EP1613976A1

EP1613976A1 - Method for the acquisition of a radio-navigation signal by satellite

Info

Publication number: EP1613976A1
Application number: EP04720009A
Authority: EP
Inventors: Nicolas Thales Intellectual Property MARTIN; Valéry Thales Intellectual Property LEBLOND
Original assignee: Thales SA
Current assignee: Thales SA
Priority date: 2003-04-15
Filing date: 2004-03-12
Publication date: 2006-01-11
Also published as: WO2004092761A1; FR2853967B1; US20060097915A1; FR2853967A1; CA2522059A1

Abstract

A method for the acquisition of radioelectric signals, more particularly by a satellite positioning system, comprising at least one sub-carrier. The acquisition of said signals occurs by means of a receiver comprising: an in-phase and ninety-degree-phase-shift carrier correlation path (10, 30, 110) between the received signal and two respective local in-phase and ninety-degree-phase-shift carriers; a sub-carrier correlation path from signals at the output of the carrier correlation path with a local sub-carrier; a code correlation path (16, 40, 114) from signals at the output of the sub-carrier correlation path with local codes formed by a digital local code generator (19, 36); in a first acquisition phase the sub-carrier correlation path (34) comprises two in-phase and ninety-degree-phase-shift paths between the signals at the output of the carrier correlation path and two respective local in-phase and ninety-degree-phase-shift sub-carriers in relation to the local code generated by a digitally controlled local sub-carrier oscillator, whereupon an energy search is carried out by detection of a correlation peak.

Description

METHOD FOR ACQUIRING A SATELLITE RADIONAVIGATION SIGNAL

The invention relates to a method for acquiring radio signals in particular those emitted by satellite positioning systems of the GPS (Global Positioning System), Galileo, GLONASS (Global Navigation Satellite System, Russian definition) type.

Satellite positioning systems use, for localization, several satellites transmitting radio signals and a receiver placed at the position to locate estimating the so-called pseudo-distances, which separate it from the satellites from the signal propagation times. satellites captured and performing localization by triangulation. The more precisely the positions of the satellites are known to the receiver and the more precise the measurements of the pseudo-distances made by the receiver, the more precise the location obtained.

The positions of the satellites are determined from a network of tracking ground stations independent of the positioning receivers. They are communicated to the positioning receivers by the satellites themselves by data transmission. The pseudo-distances are deduced by the positioning receivers from the apparent delays presented by the signals received with respect to the clocks of the satellites which are all synchronous.

If the accuracy of the knowledge of the positions of the satellites of the positioning system is independent of the performance of a positioning receiver, this is not the case for that of the pseudo-distance measurements which depend on the accuracy of the time measurements of signal propagation at the receiver.

The radio signals emitted by satellites traveling long distances, and being emitted with limited powers, arrive with very low powers at the receivers, drowned in a radio noise due to the physical environment. In order to facilitate their reception, attempts have been made to make them as less sensitive as possible to narrow band parasites, by increasing their bandwidths by means of the spread band technique. The signals transmitted by the satellites are formed by modulation of the signal carrier with a spreading code formed by a pseudo-random binary sequence. Thus, satellite signals allow two types of measurement in order to locate the receiver. In addition, the modulation of the carrier by a spreading code spreads the spectrum, which increases the resistance of the system to interference. And, in addition, it allows the satellites to be dissociated (using a different code per satellite).

On reception, the binary information contained in a satellite radio signal from a positioning system is extracted by two demodulations carried out simultaneously, a first demodulation using a carrier generated locally by an oscillator controlled by a loop. tracking in frequency or in so-called PLL phase (acronym taken from the Anglo-Saxon: "Phase Lock Loop") allowing the signal received to be transposed into baseband and a second demodulation using the pseudo-random binary sequence generated locally by a pseudo-random binary sequence generator controlled by a code tracking loop called DLL (acronym drawn from the Anglo-Saxon: Delay Lock Loop) allowing despreading of the received signal.

The propagation times of the received signals are manifested, on reception, by delays affecting the pseudo-random binary sequences present in the received signals and the carrier modulating the received signal.

The delays affecting the pseudo-random binary sequences are accessible, modulo the period of one of their binary sequences, at the level of the control signals of the tracking loops in code or DLL. The delays observed by these loops allow unambiguous or weakly ambiguous measurements, propagation times of the pseudo-random binary sequences because the number of entire pseudo-random sequences flowing during the signal paths is relatively small. We are talking about code measures. Generally the modulation used in satellite navigation systems is a BPSK type modulation, "Binary Phase Shift Keying" in English or square modulation whose spectrum has a single main lobe with adjacent secondary lobes. In order to improve navigation performance, inter alia resistance to interference and accuracy in measuring the position of the receiver, the new satellite navigation systems propose using a BOC “Binary Offset Carrier” modulation in English, or double shift carrier modulation, the spectrum of which has two main lobes apart. FIG. 1 a represents such a modulation modulation spectrum. BOC type and Figure 1b shows the form of the auto-correlation function of such a BOC signal. BOC type modulation should be preferred to BPSK modulation because it allows different use of the available band. military applications, this allows energy to be recovered when the band used by the BPSK modulation in the center is scrambled For civilian applications, it makes the radionavigation system compatible with American systems which use different bands In addition, with modulation BOC type, receiver performance is improved because the spectrum is more spread out

Each signal transmitted by a visible satellite and received by the antenna must be demodulated by the receiver, in order to deduce therefrom a measurement of propagation time, of Doppler, and possibly of transmitted data.

