DE19630722A1 - Satellite navigation receiver device for reprocessing satellite signals - Google Patents

Abstract

A receiver for satellite navigation signals (ZF) comprises a variable carrier frequency generator (3), for creating a signal with an appropriate carrier frequency. There are two mixers (1,2), a variable code generator (9,21) for producing a code after phase and frequency are agreed, and four signal correlator analysers (10,11,12,13). To achieve better correspondence with the satellite signal, the carrier frequency and the code generators are linked to three regulators (14,15,16). The first regulator (14) corrects the initial phase errors in the code signal. The second regulator (15) corrects the initial phase errors in the carrier signal. The third regulator corrects the actual frequency changes occurring between two time points (1).

Description

The invention relates to a satellite navigation receiver a receiver level for receiving and processing a Satellite signal with a changeable carrier frequency genes rator for generating one of the carrier frequency of the refurbished th satellite signal corresponding frequency with which Satellite signal in at least one carrier frequency mixer is mixed with a changeable code generator for Er generation of one with a code in the satellite signal according to Fre codes and phase matching codes, with an extractor level to determine the conformity of the gerfrequenzgenerator and signals generated by the code generator with the satellite signal, with a control circuit for ex experimental finding one roughly with the satellite signal matching carrier frequency and a suitable one Codes based on frequency and phase approximately with the Satelli tens signal matches, and with control stages to connect the readjustment of the carrier frequency generator and the code generator to produce an improved match with the satellite signal.  

The satellites emit signals for satellite navigation on a fixed carrier frequency. In the GPS system this is Carrier frequency 1,575.42 MHz. For military applications a second carrier frequency of 1,227.60 MHz is used, whose code information is not accessible to civil applications is.

A code signal is transmitted on this carrier frequency is also transmitted at a predetermined frequency, and for codes with a frequency of 1.023 MHz. The code formed as a gold code in the GPS system consists of binary pulses, being a digital information is contained in phase jumps of the sine waves. Every satellite lit emits its own code that recognizes this Satellites and the assignment of the signal to that in the receiver known position of the satellite is used.

The evaluation of the satellite signal in the receiving device sets proper processing of the satellite signal ahead. For this it is necessary to use the satellite signal as much as possible to get rid of the carrier frequency exactly. For a parallel Evaluation of signals from several satellites has one Receiving device usually several receiving channels, whereby in a carrier frequency generator and a code generator for each channel is used. The carrier frequency generator is each adjusted to the carrier frequency of the satellite signal in order to by multiplying the satellite signal by the carrier frequency quenz to generate the demodulated code signal. In the receiver there are several code generators in parallel Ka the different codes for the receiving device in Be to produce traditional satellites. The one for navigation necessary determination of the distance to the satellite ge happens via a runtime determination of the satellite signal by measuring a phase shift. Use the one for this algorithms and measures are generally known and can For example, the book Frank Schrödter "GPS satellite Navigation ", Poing 1994.  

The code chosen to identify the satellites, at for example gold code, is designed so that it has a characterized autocorrelation function. The determination of whether the code generated in the code generator with the code in the satellite signal matches, can therefore be determined by a correlation in an evaluation level. The readjustment of the in the Emp Carrier frequency and code frequency generated for a longer period is required Lich because the satellite signal can be disturbed on the one hand on the other hand, due to a relative movement between satel lit and receiving device Doppler shifts occur.

The received satellite signals are processed regularly after a conversion to an intermediate frequency one for each receiving channel with a micropro processor related correlator. A description of a derar The correlator and its evaluation functions is the ad vance Information GP 2021 from GEC Plessey Semiconductors from June 1995. After that it will be in between frequency-converted satellite signal in two branches with one carrier frequency signal generated by a carrier frequency generator nal on the one hand and a π / 2 shifted carrier frequency signal mixed on the other hand so that an I signal (in-line signal) and a Q signal (quadrature signal) arises. In the matched NEN case, the I signal gives the signal amplitude of the useful signal and the Q signal the strength of the noise again.

Both signals are output with a code generator mixed. For this purpose, a regulated output signal is used (punctual) and a modified signal (tracking signal) ver turns, with the modified signal forward by π / 2 (early) or postponed (late) or out of the dif Reference of these two signals can be formed signal.

