjusted in step 60 and the search continued without lowering the threshold. In this way, the offset would be changed systematically to effect a search over the possible frequency range. The systematic search would terminate on the acquisition of one or more satellites at the highest possible threshold.
  • a lower threshold would be set (e.g. 6dB lower) and a longer integration period would be used (e.g. 4 times as long).
  • the reference frequency offset would be set back to the previously assumed value and the frequency search would be restarted using these parameter values.
  • the measured carrier to noise ratio of the strongest signal acquired would be used to determine both the integration period to be used for a subsequent search in step 14 and the acquisition threshold to be applied during the search. For example, if a signal of greater than 50dBHz was acquired then the integration period for the subsequent search need not be any more than 32ms. This is because it is possible to detect signals of 30dBHz or more with an integration period of 32ms and the threshold would have to be set at 30dBHz or higher to avoid the dynamic range problems previously described.
  • the reference frequency offset would also be estimated using the approximate location of the SPS Receiver 1 together with the almanac data and the measured carrier frequency of the acquired signal. This should obviate the need for further frequency searches. Since at least one satellite has been acquired, a restricted search regime can be conducted for the remaining satellites using one channel per satellite since the approximate code phase differences between all of the remaining visible satellite signals and the first one can be estimated. If the accuracy of the approximate position estimate is assumed to be ⁇ 10km then the code phase differences can be estimated to within ⁇ 25 ⁇ s, approximately. In the GPS case this would permit all of the remaining satellites to be acquired within 50 * 0.128s or 6.4s. However, the search could be terminated once sufficient satellites had been acquired to permit the location to be determined.
  • the maximum time taken to acquire enough satellites would thus be Is for the first search for strong signals plus 4s for the second search for the first satellite plus 6.4s for the subsequent search for the remaining satellites. This adds up to 11.4s. However, typically, acquisition would take less than this (e.g. Is + 4s + 3,2s or 8.2s).
  • the second frequency search to acquire the first satellite signal could fail because all of the signals are even weaker than the adjusted threshold.
  • One appropriate method for failure would be to conduct additional searches using sequentially lowered threshold values adjusted in step 13 until some final threshold is utilized.
  • the approximate receiver location could be less accurate than the assumed ⁇ lOkm and, as a result, insufficient satellite signals are acquired during the first pass of the weak signal search.
  • One method to address this failure is to perform further frequency search passes with sequential increases in the search range performed in step 15. This scheme would degrade gracefully with an increase in acquisition time. However, it would ensure that any satellite signals that can be acquired would eventually be acquired. On average, the use of approximate receiver locations of greater inaccuracy will result in longer acquisition times.
  • the aiding message may not be received. This could mean that the selection of visible satellites is wrong because the receiver could be many thousands of km from the assumed location. This would also prevent any of the searches from being restricted as described. More importantly, it would prevent the pseudorange differences from being unambiguously determined from the code phases once measurements were made. It would also imply that the second aiding message will not be available when required and this would prevent a location solution from being computed.
  • One response to this failure is to simply revert to standard SPS receiver operation as shown by step 16. Thus, in the GPS case, an integration period of 32ms could be used and the last known location would be assumed with searches for a first satellite and subsequent satellites as described above and failure responses as described above.
  • an SPS Receiver 1 of the present invention requires the ability to determine the code phases for multiple satellite signals at the same instant.
  • FIG. 3 depicts one such SPS Receiver 1.
  • the SPS Receiver 1 can be broken down into roughly three parts.
  • the device has a front-end circuit 17, three or more correlators 18 and a microprocessor 20 with memory. The following describes the functions of each.
  • the front-end circuit 17 serves as the initial signal processor as follows.
  • the front-end circuit 17 amplifies, filters, down converts and digitizes the signal from an antenna so that it is suitable for processing in a digital correlator 18 and such that the signal to noise and signal to interference ratios are minimized subject to economic and practical realization requirements.
  • the front-end output 19 of the front-end circuit could be a complex signal centered at tens of KHz (in the case of GPS) or a real signal centered at around 1.3MHz or higher.
  • the sampling rate would typically be several MHz and the digitization would be at least 2 bits per sample.
  • an AGC circuit keeps the level of the digitized signal constant. Since the true signals are spread over 2MHz in the case of GPS and are weak signals in any case, this signal is dominated by noise and the AGC maintains a constant noise level at the output of the front- end.
  • Hardware correlators 18 each representing a processing channel for a particular satellite signal are used separately to further process the front end output 19 under the control of the microprocessor 20.
  • a further down conversion 21 quadrature in this case
  • the resulting complex down converted signal 22 is mixed with (i.e. multiplied by) a real binary pseudorandom code signal 23 chosen to match that of a particular satellite signal and generated by a code generator 24.
  • the code generator 24, controlled by the microprocessor 20, generates the pseudorandom code signal 23 at a selected rate set to match the estimated signal Doppler offset given the estimated crystal oscillator offset.
  • the code generator 24 also generates a late pseudorandom code signal 25 that is the same as pseudorandom code signal 23 but at a fixed lag with respect to the former.
  • This late pseudorandom code signal 25 is also mixed with the down converted signal 22.
