IL149659A

IL149659A - System for tracking a craft

Info

Publication number: IL149659A
Application number: IL149659A
Authority: IL
Original assignee: Thales Sa
Priority date: 2000-10-20
Filing date: 2002-05-14
Publication date: 2007-06-03
Also published as: EP1327159B1; EP1327159A1; FR2815792A1; WO2002033437A1; FR2815792B1; IL149659A0; DE60135566D1

Description

SYSTEM FOR TRACKING A CRAFT THALES C: 45753 SYSTEM FOR TRACKING A CRAFT The present invention relates to a system for tracking a craft from a monitoring station which may also be able to move, by means of signals transmitted by satellites of a positioning system and relayed by a transponder which is mounted on the craft and which performs, on the move, a change of frequency on the signals originating from the satellites of the positioning system.

The satellites of a satellite-based positioning system such as the American GPS system or the Russian GLONASS system, transmit signals which result from the modulation of a carrier by a spreading code consisting of a pseudo-random binary string itself modulated by data relating to the positions of the satellites, to their speed vectors and to the synchronizing carrier system clock, spreading code and data transmission rate. The spreading code delays and the Doppler shifts affecting the carrier frequency of these signals, on reception, provide information regarding the relative distances or pseudo-distances and the relative speeds or pseudo-speeds of movement between the receiver and the satellites and make it possible, as is well known, to determine the position of the receiver and the components of its speed vector through triangulation operations.

There are a number of situations in which the information contained in the signals from a satellite-based positioning system are not processed in the craft which receives them but elsewhere, in one or more monitoring stations aboard another craft or placed in a fixed position on the ground. This may be the case for a missile which receives its guidance orders from a station aboard an aircraft or placed on the ground in a fixed or moveable position, or the case for a projectile whose trajectory from a firing center one wishes to ascertain.

In these situations, the craft whose position is monitored by means of a satellite-based positioning system forwards to its monitoring station or stations, the information about its position and its speed vector given by the satellite-based positioning system, either in the raw form of the signals received from the satellites of the positioning system, or in an explicit form, after more or less successful processing of these signals.

When the information contained in the signals from the satellites of the positioning system which are received by the craft being monitored, is not directly utilized onboard the craft, it is preferred that these signals be forwarded in their raw form to the monitoring stations so as to limit the onboard computational power, doing so for reasons of payload and cost, especially when the craft is not reusable, as is the case for a missile or an artillery projectile.

To forward to a monitoring station the signals which it receives from the satellites of a positioning system, a craft cannot just simply amplify these signals and retransmit them, since a monitoring station would not be able to easily distinguish them from those which it receives directly from the satellites of the positioning system. It is therefore customary, when forwarding these signals, to change their frequencies so as to shift their frequency with respect to the signals of the satellites of the positioning system.

Various items of transponder equipment carrying out this function of retransmission, with change of frequency, toward monitoring stations, of the signals of a satellite-based positioning system which are picked up onboard a craft are known in the art. Such equipment is often designated by the term "GPS translator" . Some of this transponder equipment retransmits the signals received from the satellites of the positioning system after a simple operation of transposition within the frequency band used by the satellites of the positioning system so that they are frequency shifted with respect to those of the satellites but can however be picked up and processed by a conventional receiver of a satellite-based positioning system, others retransmit the signals received from the satellites of the positioning system after having transposed them to video band and use them to modulate an auxiliary transmission carrier.

In a receiver of a satellite-based positioning system, it is customary to liken the delay on reception of a pseudo-random binary spreading code to the delay affecting a local code recovery loop of the DLL type (standing for Delay Locked Loop), and the Doppler shift affecting on reception the carrier of a signal transmitted by a satellite to the phase shift and frequency shift affecting a local loop for carrier recovery of the PLL type (standing for Phase Locked Loop) .

In the case of a receiver of a satellite-based positioning system directly processing the signals from the satellites, the delays of the spreading codes and the Doppler shifts of the carriers affecting on reception the signals from the satellites make it possible to calculate a position and a speed in three dimensions (latitude, longitude, altitude) and also to acquire the absolute time reference of the positioning system.

In the case of a satellite-based positioning system receiver indirectly receiving the signals from the satellites after they have been relayed by a transponder carried by a craft, the delays of the spreading codes and the Doppler shifts of the carriers of the signals from the satellites are affected by the relaying .

The delays affecting the spreading codes contained in the signal originating from the transponder no longer correspond to the times taken by the signals to travel the distances separating the receiver from the satellites, but to the times taken by the signals to travel the paths going from their satellites of origin to the carrier of the transponder and from the carrier of the transponder to the monitoring station where the receiver is situated. As the second part of the paths going from the carrier of the transponder to the monitoring station where the receiver is situated is the same for the signals from all the satellites, it gives rise to an identical delay for all these signals which constitutes a common-mode error in the same guise as the shift of the reception clock. This delay can be eliminated by the customary processing performed on the pseudo-distances so as to take account of the shift of the reception clock so that in the end the delays in the spreading codes originating from the signal from a transponder make it possible to locate the carrier of the transponder as would have been possible, onboard the transponder, on the basis of the signals from the satellites of the positioning system received directly. However, this solution has the drawback of no longer allowing acquisition on reception of the absolute time reference of the positioning system.