There are thus four signals, the code correlation levels be fed. By evaluating the correlation signals are control signals for the control stage of the carrier frequency genes rators and for the control level of the code generator. There  the input variables of both control levels both from the adjustment the carrier frequency and the code frequency are dependent, there may be interference between the two control levels. In addition, the short response times of the control stages can increase Overshoot reactions cause the receiver to instabi lities and thus to a reduced measuring accuracy and prolonged settling phase of the control stages.

The invention is based on the problem, the so-called ten disadvantages of the known satellite navigation receiving advise at least to decrease.

Starting from this problem is a satellite naviga tion device of the type mentioned according to the invention characterized in that a first control stage for correcting a Initial phase error of the code signal of the code generator, one second control stage for correcting an initial phase error of the Carrier frequency signal of the carrier frequency generator and one third control stage for the correction of current frequency changes between two times of the query.

The separation according to the invention between phase control and Fre Sequence control avoids major disadvantages of the previous one Regulations. After experimenting through a search algorithm roughly matching carrier frequency signals and code signals have been determined, an initial phase error determined for the carrier frequency signal and the code signal and a correction signal can be created from this. Since the Initial phase error during the navigation operation of the Satel navigation device does not change, the Pha controller for the code signal and the carrier frequency signal have a long reaction time, ie "static" Form regulator. Frequency changes, however, are particularly due to the Doppler frequency shift in relatively moving Systems generated so that the control stage for the frequency must be carried out with a short response time, thus represents a "dynamic" controller.  

Since the Doppler shifts both in the same way affect the carrier frequency as well as the code frequency, can the third control stage ge by a common controller be educated.

The correlations are for the usual code regulation signals resulting from the difference of the Mixed signals "early" and "late" have resulted. This creates a linear dependency between the deviation and the rule signal, however, due to signal loss usually loop is bought because the difference in correlation sizes "early minus late" only about 7/8 of the maximum correlation performance allows. According to the invention, it is therefore advantageous to Set the code phase controller so that the control is set to a Match of one of the two signals "early" or "late" is set with the code signal of the satellite signal. In the "late" signal thus corresponds to this case, for example the current signal ("punctual").

Since the use of the conventional controllers previously used are optimized only for linear systems, the satellite signal behaves stochastically due to disturbances and the over load functions of the code correlators are not linear are the system errors of the previous controllers in the border area rich no longer negligibly small. With higher dynamics arise larger measurement errors that up to the instability of the Re can lead circles. In addition, by own dynamics of the controller the natural frequency of the control loops limits, so that response time and measuring accuracy of Affect each other. These drawbacks will be avoided in the present invention when the control levels in a preferred embodiment as a fuzzy controller forms are.

The invention is intended to be based on one in the drawing illustrated embodiment are explained in more detail.  

In accordance with the technique customary hitherto, the satellite signal is converted into an intermediate frequency signal IF by a conventional receiving stage (not shown). The satellite signal thus converted arrives in two parallel branches, each on a carrier frequency mixer 1, 2 . Output signals of a carrier frequency generator 3 , which is designed as a digitally controlled oscillator (DCO - digitally controlled os cillator), arrive at the respective other input of the carrier frequency mixer 1, 2 . The output signal of the carrier frequency generator 3 arriving at the second carrier frequency mixer 2 is shifted in a phase shift stage 4 by 90 ° so that at the output of the first carrier frequency mixer 1 an I signal (in line signal) and at the output of the second carrier frequency mixer 2 Q signal (quadrature signal) is present. In the adjusted state, the mixing of the satellite signal with the carrier frequency leads to the carrier frequency being eliminated from the satellite signal, so that the satellite signal is in demodulated form.

The two output signals of the two carrier frequency mixers 1 , 2 are supplied to a total of four code mixers 5 , 6 , 7 , 8 in two branches each. The other inputs of these code mixers 5 , 6 , 7 , 8 are fed by a code generator 9 , which has two different outputs, which are shifted by 90 ° to one another as "early" and "late" signals. Since, according to the invention, the regulation takes place in such a way that the "late" signal corresponds to the code signal in the satellite signal IF, the "late" signal in the adjusted state represents the current code signal ("punctual").