  • the resulting mixed signals 26 are then separately processed by decimators 28.
  • Decimators 28 low pass anti-alias filter and down sample the mixed signals 26 to a reduced sampling rate. In the case of GPS, the reduced sampling rate is approximately lKHz. This sampling rate may be derived from the local code rate such that a single sample will be obtained for each code epoch.
  • the processor 20 either causes the code generator 24 to step instantaneously by the required amount at the start of each integration period or changes the code frequency by a known amount for a precise period of time so as to effect a rapid step in the code lag.
  • This is the preferred embodiment although in an alternative scheme the code frequency may be deliberately offset while searching so that the code slews continuously relative to that of the incoming signal.
  • the processor 20 constantly adjusts the code lag as just described so as to keep the pseudorandom code signal 23 and late pseudorandom code signal 25 from the code generator 24 running one ahead (early) and one behind (late) the code of the incoming signal.
  • the code generator 24 may generate a third signal (prompt) that is kept running synchronously with the code of the incoming signal or, indeed, there may be several more signals spanning a lag interval of up to 1 chip (smallest code elements) early and late of the incoming code.
  • the correlator output samples 29 are read into the processor 20 where they are further processed by a signal processing algorithm described later in this specification to estimate the amplitude, frequency and phase of the carrier signal. Then, if data is to be extracted because the signal is strong enough for that, the phase and frequency are utilized by a separate algorithm that operates on the raw samples to extract the data. Methods for the extraction of this data will be obvious to one skilled in the art.
  • the frequency of the tracked carrier signals are then used to estimate the Doppler offset of the carriers and the crystal oscillator offset.
  • the former Doppler offset values are subsequently used to estimate the velocity of the receiver (and the vehicle in which it may be travelling) .
  • the amplitude of the early and late correlator output samples 29 represent estimates of the carrier to noise ratio of the satellite signal since the noise level is maintained constant by the AGC of the front-end.
  • the amplitudes are compared to a threshold to determine if the signal has been detected. If it has, then an acquisition procedure is commenced. The steps of an appropriate acquisition procedure will be obvious to one skilled in the art.
  • the code phase 30 for each correlator 18 are simultaneously latched by a latch element 31 within the hardware correlator.
  • the resulting signal represents the code phase measurement 32.
  • These code phase measurements 32 are then made available to the processor 20.
  • the processor 20 then applies a smoothing algorithm to the code phase measurements 32 together with the carrier frequency estimates. This algorithm is used to reduce the random error in the code phase measurements 32 over time by making use of the precision in the carrier frequency estimates to predict the changes in code phase from one integration period to the next.
  • the algorithm also filters the carrier frequency estimates to reduce their random errors.
  • the carrier smoothing algorithm is described later in this document.
  • the carrier smoothed code phase measurements and the filtered carrier frequency estimates are passed to the location solver which estimates the SPS Receiver 's 1 location and velocity.
  • the algorithm makes use of precise satellite position data and the approximate location received from the aiding source. This calculation is described in more detail later in this document.
  • the signal processing, carrier smoothing and location solving algorithms are all executed by the processor 20.
  • the invention utilizes an algorithm to measure amplitude, frequency and phase of the satellite signal.
  • the invention also applies a smoothing procedure to the code phase measurements and the carrier frequency estimates to reduce random errors occurring over time.
  • a formula is applied to convert the code phase differences into a precise position and velocity solution.
  • any SPS Receiver 1 employing a hardware correlator 18 with several signals in each channel is required to estimate the amplitude of the carrier in each of those several signals in the presence of the data.
  • a phase locked loop or a frequency locked loop and a delay locked loop control the frequency of the final down converter 21 and the code generator 24 respectively.
  • the signals are too weak to permit lock-in without the use of some sort of aiding from an auxiliary algorithm.
  • the signal processing algorithms of the present invention could be used as auxiliary algorithms for acquisition or they may be used independently of any phase-locked or frequency-locked loop for both acquisition and tracking.
  • the correlator output samples 29 are so noisy that the signal is indistinguishable from the noise unless the correlator output sampling rate is extremely low (e.g. 8 Hz for GPS).
  • the correlator output sampling rate is extremely low (e.g. 8 Hz for GPS).
  • the Doppler frequency and the crystal oscillator offset would have to be known to an impractical high precision. Accordingly, a higher correlator output sampling rate needs to be retained and a suitable algorithm is required to estimate the amplitude of the residual carrier signals in the code phase measurements 32.
  • FIG. 4 depicts a flow chart for an algorithm used to measure amplitude, frequency and phase of the signal from each of the early and late mixed signals 29 of the correlator 18 according to one embodiment of this invention.
  • the procedure involves the use of a Fast Fourier Transform 33 applied to a block of samples from the code phase measurements 32 of the correlator 18.
  • the effect of this algorithm is to compress the residual carrier signal into a few bins of the FFT output 34.
  • the satellite signal is undetectable when its energy is spread across all of the time domain samples contained in the code phase measurements 32, it is detectable in the bins of the FFT output 34.
  • its amplitude is not readily estimated because the data modulation splits the satellite signal between several adjacent bins in a semi-random manner depending on where the transition falls in relation to both the integration period and the phase of the residual carrier at that instant.
