CN110068842B

CN110068842B - High-precision satellite signal capturing method

Info

Publication number: CN110068842B
Application number: CN201910370920.5A
Authority: CN
Inventors: 邓一鹗; 汤加跃; 师强强; 张柏华
Original assignee: Xi'an Kaiyang Microelectronic Co ltd
Current assignee: Xi'an Kaiyang Microelectronic Co ltd
Priority date: 2019-05-06
Filing date: 2019-05-06
Publication date: 2021-01-29
Anticipated expiration: 2039-05-06
Also published as: CN110068842A

Abstract

The invention discloses a high-precision satellite signal capturing method, which comprises the following steps: step 1: selecting a satellite, generating a corresponding main code FFT sequence, and initializing Doppler frequency; step 2: generating a sampling FFT sequence according to satellite data; and step 3: conjugating the sampling FFT sequence, multiplying the conjugated sampling FFT sequence by the main code FFT sequence, performing IFFT on the multiplied result, and then performing modulus extraction to obtain an initial modulus value; and 4, step 4: repeating the step 3, and accumulating the initial module values obtained by each repetition at intervals to obtain accumulated module values; and 5: moving the Doppler frequency step by step; repeating steps 3 and 4 once every step; step 6: selecting the maximum value of all accumulated module values and reserving the maximum value; and 7: and taking the maximum value and the index subsequent value of the maximum value, performing combined operation to obtain and output more accurate code phase estimation and Doppler frequency offset estimation. The method greatly improves the capturing success rate and the tracking success rate and accelerates the tracking bit synchronization speed.

Description

High-precision satellite signal capturing method

Technical Field

The invention belongs to the field of signal processing, and relates to a high-precision satellite signal capturing method.

Background

In the process of positioning through a satellite, firstly, a satellite signal needs to be captured, tracking is carried out according to a capture result after the satellite signal is captured, and the satellite signal is output according to the tracking result to be used for positioning. The acquisition of satellite signals becomes a large factor affecting positioning.

The existing capture scheme is to adopt a maximum value mode to judge under the condition of low carrier-to-noise ratio, namely, to select a maximum value according to the long integral results of all Doppler, track the position of the maximum value, determine the corresponding Doppler frequency and code phase, then carry out long integral on the basis, find the initial sampling point of tracking and the initial value of bit, carry out long integral traction on the basis by using the Doppler frequency, and then track.

However, the above prior art schemes may cause a relatively large probability of missing capture and unsuccessful pulling, because when low carrier-to-noise ratio data enters the tracking, the farther the doppler frequency deviates, the greater the probability of unsuccessful pulling, and thus the tracking failure.

Disclosure of Invention

The present invention is directed to overcome the above-mentioned shortcomings of the prior art and to provide a method for acquiring satellite signals with high accuracy.

In order to achieve the purpose, the invention adopts the following technical scheme to realize the purpose:

a high-precision satellite signal acquisition method comprises the following steps:

step 1: selecting a satellite, generating a corresponding main code FFT sequence, and initializing Doppler frequency;

step 2: reading satellite data with the length 10 times of the main code of the satellite signal, and generating a sampling FFT sequence according to the satellite data;

and step 3: conjugating the sampling FFT sequence, multiplying the conjugated sampling FFT sequence by the main code FFT sequence, performing IFFT on a multiplication result, and then performing modulus extraction to obtain a modulus sequence;

and 4, step 4: repeating the step 3 of the preset times, accumulating the module value sequences obtained when the repeated times are odd numbers to obtain odd number accumulated module values; accumulating the module value sequence obtained when the repetition times are even numbers to obtain even number accumulated module values;

and 5: moving the doppler frequency stepwise from a minimum value to a maximum value of the doppler frequency; repeating steps 3 and 4 once every step;

step 6: selecting and reserving the maximum value of all the odd accumulated modulus values and the even accumulated modulus values, recording the maximum value as a first accumulated modulus value, reserving the accumulated modulus values corresponding to the Doppler frequency before and after stepping and corresponding to the first accumulated modulus value, and recording the accumulated modulus values as a second accumulated modulus value and a third accumulated modulus value;