Demodulation consists in slaving a locally generated signal, image of the signal received from the satellite considered characterized by a clean spreading code and a carrier, by seeking the maximum correlation between this received signal and the local signal

The control is carried out by a carrier loop, which controls the phase of the local carrier, and by a code loop which controls the position (or phase) of the local code. The carrier loop measures a carrier phase difference between the local signal and the received signal thanks to the correlation with a local signal in quadrature of carrier The code loop measures a phase difference of code between the local signal and the received signal thanks to the correlation with local signals modulated by drift codes (advance, delay or delta)

As soon as the servo has converged, the Doppler and propagation time measurements are developed from the frequency of the local carrier and the position of the local code respectively.

Measurement errors come from the presence in the received signal Sr, in addition to the useful signal of the satellite considered, signals from other satellites and noise of various origins (thermal, quantification, interference etc.) which disturb the servo-control and cause synchronization errors between the local signal and the received signal.

The purpose of the acquisition phase is to initialize the operation of the tracking loops, because at the beginning we do not know precisely the position of the received code, nor the value of the Doppler. However, the loops only work if the position of the code and the Doppler are close to that of the useful signal of the satellite considered. If one of the deviations is too large, the zero correlation gives no more information (no energy detected E), and the servo can no longer function. For this, a search for a correlation peak between the local signal and the received signal is carried out during a first so-called acquisition phase, in a two-dimensional space, by testing several hypotheses on the phase of the code and on the Doppler value, with a sufficiently fine sampling step so as not to miss the peak. Once a peak has been found, the code and Doppler search is refined by decreasing the sampling step, around the detected peak. When the precision obtained is deemed sufficient, the loops, which converge towards the maximum correlation, are closed: we then go into the tracking phase.

FIG. 2 shows the block diagram of a state-of-the-art satellite positioning receiver during a first acquisition phase with a received signal of the BPSK type. The receiver comprises a carrier correlation channel 10 in phase and in quadrature between the received signal Sr and two respective local carriers Fi, F _Q. These local carriers in quadrature (sin, cos) are generated by an oscillator with numerical control of carrier 12 (NCO p) of the receiver.

The signals I, Q at the output of the carrier correlation channel are then correlated in a code correlation channel 16 with the local, point and delta code, supplied by a digitally controlled NCO c code carrier oscillator 18 and a local code generator Gc 19. The signals at the output of the code correlation channels 16 are then integrated by a respective code integrator 20, 22 to supply signals l _P and Q _P to an energy detection DEng 24 for detection signal acquisition

The sum of the energies supplied by the correlation channels of the receiver in FIG. 2 is given by the relation: E = Σ (l _P ² + QP ² ) Signal detection is considered to be obtained when this energy E exceeds a predetermined energy threshold SI.

However, modulation of the BOC type has drawbacks. Indeed, the acquisition of a BOC type signal is more difficult than that of a BPSK type signal because of the oscillations of the autocorrelation function. On the one hand, the zeros z of the autocorrelation function (see Figure 1b) risk generating missed detections (no energy detected). On the other hand, the multiple p peaks induce an ambiguity, when one seeks to enslave on a local maximum of correlation, which must be resolved later.

One solution to overcome this drawback consists in treating only one main lobe Lb after analog filtering. Figure 3a shows the spectrum of the resulting signal after filtering and Figure 3b the resulting autocorrelation function after off-center of the local frequency. The treatment of a single lobe makes it possible to recover a correlation function without oscillation. However, this solution leads to a loss of half of the signal energy, which increases the acquisition threshold all the more. In addition, this requires filtering the signal and reviewing the signal processing (offset carrier)

In order to overcome the drawbacks of state-of-the-art radio navigation receivers, the invention provides a method of acquiring radio signals transmitted in particular by a satellite positioning system comprising at least one subcarrier, the acquisition signals carried out by a receiver having:

a carrier correlation channel, in phase and in quadrature between the received signal and two respective local carriers in phase and in quadrature generated by a local carrier oscillator with digital control; a subcarrier correlation channel from the signals output from the carrier correlation channel with a local subcarrier;

a code correlation channel from the signals output from the subcarrier correlation channel with the local codes supplied by a digital local code generator; characterized in that in a first acquisition phase, the subcarrier correlation channel comprises two channels in phase and in quadrature between the signals at the output of the carrier correlation channel and two respective local subcarriers in phase and in quadrature with respect to the local code generated by a local subcarrier oscillator with digital control, the receiver being configured so that in this first phase of signal acquisition an energy search is carried out by the detection of a correlation peak.

In a variant of the acquisition method according to the invention, the receiver is configured so that in the first phase of signal acquisition, the phase of the subcarrier of the received signal is eliminated by summing the powers in phase and in quadrature of subcarriers at the output of correlation channels, then in the same way a search for an unambiguous correlation peak is carried out. In a second phase of acquisition of the received signal, the loops are controlled from the outputs of the correlators causing the local code to converge to the maximum of the code correlation peak, independently of the subcarrier.

The new idea is to eliminate the subcarrier in the same way as one eliminates the carrier, after coherent integration, by summation of the energies collected on the correlation channels in phase and in quadrature. For this purpose, two local subcarriers are generated in phase and in quadrature in addition to the two local carriers in phase and in quadrature and local codes (punctual, advance, delay or delta). The method according to the invention can be implemented according to two methods:

- in a first method the local code and the local subcarrier are synchronous. The phase of the local subcarrier is a multiple of the local code. The two phases come from the same digital controlled local oscillator (NCO) controlled in speed and operating as an integrator.

- in a second method, the local code and the local subcarrier are asynchronous.

