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
  • G01S19/30—Acquisition or tracking or demodulation of signals transmitted by the system code related

Definitions

• the invention relates to the reception of a radionavigation signal from a satellite positioning system such as the GPS system (acronym for the English expression "Global Positioning System”).
• a satellite positioning system such as the GPS system (acronym for the English expression "Global Positioning System”).
• the GPS system consists of a constellation of 28 satellites and a terrestrial network of ground-based reference stations. Each satellite gravitates 20 000 km from the earth with a period of revolution of 12 hours. They emit two signals, one at 1575.452 MHz for civil applications and the other at 1227.6 MHz for reserved access applications. In the following, we only consider the civil frequency.
• the signal emitted by a satellite consists of a 1575.452 MHz frequency carrier, modulated by a known spreading code and possibly by unknown data also known as databits.
• the satellites all transmit on the same frequency and the emitted signals are differentiated by their code.
• T for example 1 ms and typically consist of 1023 chips.
• the positioning of the receiver is obtained by measuring the distance between a satellite and the receiver from the propagation time of the signal between this satellite and the receiver.
• a replica of the transmitted code is generated locally; the offset between the received signal and the local signal (i.e. the replica) corresponds to the desired propagation time.
• This offset is measured by phasing the received signal and the local signal; the phasing criterion corresponds to the maximization of the correlation function of the two signals, that is to say to the search for a peak of correlation.
• This result is obtained by using for example a correlator structure known as the "Matched Filter”: the local code is moved in front of the signal received at each correlation.
• This structure actually comprises two identical substructures in parallel, one for the real component of the sampled signal, the other for the imaginary component.
• the sampled signal is the received signal converted into baseband and sampled at N kHz, where N is the number of code chips considered over the period of the code.
• the received signal whose frequency is compensated to take into account the Doppler effect, is loaded into a non-rotating shift register 1 and the code is loaded into a rotating shift register 2.
• the length N of the shift register represents the number code chips considered over the period of the code, ie 1 ms for example for the GPS. To fully load the signal into its shift register 1, it is necessary to wait N clock cycles.
• This first step of loading register is represented on the timing diagram of Figure 2, by the designation "shift register".
• the code must be rotated about 2046 times the sampling frequency, which is difficult to implement and requires a large number of logic gates.
• An important object of the invention is therefore to optimize the signal processing time to allow to increase the integration times and therefore the sensitivity of the receiver, especially when the reception is degraded.
• the invention proposes a method of receiving a radionavigation signal which comprises, at a first determined time, a correlation calculation step of the sampled received signal with a locally generated period code T, the received signal and the code being shifted with respect to each other from a correlation to the next correlation; the method is mainly characterized in that, over the determined time, the signal is shifted from one correlation to the next, the code being fixed. This determined time is typically equal to T.
• One of the advantages of this method is that it makes it possible to parallel two phases of the conventional solutions (input of the signal in a shift register and calculation of the correlations). The time of the signal processing, therefore search satellites is divided by 2.
• This method has another advantage which appears when the signal includes data (also called data bits) in addition to the code.
• data also called data bits
• These data are transmitted by the satellite for example every 20 ms, that is to say at 50 Hz.
• these data change their value (for example, from +1 to -1 or the opposite), this translates into by a phase inversion in the signal.
• This phase inversion can occur during the integration interval, when the signal is fixed and the code is rotating on this interval. In this case, calculating a correlation peak over this interval is difficult if not impossible.
• the phase inversion is necessarily synchronous from the beginning of the integration interval because it is synchronous with the code which is fixed: the inversion of phase then no longer affects the correlations calculated during a code period.
• the sampled signal comprising a real component I and an imaginary component Q
• the samples of the two components I and Q are put in series alternately in the same memory and the correlation calculations of the code with I and with Q are performed alternately.
• the results of correlation calculations of the same component and having the same offset n between the signal and the code, and obtained over Tc code periods are added so as to obtain for each component a coherent integration respectively designated I ⁇ c (n) and Q ⁇ c (n).
• the method comprises a step of storing the signal in another memory during a second determined time, according to a write frequency fe and the signal is read at a reading frequency fl such that fl> fe.
• post-processing mode the signal is sampled and stored in memory preferably during the noncoherent time Tnc which can reach 16s; then the same samples are read into memory for each satellite and each frequency assumption. The samples are then read at a much higher frequency fl at their write frequency in memory fe.