and 7: when the second accumulated modulus value and the third accumulated modulus value are both greater than a preset threshold, F1+ Q/2 (P3-P2)/(P3+ P2);

when the second accumulated modulus is greater than a preset threshold and the third accumulated modulus is less than a preset threshold, F is F1-Q/2+ Q/2 (P1-P2)/(P1+ P2);

when the third accumulated modulus is greater than a preset threshold and the second accumulated modulus is less than a preset threshold, F is F1+ Q/2-Q/2 (P1-P3)/(P1+ P3);

when the second accumulated modulus value and the third accumulated modulus value are both smaller than a preset threshold, F is F1;

wherein: f is the captured Doppler frequency, F1 is the Doppler frequency corresponding to the first accumulated modulus, P1 is the first accumulated modulus, P2 is the second accumulated modulus, P3 is the third accumulated modulus, and Q is the Doppler frequency step value;

and 8: and outputting the captured Doppler frequency and the code phase corresponding to the first accumulated modulus value, and finishing the capturing.

The invention further improves the following steps:

step 6 further includes reserving accumulated modulus values corresponding to the code phases of the first half chips of the code phases corresponding to the first accumulated modulus values, and recording as fourth accumulated modulus values; keeping the accumulated modulus corresponding to the code phase of the second half chip of the code phase corresponding to the first accumulated modulus, and recording as a fifth accumulated modulus;

the step 7 further includes, when the fourth accumulated modulus value and the fifth accumulated modulus value are both greater than a preset threshold, M-M1 +1/2 ((L1-E1)/(L1+ E1));

when the fourth accumulated modulus value is greater than a preset threshold value and the fifth accumulated modulus value is less than the preset threshold value, M is M1-1/2+1/2 ((P1-E1)/(P1+ E1));

when the fourth accumulated modulus value is smaller than a preset threshold value and the fifth accumulated modulus value is larger than the preset threshold value, M is M1+1/2-1/2 ((P1-L1)/(P1+ L1));

when the fourth accumulated modulus value and the fifth accumulated modulus value are both smaller than a preset threshold value, M is M1;

wherein: m is the captured code phase, M1 is the code phase corresponding to the first accumulated modulus, E1 is the fourth accumulated modulus, and L1 is the fifth accumulated modulus;

step 8 is replaced by step 9;

and step 9: and outputting the acquired Doppler frequency and the acquired code phase, and finishing the acquisition.

The specific method of the step 2 comprises the following steps:

reading satellite data of a satellite signal main code length, performing code phase compensation, carrier phase compensation, intermediate frequency removal and down sampling on the satellite data through Doppler frequency, and performing FFT (fast Fourier transform) processing on the satellite data to generate a sampling FFT sequence.

The down-sampling is performed by down-sampling the frequency of the satellite data to 1/2 times the frequency of the primary code.

The satellite signal is a GPS signal.

The step value of the step movement in the step 5 is 60 Hz.

Compared with the prior art, the invention has the following beneficial effects:

the accumulated modulus values corresponding to the Doppler frequency before and after the step corresponding to the first accumulated modulus value are reserved, and then Doppler frequency shift is carried out on the first accumulated modulus value, so that the capturing success rate and the tracking success rate are greatly improved, and the speed of tracking bit synchronization is accelerated. Accumulating the accumulated module values at intervals to respectively obtain odd accumulated module values and even accumulated module values, thereby ensuring that coherent accumulated integration of the sampled satellite data is complete without being reversed and offset.

Furthermore, accumulated modulus values corresponding to the code phases of the first half chips of the code phases corresponding to the first accumulated modulus values are reserved, and then code phase offset is carried out on the first accumulated modulus values, so that sampling points of integral used for starting tracking are more accurate and closer to true values, and the capturing success rate and the tracking success rate are further improved.

Drawings

FIG. 1 is a block flow diagram of the method of the present invention;

FIG. 2 is a flow chart of Doppler frequency shift according to the present invention;

FIG. 3 is a block diagram of the code phase shifting process of the present invention;

FIG. 4 is an IQ time output plot for a prior art method;

FIG. 5 is an IQ plane output diagram of a prior art method;

FIG. 6 is an IQ time output diagram of the present invention;

FIG. 7 is an IQ plane output diagram according to the present invention.

