CN102571294A - Cyclic redundancy check codes (CRC)-based method for correcting satellite navigation message errors - Google Patents

Abstract

The invention relates to a method for correcting satellite navigation message errors by using cyclic redundancy check codes (Cyclic Redundancy Check codes, CRC), and belongs to the technical field of satellite navigation. The specific implementation method first constructs a CRC check module, then constructs a CRC error correction code table, performs error mode detection according to the CRC error correction code table, and corrects the correctable error mode obtained after detection. The present invention can not only check whether the message is wrong, but also correct the 1-bit error mode in the wrong navigation message. Compared with the application situation without CRC error correction, it can effectively reduce the frame error rate of the navigation message and improve the accuracy of positioning under weak signal conditions. Continuity and improved positioning performance; suitable for any satellite navigation system that uses CRC as the navigation message check code.

Description

A Method of Error Correction for Satellite Navigation Messages Based on CRC Coding

technical field

The invention relates to a method for correcting satellite navigation message errors by using cyclic redundancy check codes (Cyclic Redundancy Check codes, CRC), which belongs to the technical field of satellite navigation.

Background technique

With the continuous expansion of the application field of the satellite navigation system represented by the GPS system, new applications put forward higher requirements on positioning accuracy and reliability. GPS L2C, L5, and L1C are three new civilian signals. The new signals adopt many new technologies in terms of pseudo code design, modulation mode, error correction coding and message format. Europe's Galileo satellite navigation system and China's Beidou COMPASS satellite navigation system also design multiple civil signals for different purposes according to the principle of service differentiation. technology experiment.

The design of the new navigation message of the GPS system has been greatly improved compared with the traditional navigation message. An important improvement is the change of the error detection code of the message. The new message generally uses the cyclic redundancy check code CRC with stronger error detection ability to replace Hamming code of GPS NAV message. GPS L2C and L5 adopt CNAV message (IS-GPS-200E, IS-GPS-705A), the frame length of the message is 300 bits, and the CRC-24 check code is used. The encoding generation polynomial is:

g(x)＝x ²⁴ +x ²³ +x ¹⁸ +x ¹⁷ +x ¹⁴ +x ¹¹ +x ¹⁰ +x ⁷ +x ⁶ +x ⁵ +x ⁴ +x ³ +x+1;

L1C signal adopts CNAV-2 message (IS-GPS-800A), message includes 3 subframes, subframe 1 is used for synchronization, subframe 2 is 600 bits long, subframe 3 is 274 bits long, and subframe 2 and 3 are both CRC-24 is used as the check code. The European Galileo satellite navigation system includes messages in two formats: F/NAV and I/NAV (European GNSS (Galileo) Open Service Signal In Space Interface Control Document, 2010). The F/NAV format messages have a frame length of 244 bits and use CRC- 24 check codes, applied to E5a navigation signals, I/NAV format message frame length 120 bits, using CRC-24 check, applied to E5b and E1b navigation signals. The new civil navigation message design of China's Beidou COMPASS satellite navigation system and Russia's GLONASS system also uses a similar error detection coding scheme.

The purpose of the error detection code in the navigation message is to check the correctness of the navigation message and reduce the impact of the error message on the positioning. The conventional processing principle is to discard the error message once an error occurs in the message. When the navigation receiver is in a weak signal environment or a high dynamic platform, the probability of errors in the message will be very high, and the discarding of the wrong message will lead to too few available messages, which will affect the continuity of positioning and even make it impossible to locate. According to the theory of error correction coding, CRC coding belongs to shortened cyclic codes in linear block codes, which can detect and correct errors. Its error detection and correction performance is determined by the minimum Hamming distance, and the minimum Hamming distance of CRC is determined by the generator polynomial and The length of the code word is determined. When the minimum Hamming code distance d≥3, using CRC can correct one or more errors in the code word ([Yang Jie 2005] Application extension of cyclic redundancy check code error correction in wireless transmission), Using CRC error correction can effectively reduce the frame error rate of navigation messages and improve satellite positioning performance.

Contents of the invention

The purpose of the present invention is to overcome the problems of high frame error rate and poor positioning continuity of the navigation message in the weak signal environment of the existing satellite navigation receiver, and propose a satellite navigation message error correction based on the CRC error detection coding structure in the navigation message method.