The first code mixer 5 receives the I output signal of the first carrier frequency mixer 1 and the "punctual" output signal of the code generator 9 . The second code mixer 6 mixes the I signal with the "early" signal of the code mixer 9 . The third code mixer 7 mixes the Q signal with the "punctual" output signal of the code signal generator 9 and the fourth code mixer 8 mixes the Q signal with the "early" signal. The output signals from the four code mixers 5 , 6 , 7 , 8 arrive at four correlation stages 10 , 11 , 12 , 13 , which form the correlation signals I-Prompt, I-Track, Q-Prompt and Q-Track.

The correlation signals of the correlation stages 10 , 11 , 12 , 13 represent a measure of the control state and are therefore suitable as input signals for the control stages 14 , 15 , 16 provided according to the invention. The output signals of the correlation stages 10 , 11 , 12 , 13 are fed to the three control stages 14 , 15 , 16 via three function generators 17 , 18 , 19 , the control function of which is explained in more detail below.

The first control stage 14 is used to correct an initial phase error, thus creating a correction signal Δf ^o _code which is added to an initial code frequency f _{N, code} determined by the aforementioned search algorithm. The correction signal Δf ^o _code does not change regularly during a navigation measurement, so that the first control stage represents a "static" controller. The aforementioned addition takes place in an addition stage 20 , the output signal of which is fed to a code frequency generator 21 as part of a code generator 9 , 21 .

The second control stage 15 is used to correct an initial phase error of the initial carrier frequency f _{N, carrier} . The correction signal Δf ^o _carrier is obtained by subtracting two signals Δf ⁱ _carrier and Δf ⁱ _{D, L1} and then dividing by 2, as will be explained in more detail below. The correction signal Δf ^o _carrier is added to the initial _carrier frequency f _{N, carrier} in an addition stage 22 and also represents a practically unchangeable variable, so that the second control stage 15 is also a static controller.

The third control stage 16 corrects detected frequency deviations between polling times i, which are caused, for example, by Doppler frequency shifts. The control time of the third control stage 16 is therefore comparatively short, so that the third control stage 16 represents a dynamic controller. The summed up in a summation stage 23 correction signals for dynamic frequency deviations are on the one hand added to the signal Δf ^o _carrier in an addition stage 24 and influence the frequency of the carrier frequency generator 3 via the addition stage 22 . On the other hand, the total frequency deviation reaches a divider stage 25 , which divides by 1540, since the carrier frequency is 1540 times the code frequency. The output signal of the frequency divider 25 also reaches the addition stage 20 , so that the frequency of the code frequency generator 21 is also influenced by the dynamic frequency deviations.

The functions of the three control loops are explained below:
The code phase controller formed with the first control stage has the task of reducing the phase shift between the incoming code signal and the internally generated code signal to 0.

In the well-known satellite navigation system GPS (Global Positioning System), a C / A code sequence is a binary periodic square wave and lasts 1023 chips. You ent speaks exactly 1 ms at a clock frequency of 1.023 MHz. Of the For example, internal code can be from a 10-bit slider ghosts are generated.

There is one between the incoming and the internal code Phase shift τ. The task of the code phase controller be is to reduce this phase shift to 0.

The autocorrelation function is used for control, the is represented as follows:

The ideal autocorrelation for such a pseudo stochastic binary code is 0 if the phase shift <than 1 chip and <than 1022 chips. Only in one phase shift τ within ± 1 chip is the correlation size noticeably large. The maximum arises with the phase shift Zero. By shifting the internal code phase in steps of half chips in relation to the incoming code signal this state can be reached (search algorithm). After that over the code phase controller takes over the control function.

A delay lock loop is used. The code generator generates two C / A code sequences synchronously, which by a con constant phase shift to each other.

In order to reduce the influence of the error from the carrier frequency control loop, the total signal powers R _Early and R _{Late are used} as the input variable.

The autocorrelation function can be performed by the following Equations are represented:

The so-called early-minus-late method used so far for the code control has the advantage that the input variable R _Early minus R _{Late is} linear to the code phase error. The disadvantage, however, is that the steady code phase controller only uses 7/8 of the maximum correlation power, which results in a signal loss for the control loop.

According to the invention, the code phase control is set so that R _{Late reaches} the maximum value. In this case, R _{Late is} identical to R _Punctual . According to the invention, the ratio D of τ of the two correlation variables is used as the input variable of the code phase controller:

This means here:

Fig. 2 shows the transfer function of the code phase controller, wherein the ratio D (τ) in dB unit is specified. In the zero phase (τ = 0), the input variable D of the controller is exactly 0.5, which also corresponds to -6 dB. Conventional linear controllers have difficulties with such a non-linear function, which is very difficult to reproduce without loss of accuracy. This problem can easily be solved by using a non-linear fuzzy controller.