Detailed Description

In order to make the technical solutions of the present invention better understood, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.

It should be noted that the terms "first," "second," and the like in the description and claims of the present invention and in the drawings described above are used for distinguishing between similar elements and not necessarily for describing a particular sequential or chronological order. It is to be understood that the data so used is interchangeable under appropriate circumstances such that the embodiments of the invention described herein are capable of operation in sequences other than those illustrated or described herein. Furthermore, the terms "comprises," "comprising," and "having," and any variations thereof, are intended to cover a non-exclusive inclusion, such that a process, method, system, article, or apparatus that comprises a list of steps or elements is not necessarily limited to those steps or elements expressly listed, but may include other steps or elements not expressly listed or inherent to such process, method, article, or apparatus.

The invention is described in further detail below with reference to the accompanying drawings:

referring to fig. 1-3, the invention relates to a method for capturing satellite signals with high precision, comprising the following steps:

step 2: reading satellite data of a satellite signal main code length, performing code phase compensation, carrier phase compensation, intermediate frequency removal and down-sampling on the satellite data through Doppler frequency, down-sampling the frequency of the satellite data to 1/2 of the main code frequency during down-sampling, and then performing FFT on the satellite data to generate a sampling FFT sequence;

and step 3: conjugating the sampling FFT sequence, multiplying the conjugated sampling FFT sequence by the main code FFT sequence, performing IFFT on a multiplication result, and then performing modulus extraction to obtain an initial modulus value;

and 5: moving the Doppler frequency in a stepping mode, wherein the stepping value is 60Hz and is from-4980 Hz of the minimum value of the Doppler frequency to 4980Hz of the maximum value; repeating steps 3 and 4 once every step;

step 6: selecting a maximum value in all accumulated modulus values, reserving the maximum value, recording the maximum value as a first accumulated modulus value, reserving accumulated modulus values corresponding to Doppler frequencies before and after the Doppler frequency stepping corresponding to the first accumulated modulus value, and recording the accumulated modulus values as a second accumulated modulus value and a third accumulated modulus value; meanwhile, the accumulated modulus of half a chip in the morning and evening of the first accumulated modulus is reserved and recorded as a fourth accumulated modulus and a fifth accumulated modulus; keeping the accumulated modulus value corresponding to the code phase of the first half chip of the code phase corresponding to the first accumulated modulus value, and recording as a fourth accumulated modulus value; keeping the accumulated modulus corresponding to the code phase of the second half chip of the code phase corresponding to the first accumulated modulus, and recording as a fifth accumulated modulus;

when the fourth accumulated modulus value and the fifth accumulated modulus value are both greater than a preset threshold, M-M1 +1/2 ((L1-E1)/(L1+ E1));

and 8: and outputting the acquired Doppler frequency and the acquired code phase, and finishing the acquisition.

Some of the principles of the present invention are described in detail below:

firstly, the operation of removing the intermediate frequency of the GPS signal is as follows:

obtaining formulae (3) and (4) by formulae (1) and (2):

I(t)＝d(t)*cos(2πf_IF+dopplert) (1)

Q(t)＝d(t)*sin(2πf_IF+dopplert) (2)

wherein: t is time; i is I branch data; q is Q branch data, and Q is orthogonal to I branch; f. of_IF+dopplerThe doppler frequency is added to the intermediate frequency.

The fixed-point operation process is carried out by the following equations (3) and (4):

I (k)＝d(k)*cos_table(index_k) (3)

Q(k)＝d(k)*sin_table(index_k) (4)

wherein: index _ k is a cos table index corresponding to the discrete time; and calculating a sine coefficient and a cosine coefficient according to the index in a sine table and a cosine table through a carrier wave stepping mode, so as to realize the removal of the intermediate frequency of the GPS signal.