The present invention is achieved through the following technical solutions:

A method for correcting errors in satellite navigation text based on CRC encoding, comprising the steps of:

Step 1, constructing a CRC check module.

According to the CRC code generator polynomial g(x) in the satellite navigation message specification:

g(x)＝x ²⁴ +x ²³ +x ¹⁸ +x ¹⁷ +x ¹⁴ +x ¹¹ +x ¹⁰ +x ⁷ +x ⁶ +x ⁵ +x ⁴ +x ³ +x+1,

According to the error detection principle of the cyclic redundancy check code, the CRC check module is constructed. The function of the CRC check module is to calculate the check remainder of the navigation message and construct the CRC error correction code table.

Step 2, constructing a CRC error correction code table.

According to the message frame length N stipulated in the satellite navigation message specification, construct N different 1-bit error pattern data, and sequentially send N kinds of 1-bit error pattern data to the CRC check module in step 1 to obtain N different CRCs Check remainder S _i , i represents the sequence number of 1-bit error pattern data, i=1...N. The check remainder S _i and error position P _i of the i-th 1-bit error pattern data form the i-th entry in the error correction code table, and the N entries generated by N different 1-bit error pattern data are stored in CRC in order Error correction code table.

The specific characteristics of 1-bit error mode data are: a string of binary bit streams with a length of N, in which there are 1 "1" and N-1 "0"s in N bits, where "1" represents an error, "0" means no error occurred. The construction method is as follows: the i-th bit of the i-th 1-bit error pattern data is 1, the others are 0, and the error position P _i =i.

The specific characteristics of the CRC error correction code table are: the code table has N entries in total, each entry contains 1 error position and 1 check remainder, and the error position P _i in the 1st to N entries is in accordance with the order from 1 to N The order is incremented.

Step 3, error mode detection.

Send the received satellite navigation message to the CRC check module in step 1 to perform a check operation. If the check remainder S=0, the message is considered correct, and subsequent positioning calculation operations are performed; if the check remainder S≠0, Then it is considered that there is an error in the message, and the CRC error correction code table is searched to determine whether the error mode can be corrected.

The specific judgment principle is: compare the check remainder S of the current message with the check remainder S _i of the N entries in the CRC error correction code table in step 2, and if S _i = S is found, the error mode can be corrected , keep the error position P _i in the same entry as S _i ; otherwise, the error mode is considered uncorrectable, and the current message is discarded.

Step 4, correcting the correctable error pattern obtained after the detection in step 3.

The specific method is: according to the error position P _i obtained in step 3, flip the bit P _i of the navigation message, so as to correct a 1-bit error.

So far, the process of using CRC to correct errors in navigation messages is over.

Beneficial effect

A satellite navigation message error correction method based on CRC encoding proposed by the present invention can not only check whether the message is wrong, but also correct the 1-bit error mode in the wrong navigation message. Compared with the application situation that does not use CRC error correction, it can effectively reduce The frame error rate of the navigation message improves the continuity of positioning under weak signal conditions and improves positioning performance.

Description of drawings

Fig. 1 is a typical satellite navigation receiver processing block diagram in the prior art;

Fig. 2 is the GPS CNAV navigation message frame structure diagram in the specific embodiment;

Fig. 3 is the flow chart of error correction of satellite navigation message of the present invention;

Fig. 4 is the comparison figure of frame error rate before and after using CRC error correction to GPS CNAV message in the specific embodiment.

Detailed ways

In order to better illustrate the purpose and advantages of the present invention, the technical solutions of the present invention will be further described below in conjunction with the accompanying drawings and embodiments. In this specific implementation manner, the GPS CNAV message will be used as an example description.

The structure of a typical satellite navigation receiver is shown in Figure 1. Satellite signals are despread, demodulated, and decoded to obtain navigation messages. The navigation messages use CRC check codes to check the correctness of the messages. When the check remainder is 0, it is considered If the message is correct, perform subsequent positioning calculation operations. When the check remainder is not 0, the message is considered to be wrong, and the wrong message is discarded.