The receiver also supplies, in known manner, from the integrated signals at the output of the code correlation channel, the speeds of carrier, subcarrier and code to control the respective numerically controlled oscillators generating the carriers, subcarriers and local codes

The invention will be better understood using examples of receiver implementations implementing the acquisition method according to the invention, with reference to the accompanying drawings, in which

- Figures 1a and b, already described, respectively show a BOC type signal and the auto-correlation function of a state-of-the-art receiver, - Figure 2, already described shows the block diagram d '' a state-of-the-art satellite positioning receiver during the acquisition phase,

- Figures 3a and 3b, already described, respectively show the spectrum of the BOC type signal after filtering of one of the lobes and the resulting auto-correlation function after decentralization of the local frequency, - Figure 4 shows the block diagram d 'a receiver according to the invention during the acquisition phase,

FIGS. 5a, 5b respectively show the code received of BPSK type without modulation by the subcarrier and the code received of BOC type with modulation by the subcarrier of the receiver of FIG. 4, according to the invention,

FIGS. 5c 5d and 5e respectively show the local code and the two local subcarriers in phase and in quadrature of the receiver of FIG. 4, according to the invention,

FIGS. 5f, 5g and 5h respectively represent the auto-correlation function with the subcarrier in phase, with the subcarrier in quadrature and the envelope of the energy detection at the output of the correlation channels,

FIG. 6 shows curves representing the phase of the local code Φc as a function of time t in the acquisition phase of the receiver according to the invention,

FIG. 7 shows another receiver, according to the invention, with a local code and asynchronous local subcarriers,

FIG. 8 shows the receiver of FIG. 4 during the transition phase to the pursuit in the case where the local code and the local subcarrier are synchronous, - Figure 9 shows a receiver, according to the invention, comprising three digitally controlled oscillators during the transition phase to tracking in the case where the local code and the local subcarrier are asynchronous; FIGS. 10 and 11 represent two receivers in which the servo-control of the carrier and subcarrier phases is carried out independently at the same time as the code;

- Figure 12 shows a receiver in a final phase of tracking without elimination of the subcarrier; - Figure 13 shows a variant of the receiver of Figure 12;

- Figure 14a shows the minimum step P 1 required for code scanning to obtain energy detection with elimination of the subcarrier;

- Figure 14b shows the minimum step P2 required without elimination of the carrier.

We will subsequently describe receivers implementing the method of acquiring a BOC signal according to the invention and by the two methods mentioned above.

FIG. 4 shows a receiver implementing the acquisition method according to the invention, when receiving a spread-band signal of BOC type, by the first method, with a local code and synchronous local subcarriers : according to this first method, the phase of the local subcarrier is a multiple of the local code. Figure 4 shows the elements required during the acquisition phase. The receiver includes:

a carrier correlation channel 30 in phase and in quadrature between the signals received Sr from the positioning satellites and two respective local carriers F | _P , F _QP . These local carriers in quadrature (cos, sin) are generated by an oscillator with numerical control of carrier 32 (NCO p) of the receiver;

a subcarrier correlation channel 34 in phase and in quadrature between the signals I _PT and Q _PT at the output of the carrier correlation channel and two respective local subcarriers Fis, FQS in phase and in quadrature; a code correlation channel 40 between the signals at the output of the subcarrier correlation channel and the local codes supplied by the digital local code generator 36.

- an NCO code oscillator c 38 driving a generator of local subcarriers Gsp 42 and the local code generator Gc 36;

- energy detection 44 of signals II, IQ _P , Q | _P , QQP at the output of the code correlation channel after integration by respective integrators 46, 47, 48, 49.

We will then describe how the receiver works. FIGS. 5a, 5b, 5c, 5d and 5e respectively show the code received of the BPSK type without modulation by the subcarrier and the code received of the BOC type with the modulation by the subcarrier, the local code generated by the code generator local Gc 36 and the two local subcarriers in phase and in quadrature. FIGS. 5f, 5g, 5h respectively represent the auto-correlation function with the subcarrier in phase, with the subcarrier in quadrature and the envelope Ev of the energy detection at the output of the correlation channels.

In a first acquisition phase, the signals at the output of the carrier correlation channel 30 comprising the subcarrier of the signal BOC, are applied to the subcarrier correlation channel 34 demodulating the subcarrier. The signals at the output of the subcarrier correlation channel 34 are applied to the code correlation channel 40 providing, after integration, the signals IIP, IQP _, Q | _P , Q _QP at the energy detector 44.

The sum of the energies collected on each of the subcarrier channels in phase and in quadrature makes it possible to detect a single energy peak Pu (see FIG. 5h) and unambiguous identical to that which one would have with a signal not comprising subcarrier. The sum of the energies E is given by the following relation:

E = Σ (IIP ² + IQP ² ₊ Q _IP ² + QQP ² )

The sum E being a non-coherent sum of several samples over a time T multiple of a coherent time Te.

Two solutions to find the energy shown in Figure 6: First solution: we test the code hypotheses by continuously dragging the local code (scanning, curve Bc in Figure 6). In this case, the subcarrier also slides and a coherent integration time is required which is less than the duration of scanning of a portion of a subcarrier peak (we will take a quarter wavelength of sub- carrier) so as not to lose too much energy and reduce the ability to detect the signal in a noisy environment.

Second solution: we test the fixed code hypotheses (curve Bi in Figure 6) by making phase jumps Δφ (time Td1, Td2, Td3, ... Tdn) between integrations. In this case, the subcarrier phase remains constant and there is no loss of energy. Phase jumps Δφ can be generated by accelerating the speed of the local code oscillator (NCO c) over short durations Δt between two integrations, or by another means consisting in instantaneously changing the phase at the output of NCO c and in incrementing the code generator. An energy detection test is carried out after integration with each incrementation or phase jump Δφ.

