Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
  • G01S19/00—Satellite radio beacon positioning systems; Determining position, velocity or attitude using signals transmitted by such systems
  • G01S19/01—Satellite radio beacon positioning systems transmitting time-stamped messages, e.g. GPS [Global Positioning System], GLONASS [Global Orbiting Navigation Satellite System] or GALILEO
  • G01S19/13—Receivers
  • G01S19/24—Acquisition or tracking or demodulation of signals transmitted by the system
  • G01S19/29—Acquisition or tracking or demodulation of signals transmitted by the system carrier including Doppler, related
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
  • G01S19/00—Satellite radio beacon positioning systems; Determining position, velocity or attitude using signals transmitted by such systems
  • G01S19/01—Satellite radio beacon positioning systems transmitting time-stamped messages, e.g. GPS [Global Positioning System], GLONASS [Global Orbiting Navigation Satellite System] or GALILEO
  • G01S19/13—Receivers
  • G01S19/32—Multimode operation in a single same satellite system, e.g. GPS L1/L2
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
  • G01S19/00—Satellite radio beacon positioning systems; Determining position, velocity or attitude using signals transmitted by such systems
  • G01S19/38—Determining a navigation solution using signals transmitted by a satellite radio beacon positioning system
  • G01S19/39—Determining a navigation solution using signals transmitted by a satellite radio beacon positioning system the satellite radio beacon positioning system transmitting time-stamped messages, e.g. GPS [Global Positioning System], GLONASS [Global Orbiting Navigation Satellite System] or GALILEO
  • G01S19/40—Correcting position, velocity or attitude

Definitions

• the satellites, numbering 24, are distributed over six fixed orbital planes of 1 2 hours in order to ensure the most regular terrestrial coverage possible at the rate of four satellites per orbital plane. Their positions are known precisely at all times. They are provided with clocks synchronized with each other and emit signals which make it possible, when they are in direct vision of a receiver, to determine the distances between them and the receiver and therefore, knowing their positions, to deduce that of the receiver by triangulation.
• a source of inaccuracy is due to the crossing of the ionosphere by the waves coming from the satellites in orbit at more than 20,000 kilometers of altitude. In fact, when passing through this medium charged with electrons, the waves undergo refractions which decrease their apparent speed. It is known to measure the effect of the ionosphere on the propagation of waves from satellites from the delay appearing in reception between two waves of different frequencies transmitted in a coherent manner because the propagation delay due to the ionosphere which depends of the electron concentration, varies, as a first approximation, as an inverse function of the square of the frequency. This measurement of the effect of the ionosphere is one of the justifications for the fact that each GPS satellite transmits on two different frequencies in the L-band.
• each GPS satellite transmits on two carriers in the L band: an L1 carrier at 1,575.42 MHz and an L2 carrier at 1,227.60 MHz.
• These carriers are phase modulated according to the BPSK (Binary Phase Shift Keying in English language) technique. Saxon) by a pseudo-random binary sequence called spread spectrum code or PRN (Pseudo Random Noise in Anglo-Saxon language), and by a satellite navigation signal called data.
• PRN pseudo-random binary sequence
• the carrier L1 is doubly modulated, in phase by a spreading code C / A and the data, and in quadrature by a spreading code P or Y if it is scrambled, and the data while the carrier L2 is simply modulated by the spreading code P (Y) and the data.
• the modulation of the carriers L1 and L2 by pseudo-random binary sequences causes spreading of the frequency spectrum of the signals transmitted which are then less sensitive to interference and interference.
• all the codes are synchronous. All the signals: carriers, spreading codes and data, are coherent, the transitions of the navigation messages corresponding exactly to the possible transitions of the pseudo-random codes which themselves have a very precise phase relationship with the carriers which same derive from a very stable single clock.
• the spreading codes C / A and P (Y) are individualized and different for each satellite in order to distinguish the satellites from each other.
• the spreading code C / A has a length of 1.023 bits with a bit rate of 1.023 MHz which gives it a duration of 1 millisecond and an occupied frequency bandwidth close to 2 MHz.