The frequency shifts of the carriers contained in the signal of a transponder result, as regards the signal from each satellite, from the combination: of the frequency error affecting the initial carrier transmitted by the satellite, of a first Doppler effect which is due to the speed of relative movement between the transmitting satellite and the carrier of the transponder and which affects the initial carrier transmitted by the satellite, on reception thereof at transponder level, of the shift of the clock reference used on reception, - of the frequency error made by the transponder during the change of frequency, and of a second Doppler effect which is due to the relative movement of the carrier of the transponder with respect to the receiver and which affects the initial carrier of the signal after transposition thereof by the transponder, on reception thereof at receiver level.

The first three causes from which the noted frequency shift stems are conventional and are found again in the signals picked up directly from the satellites. On the other hand, the other two, the frequency error made by the transponder during the frequency shifting and the second Doppler effect affecting the carrier of the signal after transposition, are new and need to be taken into account .

It is very difficult to act at transponder level on the error which it makes during the change of frequency, the only possibility of action being to improve the accuracy of its clock and rapidly having a limit. Thus, we seek to correct this error on reception. To do this, we must have an estimate thereof on reception. One way of obtaining this estimate consists in having the transponder transmit a transposition driver which is phase locked with its clock. This transposition driver makes it possible, on reception, to recover the phase and the frequency of the clock of the transponder and hence to ascertain precisely the frequency transposition actually performed by the transponder on the carrier of the signals from the satellites. It is thus possible to correct the frequency gap measured by the error signal of the carrier recovery loop before using it to extract information about the speed of the carrier of the transponder.

The second Doppler effect affecting the carriers after their frequency transpositions by the transponder on account of the relative motion between the carrier of the transponder and the receiver, is common to all the signals from the satellites and gives rise to a common- mode error in the measurement of the speed of the carrier of the transponder which can be eliminated by mathematical processing by involving an additional satellite .

However, the frequency gap noted on reception in the carrier recovery loop remains, even after having been corrected of its component due to the inaccuracy in the change of frequency of the transponder, biased by the second Doppler effect. This bias is weaker in the case of transponders performing a transposition of the signals received to video band before retransmitting them by modulation of a transmission carrier but it persists and brings about an inconsistency between the pseudo-distances derived from the code delays and the pseudo-speeds derived from the carrier frequency gaps. This inconsistency implies that the integral of the pseudo-speeds does not correspond to the pseudo- distances measured although this property is sought since it makes it possible to carry out in a simple manner tests of proper operation of a receiver of a satellite-based navigation system.

The aim of the present invention is to solve this problem and more generally, to improve the accuracy of the position information and speed information which it is possible to extract from the signals from the satellites of a positioning system after they have passed through a transponder mounted on a craft.

Its subject is a system for tracking a craft from a monitoring station by means of signals transmitted by satellites of a positioning system and frequency- transposed and relayed by a transponder placed onboard the craft, said signals transmitted by the satellites of the positioning system each resulting from the modulation of a carrier by a spreading code.

This craft tracking system comprises: onboard the craft, a transponder retransmitting the signals from the satellites of the positioning system after an infradyne frequency transposition by means of a transposition carrier of higher frequency KFQ than that F0 of the satellite signals, by associating with the retransmitted signals a transposition driver FQ representative of the phase and frequency of the transposition carrier KFQ, and, at the monitoring station, a receiver responsive to the signals retransmitted by the transponder, with: a signal processing part incorporating a demodulation stage with a spreading code recovery loop nested within a carrier recovery loop, each slaved to its own error signal, the error signal of the code recovery loop being representative of the delay affecting, on reception, the spreading code of the signal undergoing demodulation and the error signal of the carrier recovery loop being representative of the frequency gap affecting, on reception; the carrier of the signal undergoing demodulation, a part for determining the frequency gap KAF2, with respect to its preset value, of the transposition carrier used by the transponder, operating on the basis of the transposition driver received from the transponder, means for correcting the error signal of the carrier recovery loop, of the bias caused by the frequency gap KAF2 determined by the part for determining frequency gap, and a part for signal utilization, extracting information regarding the position and the speed of the craft from the error signals of the code recovery loop and of the carrier recovery loop of the processing part, the error signal of the carrier recovery loop being corrected beforehand of the frequency gap KAF2 delivered by the frequency gap determining part .

It is noteworthy in that it comprises, at the level of the monitoring station, means for interpreting the error signal of the carrier recovery loop, after correction of the bias caused by the frequency gap KAF2 determined by the frequency gap determining part as a measurement representative of the Doppler effect caused, on the initial frequency Fo of the signal relayed by the transponder, by an approach speed corresponding to the sum of the craft/satellite V0s and craft/receiver V0R relative retreat speeds.