FIG. 7 shows another receiver implementing the acquisition method according to the invention, when receiving a spread-band signal of BOC type, by the second method, with a local code and local subcarriers asynchronous.

The receiver has three oscillators, a 50 NCO p local carrier oscillator digitally controlled generating the two local carriers FI, FQ _P in phase and quadrature for the carrier correlation channel 30, a 52 NCO sp subcarrier oscillator with control digital generator, by a generator of local subcarriers Gsp, the two local subcarriers F | _S , FQ _S in phase and in quadrature for the subcarrier correlation channel 34 and a code oscillator 54 supplying by a code generator Gc the local code of the code correlation channel 40 of the receiver. The receiver of FIG. 7 like that described previously comprises:

the carrier correlation channel 30 in phase and in quadrature between the signals received Sr from the positioning satellites and the two respective local carriers FIP, FQP generated by the oscillator with digital carrier control 50 (NCO p) of the receiver. the subcarrier correlation channel 34 in phase and in quadrature between signals at the output of the carrier correlation channel and the two respective local subcarriers F | _S , F _QS in phase and in quadrature generated by the local oscillator of subcarrier 52 and of local code with numerical control;

the code correlation channel 40 between the code of the satellite received and the local codes supplied by the digital local code generator 54.

- energy detection 44 of the signals l | _P , l _QP , Q | _P , Q _QP at the output of the code correlation channel after integration by respective integrators 46, 47, 48, 49.

As described above, in a first acquisition phase, the signals at the output of the carrier correlation channel 30 comprising the subcarrier of the BOC signal, are applied to the subcarrier correlation channel 34 demodulating the subcarrier . The signals at the output of the subcarrier correlation channel are applied to the code correlation channel 40 providing, after integration, the signals l | _P , l _QP, QIP, QQP at the DEng 44 energy detector.

The sum of the energies collected on each of the subcarrier channels (in phase and in quadrature) makes it possible to detect a unique and unambiguous energy peak identical to that which one would have with a signal not comprising a subcarrier.

The sum of the energies E is given by the following relation: E = ∑ (I _IP ² + l _QP ² ₊ Q, p ² + Q _QP ² )

The signal acquisition is carried out by dragging the code to scan the hypotheses to be tested regardless of the phase of the subcarrier. The latter is made consistent with the carrier phase speed to take account of the Doppler. In a variant of the receiver in FIG. 7, the local subcarrier oscillator is saved and a single oscillator (NCO) is used for the carrier and the subcarrier, by dividing the carrier phase by the ratio of the lengths d wave to obtain the phase of the subcarrier.

The receivers are configured to perform the following correlation operations: l | P = J [nT, (n + 1) η Sp _eç u - C0S (φ (t)). SP | n phase (t) • Cθdθp _on otuel (t) dt

IQP = J [nT, (π + 1) T] SReceived • G0S (φ (l)). SPQ _Ua drature (t) - Cθdep ₀ nctual (t) dt

Qlp JINT ≈ (π + 1) η S _Re çu • Sin (φ (t)) SPjn phase (t) - Codeponctuel (t) dt

QQP ≈ J [nT, (n + 1) η S _R eceived • Sin (φ (t)). SP _Qu adrat_re (t) - Cθdep _onc tuel (t) dt With

T Duration of coherent integration cos (φ (t)), sιn (φ (t)) Local carriers in phase and in quadrature

SPin p ase, SPouadraiure Local subcarrier in phase and in quadrature

Codep ₀ notuei (t) Local point code

For the control of the phase of the code (transition and pursuit) the same operation is carried out but with a local code in advance Cav, in delay Crt, or “delta”, the delta code being the advance code minus the delay code multiplication being associative and commutative, one can carry out this operation in several ways

the signal received is successively multiplied by the local carrier, the local subcarrier then the local code,

- the signal received is multiplied by the product of the local carrier, the local subcarrier and the local code

- etc

Coherent and inconsistent integration Definition ^•

Coherent integration l _n = J [nτ, (n + i) T] S _Received (t). S _oC al in phase (t) dt

Qn = J [nT, (n + 1) T] Sp _eou (t) SLocal Quadrature (t) dt

Non-coherent integration E = ∑ _n _ •) _{a N} (i _n ² + Q _n ² ) Energy losses: sinc ² ( _{ΔD 0pp} ι _er .T / 2)

With:

SLocal In phase (t) = COS (û ⁾ t). SP | _{π p} hase (t). Cθde _Pon ctuel (t) S ocal Quadrature (t) = Sin (ωt). SP | _n phase (t) - Cθdep _onc tuel (t)

Δ _D oppier: Doppler error between the local carrier and the received carrier

The coherent integration time T is limited by the Doppler which induces energy losses. A coherent integration time that is too short induces quadratic losses which degrade the signal to noise ratio and requires a longer total (non-coherent) integration time.

A long integration time reduces the width of the Doppler peak (in practice the width of the Doppler peak at 3 dB is equal to 1 / 2T) and therefore requires the processing of more Doppler hypotheses.

The choice of the coherent integration duration results from an optimization of the energy search time by a compromise between the time spent on each Doppler hypothesis and the number of Doppler hypotheses. In the case where the subcarrier is dragged with the code, energy losses must also be taken into account. The coherent integration time may need to be reduced if the scanning speed causes the subcarrier phase to travel more than a quarter turn during this integration time. Hence the advantage of proceeding by jumping (first method) or not sliding the subcarrier (second method).

Next, we will describe the transition phase to the receiver tracking phase. Indeed, once energy has been found, it is necessary to refine the synchronization of the carrier frequency and of the subcarrier and local code phases in order to be able to switch to nominal search and benefit from the advantages of BOC modulation ( precision).