• the spreading code P (Y) has a duration greater than 7 days with a bit rate of 1 0.23 MHz, which gives it an occupied frequency bandwidth of about 20 MHz.
• the spreading codes C / A and P are known.
• a first method consists in performing an intercorrelation between the signals L1 and L2 since the signals L1 and L2 transmitted by the same satellite are modulated coherently by the same spreading code Y (encrypted P).
• This intercorrelation has the drawback of considerably increasing the noise power when the signal-to-noise ratio of the signals received is already very low.
• it does not make it possible to separate the different GPS satellites, nor to obtain a measurement on the carrier (speed).
• a second more efficient method takes into account the fact that a spreading code Y (encrypted P) results from the product of the spreading code P not encrypted occupying a frequency band of the order of 20 MHz by a binary code of W encryption occupying a frequency band forty times weaker, of the order of 500 KHz. It consists in demodulating the carrier L2 modulated by a spreading code Y (encrypted P) by means of the code P itself unencrypted locally generated in reception by means of a spreading code generator controlled in phase so as to obtain maximum power for the demodulated signal obtained limited to a frequency band of 500 kHz and to square the signal from the demodulation to eliminate any two-phase modulation.
• This technique is implemented using a servo comprising, in a loop:
• a band filter limiting the pass band of the demodulator output signal to a width of 500 kHz around the carrier of the L2 signal translated into the lower band, - a circuit for squaring the demodulated and filtered signal eliminating the two-phase modulation,
• a loop control circuit commanding the local generator of spreading code P not encrypted, with an output of half a binary element in advance and another of a binary half-delay in delay, and controlling the phase shift of the local generator spreading code so as to obtain, in both the advance and delay cases, equal powers for the frequency line at twice the carrier of the L2 signal translated into lower band appearing in the demodulated, filtered and put signal squared.
• the present invention aims to improve the signal to noise ratio in the phase control loop of the local spreading code generator P not encrypted in order to obtain greater efficiency in the control and to achieve more easily synchronizing the locally generated unencrypted spreading code P with the spreading code Y (encrypted P) modulating on transmission a signal L2.
• Its subject is a method for the reception processing of a signal L2 from a GPS satellite modulated by a spreading code Y (encrypted P) from which the encryption signal W is ignored to deduce a pseudodistance. This process consists of:
• the phase control of the local spreading code generator P not encrypted is done by means of a loop with three parallel parallel channels operating simultaneously and not sequentially, which improves the signal to noise ratio of the useful signal in the loop, of a ratio four decomposing into a ratio two due to the simultaneous operation of the advance and delay channels and into a ratio two due to the fact that one no longer carries out a simple squaring of the demodulated and filtered signals of the channels early and late but to the products of these signals by the demodulated and filtered one of the point channel which has the advantage of presenting a signal to noise ratio twice better.
• the estimation of the Doppler effect affecting the signal L2 consists in extracting from the signal L1 of the GPS satellite considered, modulated by a known C / A spreading code, the Doppler shift affecting the carrier of the signal L1 and to apply to this Doppler shift a proportionality ratio of 1 20/1 54 èm ⁇ .
• the invention also relates to a device for implementing the above method.
• FIG. 1 is a diagram of a GPS receiver according to the invention, ensuring processing of the signal L2 in the intermediate frequency band, and
• FIG. 2 is a diagram of another GPS receiver according to the invention, ensuring processing of the L2 signal in baseband.
• FIG. 1 shows the diagram of a GPS receiver according to the invention, detailing in particular its circuits ensuring the processing of the signal L2 in the intermediate frequency band.
• a reception antenna 1 through which the receiver receives all of the signals L1 and L2 emitted by the GPS satellites which are in direct vision.
• This reception antenna 1 is connected to a diplexer 2 which separates by their distinct frequency bands the signals L1 and L2.