• This post-processing mode makes it possible to reduce the overall search time of the satellites.
• the subject of the invention is also a receiver of a radionavigation signal comprising means for implementing the method.
• the correlator capable of correlating the received signal with a code
• the correlator comprises a non-rotating shift register for the code and at least one non-rotating shift register for the signal.
• the non-rotating shift register for the code comprises N cells, where N is the length of the code
• the non-rotating shift register for the signal comprises 2N cells and it comprises N multipliers each connected on the one hand to a cell of the register of the code and on the other hand to a cell of the signal register and an adder with N inputs, each input being connected to a multiplier.
• the structure has only one adder and one set of elementary multipliers.
• the silicon surface is practically divided by a factor of 2 and the cost of the circuit is reduced.
• it further comprises connected to the correlator means capable of storing the 2N results from the correlator and incrementing them respectively as and when the results provided by the correlator for a duration Tc to achieve 2N consistent integrations.
• it comprises means able to memorize the N amplitudes or powers of coherent integrations, and to increment them respectively during a duration Tnc, as the amplitudes or the powers of coherent integrations are obtained, to realize N inconsistent integrations.
• FIG. 1, already described schematically represents a known correlator of the matched filter type for an I or Q component of the signal
• FIG. 2 already described schematically represents a timing diagram of the processing of the signal received with rotation of the code
• FIG. 4 schematically represents a timing diagram of the processing of the signal received with signal rotation
• FIG. 5 schematically represents another example of a correlator according to FIG. the invention of "matched filter" type in which the two substructures are combined
• FIG. 6 schematically represents an optimization example of the example of FIG. 5
• FIG. 7 schematically represents an example of a correlator according to FIG. invention associated with means for mixing coherent integrations and non-coherent integrations your.
• the principle of finding a correlation peak lies in a series of correlation tests between the signal and the code by modifying the offset of one with respect to the other.
• Conventional solutions have opted for code shifting, as shown in the preamble. This technique proceeds in 2 phases. The first phase loads the shift register with the signal over a period of code. The second phase calculates a correlation with each rotation of the code on itself.
• the invention is based on the same principle of correlation but adopts the shift of the signal. From one correlation to another, the code is now fixed and only the signal is shifted: it slides in its shift register at each clock cycle.
• the slice of signal is always equal to a period of code but this slice is slippery.
• This correlation method is for example obtained by means of an improved "Matched Filter” type structure.
• the code is entered in a non-rotating shift register 2 'and the component of the compensated signal of its Doppler is continuously fed into a non-rotating shift register 1.
• the length N of the shift register represents the number of code chips considered over the period of the code, ie 1 ms for example for the GPS. To calculate all the correlations, it is necessary to wait N clock cycles. At each clock pulse, the component of the signal is shifted into shift register 1 and a new correlation is calculated. After N offsets the signal, all the correlations over the period of the code are calculated.
• This correlator 20 is provided with a non-rotating shift register 2 'for the code, common to the two components, of length N.
• the other elements were already present in the example of FIG. 3, namely two shift registers 1 of length N (one for each real and imaginary component of the signal), 2N multipliers 3 (N for each component) and 2 adders 4 to N entered each (1 adder for each component).
• An adder requires a very large number of logic gates and constitutes the main silicon surface of the invention.
• the samples of the two real and imaginary components of the signal are serially stored alternately in order to limit the number of doors required.
• the number of gates for the signal and code shift registers remains unchanged.
• the structure has only one adder and one set of elementary multipliers.
• the silicon surface is practically divided by a factor of 2 and the cost of the circuit is reduced.
• An example of such a correlator 20 is shown in FIG. 6. It comprises a non-rotating shift register 2 ', of length N for the code, a non-rotating shift register 1', of length 2N for the signal, N multipliers 3 and an adder 4 with N inputs.
• the adder 4 makes it possible to obtain the correlations for a component, real or imaginary. Then, at the next clock cycle, the signal slides in the register 1 by shifting one cell and the adder 4 then makes it possible to obtain the correlations for the other component and so on as the slip progresses. of the signal.
• the code chips can be divided by 2 as in this example or by 3, 4, ..., even 3.3, etc.
• the two real and imaginary components of the signal are alternately put in series in the shift register and can for example be expressed in the following way:
• SR (2i, tk, n ) are the samples of the real component of the signal at the instant ix 1 ms / 2046 + t k , n ,