Secondly, capturing a result output description:

the capture results form a 3X6 matrix, resultX, as shown in table 1:

TABLE 1 Capture results Table

index1	E2	P2	L2	F2	valid1
						index2	E1	P1	L1	F1	valid2
index3	E3	P3	L3	freq3	valid3

Wherein: index1 indicates the index of the corresponding value of the frequency immediately preceding the frequency at which the maximum value is located; e2 represents the maximum value corresponding to the previous value of the frequency preceding the frequency at which the maximum value is located; p2 represents the current value corresponding to the maximum value of the frequency preceding the frequency at which the maximum value is located; l2 indicates that the maximum value of the frequency preceding the maximum value corresponds to the latter value; f2 represents the value of the frequency preceding the frequency at which the maximum value is located; valid1 indicates whether a value indicating that the frequency at which the maximum value is located is valid or not, which is obtained one frequency ahead. index2 indicates the index of the frequency corresponding value where the maximum value is located (determines the current code phase); e1 represents the frequency maximum at which the maximum value corresponds to the previous value; p1 denotes the frequency maximum at which the maximum lies; l1 indicates the frequency maximum at which the maximum value corresponds to the latter value; f1 represents the value of the frequency at which the maximum value is located; valid2 indicates whether the value extracted by the frequency at which the maximum value is found is valid, and is valid by default. index3 indicates the index of the corresponding value of the frequency following the frequency at which the maximum value is located; e3 represents the maximum value corresponding to the previous value of the frequency following the frequency at which the maximum value exists; p3 represents the current value corresponding to the maximum value of the frequency following the frequency at which the maximum value is located; l3 indicates that the maximum value corresponds to the latter value of the latter frequency of the frequency at which the maximum value is located; freq2 represents the value of the frequency subsequent to the frequency at which the maximum value is located; valid3 indicates whether the value obtained by extracting the frequency of the maximum value is valid or not, and is invalid by default.

The results of the odd-even non-coherent integration sequences all obtain a 3X6 matrix, wherein when the P2 value is large, the corresponding matrix is selected, and the other matrix is discarded.

The specific steps for constructing resultX are as follows:

s1: the 2 × 4 line data is generated first, and the result of the first line is generated as a first line preparation.

The frequency is initialized and the result is obtained based on this frequency and if it is greater than the threshold, see table 2:

TABLE 2 initialization frequency results table

index01	E02	P02	L02
				index02	E01	P01	L01

And one of the rows is transferred into the resultX first row according to the sizes of P01 and P02, where index02 cannot be adjacent to index 01. When P01 is greater than P02, and P02 is the next largest value, the corresponding value is passed into ResultX first row, with the results shown in Table 3:

table 3 resultX first row assignment table

index1

E2＝E02

P2＝P02

L2＝L02

S2: when the doppler frequency is changed in steps, if the obtained P value is greater than P2, after index2 is obtained, index01 and index02 are reassigned, and the latest value is written into the second row, the results are shown in table 4:

table 4 resultX second row assignment table

index2

E1

P1

L1

valid2

And put valid2 to 1.

S3: if valid2 is equal to 1, each time when the maximum value is greater than P1, index2 is reassigned, the row of index1 is assigned the value of index2 retained last time, and index3 is discarded.

If valid2 is 1, the line of index3 is assigned and the line of index1 has no value when the maximum value obtained in each calculation is less than that of P1 and index3 has no value; when index3 has a numerical value, the value calculated this time is discarded.

S4: the steps S2 and S3 are repeated until all the preliminary search values of the doppler frequency are operated. Each frequency hopping determines whether to update the resultX according to a threshold, and the final result is to perform offset according to data in the resultX.

And obtaining more accurate code phase value and frequency value according to the result obtained by the matrix, and outputting the more accurate code phase value and frequency value to a tracking channel.

1. Centered on the row of data where P1 is located. This is the result given by the original algorithm. This result may be biased to be larger, and higher than the design, which may also result in capture failure, and the comparison and supplement with adjacent values may make the obtained output result more stable.