We start by closing the code loop using advance correlation and delay correlation channels.

FIG. 8 shows the receiver of FIG. 4 during the transition phase to the pursuit in the case where the local code and the local subcarrier are synchronous. In this tracking phase, the receiver of FIG. 8 generates, from the signals lι _A , IIR, IQA, IQR, QIA, QIR, QQA, QQR, at the output of integrators

80 of the respective code correlation channels, through a code discriminator 90 followed by a code corrector 92, commands to the code oscillator 38 aided by the carrier speed Vp.

The Doppler speed (Vp) applied to the digitally controlled carrier oscillator (NCO p) 32 is that found at the end of the search for energy in the first acquisition phase. In this case, the coherent integration time must be compatible with the residual Doppler error at the end of the energy search phase and also with the joining speed applied to the subcarrier.

FIG. 9 shows a receiver comprising the three digitally controlled oscillators 50, 52, 54, during the tracking phase in the case where the local code and the local subcarrier are asynchronous. In this tracking phase, the receiver of FIG. 9 generates, from the signals IIA, IIR, IQA, IQR. QIA, QIR, QQA, QQR, at the output of integrators 80 from the respective code correlation channels, through a code discriminator 90 followed by a code corrector 92, commands to the code oscillator (NCO c) 54 aided by the carrier speed Vp. The Doppler speed (Vp) applied to the carrier oscillator

(NCO p) 50 and digitally controlled subcarrier (NCO sp) 52 is that found at the end of the energy search in the acquisition phase.

The duration of coherent integration is also unchanged. In this case the speeds of the carrier and subcarrier NCO oscillators are identical. We can also have only one NCO.

The code discriminator provides a signal:

ε _∞dB = (I | _A ² + IQA ² + QIA ² + QQA ² - IIR ² + IQR ² + QIR ² + Q _QR ² ) / Energy

Energy = I | _A ² + I _QA ² + QIA ² + QQA ² + IIR ² + IQR ² + QIR + QQR ² FIGS. 10 and 11 show variants of the receivers of FIGS. 8 and 9 respectively, for the variant of FIG. 10, with a local code and synchronous subcarriers and, for the variant of FIG. 11, with local code and asynchronous subcarriers. In these variants, the servo and subcarrier phases are independently controlled at the same time as the code (processing carried out in parallel). The interest of the method is to refine the measurement of the Doppler and of the carrier phase to help the code loop and to be able to reduce the predetection band (inverse of the coherent integration time) and noise. A better final precision of the code is thus obtained, which reduces the risk of going into nominal BOC tracking on a lateral peak of the autocorrelation function inducing a bias on the measurement.

In the variant of FIG. 10, (with a local code and synchronous subcarriers) the receiver comprises:

the carrier correlation channel 30 in phase and in quadrature between the signals received Sr from the positioning satellites and the two respective local carriers F | _P , F _QP generated by the digital carrier oscillator (NCO p) 32 of the receiver. the subcarrier correlation channel 34 in phase and in quadrature between signals at the output of the carrier correlation channel and the two respective local subcarriers Fis, FQS in phase and in quadrature generated by the local oscillator of Gsp subcarrier and the local code generator Gc controlled by the digitally controlled code oscillator (NCO c) 38;

the code correlation channel 40 between the code of the satellite received and the local code provided by the digital local code generator Gc controlled by the digitally controlled code oscillator (NCO c) 38;

- a carrier discriminator 94 (Dsp) followed by a carrier loop corrector 96 (Crp) supplying from signals l | _P , IQP, QIP, QQP at the output of the code correlation channel after integration by respective integrators 46, 47, 48, 49 a control signal from the carrier oscillator aided by the speed of Doppler Vp. The carrier discriminator provides a signal: Eporteuse = (Ql-I | + QQ-IQ) I ('| P + IQP + Q | P + QQP ~) in a variant:

Eportousβ = Arctan [2 (Q | .l) + QQ.IQ) / (l | .l | + l _Q .l _Q - Q | .Q _t - QQ.QQ) J

In the variant of FIG. 11, (with a local code and asynchronous subcarriers) the receiver comprises:

the carrier correlation channel 30 in phase and in quadrature between the signals received Sr from the positioning satellites and the two respective local carriers FIP, F _QP generated by the numerically controlled oscillator of carrier 50 (NCO p) of the receiver.

the subcarrier correlation channel 34 in phase and in quadrature between signals at the output of the carrier correlation channel and the two respective local subcarriers Fis, FQS in phase and in quadrature generated by the local oscillator of Gsp subcarrier controlled by the numerically controlled subcarrier oscillator (NCO sp) 52.

the code correlation channel 40 between the code of the satellite received and the local codes supplied by the digital local code generator Gc controlled by the numerically controlled code oscillator (NCO c) 54.

a carrier discriminator 100 (Dsp) followed by a carrier loop corrector 106 (Crp), a subcarrier discriminator 102 (Dssp) followed by a subcarrier loop corrector 104 (Crsp) providing respectively from the signals IIP, IQP, QIP, QQP at the output of the code correlation channel, after integration by respective integrators 46, 47, 48, 49, a control signal from the carrier oscillator 50 aided by the speed of Doppler Vp and a control signal of the subcarrier oscillator 52. The carrier discriminator provides a signal:

Subcarrier = (IQ-I | + QQ-Q |) / (l | P + IQP + Q | P + QQP) in a variant: βsubcarrier = Arctan [2 (IQ.I | + QQ.QI) / (l | .l | + Q | .Q | - IQ. IQ - QQ.QQ) l

Rationale: Let φ be the carrier phase deviation and 0 the subcarrier phase deviation (assimilated to a sinusoidal signal)