• the signal L1 available at output 3 of the diplexer 2 is applied to a conventional processing circuit 4 which demodulates it by a spreading code C / A and derives therefrom:
• the adjustment phase of the C / A spreading code generator used locally in the receiver gives an indication of the pseudodistance (psd) separating the receiver from the GPS satellite in direct vision using the C / A spreading code considered.
• the satellite data D obtained after demodulation of the signal L1 by a spreading code C / A made up of ephemeris and an almanac make it possible to very precisely calculate the position of the GPS satellite at a given instant.
• the Doppler frequency offset (psv) affecting the received L1 signal makes it possible to know the relative speed of the receiver with respect to the GPS satellite using the C / A spreading code considered.
• the signal L2 available at output 5 of the diplexer 2 is applied at the input of an intermediate frequency converter 6 which has the role of transposing the signal L2 into a lower frequency band in order to facilitate its processing.
• the last stage of this intermediate frequency converter 6 is controlled by an oscillator with digital phase control NCO 7, the frequency of which is mobile and offset, around a fixed value, by the Doppler effect affecting the signal L2.
• This Doppler effect affecting the L2 signal is deduced from the Doppler effect affecting the L1 signal by a scaling taking into account the proportionality ratio of 120/1 54 th which exists between the carriers of the L1 signals. and L2.
• the signal (psv) at the output of the processing circuit 4 which represents the Doppler frequency difference affecting the signal L1
• a multiplier 8 which implements the proportionality ratio 1 20 / 1 54 é e and the result is used to control the phase control input of the oscillator 7.
• the signal L2 which has a bandwidth of the order of 20 MHz is centered on a fixed intermediate frequency Fi.
• demodulators 1 0, 1 1, 1 2 which receive as demodulation signals a point version P and two advanced versions A and delayed R of a binary half-element of a spreading code P unencrypted generated by a local generator 1 3 of spreading code P.
• the three resulting demodulated signals are filtered by bandpass filters 1 4, 1 5, 1 6 with a bandwidth of 500 KHz which are centered on the fixed intermediate frequency Fi and adapted to the phase and frequency of the binary encryption signal W.
• the demodulated signals and filters available at the output of the bandpass filters 1 5 and 1 6, resulting from demodulations by the advanced versions A and delayed R of the spreading code P not encrypted generated locally are again subjected to two demodulators 1 7, 1 8 which demodulated once again by the demodulated signal available at the output of the bandpass filter 14 resulting from the demodulation of the signal L2 by the point version P of the spreading code P not encrypted locally generated.
• the two doubly demodulated signals available at the output of the demodulators 1 7, 1 8 are then applied to circuits 1 9 and 20 for measuring the powers EA, ER of their component at the intermediate fixed frequency Fi which operates over a horizon of a few milliseconds.
• These power measurements EA and ER are then applied as inputs of a discriminator 21 which controls the phase shift control input of an oscillator with digital phase control NCO 22 delivering a binary element timing signal to the local generator 1 3 of spreading code P not encrypted.
• FIG. 2 illustrates a variant of a GPS receiver in which the signal L2 is brought back to baseband before being used to synchronize a local spreading code generator P not encrypted.
• a reception antenna 1 leading to a diplexer 2 which separates the distinct frequency bands of the signals L1 and L2 from a GPS satellite.
• the signal L1 available at output 3 of the diplexer 2 is always applied to a conventional processing circuit 4 which demodulates it by a spreading code C / A suitable for the GPS satellite which it is desired to receive and which draws the usual information therefrom.
• the signal L2 available on the output 5 of the diplexer 2 is applied to an intermediate frequency converter 30 which translates the signal L2 into the lower band in one or more steps using fixed pilot frequencies.
• the signal L2 is subjected to a quadrature demodulator translating it into baseband.
• This quadrature demodulator is composed of two demodulators 31, 33 and a ⁇ / 2 phase shifter 32.
• the two demodulators 31, 33 receive the phase and quadrature versions of a carrier slightly offset from the intermediate frequency Fi to hold account for the Doppler effect on the L2 signal.