2. The specific method comprises the following steps:

setting a threshold pThresh ═ 0.95 × thresh, where: thresh is a threshold corresponding to the capture length, below which the probability of false capture is below 5%. When P2 or P3 is greater than pThresh, it is judged to be effective, and the result corresponding to P2 is shifted in the direction of freq1 or freq 3. If P1 is greater than pThresh, valid1 is set to 1; if P3 is greater than pThresh, valid3 is set to 1;

and judging the code phase shift situation according to P2 and P3, and shifting the final result of the Doppler input tracking loop within a certain range according to the sizes and the effectiveness of P2 and P3 on the basis that P1 corresponds to the optimal quality. And judging the shift condition of the code phase according to E1 and L1, and shifting the final result of inputting the code phase into the tracking loop within a certain range according to the size and the effectiveness of E1 and L1 on the basis of the maximum value corresponding to P1 so as to shift the initial value of the number of the tracked sampling points by a few.

Examples

In this embodiment, GPS satellite data is used, and the data power: -146dBm, sample rate: 16.367667MHz, intermediate frequency: 4.123968 MHz. The length of data used for capturing is 800ms, and compared with the difference between the original method and the high-precision method, the difference of stability (whether the data is stable or the time used for stabilizing) after tracking is entered due to the difference of initial conditions (code phase and Doppler) after capturing, so that the quality of high precision is judged. At-146 dBm power, 1ms tracking must not keep up, and a 20ms long integration tracking mode must be used. In the long integration tracking mode of 20ms, both the initial tracking sample point (depending on the acquired code phase) and the initial doppler frequency have an effect on whether the tracking can be stable.

Capture using this method resulted in two arrays of 3X3 as shown in tables 5 and 6:

TABLE 5 odd cumulative modulus results table

E2＝1.14863927355656	P2＝1.12480841613200	L2＝1.10565722947970
			E1＝2.36978665077273	P1＝2.77821838045199	L1＝1.13267266758376
E3＝1.82495324351604	P3＝1.94412798545407	L3＝1.27638522165765

TABLE 6 table of even-numbered accumulated modulus results

E2＝1.05321657127890	P2＝1.04144706747548	L2＝0.991468736457175
			E1＝1.14282125887393	P1＝1.53964169437592	L1＝0.953468154761476
E3＝1.03139513209542	P3＝1.18497262057003	L3＝1.14845513605902

According to the magnitude of P1, an odd accumulated modulus is selected, the Doppler frequency corresponding to the first accumulated modulus is 2580Hz, and the half chip count is 1045.

The preset threshold is 2.02, multiplied by 0.95 and equals 1.9190, and it can be seen from the result that there are only three valid values: e1, P1, P3. The acquisition result is modified after the calculation by adopting the method as follows: the doppler frequency was 2604.7Hz and the half chip count was 1044.54.

The calculation steps are as follows:

comparing the tracking results of the 20ms integration, referring to fig. 4 and 5, the tracking derived from the results of the existing method is not stable all the time, and the correct traction is not successful; referring to fig. 6 and 7, it can be seen that the high accuracy acquisition result has entered the tracking state and has begun to converge.

The above-mentioned contents are only for illustrating the technical idea of the present invention, and the protection scope of the present invention is not limited thereby, and any modification made on the basis of the technical idea of the present invention falls within the protection scope of the claims of the present invention.

Claims

1. A method for acquiring satellite signals with high precision is characterized by comprising the following steps:

2. The method for acquiring satellite signals with high precision according to claim 1, wherein the step 6 further comprises retaining an accumulated modulus value corresponding to a code phase of a first half chip of the code phase corresponding to the first accumulated modulus value, which is denoted as a fourth accumulated modulus value; keeping the accumulated modulus corresponding to the code phase of the second half chip of the code phase corresponding to the first accumulated modulus, and recording as a fifth accumulated modulus;

step 8 is replaced by step 9;

3. The method for acquiring satellite signals with high precision according to claim 1, wherein the specific method of the step 2 is as follows:

4. The method for high-precision satellite signal acquisition according to claim 3, wherein the down-sampling is performed by down-sampling the frequency of the satellite data to 1/2 times the frequency of the main code.

5. The method for high-precision satellite signal acquisition according to claim 1, wherein the satellite signal is a GPS signal.

6. The method for acquiring satellite signals with high precision according to claim 1, wherein the step value of the step movement in the step 5 is 60 Hz.