IIP = A.cosφ.cosO l _QP = A. cosφ. sinO QIP = A.sinφ.cosθ Q _QP = A.sinφ.sinθ

(A: amplitude after correlation with the local point code)

IQ-II + QQ-QI = A .sin0. cosθ (cosφ ² + sinφ ² ) = A ² .sinθ.cosθ = A ² . ^1/2 sin20 l |. + Q | .Qι = A ² .cos0.cos0 (cosφ ² + sinφ ² ) = A ² . cos. cos

IQ.IQ + QQ.QQ = A ² .sin0 .sinθ (cosφ ² + sinφ ² ) = A ² .sinθ.sinθ

li.li + QI.QI - IQ.IQ - QQ.QQ = A ² (cosθ.cosθ - sinθ.sinO) = A ² cos2θ

Qi.li + QQ.IQ = A ² . sinφ. cosφ (cosθ ² + sinθ ² ) = A ² .sinφ.cosφ = A ² . ¹ έ sin2φ li.li + IQ.IQ = A ² .cosφ.cosφ (cosθ ² + sinθ ² ) = A .cosφ.cosφ

QI.QI + QQ.QQ = A ² .sinφ .sinφ (cosO ² 4- sinθ ² ) - A ² .sinφ.sinφ

l) -lι + IQ.IQ - QI.QI - QQ.QQ = A ² (cosφ.cosφ - sinφ sinφ) = A ² .cos2φ IIP ² + IQP + QIP ² + Q _QP ² = A ²

After the transition phase to tracking, the receiver goes to the final tracking phase.

After a convergence time to be determined which depends on the characteristics of the dynamics, the noise level and the gains of the loops, and if the accuracy of the code phase enslavement is deemed sufficient, we go into nominal BOC tracking: we replace the code by the code modulated by the subcarrier.

FIG. 12 shows the receiver in this final phase, without elimination of the subcarrier. The receiver essentially comprises in this tracking phase:

- a correlation channel carrier ^'110, in-phase and quadrature between the received signal Sr and two respective local carriers Fi FQ quadrature generated by a local oscillator carrier CNC 112;

a code correlation channel 114 comprising the subcarrier (BOC type signal as in FIG. 5b), a code generator 116 controlled by the code oscillator 118 supplying the code correlation channel 114 with the signals of advance code Cav, delay Crt and punctual code Cp.

- a carrier discriminator 120 (Dsp) followed by a carrier loop corrector 122 (Crp), a code discriminator 124 (Dsc) followed by a code loop corrector 126 (Crc) respectively supplying from signals l | _P , IQ _P , Q | _P , Q _QP at the output of the code correlation channel after integration, a carrier speed signal to control the carrier oscillator 112 and a code speed signal to control the code oscillator 118 aided by the speed of carrier.

The operation in this last phase is that of a BOC type receiver. The code discriminator providing a signal: εoo β = [(IA- IR) • Ip + (QA - QR) • Qp)] / (lp ² + Qp ² )] or εcode = [(IA - IR) ² + ( QA - QR) ² ] / [(l _A + IR) ² + (QA + Q _R ) ² ]

FIG. 13 shows the BOC type receiver in a variant of the receiver of FIG. 12, in the final phase, without elimination of the subcarrier. In this variant of FIG. 13, the correlation by the advance and delay codes is replaced by a correlation by a delta code CΔ, obtained by differentiating the advance codes Cav and delay Crt.

In the configuration of the receiver in FIG. 13, the code correlation channel 114 comprising the subcarrier (BOC type signal as in FIG. 5b), a code generator 130 controlled by the code oscillator 118 provides the code correlation channel 114, the delta code signals CΔ and point code Cp

The code discriminator providing a signal: ε _codβ = (. Lp + Q _Δ - Qp) / (lp ² + Qp ² ) It can be noted that the local advance and delay BOC codes, obtained by coherently advancing or delaying the local code and the local subcarrier can be replaced by a local punctual code modulated by an advanced and delayed subcarrier.

In the method according to the invention, the integration time of the correlation depends on the duration of the acquisition phase and the ability to find the useful signal in a noisy environment. The best compromise will be obtained by maximizing the signal to noise ratio at the output of the energy detection (the higher the signal to noise ratio, the shorter the total integration time). From where the interest of the method compared to the treatment considering only one lobe, which makes lose 3 dB.

The duration of the acquisition also depends on the sampling step: a fine sampling step increases the number of hypotheses to be tested. From where the interest of the method compared to the scanning without elimination of subcarrier which would impose a step of sampling in code equal to the half width of the principal peak of the function of autocorrelation.

FIG. 14a shows the minimum step P1 necessary for code scanning to obtain an energy detection with elimination of the subcarrier. FIG. 14b shows the minimum step P2 necessary without eliminating the carrier. The minimum step P1 required is much larger than the minimum step P2, so fewer code hypotheses are needed to find energy in the case of elimination of the subcarrier.

Claims

1. Method for acquiring radioelectric signals emitted in particular by a satellite positioning system comprising at least one subcarrier, the acquisition of the signals being carried out by a receiver having:

- a carrier correlation channel (10, 30, 110), in phase and in quadrature between the received signal and two respective local carriers in phase and in quadrature generated by a local carrier oscillator (12, 32, 50, 112) digitally controlled;

a subcarrier correlation channel from the signals output from the carrier correlation channel with a local subcarrier;

- a code correlation channel (16, 40, 114) from the signals output from the subcarrier correlation channel with the local codes supplied by a digital local code generator (19, 36); characterized in that in a first acquisition phase, the subcarrier correlation channel (34) comprises two channels in phase and in quadrature between the signals at the output of the carrier correlation channel and two respective local subcarriers in phase and in quadrature compared to the local code generated by a local oscillator of subcarrier with numerical control, the receiver being configured so that in this first phase of acquisition of the signals an energy search is carried out by the detection of a correlation peak.