  • window operation 35 on the bins of the FFT output 34 centered on the peak value. More distant bins may be discarded completely. This is simply a filtering operation and significantly improves the signal-to-noise ratio prior to nonlinear processing 36 of the window-filtered signal 37 to eliminate the data transitions.
  • the remaining windowed bins in the window-filtered signal 37 may be processed in one of several ways in order to estimate the amplitude in the presence of the noise and data as follows: 1.
  • the discarded bins may be zero filled and the complete set of bins can then be inverse transformed back to the time domain.
  • the vector of remaining windowed bins may be autoconvolved to remove the data in an autoconvolution process 36. This is equivalent to squaring in the time domain but, if the number of bins in window filtered signal 37 is small compared to the size of the FFT 33, then the autoconvolution process 36 may be less computationally costly than the process described in option 2 above.
  • the autoconvolved samples may be less computationally costly than the process described in option 2 above.
  • Fc is the RF carrier frequency
  • Fdl is the total frequency shift due to the front-end down conversion
  • Fd2 is the frequency shift due to the down conversion in the correlator
  • Npl is the bin number (between -N/2 and (N/2+1)) for an N- point FFT) of the peak in the FFT (which is the center bin of those extracted for performing the autoconvolution);
  • Np2 is the bin number of the peak bin in the autoconvolved bins computed (which is the center bin of those extracted for estimating the amplitude and frequency) ;
  • Nnom is the bin number of the nominal peak bin in the autoconvolved bins computed (i.e. corresponding to zero lag);
  • N ⁇ is the frequency adjustment (in bins and fractions of bins relative to Np2) estimated by analysis of several adjacent bins of the autoconvolved bins computed;
  • Fbin is the bin width of the original FFT;
  • ⁇ Fxo is the crystal oscillator offset from its nominal frequency
  • Fnom is the nominal RF carrier frequency of the signal
  • Fxo is the nominal crystal oscillator frequency.
  • the algorithm processes differences between the estimates from one satellite and those from all the others rather than processing the absolute estimates for individual satellites.
  • differences rather than absolute estimates are used because differences can be processed without the need to take account of the frequency offset of the reference oscillator.
  • All of the frequency difference estimates 40 corresponding to a particular satellite signal are simply averaged in step 42.
  • the average carrier frequency difference 43 is then used to predict forward in prediction step 44.
  • This prediction step 44 is used for all but the latest code phase differences 41 for that satellite to the latest measurement instant before averaging them in step 45.
  • the prediction step 44 uses an estimated Doppler offset of the code 47 determined from step 46 which is based upon the difference between the Doppler offsets of the carrier frequency (Fcl-Fc2).
  • Tip is the nominal integration period
  • Te is the code epoch period as determined from the tracking algorithm; and All other quantities are as previously defined.
  • the predicted code phase differences are then averaged in step 45 to produce an average code phase difference 49.
  • FIG. 6 illustrates the PV solution process used by a preferred location solver.
  • the location solver which computes the location and velocity using the refined average carrier frequency differences 43, the average code phase differences 49, the precise satellite positions 52 and the approximate receiver location 54 as follows.
  • step 50 approximate ranges 51 to all of the satellites for which there are measurements are computed, Since the satellite positions 52 are supplied by the Aiding Source 2, this step simply involves the vector difference of the Cartesian coordinates of the satellite positions and the approximate location 54, which was also supplied by the Aiding Source 2. The vector magnitudes of these vector differences are approximate ranges 51.
  • step 55 the epoch ambiguities are resolved and the average code phase differences 49 are converted into pseudorange differences 56. All of the approximate ranges 51 are subtracted from the approximate range of a reference satellite (as selected for computing the code phase differences) to give approximate range differences. These are saved for later use as well as being used in resolving the epoch ambiguities according to the following formula:
  • P1-P2 int[(Rl-R2)/c*Te-( ⁇ l- ⁇ 2) + 0.5] + ( ⁇ 1- ⁇ 2)
  • C is the speed of light
  • Te is the nominal epoch period
  • Pi and P2 are the pseudoranges 56 of satellites 1 and 2; Rl and R2 are the approximate range estimates for the same two satellites;
  • ⁇ 1- ⁇ 2 is the code phase difference between the same two satellites.
  • the range rate differences 57 are calculated from the Doppler affected average carrier frequency differences 43.
  • the current location estimate 60 and current velocity estimate 61 are computed in step 59 by a method similar to that for a single update of a Kalman navigation filter in a traditional SPS receiver. In fact, if time allows, a true navigation filter can be run for several updates to further refine the estimated PV solution.
  • the initial state vector, X ⁇ x * is set to the approximate location from the aiding source with zero velocity.
  • the first prediction vector, Y PRED is set to the approximate range differences derived from the approximate ranges 51 and zero for the range rate differences 57.
  • the state covariance matrix, P is initialized to a diagonal matrix with the entries representing the estimated variances of the approximate location and velocity estimates.
  • the location variance estimates may be obtained from the Aiding Source 2 or a fixed value could be used.
  • the initial velocity estimate is zero and its variance depends on the application.
  • the measurement variances can be estimated as in a conventional receiver except that the measurement variance matrix, R, is no longer diagonal.
  • the fact that differences with a reference satellite were computed means that the covariance terms between the satellites are half of the value of the variance estimates rather than zero as in a conventional receiver.
  • the approximate location (54) and the satellite positions (52) can be used to determine the direction cosines and the direction cosine differences form the rows of the measurement matrix, M.

