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
  • G01S19/243—Demodulation of navigation message
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
  • G01S19/00—Satellite radio beacon positioning systems; Determining position, velocity or attitude using signals transmitted by such systems
  • G01S19/01—Satellite radio beacon positioning systems transmitting time-stamped messages, e.g. GPS [Global Positioning System], GLONASS [Global Orbiting Navigation Satellite System] or GALILEO
  • G01S19/13—Receivers
  • G01S19/35—Constructional details or hardware or software details of the signal processing chain
  • G01S19/37—Hardware or software details of the signal processing chain

Definitions

• the present invention relates to a method for demodulating radio navigation signals comprising a data channel modulated by a radio navigation message and an unmodulated pilot channel, these signals being transmitted in spread spectrum using pseudo-random spectrum spreading codes.
• a radio navigation signal receiver In a satellite navigation system, a radio navigation signal receiver has several reception channels for simultaneously receiving radio navigation signals from several satellites (at least three). Each reception channel performs a measurement on the spreading code and a measurement of the frequency of the received carrier. These measurements make it possible to determine the distance and the radial speed between the receiver and the satellite and to recover the radio navigation message containing in particular information relating to the transmitting satellite, namely its trajectory, its state and corrective terms to be applied to its clock, as well as global information relating to the trajectories of all the satellites in the navigation system.
• the demodulation of a radionavigation signal in spread spectrum is carried out using two operations, namely an operation of despreading of the signal by a reference code, and an operation of estimation of the phase of the signal to raise ambiguity of the signal, i.e. estimate the sign of the message symbol.
• the first operation is performed by a loop of DLL code
• Delay-Lock Loop coherent or not
• the second operation is generally carried out using a PLL phase loop (Phase-lock loop) or a Costas loop, allowing to reconstruct a replica of the carrier which is multiplied with the received signal.
• the resulting signal is filtered so as to extract the information modulating the carrier therefrom.
• demodulation can only be performed when these two loops are hooked. Since the code loop is generally more robust than the phase loop, demodulation can only be carried out when the signal to noise spectral density ratio of the received signal exceeds the hooking threshold of the phase loop or of Costas.
• This attachment threshold is located below the readability threshold of the navigation message for the purpose of position calculation. It is in fact considered that above a TEB binary error rate of 10 "4 to 10 " 5 , the message is no longer usable for performing a position calculation.
• This technique therefore has the drawback of no longer being able to demodulate the message as soon as the phase loop has stalled.
• the navigation message can be used for tracking, using techniques known as" data-iping ". These techniques use the removal of ambiguity. on the sign of the symbol to continue the predetection of the signal in bands much smaller than that of the navigation message. However, these techniques can be applied only if the estimation of the symbol is possible, that is to say as long as the carrier loop remains attached.
• some receivers include a device allowing them to continue the signal code (data or pilot) even when the carrier detection loops no longer work.
• This operating mode commonly called “code only” mode, makes it possible to continue the signal in the event of unfavorable link balance, but does not make it possible to demodulate the navigation message.
• the present invention aims to eliminate these drawbacks.
• This objective is achieved by providing a method for demodulating radionavigation signals transmitted in spread spectrum and comprising a data channel modulated by a navigation message, and a pilot channel not modulated by a navigation message, the data channel and the pilot channel being combined in a multiplexing scheme in order to modulate a carrier, this method consisting in applying to the signals of the pilot and data channels a despreading treatment and in demodulating the despread data signal to obtain the navigation message.
• the demodulation of the despread data signal to obtain the navigation message is carried out using the carrier obtained by the despreading processing of the pilot channel.
• the demodulation can be carried out in "code-only" mode, when for example the carrier phase is estimated by an external navigation system (for example an inertial system) or internal (for example a Kalman filter using measurements made on the spreading code);
• an external navigation system for example an inertial system
• internal for example a Kalman filter using measurements made on the spreading code
• the receiver can include only one FLL loop for the tracking of the carrier, thus offering a simplification of the architecture of the receiver while offering better robustness than a PLL loop;
• the removal of ambiguity on the message symbol allows the use of an FLL loop discriminator based on the extended arctangent function.
• the demodulation of the navigation message is no longer dependent on the tracking threshold of a phase loop (PLL).
• the data hooking threshold depends on the threshold for following the code loop or on the conditions for implementing a symbol ambiguity removal technique (bit error rate less than or equal to 10%).
• the pilot channel and the data channel of the signal to be demodulated are time-multiplexed.
• pilot channel and the data channel of the signal to be demodulated are multiplexed in phase.