2. A method of acquiring radiofrequency signals according to claim 1, characterized in that the receiver is configured so that, in the first phase of signal acquisition, the phase of the subcarrier of the received signal is eliminated by summing the powers in phase and in quadrature of subcarriers in outputs of correlation channels then in the same way one carries out a search for a peak of unambiguous correlation.

3. A method of acquiring radio frequency signals according to one of claims 1 or 2, characterized in that, in a second phase of acquisition of the received signal, the loops are controlled. from the outputs of the correlators making the local code converge towards the maximum of the code correlation peak, independently of the subcarrier.

4. A method of acquiring radiofrequency signals according to one of claims 1 to 3, characterized in that the local code and the local subcarrier are synchronous, the phase of the local subcarrier being a multiple of the local code, the two phases being from the same local numerically controlled oscillator (NCO) controlled in speed and operating as an integrator.

5. A method of acquiring radiofrequency signals according to one of claims 1 to 3, characterized in that the local code and the local subcarrier are asynchronous.

6. A method of acquiring radiofrequency signals according to claim 4, characterized in that in the first acquisition phase, the signals at the output of the carrier correlation channel (30) comprising the subcarrier of the BOC signal, are applied to the subcarrier correlation channel (34) demodulating the subcarrier, the signals output from the subcarrier correlation channel being applied to the code correlation channel (40) providing after integration of the signals lι _P , l _QP , Qι _P , Q _QP to an energy detector (44), the sum of the energies collected on each of the subcarrier channels in phase and in quadrature making it possible to detect a single and unambiguous energy peak identical to that which one would have with a signal not comprising a subcarrier, the sum of the energies E being given by the relation:

E = Σ (I | _P ² + IQP ² ₊ QIP ² + QQP ² ), the sum E being a non-coherent sum of several samples over a time T greater than or equal to a coherent time Te.

7. A method of demodulating radiofrequency signals according to claim 6, characterized in that to find the energy E one tests the code hypotheses by continuously sliding the local code, the sub- carrier also sliding and in that the coherent integration duration is less than the duration of scanning of a portion of a subcarrier peak

8 A method of demodulating radiofrequency signals according to claim 6, characterized in that to find the energy E, the fixed code hypotheses are tested, by making phase jumps (Δφ) between integrations, the phase of the subcarrier remaining constant

9 A method of acquiring radio frequency signals according to claim 8, characterized in that the phase jumps (Δφ) can be generated by accelerating the speed of the local code oscillator (NCO c) over short durations (Δt) between two integrations

10 Acquisition of radio frequency signals according to claim 8, characterized in that the phase jumps (Δφ) can be generated by a means consisting in instantly changing the phase at the output of the NCO and by incrementing the code generator Gc and in what an energy detection test is carried out at each incrementation or phase jump.

11 Acquires radio frequency signals according to claim 5, characterized in that the receiver comprises

- three oscillators, a digitally controlled local carrier oscillator (50) NCO p generating the two local carriers F _{! P} , FQP in phase and in quadrature for the carrier correlation channel 30, an NCO subcarrier oscillator (52) sp with numerical control generating, by a generator of local subcarriers Gsp, the two local subcarriers F | _S , F _αs in phase and in quadrature for the subcarrier correlation channel (34) and a code oscillator (54) supplying by a code generator Gc the local code of the code correlation channel (40) of the receiver,

- an energy detection DEng (44) of the signals l | _P , l _QP , Q | _P , Q _QP at the output of the code correlation channel after integration by respective integrators (46, 47, 48, 49) and in that in the first acquisition phase, the signals at the output of the correlation channel of carrier (30) comprising the BOC signal subcarrier, are applied to the subcarrier correlation channel (34) demodulating the subcarrier, the signals output from the subcarrier correlation channel being applied to the subcarrier correlation channel code (40) providing after integration the signals p, IQP, QIP, QQP to the energy detector DEng (44), the sum of the energies collected on each of the subcarrier channels (in phase and in quadrature) making it possible to detect a unique and unambiguous energy peak identical to that which one would have with a signal not comprising a subcarrier.

12. A method of acquiring radiofrequency signals according to claim 11, characterized in that the sum of the energies E is given by the following relation:

E = ∑ (IIP ² + IQP ² ₊ QIP ² + QQP ² ) the sum E being a non-coherent sum of several samples over a time T greater than or equal to a coherent time Te and in that the acquisition of the signal is performed by dragging the code to scan the hypotheses to be tested independently of the phase of the subcarrier, the latter being made consistent with the speed of the carrier phase to take account of the Doppler.

13. A method of acquiring radiofrequency signals according to claim 5, characterized in that a single oscillator (NCO) is used for the carrier and the subcarrier.