• t k , n represents the (k + 1) th code period, n varying from 0 to 2045 and represents the delay index between the signal and the code.
• the signal generally includes data (also called data bits) in addition to the code. These data are transmitted by the satellite for example every 20 ms, that is to say at 50 Hz. When these data change their value (for example from +1 to -1, or vice versa), this is translated by a phase inversion in the signal. This phase inversion can occur during the integration interval, when the signal is fixed and the code is rotating on this interval. In this case, calculating a correlation peak over this interval is difficult if not impossible.
• the signal is sliding and the code is fixed on this interval, the phase inversion is necessarily synchronous from the beginning of the integration interval because it is synchronous with the code which is fixed: the inversion of phase then no longer affects the correlations calculated during a code period.
• the coherent integration requires taking into account the phase of the data bits by means of a known device 5 represented in FIG. 7: the result of each correlation is multiplied by ⁇ 1 according to the value of the databit on the code period during which the correlations are made.
• the characteristic of coherent integration is to keep the phase information of the signal. Integration consistent with the consideration of the phase of the data bits is the most efficient signal processing in terms of detection sensitivity. An important coherent time makes it possible to have a narrow bandwidth and to bring out the signal of the noise. On the other hand, it is not possible to greatly increase the coherent time. This one is very quickly limited by: the number of assumptions of frequencies to be considered to compensate the Doppler effect, the potential acceleration of the mobile, the characteristics and the price of the pilot oscillator in the receiver.
• the reception sensitivity for a coherent time of 20 ms is located around -144 dBm according to the probabilities of absence and false detections of the radionavigation signal.
• a coherent integration is mixed over a time Tc and a non-coherent integration over a time Tnc.
• This mixing is implemented for example by means shown in FIG.
• the process according to the invention proceeds in 3 steps.
• the first step calculates a coherent integration over a time Tc with its 2 real and imaginary components.
• the second step calculates an amplitude or a power from the 2 components.
• the third step calculates over a time Tnc the sum of the amplitudes or powers having the same offset.
• the coherent integration (1 st step) is performed during a time Tc, multiple of the period of the code, in the multiple occurrence of 1 ms; it is performed separately on both the real and imaginary components.
• n to + kx 1 ms + nx 1 ms / 2046.
• This coherent integration can be obtained by means of a microprocessor or by means of an adder 6 associated with a memory 7 to 2N boxes able to be read and written such as a RAM memory.
• the memory 7 is read and written as and when the results provided by the correlator 20. An amplitude or a power is then calculated from the
• This second step is obtained in known manner by software means or a device 8.
• the non-coherent integration consists of calculating over the period of the code, the amplitude or the power of the correlations on the two components, and then summing them.
• the signal is sampled and processed for a satellite and a frequency assumption (considered to compensate for the Doppler effect) at a time.
• the samples are directly used for the processing of the signal which lasts Tnc.
• the signal is sampled and stored in memory 11 preferably during the noncoherent time Tnc which can reach 16 s. Then the samples are read into memory for each satellite and each Doppler frequency hypothesis; that is, the I and Q signals from the memory 11 are converted to baseband and compensated for a Doppler frequency by the element 12 before being inserted into the shift register 1 of the correlator 20. The samples are read at a frequency f1 much higher than their write frequency in memory fe.
• This post-processing mode makes it possible to reduce the overall search time of the satellites as it appears on the following comparison. We design :
• NBSAT the number of satellites
• Tnc the non-coherent integration time
• Tc the coherent integration time
• T tr Real time global processing time
• Tpt Total treatment time post-treatment We have:
• Ttr NBSAT x BPDOPPLER x Tc x Tnc
• Tpt NBSAT x BPDOPPLER x Tc x Tnc / K + Tnc,
• two-state signals ( ⁇ 1) were considered; of course, the invention applies in the same way to signals with more than two states. So far it has been considered that only one code is generated locally. When it is desired to consider other codes, these can be considered in series by reprogramming the local code and the associated Doppler frequencies, as one changes code.
• the received signal may be a satellite and / or pseudolite radionavigation signal.
• GPS Global Positioning System
• GALILEO Global System for Mobile Communications

Landscapes

• 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)
• Electromechanical Clocks (AREA)
• Noise Elimination (AREA)

Applications Claiming Priority (2)

Publications (1)

Family

ID=35058198

Family Applications (1)

Country Status (4)

Families Citing this family (1)

Citations (3)

Family Cites Families (7)

• 2004
  • 2004-10-15 FR FR0410953A patent/FR2876845B1/fr active Active
• 2005
  • 2005-10-11 US US11/577,322 patent/US7826518B2/en active Active
  • 2005-10-11 WO PCT/EP2005/055169 patent/WO2006040325A2/fr active Application Filing
  • 2005-10-11 EP EP05797254A patent/EP1807711A2/de not_active Withdrawn

Patent Citations (3)

Also Published As

Similar Documents

Legal Events