• pilot channel and the data channel of the signal to be demodulated are multiplexed according to an ALTBOC scheme.
• the pilot channel and the data channel of the signal to be demodulated are multiplexed according to a scheme in which the carrier contains at least the data channel and the pilot channel of the signal to be demodulated.
• the despreading processing is carried out by an estimation or tracking code processing, associated with an estimation or tracking processing of the frequency or of the carrier phase.
• the carrier tracking processing is performed using a frequency locked loop
• the code tracking processing is performed using a code loop.
• this method is applied to the demodulation of satellite navigation signals of the GPS-IIF L5, L2C type, or to the demodulation of satellite navigation signals transmitted by the GALILEO system, or transmitted by ground stations, by modernized GLONASS satellites, or by COMPASS or QZS satellites.
• the invention also relates to a receiver of radionavigation signals transmitted in spread spectrum and comprising a data channel modulated by a navigation message, and a pilot channel not modulated by a navigation message, the receiver comprising a despreading and tracking device.
• a spreading code generator delivering spreading codes and means for applying the spreading codes to the signals of the pilot channel and the data channel in order to obtain pilot and despread data signals.
• this receiver comprises a demodulator using the despread pilot signal to demodulate the despread data signal, in order to obtain the navigation message.
• this receiver comprises means for estimating or tracking the frequency or the phase of the signal from the despread pilot channel.
• this receiver comprises a frequency locked loop to continue the pilot signal and a code loop controlling the spreading code generator.
• the frequency locked loop comprises a discriminator of the extended arctangent form.
• the frequency locked loop comprises a loop filter of order 1 or 2 adapted to the dynamics of the signals received.
• the output of the filter of the frequency locked loop is coupled to the code loop, the code loop comprising a loop filter of order 0.
• the code loop comprises a discriminator applied to the pilot signals and to the data signals, the data signals being weighted by a coefficient depending on the signal to noise spectral density ratio of the signals received.
• the frequency locked loop is designed to receive doppler speed assistance from a navigation system.
• Figure 1 schematically illustrates in the form of a block diagram the general principle of the invention
• FIG. 2 schematically represents a receiver adapted to the reception of GPSIIF signals in L5 band, applying the general principle of the invention
• Figures 3 and 4 show in more detail some components of the receiver shown in Figure 2.
• s (t) be a radionavigation signal transmitted in spread spectrum, composed of the sum of a pilot signal s p (t) made up of an unmodulated carrier signal, and a data signal S (t) made up a navigation message modulating the carrier.
• C p and ca are the spreading codes applied respectively to the pilot channel and the data channel, d the symbol of the navigation message, and mux_p and mux_d the multiplexing functions of the pilot channel and of data.
• the multiplexing of the pilot and data channels can be carried out in phase or over time, or alternatively according to the alternative ALTBOC or BOC (Binary Offset Carrier) scheme, or else according to a scheme in which the carrier contains at least the data and pilot.
• the data channel is for example in phase and the pilot channel in quadrature.
• the functions mux_d (t) and mux_p (t) are then equal to 1 and j respectively (in complex notation).
• mux_d (t) and muxjp (t) are worth respectively h c (t) and 1 - h c (t).
• the signals at the output of the phase correlators can be written in the form: or :
• Rp ( ⁇ ) and R d ( ⁇ ) are the autocorrelation functions of the code for a phase shift of ⁇
• - ê d (t) and ⁇ p (t) are the phase estimates, respectively of the data and pilot signals supplied by a device for estimating the carrier frequency.
• an FLL Frequency Lock Loop
• an external navigation device For carrier estimation, an FLL (Frequency Lock Loop) or an external navigation device can be used.
• the signal s (t) is then a signal modulated according to the phase modulation technique with four states QPSK (Quaternary Phase-Shift Keying), the channel in phase being modulated by the data signal and the quadrature channel being modulated by the pilot signal.
• QPSK Quadrature Phase-Shift Keying
• FIG. 1 represents a device for receiving such navigation signals implementing this principle.
• This device includes a tracking device
• This device provides an estimate of the despreading codes c d and p p , as well as an estimate of a pseudo-distance between the receiver and the transmitter of the radio navigation signal, on the basis of a difference between the clocks of the transmitter and receiver.
• the code estimates are applied to the input of mixers 3 and 6 to respectively despread the components S d (t) and s p (t) of the received composite signal s (t).