14. A method of acquiring radiofrequency signals according to claims 1 to 13, characterized in that the receivers are configured to perform the following correlation operations:

IIP = | nτ, ₍ n ₊ i) η S _Received . cos (φ (t)). SPι _{π phase} (t). _PonotUΘ code ι (t) dt

IQP = J [nT, (n + 1) T] Speçu - C0S (φ (t)). SPQuadrature (t) - Cθdβp ₀ nctual (t) t

QlP = J [nT, (n + 1) η S _Received . Sin (φ (t)) - SP | _n phase (t) • CodΘponotuel (t) dt QQP - J [nT, (n + 1) TJ Sp _ΘÇU . Sin (φ (t)). SPQ _Ua d ature (t) • Shortcode (t) dt

With: T Duration of coherent integration cos (φ (t)), sin (φ (t)) Local carriers in phase and in quadrature

^> P | n phase, t> "Quadrature Local subcarrier in phase and quadrature Cθdepoπctuel (t) Local point code

15. A method of acquiring radiofrequency signals according to claim 4, characterized in that, in a transition phase towards the tracking phase of the receivers, in the case where the local code and the local subcarrier are synchronous and once that energy has been found, we begin by closing the code loop using advance correlation and delay correlation channels, the receiver generating, from the signals IIA, IIR, IQA, IQR, QI _A , QIR, QQA, QQR, at the output of integrators (80) of the respective code correlation channels, through a code discriminator (90) followed by a code corrector (92), commands to the code oscillator (38 ) aided by the carrier speed (Vp), the Doppler speed (Vp) applied to the digitally controlled carrier oscillator (NCO p) 32 being that found at the end of the energy search in the first phase d 'acquisition.

16. A method of acquiring radiofrequency signals according to claim 15, characterized in that the coherent integration time must be compatible with the residual Doppler error at the end of the energy search phase and also with the joining speed applied to the subcarrier.

17. A method of acquiring radio frequency signals according to claim 5, the receiver comprising the three digitally controlled oscillators (50, 52, 54), characterized in that during the tracking phase in the case where the local code and the local subcarrier are asynchronous, the receiver generates, from the signals l | _A , IIR, IQ _A , IQR, QI _A , QIR, QQ _A , QQR, at the output of integrators (80) of the respective code correlation channels, through a code discriminator (90) followed by a corrector of code (92), of commands to the code oscillator (NCO c) (54) aided by the carrier speed (Vp), the Doppler speed (Vp) applied to the carrier oscillators (NCO p) (50) and subcarrier ( NCO sp) (52) digitally controlled being that found at the end of the search for energy in the acquisition phase, the coherent integration time also being unchanged, the code discriminator providing a signal:

εoode = (IIA ² + IQA ² + QIA ² + QQA ² - IIR ² + IQR ² + QIR ² + Qα _R ² ) / Energy with Energy = IIA ² + IQA ² + QIA ² + QQA ² + IIR ² + IQR ² + QIR ² + QQR ²

18. A method of acquiring radiofrequency signals according to claim 5, characterized in that the receiver comprises a single NCO, the speeds of the carrier and subcarrier NCO oscillators being identical.

19. A method of acquiring radio frequency signals according to claim 4, characterized in that with a local code and synchronous subcarriers the receiver comprises:

- a carrier discriminator (94) (Dsp) followed by a carrier loop corrector (96) (Crp) supplying from the signals l | _P , l _QP , Q | _P , QQP at the output of the code correlation channel after integration by respective integrators (46, 47, 48, 49) a control signal from the carrier oscillator aided by the speed of Doppler Vp.

20. A method of acquiring radiofrequency signals according to claim 19, characterized in that the carrier discriminator provides a signal:

Sportswoman = (Q | -I | + QQ-IQ) / (l | P + IQP + Q | P + QQP ^" )

21. A method of acquiring radiofrequency signals according to claim 19, characterized in that the carrier discriminator provides a signal: εcarrier = Arctan [2 (Qi.ij + QQ.IQ) / (l | .l | + IQ.IQ - Q | .Q | - QQ.QQ) I

22. A method of acquiring radio frequency signals according to claim 5, characterized in that, with a local code and asynchronous subcarriers the receiver comprises:

- a carrier discriminator (100) (Dsp) followed by a carrier loop corrector (106) (Crp), a subcarrier discriminator (102) (Dssp) followed by a subcarrier loop corrector 104 (Crsp) providing respectively from the signals l | _P , IQ _P , Q _iP , QQ _P at the output of the code correlation channel, after integration by respective integrators (46, 47, 48, 49), a control signal from the carrier oscillator (50) helped by the speed of Doppler Vp and a control signal from the subcarrier oscillator (52).

23. A method of acquiring radio frequency signals according to claim 22, characterized in that the subcarrier discriminator provides a signal: εsubcarrier = (IQ-I | + QQ-Q |) / (l | P + IQP + Q | P + QQP ~)

24. A method of acquiring radiofrequency signals according to claim 22, characterized in that the subcarrier discriminator provides a signal:

Ësubcarrier = Arctan [2 (IQ.I | + QQ.QI) / (l | .l | + QI.QI - IQ.IQ - QQ.QQ) l

25. A method of acquiring radiofrequency signals according to one of claims 17 to 24, characterized in that after the transition phase to tracking, the receiver goes to the final phase of nominal BOC tracking by replacing the code with the code modulated by the subcarrier.

26. A method of acquiring radio frequency signals according to claim 25, characterized in that the receiver comprises a code correlation channel (114) comprising the subcarrier, a code generator (116) controlled by the code oscillator (118) providing code correlation (114), the advance code signals Cav, delay Crt and punctual code Cp, the code discriminator providing a signal: εoo e ≈ [(IA- IR). l _P + (Q _A - QR). Q _P )] / (l _P ² + Q _P ² )] or ε _code = [(l _A - IR) ² + (QA - QR) ² ] / [(+ IR) ² + (QA + QR) ² ]

27. A method of acquiring radio frequency signals according to claim 25, characterized in that the receiver comprises a code correlation channel (114) comprising the subcarrier, a code generator (130) controlled by the code oscillator (118) supplying to the code correlation channel (114), the signals of delta code CΔ and point code Cp, the delta code CΔ being obtained by differentiating the advance codes Cav and delay Crt, the code discriminator providing a signal: ε≈de = (• Ip + QΔ - QP) / (IP ² + QP ² )