Advantageously, the receiver part relating to the determination of the frequency gap affecting the transposition carrier of the transponder comprises: a loop for recovering the transposition driver, a circuit for measuring the frequency gap of the transposition driver with respect to a preset value, and a first scaling circuit operating on the error signal delivered by the gap-measuring circuit so as to take account of the ratio K existing between the transposition driver and the transposition carrier.

Advantageously, the signal estimating the frequency error made by the transponder, produced by the gap determining part of the receiver is added to the error signal of the carrier recovery loop before its use by the latter.

As a variant, the signal estimating the frequency error made by the transponder, produced by the gap determining part of the receiver is deducted from the error signal of the carrier recovery loop before its use by the signal utilization part of the receiver.

Advantageously, the spreading code recovery loop is aided by the error signal of the carrier recovery loop, weighted in the ratio existing between the frequency of the spreading code and the carrier frequency of the signal from which it stems in the satellite.

Other characteristics and advantages of the invention will emerge from the following description of an embodiment given by way of example. This description will be given in conjunction with the drawing in which: a figure 1 is a situation diagram illustrating the positions, speed vectors and movements of the main actors participating in a system for tracking a craft implementing the signals from satellites of a positioning system relayed by a transponder placed onboard the craft and received by a ground station, a figure 2 is a chart, in the frequency domain, illustrating the problem posed, on reception, by Doppler effects of diverse origins and by the clock bias of a transponder, a figure 3 is a diagram of a first embodiment of a demodulator belonging to the receiver of a system for tracking a craft according to the invention, a figure 4 is a diagram of a second embodiment a demodulator belonging to the receiver of system for tracking a craft according to invention, and a figure 5 is an exemplary diagram transponder .

Figure 1 diagrammatically illustrates the case of a shell 3 fired by a gun 4, whose trajectory is measured in a monitoring station 5, placed on the ground, using a satellite-based positioning system whose four visible satellites 6, 7, 8, 9 are represented. The shell 3 is equipped with a transponder allowing it to forward the signals from the visible satellites 6, 7, 8, 9 of the positioning system to the monitoring station, in a form making it possible to distinguish the signals forwarded from the original signals. The monitoring station 5 is capable of processing the signals from the satellites 6, 7, 8, 9 of the positioning system whether they come directly from it or by way of the transponder of the shell 3, the signals received directly allowing it to self-locate accurately and hence also to locate the gun 4 accurately, and the signals relayed by the transponder of the shell 3 allowing it to locate the shell. In what follows, we shall be more especially interested in the signals relayed by the transponder of the shell 3.

To reach the monitoring station 5, the signals from the satellites 6, 7, 8, 9 of the positioning system which are relayed by the transponder of the shell 3 follow a first path part leading from the relevant satellite 6, 7, 8 or 9 to the shell 3, then a second path part going from the shell 3 to the monitoring station 5. Over the two parts of their path they undergo Doppler effects.

As is well known, the Doppler effect causes on reception of a wave of frequency f, a frequency shift Δί which, when the speeds of movement of the transmitter of the wave and of the receiver of the wave are low with respect to that C of light adopted as wave propagation speed, is equal to the product of the frequency f of the wave times the speed Vrap of approach of the transmitter and the receiver, divided by the speed C of light: Over the first path part going from the relevant satellite 5, 6, 7, 8 or 9 to the shell 3, the speed of approach to be taken into account for the Doppler effect is the speed of approach between the shell 3 and the relevant satellite. If the shell 3 is at a position 0 with a speed vector VQ and the relevant satellite 6, → 7, 8 or 9 at a position Si with a speed vector ■ (the two speed vectors being defined with respect to a common benchmark tied to the terrestrial globe) , this speed of approach Vrap osi is expressed by the relation: V r.pOSi which may be cast into the form: The frequency of the wave to be considered is either the rate of throughput of the pseudo-random binary sequence of the spreading code if we are interested in the Doppler effect affecting the code clock on reception, or the frequency of the carrier transmitted by the relevant satellite if we are interested in the Doppler effect affecting the carrier of the signal on reception.

Over the second path part going from the shell 3 to the monitoring station 5, the speed of approach to be taken into account for the Doppler effect is the speed of approach between the monitoring station 5 and the shell 3. Assuming that the monitoring station 5 is motionless on the ground at a point R, this speed of approach rap Ro is expressed by the relation: The frequency of the wave to be considered is either, as before, the rate of throughput of the pseudo-random binary sequence of the spreading code, or the frequency of the initial carrier originating from the satellite after its transposition in the transponder performing the relaying.

Figure 2 illustrates the consequences of the clock bias of a transponder on the expected spectral position of the satellite signals from a GPS positioning system after they have passed through a transponder when the latter performs a change of frequency.