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  • Engineering & Computer Science (AREA)
  • Radar, Positioning & Navigation (AREA)
  • Remote Sensing (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Signal Processing (AREA)
  • Position Fixing By Use Of Radio Waves (AREA)

Abstract

L'invention concerne un procédé, un dispositif et un système destinés à déterminer un emplacement de récepteur par satellite avec émissions à signal faible. Le procédé consiste à mettre en oeuvre une séquence d'échanges entre une source additive et un récepteur fournissant une information additive au récepteur de façon que l'emplacement du récepteur puisse être déterminé en présence d'émissions satellite faibles. Avec l'information additive, le récepteur détecte, acquiert et suit les signaux satellite faibles et calcule des solutions de position à partir de distances fictives calculées malgré l'incapacité à réaliser un extraction de date de synchronisation temporelle à partir des signaux satellite faibles.
EP01929105A 2000-05-08 2001-05-07 Recepteur de systeme de positionnement par satellite pour un fonctionnement en signal faible Withdrawn EP1299745A4 (fr)

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US20246400P 2000-05-08 2000-05-08
US202464P 2000-05-08
PCT/AU2001/000519 WO2001086318A1 (fr) 2000-05-08 2001-05-07 Recepteur de systeme de positionnement par satellite pour un fonctionnement en signal faible

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JP (1) JP2003532903A (fr)
KR (1) KR20030013405A (fr)
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KR100438396B1 (ko) * 2001-11-06 2004-07-02 지규인 위성을 이용한 위치결정수신기의 실시간 다중경로오차검출방법 및 그 방법을 이용한 위치결정 장치
JP3726897B2 (ja) 2002-02-27 2005-12-14 ソニー株式会社 Ｇｐｓ受信機およびｇｐｓ衛星信号の受信方法
EP1420380A1 (fr) * 2002-11-18 2004-05-19 Owasys Advanced Wireless Devices, S.L.L. Dispositif de navigation et procédé
US7729457B2 (en) 2005-07-25 2010-06-01 Mstar Semiconductor, Inc. Method of weak signal acquisition and associated apparatus
US7630430B2 (en) 2005-07-25 2009-12-08 Mstar Semiconductor, Inc. Method and apparatus for accelerating correlation processing of GPS signal
JP2016048246A (ja) * 2015-10-30 2016-04-07 セイコーエプソン株式会社 電子機器

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IL152708A0 (en) 2003-06-24
KR20030013405A (ko) 2003-02-14
JP2003532903A (ja) 2003-11-05
NZ523031A (en) 2004-06-25
WO2001086318A1 (fr) 2001-11-15
EP1299745A1 (fr) 2003-04-09
CA2411607A1 (fr) 2001-11-15
AU5598801A (en) 2001-11-20

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