• This baseband transposition carrier is delivered by a digital phase control oscillator NCO 34 whose digital phase control input receives, via a multiplier 8 introducing a proportionality ratio 1 20/1 54 th , the signal (psv) which is available at the output of the processing circuit 4 of the signal L1 modulated by the spreading code C / A and which represents the frequency offset due to the Doppler effect affecting the signal L1.
• the two components in phase I and in quadrature Q of the signal L2 in baseband are subjected in parallel to three double demodulators (complex signal) 35, 36, 37 which receive, as signals for demodulating a point version P and two advanced versions A and delayed R of a binary half-element, of an unencrypted spreading code P generated by a local generator 1 3.
• the three resulting demodulated signals with two components, l one in phase and the other in quadrature are subjected to double low-pass filters 38, 39, 40 adapted to the phase and to the frequency of the encryption signal W, which limit the bandwidth of the demodulated signals to 500 KHz by integrating them over a period of 2 ⁇ s.
• phase and quadrature components of the demodulated and filtered signals available at the output of the double low-pass filters 39, 40 and resulting from the demodulation of the signal L2 by the advanced A and delayed R versions of the spreading code P not encrypted locally generated are again subjected to two double demodulators 41, 42 which demodulate them further by the in-phase and quadrature components of the demodulated signal available at the output of the double low-pass filter 38 and resulting from the demodulation of the signal L2 by the point version P of the spreading code P not encrypted generated locally.
• phase and quadrature components of each of the two doubly demodulated signals available at the output of the double demodulators 41, respectively 42 are then subjected to a power estimation circuit 43, respectively 44 which integrates them over a horizon of a few milliseconds and in extracts the module EA, respectively ER from each of the signals.
• EA and ER modules are then applied to a discriminator 45 which controls the phase shift control input of an NCO digital phase control oscillator 22 delivering a bit element timing signal to the local code generator 1 3 spread P not encrypted.
• the unencrypted spreading code generator P When the unencrypted spreading code generator P is in phase with the spreading code Y (encrypted P) modulating the signal L2 coming from a GPS satellite, we find, at the output of the demodulator 35 corresponding to the point version of the unencrypted spreading code P, the data D of the satellite modulated by the encryption code W.
• the phase and quadrature components of this signal which has a bandwidth limited to around 500 KHz, is then advantageously subjected to the double low-pass filter 38 adapted to the phase and the frequency of the encryption signal W, which limits the bandwidth of the useful signal to 500 KHz for eliminate out-of-band noise.
• the pseudodistance (psd) of the signal L2 is deduced when the phase control loop is locked.
• the comparison of the pseudoranges (psd) measured with the L1 and L2 signals gives the relative delay in reception of the L2 signal compared to the L1 signal, which makes it possible to estimate the effects of the ionosphere towards the target satellite on the propagation times. signals L1 and L2 to take them into account in localization.
• the processing operations for reception of the signals L1 and L2 are carried out in digital mode as soon as they have been translated into a sufficiently low frequency band, thanks to an analog-digital conversion practiced from the output of the converters at intermediate frequency.
• Filters adapted to the phase and frequency of the encryption signal W 14, 1 5, 1 6, 38, 39, 40 can be produced, optimally, according to the technique known by the Anglo-Saxon name “integrate and dump ", which consists in integrating the signal processed on adjacent time windows, about 2 ⁇ s wide, synchronized with the signal from the local spreader code spreader P unencrypted.

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

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• 1998
  • 1998-09-25 FR FR9812022A patent/FR2783929B1/fr not_active Expired - Fee Related
• 1999
  • 1999-09-24 EP EP99946229A patent/EP1034439A1/de not_active Withdrawn
  • 1999-09-24 US US09/554,600 patent/US6317078B1/en not_active Expired - Fee Related
  • 1999-09-24 WO PCT/FR1999/002277 patent/WO2000019229A1/fr not_active Application Discontinuation
  • 1999-09-24 CA CA002312030A patent/CA2312030A1/fr not_active Abandoned

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