The signals transmitted by a GPS satellite consist of two L band carriers, at 1,575.42 MHz and 1,227.6 MHz phase modulated, either by a spreading code consisting of a pseudo-random binary string having a throughput of 1.023 MHz, or by a spreading code consisting of a pseudo-random binary string having a throughput of 10.23 MHz, or simultaneously, in quadrature by the two above spreading codes, and overmodulated in dual-phase mode by low throughput data (50 MHz).

In what follows, consideration will be given only to the spreading code at the lowest throughput of 1.023 MHz, the line of reasoning being easily transposable to the other spreading code at the faster throughput of 10.23 MHz.

The spreading code at the throughput of 1.023 MHz occupies a bandwidth of 2.048 MHz. By phase modulating a carrier, it produces a transmission signal occupying twice the bandwidth, 4.096 MHz, centered on the carrier, either 1.575.42 MHz, or 1,227.6 MHz. To take account of the Doppler effect, it is possible to take a certain safety margin and it is assumed to occupy a window slightly wider than 4.58 MHz.

A transponder, when it retransmits such a signal, frequency translates it into a band suitable for forwarding. To do this, it uses a transposition frequency which it produces from a more or less accurate internal clock which is however independent of that of the GPS satellites. The bias and the drift of the clock cause a bias and a drift in the change of frequency carried out by the transponder, which is found again, on reception, in the error signal of a carrier recovery loop. To estimate, on reception, this bias and this drift in the central frequency of the signal retransmitted by the transponder, it is customary to have the transponder transmit a transposition driver via which a receiver can get back to the actual frequency of the clock of the transponder .

Figure 2 summarizes the situation encountered in the presence of retransmission, with change of frequency, of a GPS satellite signal, by a transponder also producing a transposition driver.

In theory, the GPS satellite signal is expected to be found in a spectral window 1 centered on a frequency Fx whereas the transponder shifts it into a spectral window 1' centered on a frequency Fi+AFi- We expect a transposition driver 2 at the frequency F2 whereas a transposition driver 2' is found at the frequency F2+AF2.

Let: AFQ be the gap with respect to the preset frequency F2 affecting the transposition driver actually produced by the transponder, Vrap OR be the radial speed of approach existing between the carrier of the transponder and the receiver of the monitoring station fixed on the ground utilizing the signals from the transponder, and C be the speed of light which is adopted, by approximation, as radio wave propagation speed.

The frequency gap AF2 affecting the transposition driver such as it is perceived from the receiver utilizing the signals from the transponder is expressed as a function of the actual frequency gap AFQ affecting the transposition driver when it is generated in the transponder by the relation: AF2=AFQ+^-x(F2+AFQ) or else: or by introducing the radial speed V0R of the shell with respect to the receiver which is taken positive when the latter is retreating: AF2=AFQ- OR x(F2+AF0) (1) This gap also takes account of the Doppler effect, on the initial frequency F2+AFQ of the transposition driver, resulting from the relative movement of the carrier of the transponder with respect to the receiver utilizing the signals from the transponder.

Let: Fo be the carrier frequency of the signal transmitted initially by a satellite of the positioning system, Vrap osi be the radial speed of approach between the signal transmitting satellite and the carrier of the transponder, and K be the ratio existing within the transponder between the carrier and the transposition driver.

The frequency gap AFi affecting the transposition frequency Fx which is the carrier frequency of the signal transmitted by the satellite after its transposition in the transponder, such as it is perceived from the receiver utilizing the signals from the transponder, depends on the side band used during the transposition. Considering the side band used during the transposition to be the lower side band, this gap AFi is expressed as a function of the frequency gap KAFQ affecting the transposition carrier and of the two successive Doppler effects by the relation : ccAF} = KAFQ +^X(F + KAFQ) -ΐ Ι Fq a being a positive sign when the transposition carrier is greater than F0 and a negative sign in the converse case : Fi = CC (KFQ - Fo) or by introducing the radial speed V0R of the shell with respect to the receiver which is taken positive when the latter is retreating, and the radial speed V0Si of the satellite with respect to the shell, which is taken positive when the satellite is retreating from the shell: aAF =KAFQ -^x( , + KAFQ) + ^- F0 (2) This gap takes into account both the Doppler effect due to the relative movement of the carrier of the transponder with respect to the receiver utilizing the signals from the transponder and the Doppler effect due to the relative movement of the carrier of the transponder with respect to the satellite from which the forwarded signal stems. It may also be cast into the form: a&FL = K FQ - -^- (CKFQ - ccF0 + K FQ ) + -^ - F0 (3) which will be useful hereinbelow.