• the signals at the output of mixers 3 and 6 are applied respectively to two low-pass filters 4 and 7 (which can be produced by summers) so as to obtain the following signals r d and r p , corresponding to the components S d (t ) and s p (t) despread:
• ⁇ is the phase shift between the received signal s (t) and the local replicas
• R is the correlation function of the pilot and data codes
• (d (t) is the estimation of the symbol of the navigation message.
• GPS-IIF signal in L5 band is a special case of the signal s (t) described above, with
• the GPS-IIF signal in the L2 band called L2C is an example of a signal in which the signals S d (t) and s p (t) are multiplexed in time.
• FIG. 2 shows in more detail an example of a GPS-IIF signal receiver in L5 band implementing the principle according to the invention, described above.
• This receiver comprises a reception antenna 12 connected to a stage 13 for converting the RF frequency of the signals received into an intermediate frequency IF.
• the output of this stage is connected to a stage for removing the carrier, comprising for example two mixers 14, 15 receiving respectively on another input the imaginary and real parts of the carrier generated locally by the receiver.
• the signal received from which the locally estimated frequency has been withdrawn from the carrier is applied to a set 11 of correlators, receiving on another input estimated values of six spreading codes and two Neuman-Hoffman codes, produced by a code generator 23.
• the set of correlators 11 delivers on separate outputs twelve signals comprising the real I and imaginary parts Q of three output signals, respectively in phase, early and late, for each of the pilot and data signals contained in the received signal .
• the estimate d of the received message symbol provided by the assembly 16 is applied to a convolutional decoder 17, for example of the Viterbi decoder type.
• This convolutional decoder can call upon a priori knowledge of the navigation message to improve the decoding.
• the phase signals of the pilot channel coming from the set 16 are applied to a discriminator 19 of the FLL (Frequency-Lock Loop) loop, while the set of the twelve signals coming from the set 16 are applied to a discriminator 21 DLL (Delay-Lock Loop).
• the phase signals from assembly 16 can also be applied to a noise estimator 18 delivering an estimate of the C / N 0 ratio of the signal received over the noise spectral density, this ratio transformed by a function F being applied as input. of the DLL loop discriminator 21.
• the output signal of the FLL loop discriminator is processed by an FLL loop filter 20 before being applied to the control input of a digitally controlled oscillator (OCN) 27 which locally generates an estimate of the carrier of the received signal. , when the FLL loop thus formed is locked on the carrier of the received signal.
• the oscillator 27 delivers on separate outputs the imaginary part (sin) and the real part (cos) of the carrier which are applied respectively at the input of the mixers 14, 15.
• the output signal of the DLL loop discriminator is also processed by a DLL loop filter 22. Furthermore, the output of the FLL filter 20 is also connected via an amplifier 26 of gain k, to the input d 'an adder 25 which also receives as input the output signal of the loop filter DLL 22.
• the value of the gain k is chosen equal to the ratio R c / R p which represents the coefficient of proportionality between the frequency of the code R c and the carrier frequency R p . In the case of GPS-IIF signals in L5 band, this ratio is 1/115.
• the output of the summator 25 is connected to the control input of another digitally controlled oscillator (OCN) 24 whose output signal drives the code generator 23.
• OCN digitally controlled oscillator
• the DLL loop filter 22 can therefore be of order 0, that is to say it can be produced by a simple amplifier having a certain gain.
• the FLL loop filter 20 has a higher order, typically of 1 or 2, which can be adapted to the dynamics of the signal to be processed.
• the gain of the DLL loop filter 22 can be adjusted to be adapted to the characteristics of the signal to be processed (signal to noise ratio, residual dynamic, effect of the local oscillator 27, etc.).
• a navigation system 29, internal or external providing an estimate of the doppler speed, that is to say the relative radial speed. of the receiver in relation to the transmitter of the radio navigation signals.
• This estimate of the doppler speed is applied at the input of a summator 28 interposed on the link between the output of the FLL filter 20 and the input of the oscillator. 27.
• the spreading and Neuman-Hoffman codes estimated by the code generator 24 as a function of the frequency of the signal from the oscillator 24 include, for example:
• the set of correlators 11 comprises a block 31 of 1 kHz correlators which perform a coherent integration over 1 ms and to which the signals from the mixers 14, 15 are applied, and a block 32 of 100 Hz correlators which perform a coherent integration over 10 ms and to which the output signals of block 31 are applied respectively.
• Block 31 also receives the spreading codes generated for the pilot channel Ep, Lp, P P , and for the data channel E D , L D , P D , while block 32 receives the Neuman-Hoffman codes NH_pilote and NH_donnee generated respectively for the pilot channel and the data channel.