The carrier phase controller formed with the second control stage has the task of regulating the initial phase difference of the carrier signal to 0. Together with the carrier frequency controller formed by the third control stage 16 , both the phase difference and the frequency difference between the incoming signal and the internally generated sine / cosine signals should therefore be regulated to zero.

The radiated satellite signal is:

S _L1 (t) = A _{C /} AC / A (t) D (t) sin (ω _L1t + θ _{0, L1} ) (11)

in which

S _L1 (t): radiated signal;
A _{C / A} : amplitude adjustment;
C / A (t): state of the gold code ∈ {-1.1};
D (t): data of the satellite message ∈ {-1.1};
ω _L1 : 2πf _L1 nominal carrier frequency L1;
θ _{0, L1} : phase error consisting of parts of the satellite oscillator.

The satellite signal received and mixed down to an intermediate frequency IF:

S _s (t) = A _s · C / A (t) · D (t) · sin (ω _S t + θ _S)
= A _s · C / A (t) · D (t) · sin [(ω _{N, Carrier} + ω _{L1, Doppler)} t + θ _S] (12)

in which

S _s (t): the S _L1 (t) received and mixed down to the IF;
A _s : signal amplitude;
C / A (t): state of the gold code ∈ {-1.1};
D (t): data of the satellite message ∈ {-1.1};
ω _{N, carrier} : IF nominal frequency z. B. 1.405396826 MHz;
ω _{L1, Doppler} : Doppler frequency of L1;
θ _S : IF phase shift through the RF mixer.

The internally generated sine and cosine signals:

S _{L, 1} (t) = A _L · sin (ω _L t + θ _L)
S _{L, 2} (t) = A _L · sin (ω _L t + θ _L + π / 2) = A _L cos (ω _L t + θ _L)
S _{L, 1} , S _{L, 2} : sine and cosine signals generated by the local carrier DCO;
A _L : signal amplitude ± 2;
ω _L : local carrier frequency from the carrier DCO;
θ _L : Local carrier phase from the carrier DCO.

The carrier DCO should be controlled so that the local frequency ω _L and phase θ _{L are} as close as possible to the frequency ω _S and phase θ _{S of} the received satellite signals. Therefore, they can be represented by the following formulas:

ω _L = ω _S -ω _Error and θ _L = θ _S -θ _Error .

in order to

S _{L, 1} (t) = A _L · sin [(ω _S -ω _Error ) t + θ _S -θ _Error ] (13)

S _{L, 2} (t) = A _L · cos [(ω _S -ω _Error ) t + θ _S -θ _Error ] (14)

In the control loop, the input signal is multiplied by the sine S _{L, 1} (t) and cosine signal S _{L, 2} (t) generated by the local quartz, so that the carrier frequency is mixed down to zero.

Then both signals are generated with the internally generated C / A code C / A ′ (t) multiplied. With the help of the code control loop, the code function C / A ′ (t) is set so that that it correlates with the C / A (t) by

C / A ′ (t) = C / A (t) or C / A ′ (t) · C / A (t) = C / A² (t) = (± 1) ² = 1;

The input signals f₁ (t) and f₂ (t) of the integrators can then be shown as follows will:

with K = A _S · A _L · C / A² (t) = A _S · A _L.

The integrators add up the input variables within a constant time period T. I _i and Q _i are the output variable of the integrators, and ω _error and θ _{error are} the frequency and phase errors at time T _i :

Since the carrier frequency ω _{S is} significantly larger than the frequency _error ω _Error , the second integration terms of (15) and (16) are neglected small.

where D (T _i ) 2 = (± 1) 2 = 1.

The square of the total signal power:

The standardized IQ product is then the same:

The angle error η _{i to} be corrected can be calculated:

The change in the setting frequency Δf ⁱ _carrier is then:

Only the entire error can be calculated from equation (23). To the Separating frequency error and phase error becomes a subsequent procedure Frequency lock loop (FLL) applied.