As far as the gap AFC in the frequency of the code as viewed from the receiver with respect to its preset value is concerned, it is due to the combination of a first Doppler effect on this same frequency at the start of the signal path, when it propagates from its satellite of origin to the carrier of the transponder and to a second Doppler effect again on this same frequency when the signal propagates from the transponder to the receiver. With the notational conventions adopted previously with regard to the speeds, it is possible to write, as a first approximation (that is to say, if no account is taken of the cross terms invoking the product of the two speeds of approach over the square of the speed of light whose influence is very small relative to that of the other terms) : ^ * rcode £ A rcode g X Γ code ¾ ' The error signal of the code recovery loop is utilized, outside the code recovery loop, for its delay or its phase shifting which is proportional to a pseudo- distance consisting of the sum of the distances separating the craft carrying the transponder on the one hand, from the satellite from which the signal stems and on the other hand, from the receiver. Its frequency component expressed by relation (4) constitutes a spurious term in respect of the processing performed outside the demodulator by the signal utilization part of the receiver.

The error signal of the carrier recovery loop, once stripped of its component due to the inaccuracy in the transposition carrier of the transponder, now stems from Doppler effects only. It therefore carries information regarding the speeds of the relative movements of the carrier of the transponder in relation to the satellite of the positioning system from which the signal processed in the receiver stems and in relation to the receiver. In order for it to be utilizable and to lead to pseudo-speed information which is consistent with the pseudo-distance information derived from the delay of the code recovery loop, it must arise from Doppler effects which apply at a known single frequency and originate from the speeds of the relative movements encountered over the paths corresponding to the pseudo-distances measured moreover by means of the phase shift of the error signal of the carrier recovery loop. Relation (3) shows that this is not generally the case. It is not therefore possible,' in the general case, to extract, in a simple manner, pseudo-speed information from the error signal of the carrier recovery loop. This difficulty is customarily sidestepped by obtaining the pseudo-speeds by differentiating the pseudo-distances, this amounting to not utilizing the information contained in the slaving signal of the carrier recovery loop and hence to a loss of information.

It is proposed that the error signal of the carrier recovery loop be rendered utilizable in respect of pseudo-speed information which is consistent with the pseudo-distance information deduced from the delay of the code recovery loop.

To render the error signal of the carrier recovery loop utilizable in respect of pseudo-speed information which is consistent with the pseudo-distance information deduced from the delay of the code recovery loop, it is necessary for the Doppler effects pertaining to just a single frequency to be highlighted in this error signal. With the aim of being able easily to correct the spurious Doppler effect affecting the slaving signal of the code recovery loop (relation (4)), it is sought to obtain, from the error signal of the carrier recovery loop, a single Doppler component of the form: This amounts to defining a corrective term AFcor, for example, by the system of relations: OAF, = KAFQ + FQ+AFC C OAF, = KLFn -^-x(aKFQ -aF0 + KAFQ)+^xF0 C from which the definition of the corrective term AFcor is derived: Wcor=(a-l)^FQ-K^-(aFQ +AFQ) (5) We note that, in the case where the sign coefficient a is positive, the corrective term AFcor corresponds to K times the Doppler effect affecting the transposition driver (relation 1) . By imparting this corrective term to the apparent frequency Fi of the carrier picked up at receiver level, it is then possible to obtain a corrected apparent carrier frequency enjoying a frequency shift corresponding to that which a Doppler effect would impart on the carrier frequency F0 if the receiver and the satellite from which the signal stems had a relative approach speed equal to the sum of the radial retreat speeds V0s and V0R. AS it is, apart from the sign, the Doppler shift affecting the code frequency Fc, we then recover the consistency between distance measurement and speed measurement, this being the objective sought.

Figure 3 illustrates a first way of using such a correction signal in a demodulation stage of a receiver processing signals from positioning satellites relayed at the level of a craft, by a transponder performing an infradyne frequency transposition, that is to say by means of a lower side band, obtained with a transposition carrier FQ greater than the carrier Fo of a signal originating from a satellite of the positioning system, this corresponding to a type of architecture for which the sign coefficient a is positive, and transmitting a transposition driver F2.

This demodulation stage is disposed, inside a satellite-based positioning receiver, ahead of a signal utilization part 50 and behind various input stages (not represented) . The signal utilization part 50 provides the pseudo-distance and pseudo-speed of the transponder carrier craft with respect to the various visible satellites of the positioning system as well as the position and the speed vector of the craft carrying the transponder and the clock of the positioning system. The various input stages of the satellite-based positioning receiver placed upstream of the demodulation stage perform a preprocessing of the signal from the transponder making it possible to remove therefrom any auxiliary transmission carrier, to reduce the signal to video band, to sample the signal and to digitize it so that it takes the form of a string of complex digital samples, that is to say one having an in-phase component and a quadrature component, which is suitable for digital processing.

To simplify the figure, the filters which are conventionally found before and after the demodulators for eliminating the spurious bands and decreasing the noise have been omitted, and the signals from the satellites considered in their complex form.