The navigation message used by the L2C and L5 signals of the GPS system is a CNAV message. The frame format is shown in Figure 2. The frame length is 300 bits, and the last 24 bits are CRC check codes. The generator polynomial is: g(x)=x ²⁴ +x ²³ +x ¹⁸ +x ¹⁷ +x ¹⁴ +x ¹¹ +x ¹⁰ +x ⁷ +x ⁶ +x ⁵ +x ⁴ +x ³ +x+1. Under the condition that the frame length of the CNAV message is 300 bits, the minimum Hamming distance d=6 is obtained through computer search, and a corresponding code polynomial is m(x)=x ⁵⁴ +x ⁴⁵ +x ⁴³ +x ³⁰ +x ⁶ + 1. According to the nature of the block code, CRC-24 can detect up to 5-bit errors or correct 2-bit errors. In a typical satellite navigation system, the bit error rate of the message after error correction and decoding is p=10 ^-3 ～10 ^-5 , if p=10 ^-3 , the probability of 1 bit error in a frame of message is FER _1bit ＝300×10 ^-3 ×(1-10 ^-3 ) ²⁹⁹ ＝0.2224, the probability of 2-bit error is FER _2bit ＝300×299×(10 ^-3 ) ² ×(1-10 ^-3 ) ²⁹⁸ /2 =0.0333, indicating that the probability of a 1-bit error is far greater than the probability of a 2-bit error. You can choose to correct a 1-bit or 2-bit error during error correction. The difference between the two lies in the required resources and complexity. This embodiment chooses to correct 1 bit error.

An error correction method for satellite navigation messages based on CRC encoding, as shown in Figure 3, the specific steps for CNAV messages to use CRC to correct errors are:

Step 1, according to the CRC-24 encoding generator polynomial g(x) of the CNAV navigation message, construct the CRC check module according to the error detection principle of the cyclic redundancy check code.

Step 2, according to the CNAV navigation message frame length of 300 bits, construct 300 kinds of 1-bit error pattern data with a length of 300 bits, the i-th bit of the i-th 1-bit error pattern data is 1, and the remaining bits are 0.

Send the i-th kind of (i=1, 2, ..., 300) 1-bit error pattern data to the CRC checking module in step 1 for processing, obtain the check remainder S _i of the i-th kind of 1-bit error pattern data, and check The remainder S _i and the error position P _i =i form an entry, and 300 entries are stored in the CRC error correction code table in the order of the error position from small to large. The structure of the error correction code table of the CNAV message is shown in Table 1.

Table 1 CRC error correction code table structure of CNAV message

error position P _i check remainder S _i 1 S ₁ 2 S ₂ 3 S ₃ ... ... 299 S ₂₉₉ 300 S ₃₀₀

Step 3. Send the received satellite navigation message to the CRC check module for processing to obtain the check remainder S. If S=0, the message is considered correct, and the subsequent positioning calculation operation is performed; if the check remainder S≠0, it is considered When an error occurs in the message, search the error correction code table to determine whether the error mode can be corrected.

The specific judgment principle is: sequentially search 300 entries in the error correction code table, compare S _i with S in the current message, if S _i = S is found, the error mode can be corrected, and the error position P _i corresponding to S _i is reserved, For example, S ₂₅ =S, P ₂₅ =25, which means that the 25th bit of the current message has an error; if S _i =S is not found in the 300 entries, the error mode is considered uncorrectable, and the current message is discarded.

Step 4, correcting the correctable error pattern obtained after the verification in step 3.

The specific method is: get the error bit position _Pi according to step 3, correct the 1-bit error in the current message, and perform subsequent positioning and calculation processing on the message after error correction. For example, if P ₂₅ = 25 is obtained in step 3, the 25th bit of the current message is reversed to obtain the error-corrected navigation message

So far, the process of using CRC to correct errors in navigation messages is over.

In order to specifically analyze the performance of the CRC error correction method, software is used to simulate the CRC error correction process of the CNAV message. The simulated message frame length is 300 bits, and the error detection code is CRC-24. Fig. 4 is the simulated CRC code error correction according to the embodiment As a result, the horizontal axis represents the message bit error rate BER, and the vertical axis represents the message frame error rate FER. Curve A represents the frame error rate of the navigation message without CRC code error correction, and curve B represents the frame error rate of the navigation message after using CRC code error correction. It can be seen from the figure that using CRC coding error correction effectively reduces the frame error rate of the navigation message, and the specific frame error rate improvement performance data is shown in Table 2.