Since the Doppler shift only causes relatively slow changes in frequency errors, it can be assumed that at times T _i and T _{i + 1} the frequency _error has approximately the same amount ω _Error :

Since the data function D (t) has a very low data rate (50 Hz), in most cases the data bits D (T _i ) are identical to D (T _{i + 1} ), so that D (T _i ) · D (T _{i + 1} ) = D² (T _i ) = 1.

The equation (14) is normalized here by the amount of I _i + Q _i ² ².

The change in the setting frequency Δf ⁱ _{D, L1} for the frequency error is then:

The equation (23) and (29) can be used to calculate the frequency change Δf⁰ _{D, L1} for the phase error as follows:

Equation (30) illustrates the formation of the signal Δf⁰ _carrier using the output signal Δf ⁱ _{carrier of} the second control stage 15 and the output signal Δf ⁱ _{D, L1 of} the third control stage 16 as it reaches the addition stage 24 .

The dynamic regulations are taken into account in that Decode and carrier frequency as the sum of the deviations from an initial frequency is understood:

f ⁱ _code = f _{N, code} + Δf⁰ _code + Σ Δf ⁱ _{D, code}

f ⁱ _carrier = f _{N, carrier} + Δf⁰ _carrier + Σ Δf ⁱ _{D, L1} .

Claims (9)

1. satellite navigation receiver with a receiver stage for receiving and processing a satellite signal, with a changeable carrier frequency generator ( 3 ) for generating a frequency corresponding to the carrier frequency of the processed satellite signal (IF) with which the satellite signal in at least one carrier frequency mixing stage ( 1 , 2 ) is mixed with a changeable code generator ( 9 , 21 ) for generating a code with a code in the satellite signal (IF) according to frequency and phase, with an evaluation stage ( 10 , 11 , 12 , 13 ) for establishing the match the signals generated by the carrier frequency generator ( 3 ) and by the code generator ( 9 , 21 ) with the satellite signal (ZF), with a control circuit for experimentally finding a carrier frequency approximately corresponding to the satellite signal and a suitable code which is approximately identical in frequency and phase with match the satellite signal mt, and with control stages ( 14 , 15 , 16 ) for subsequent readjustment of the carrier frequency generator ( 3 ) and the code generator ( 9 , 21 ) to produce an improved match with the satellite signal (ZF), characterized in that a first control stage ( 14 ) for correcting an initial phase error of the code signal of the code generator ( 9 , 21 ), a second control stage ( 15 ) for correcting an initial phase error of the carrier frequency signal of the carrier frequency generator ( 3 ) and a third control stage for correcting current frequency changes between two query times (i) . 2. Satellite navigation receiver according to claim 1, characterized in that in the evaluation stage for determining the correspondence of the signals generated by the carrier frequency generator ( 3 ) and code generator ( 9 , 21 ) with the satellite signal (IF) correlation stages ( 10 , 11 , 12 , 13th ) are provided. 3. Satellite navigation receiving device according to claim 1 or 2, characterized by two carrier frequency mixing stages ( 1 , 2 ) for mixing the satellite signal (IF), each because the satellite signal (IF) on the one hand and an output signal from the carrier frequency generator ( 3 ) directly or by π / 2 shifted, on the other hand can be fed and a mixed signal (I, Q) is present at the outputs. 4. Satellite navigation receiving device according to one of claims 1 to 3, characterized by code mixing stages ( 5 , 6 , 7 , 8 ), which on the one hand the mixed signal (I, Q) and on the other hand an output signal of the code generator ( 9 , 21 ) can be fed directly or processed is. 5. Satellite navigation receiver according to claim 4, characterized in that the processed output signal of the code generator ( 9 , 21 ) is shifted by π / 2 compared to the direct output signal. 6. Satellite navigation receiver according to one of claims 1 to 5, characterized in that the control of the code generator ( 9 , 21 ) is carried out so that the processed output signal (punctual) of the code generator ( 9 , 21 ) completely with the code of the satellite signal ( ZF) matches. 7. Satellite navigation receiving device according to one of claims 1 to 6, characterized in that the third control stage ( 16 ) from a common controller for the carrier frequency generator ( 3 ) and the code generator ( 9 , 21 ) is formed. 8. Satellite navigation receiver according to one of claims 1 to 7, characterized in that the control stages ( 14 , 15 , 16 ) are designed as fuzzy controllers. 9. Satellite navigation receiving device according to one of the An sayings 1 to 8, characterized in that the recipient a converter for conversion into an intermediate frequency (IF).