This demodulation stage essentially features a code recovery loop 20 nested within a carrier recovery loop 30. The signals from satellites relayed by the transponder and transposed into video band are first subjected to a synchronous detector 31 operating with the aid of a local carrier HL delivered by a local carrier generator 32 forming part of the carrier recovery loop 30. At the output of the synchronous detector 31, the satellite signals, which are available in baseband but still in spread band, are despread by a correlator 21 with spreading codes produced locally by a spreading codes local generator 22 forming part of the code recovery loop 20.

The spreading code recovery loop is a conventional loop having a correlation window. It derives its slaving signal from the comparison of the correlation values of the signal with two local versions of the spreading code which are shifted by a constant offset, the one A advanced, the other R delayed with respect to a local version of the spreading code P, the so-called in-phase version, actually used for the despreading of the signal. It searches for the position in time of these advanced A and delayed R versions of the local code leading to correlations of identical values with the signal to be despread. Specifically, the law of variation of the value of correlation between the signal to be despread and its assumed spreading code, as a function of the positioning error of the one with respect to the other, exhibits a triangular shape with a maximum when the error vanishes. On either side of this maximum, the value of the correlation decreases progressively. If the advanced and delayed offsets are chosen less than the semi-width of the base of the triangle of the correlation function, the search for identical correlation values in respect of the advanced and delayed versions of the local spreading code makes it possible to bracket the maximum and to make the in- phase version of the local spreading code coincide with the code of the signal to be despread.

To do this, the spreading code recovery loop 20 comprises two auxiliary correlators 21a, 21r, a spreading code local generator 22 providing the three versions mutually shifted in time, of the desired spreading code, a comparator 23 connected at the output of the auxiliary correlators 21a and 21r and a loop filter 24 providing a slaving signal used to act on the timing of the spreading code local generator 22 so that the local spreading code can be made to coincide with that of the signal to be despread.

This spreading code recovery loop with correlation window, which operates using an error criterion based on energy considerations is hardly sensitive to the phase errors affecting the local carrier.

The carrier recovery loop derives its slaving signal from the despread signal emanating from the correlator 21 operating with the in-phase version P of the spreading code by means of a circuit 33 for extracting a harmonic spectral line 1 from the carrier using for example the Costas technique. As is customary, a loop filter 34 is interposed between the phase control of the local carrier oscillator 32 and the circuit for extracting a harmonic spectral line 33.

The spreading code is specific to a satellite of the positioning system. It makes it possible to separate on reception, the signals transmitted by the various visible satellites of the positioning system. When the local spreading code and the local carrier are synchronous, the demodulation and the despreading make it possible to obtain access to data transmitted by the satellites specifying their positions and their clock. This information is not essential here since a processing station can obtain it from the signals which it receives directly from the satellites. On the other hand, the delay of the local spreading code and the frequency gap of the local carrier with respect to the transmission carrier provide information regarding the transmission offset and regarding the Doppler effects which one may hope to derive from the particulars regarding the position and the speed vector of the craft .

As was seen previously (relation 4), the code clock of the signal received has undergone, in addition to a phase shift 0C due to the transmission offset, an apparent frequency gap AFC : V +V AF c = - 05 Q 0R xF c^ode so that the slaving signal of the code recovery loop, which is accessible at the output of the loop filter 24 and which supervises the clock of the code local generator 22 with respect to an absolute phase reference consisting of the phase of the code on transmission by a satellite and derived from the data forwarded by the satellites, exhibits, when this loop is locked, a continuous component dependent on the phase shift 0C and an alternating component dependent on the apparent frequency gap AFC .

During the subsequent processing of this signal by the signal utilization part 50 of the receiver, only its continuous component is useful, since it makes it possible, in conjunction with information regarding the phase of the code generator used on transmission, forwarded by way of the data transmitted by the satellites, to assess the effective phase shift of the local code with regard to the transmission code and hence the transmission . offset. The alternating component is a parasitic spurious term.

The carrier of the signal received has undergone a phase shift Θι due to the transmission offset and an apparent frequency gap AFi which is given by the relation : derived from relation (3) by putting: a = 1 or again, by taking relation (1) into account V V oAF = KAF2 + Y OSi _^ v OR xF0 c c with: AFcor = KAF2 so that the slaving signal of the carrier recovery loop which is available at the output of the loop filter 34 and which supervises the carrier local generator 32, with respect to a relative phase reference consisting of the apparent phase of the carrier of the signal received, exhibits, when this loop is locked, an alternating component stemming from two different origins : the gap KAF2, with respect to its nominal value, affecting the apparent frequency of the transposition carrier viewed from the receiver, and a Doppler shift affecting the carrier F0 of the initial satellite signal from the positioning system on account of the relative craft/satellite and craft/receiver motions.

The component KAF2 representing the gap of the transposition carrier of the transponder as seen from the receiver, with respect to its nominal value, is a spurious term in respect of the subsequent processing of this signal by the signal utilization part 50 of the receiver which is responsible for extracting, from this carrier recovery loop slaving signal, the pseudo-speed. It is eliminated by means of a subtractor circuit 42 which receives on its other input, an estimate of its value delivered by a circuit for detecting the frequency gap of the transposition clock 40 followed by a scaling circuit 41.