Table 2 Improvement of frame error rate after using CRC error correction

BER Frame error rate before error correction Frame error rate after error correction 1×10 ^-2 0.95 0.80

1×10 ^-3 0.26 0.037 1×10 ^-4 2.9×10 ^-2 4.4×10 ^-4 1×10 ^-5 2.9×10 ^-3 4.5×10 ^-6 1×10 ^-6 2.9×10 ^-4 4.5×10 ^-8 1×10 ^-7 2.9×10 ^-5 4.5× ^10-10

The method of the present invention is applicable to the used CNAV navigation message of U.S. GPS satellite positioning system L2C, L5 signal, the used CNAV-2 navigation message of L1C signal, the used F/NAV, I/NAV navigation message of European Galileo Galileo satellite positioning system, China Beidou The navigation message used by the COMPASS satellite positioning system, and the navigation message used by the Russian GLONASS satellite positioning system.

The method of the present invention is not limited to a specific GPS satellite navigation system, but is applicable to any satellite navigation system that uses CRC as a navigation message check code.

Although the embodiment of the present invention has been described in conjunction with the accompanying drawings, for those skilled in the art, some modifications and improvements can be made without departing from the principle of the present invention, and these should also be regarded as belonging to the protection of the present invention scope.

Claims (5)

1. the satellite navigation message error correction method based on the CRC coding is characterized in that: comprise the steps:

Step 1, structure CRC check module;

According to the CRC in satellite navigation message standard coding generator polynomial g (x):

g(x)＝x ²⁴+x ²³+x ¹⁸+x ¹⁷+x ¹⁴+x ¹¹+x ¹⁰+x ⁷+x ⁶+x ⁵+x ⁴+x ³+x+1，

Error detection principles of construction CRC check module according to CRC;

Step 2, structure CRC error correction code table;

According to the text frame length N that stipulates in the satellite navigation message standard, 1 different bit error pattern data of structure N kind are sent N kind 1 bit error pattern data into the CRC check module in the step 1 successively, obtain N different CRC check remainder S _i, i representes the sequence number of 1 bit error pattern data, i=1 ... N; The verification remainder S of i kind 1 bit error pattern data _iWith errors present P _iForm i clauses and subclauses in the error correction code table, N the clauses and subclauses that the different 1 bit error pattern data of N kind are produced deposit CRC error correction code table in order in;

The building method of 1 bit error pattern data is: i bit of i kind 1 bit error pattern data is 1, and other is 0, errors present P _i=i;

The concrete characteristic of CRC error correction code table is: code table has N clauses and subclauses, and each clauses and subclauses comprises 1 errors present and a verification remainder, the errors present P in the 1st to N clauses and subclauses _iOrder according to from 1 to N increases progressively;

Step 3, error pattern detects;

The CRC check module that the satellite navigation message that receives is sent in the step 1 is carried out the verification computing,, carry out follow-up positioning calculation computing if verification remainder S=0 thinks that then text is correct; If verification remainder S ≠ 0 thinks that then text makes a mistake, carry out the search of CRC error correction code table and whether can correct with the misjudgment pattern;

Step 4 is carried out error correction to the correctable error pattern that obtains after step 3 verification;

So far, the process of using CRC to correct the navigation message mistake finishes.

2. a kind of satellite navigation message error correction method according to claim 1 based on the CRC coding; It is characterized in that: the described 1 bit error pattern data of step 2 are that length is the 2 system bit streams of N; 1 " 1 " and N-1 " 0 " is arranged in N bit; Wherein " 1 " representative makes a mistake, and " 0 " representative does not make a mistake.

3. a kind of satellite navigation message error correction method based on the CRC coding according to claim 1 is characterized in that: the method for step 3 described search CRC error correction code table and misjudgment pattern is: with the verification remainder S of N clauses and subclauses in the verification remainder S of current text and the CRC error correction code table in the step 2 _iCarry out order relatively, if find S _i=S thinks that then error pattern can correct, and keeps and S _iErrors present P in same clauses and subclauses _iOtherwise think that error pattern can not correct, current text carries out discard processing.

4. a kind of satellite navigation message error correction method based on CRC coding according to claim 1, it is characterized in that: the concrete grammar that said step 4 pair correctable error pattern is carried out error correction is: the errors present P that obtains according to step 3 _i, with navigation message P _iIndividual bit overturns, thereby corrects 1 bit mistake.

5. a kind of method of using the CRC coding to correct the satellite navigation message mistake according to claim 1 is characterized in that: said method is applicable to the satellite navigation system of any use CRC as the navigation message check code.

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