• Each of the blocks 31, 32 of correlators has twelve channels each comprising a respective input of the block connected to the input of a mixer, 35 l to 35 12 and 37 1 to 37 12 , respectively, and a summator, 36 1 to 36 12 and 38 1 to 38 12 , respectively, whose input is connected to the mixer output of the channel and the output constitutes a respective output of the block.
• the other input of mixers 35 1 to 35 12 and 38 1 to 38 12 receives a respective spreading or Neuman-Hoffman code.
• the mixers 35 1 to 35 12 are grouped in pairs each receiving the same spreading code and respectively, the real signal and the imaginary signal coming respectively from the mixers 14, 15.
• the pair of mixers 35 1 and 35 2 receives the spreading code E P
• the pair of mixers 35 3 and 35 4 receives the spreading code L P
• the pair of mixers 35 5 and 35 6 receive the spreading code P P
• the pair of mixers 35 and 35 receive the spreading code P D
• the pair of mixers 35 9 and 35 10 receive the spreading code E D
• the pair of mixers 35 and 35 receives the spreading code L D.
• the first six mixers 37 1 to 37 6 of the second block 32 receive the Neuman-Hoffman NH_pilote code from the pilot channel as input, while the other six mixers 37 to 37 of the second block 32 receive the Neuman-Hoffman code as input NH_datae of the data channel.
• the outputs of the second 32 deliver the signals noted respectively, Ip £ , QPE,
• the assembly 16 comprises a demodulator block 41 and two integrator blocks 42, 43 each comprising six summers 49 1 to 49 6 and 49 7 to 49 respectively, respectively receiving the signals at the output of the assembly 11, these blocks integrators pursuing up to 20 ms or beyond, the coherent integration carried out by the set 11 of correlators.
• the signals I P E, Qp E , Ip L , Q P , Ipp and Q PP at the output of the assembly 11 are applied to the block 42 of summers 49 1 to 49 6 comprising a summator for each input signal of the block, these summers respectively delivering the signals IP E , Q PE , IP L> Q PL5 Ipp and Q PP relating to the pilot channel.
• the signals IDP > QDP > IDE > QDE> IDL > QDL at the output of the assembly 11 are applied respectively, via mixers 48 1 to 48 6 respectively, to the summers 49 7 to 49 12 of block 43, which deliver respectively the signals I DP> Q DP , I D E > QDE, I DL and Q DL relating to the data channel.
• the signals I PP and I D p are applied to a mixer 45, while the signals Qp P and Q D p are applied to another mixer 44.
• the outputs of the two mixers 44, 45 are added in a summator 46 which thus delivers an estimate d of the symbol d of the message received.
• a 1 1 1 1 d Ipp XI DP + Qpp XQ D p (H)
• the other input of the mixers 48 to 48 receives the estimate d of the symbol d of the received message, processed by an amplifier 47 of gain k ′ which can for example be chosen as a function of the estimate of the signal to noise ratio of the received signal , k ′ being the lower the lower the signal to noise ratio.
• the proportionality factor k 'thus applied to the estimate d of the received symbol makes it possible to weight the data channel with respect to the pilot channel so as to optimize the desired performance of the receiver.
• the signals at the output of the assembly 16 represent:
• the receiver tracking device which has just been described only uses the pilot channel.
• the discriminator 19 of the carrier loop FLL can be represented by the following expression: in which :
• ATAN2 () represents the extended arctangent function, providing a result located in the interval] - ⁇ , + ⁇ [,
• X (n-1) and X (n) represent the values of the signal X at two successive instants n-1 and n, that is to say two successive signal samples at the output of the correlator blocks 42, 43. These two successive signal samples are therefore spaced from the duration of the integration (20 ms or more) carried out by the correlators of blocks 42, 43.
• the discriminator 21 of the DLL code loop uses both the data channel and the pilot channel. It can be represented by the following expression:
• Cs represents the phase shift expressed in number of chips between the early and late phases (phase shift between the signals of index E and the signals of index L, for example I PE and I PL ).
• Cs is typically expressed in inverse power of 2 and is worth for example 2 or 2 " .
• the discriminator D D applied to the data channel is thus weighted by a coefficient F (C / N 0 ) depending on the C / N 0 ratio determined by function 18.
• this weighting coefficient is close to 1, and when this ratio decreases, the coefficient F (CN 0 ) tends to 0.
• the carrier frequency and the codes are obtained using closed loop tracking devices (FLL loops and DLL loops).
• closed loop tracking devices FLL loops and DLL loops.

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

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