The circuit 40 for detecting the frequency gap of the transposition clock uses the presence of the transposition driver in the signal retransmitted by the transponder to measure its frequency gap with respect to its nominal value. It comprises a transposition driver recovery loop whose input is connected in parallel with that of the demodulator and a circuit for measuring the frequency gap existing between the recovered transposition driver and its preset value provided by a frequency standard.

The scaling circuit 41 multiples the frequency gap delivered by the detection circuit 40, by the ratio K existing between the frequency of the transposition driver F2 and the transposition carrier used by the transponder, and delivers the frequency gap KAF2 which is then applied to the subtractor 42 interposed in the link conveying the slaving signal from the carrier recovery loop 30 to the signal utilization part 50 of the receiver.

Thus, available at the output of the subtractor 42 is a signal resulting from a combination of two Doppler effects on one- and the same frequency F0 which is utilizable and affords access to the pseudo-speed V0s of the craft since it is possible to eliminate the shell /receiver radial speed of approach V0 between the signals originating from the various visible satellites by considering it to be a common-mode error.

The signal available at the output of the subtractor 42 is applied to the signal utilization part 50 of the receiver so as to extract therefrom the pseudo-speed of the craft. It is also used to eliminate, from the slaving signal of the spreading code recovery loop 20, its parasitic alternating component: before proceeding to the extraction of the pseudo- distance of the craft. To do this, the signal available at the output of the subtractor 42 is subjected to a scaling circuit 43 which assigns it a scale ratio Κχ : j _ Fcode 1 F then added to the error signal of the code recovery loop 20 by an adder 44. The pseudo-distance emanates from the registers of the accumulator 22.

In practice, when it starts up, the carrier recovery loop exhibits a certain lock-on time which implies that the correction of the slaving signal of the code loop before its utilization by the signal utilization part 50 of the receiver is efficacious only with a certain delay. To improve the efficacy of this correction, the signal originating from the carrier recovery loop is replaced momentarily, during the lock-on period of the carrier recovery loop 30, by means of a routing of signals 45, with a signal synthesized in a circuit 50a of the signal utilization part 50 of the receiver which runs a ballistic model of the motion of the craft on its departure.

Figure 4 shows a variant of the previous demodulation circuit, in which the corrections imparted to the slaving signals of the code recovery and carrier recovery loops, before their application to the signal processing part 50 of the receiver of the positioning system, are made inside the loops themselves, thereby making it possible to reduce the ranges of variation of the slaving signals of these loops and hence to improve their tracking and capture performance.

In this figure 4, the elements which are unchanged relative to the diagram of the previous figure 3 retain the same indices .

The correction KAF2 performed on the slaving signal of the carrier recovery loop 30' is made actually inside this loop by means of an adder 42' interposed ahead of the control input of the carrier local oscillator 32. In the same way, the Doppler effect correction performed on the slaving signal of the code recovery loop 20' is made inside this loop by means of a subtractor 44' interposed ahead of the control input of the code local generator 22.

Figure 5 shows an exemplary transponder architecture useable with the satellite-based positioning system receiver whose demodulation stage has just been detailed with regard to figures 3 and 4. The signals transmitted by the satellites of the positioning system at a frequency F0 are received, onboard the craft equipped with the transponder, by the latter 's reception antenna 10. They may then possibly be filtered so as to limit the noise band to the useful band, be amplified in an amplifier 11 and then subjected to a change of frequency by means of a modulator 12 followed by a low-pass filter 13 eliminating the upper side band resulting from the modulation. After having undergone the change of frequency, the signals from the satellites, which are again at the frequency Fi experience the addition by a summator circuit 14 of a transposition driver at a frequency F2, and are then used to modulate a transmission carrier in a second modulator 15. The transposition carrier is obtained by multiplying by a coefficient K, in a multiplier circuit 16, the signal originating from a local clock 17 which also serves as transposition driver. The transmission carrier is produced by a particular oscillator 18 which does not have to be highly accurate since it is not involved in the behavior of the carrier recovery and spreading code recovery loops of the demodulation stage of the receiver .

In the transponder, the transposed signal results from choosing a transposition carrier of higher frequency than that of the signal picked up and from choosing a lower side band. The transposition carrier KF2 is related to the frequency F0 of the signal initially transmitted by a satellite of the positioning system and picked up for relay by the transponder, and to the frequency Fi of transposition of the signal within the transponder by the relation: KF2 = Fo + Fi To aid comprehension, the demodulation circuits proposed in respect of the reception processing of the satellite signals from a positioning system which are relayed by a transponder placed onboard a craft, have been represented in the form of diagrams consisting of assemblages of boxes carrying out various functions. In practice, the satellite signals are often digitized ih the input stages of the receiver, upstream of the demodulation circuit, so that all the processing which they undergo in the demodulation stage is digital processing carried out by means of one or more signal processors. These signal processors, driven by software, perform various tasks corresponding to the functions eluded to, in parallel or under time sharing, in an order suited to the digital techniques implemented without necessarily complying with the split adopted for the description.

Claims

1. A system for tracking a craft from a monitoring station by means of signals transmitted by satellites of a positioning system and frequency-transposed and relayed by a transponder placed onboard the craft, said signals transmitted by the satellites of the positioning system each resulting from the modulation of a carrier by a spreading code, this craft tracking system comprising: onboard the craft, a transponder retransmitting the signals from the satellites of the positioning system after an infradyne frequency transposition by means of a transposition carrier of higher frequency KFQ than that F0 of the satellite signals, by associating a transposition driver FQ representative of the phase and frequency of the carrier used for the transposition KFQ, and, at the monitoring station, a receiver responsive to the signals retransmitted by the transponder, with: a signal processing part incorporating a demodulation stage with a spreading code recovery loop (20, 20') nested within a carrier recovery loop (30, 30')/ each slaved to its own error signal, the error signal of the code recovery loop (20, 20') being representative of the delay affecting, on reception, the spreading code of the signal undergoing demodulation and the error signal of the carrier recovery loop (30, 30') being representative of the frequency gap affecting, on reception, the carrier of the signal undergoing demodulation, a part (40, 41) for determining the frequency gap KAF2, with respect to its preset value, of the transposition carrier used by the transponder, operating on the basis of the transposition driver received from the transponder, means for correcting the error signal of the carrier recovery loop, of the bias caused by the frequency gap AF2 determined by the part for determining frequency gap, and a part (50) for signal utilization, extracting information regarding the position and the speed of the craft from the error signals of the code recovery loop (20, 20') and of the carrier recovery loop (30, 30') of the processing part, the error signal of the carrier recovery loop being corrected beforehand of the frequency gap KAF2 delivered by the frequency gap determining part (40, 41) , said craft tracking system being characterized in that it comprises: at the level of the monitoring station, means (50) for interpreting the error signal of the carrier recovery loop, after correction of the bias caused by the frequency gap KAF2 determined by the frequency gap determining part (40, 41) as a measurement representative of the Doppler effect caused, on the initial frequency F0 of the signal relayed by the transponder, by an approach speed corresponding to the sum of the craft/satellite V0s and craft/receiver V0R relative retreat speeds.

2. The craft tracking system as claimed in claim 1, characterized in that the receiver part (40, 41) relating to the determination of the frequency gap affecting the transposition carrier .of the transponder comprises: a loop for recovering the transposition driver (40) , a circuit (40) for measuring the frequency gap of the transposition driver with respect to a preset value, and a first scaling circuit (41) operating on the error signal delivered by the gap-measuring circuit (40) so as to take account of the ratio K existing between the transposition driver and the transposition carrier.

3. The system for tracking a craft as claimed in claim 1, characterized in that the receiver comprises a first subtractor (42) which is interposed on a link traversed by the error signal of the carrier recovery loop (30) so as to reach the. signal utilization part (50) of the receiver and which deducts, from the error signal of the carrier recovery loop (30) , the signal delivered by the part (40, 41) for determining the frequency gap of the transposition carrier.

4. The system for tracking a craft as claimed in claim 3, characterized in that the receiver furthermore comprises an adder (44) which is interposed on a link traversed by the error signal of the code recovery loop (20) so as to reach the signal utilization part (50) of the receiver and which adds to the error signal of the code recovery loop, the error signal of the carrier recovery loop (30) once it has been corrected of the frequency error made by the transponder and scaled in the ratio of the code frequency Fc to the carrier frequency F0.

5. The system for tracking a craft as claimed in claim 1, with a receiver comprising a carrier recovery loop (30') provided with a local oscillator (32) driven by an error signal, by way of a phase control input, characterized in that the receiver comprises an adder (42') which is interposed in the carrier recovery loop (30'), ahead of the phase control input of the local oscillator (32) and which adds to the error signal of the carrier recovery loop (30'), the signal delivered by the part (40, 41) for determining the frequency gap of the transposition carrier.

6. The system for tracking a craft as claimed in claim 1, with a receiver comprising a code recovery loop (20') provided with a local code generator (22) whose clock is driven by an error signal, by way of a phase control input, characterized in that the receiver furthermore comprises a subtractor (44') which is interposed in the code recovery loop (20') ahead of the phase control input of the clock of the local code generator (22) of the receiver and which deducts from the error signal of the code recovery loop (20'), the error signal of the carrier recovery loop (30) once it has been corrected of the frequency error made by the transponder and scaled in the ratio ' of the code frequency Fc to the carrier frequency F0, as delivered by .the part (40, 41) for determining the frequency gap of the transposition carrier. For the Applicant, C: 45753