CN110007325B - Rapid frame synchronization method for satellite-based enhanced L5 signal - Google Patents

Abstract

The invention provides a fast frame synchronization method for a satellite-based enhanced L5 signal, which comprises the steps that data acquired by an SBAS L5 receiver through a carrier wave are subjected to Manchester decoding to generate first-time decoded data; decoding the FEC by using a Viterbi decoding algorithm for the data subjected to Manchester decoding to generate original navigation message data; finding the position or offset of the frame identifier in the original navigation message data; and adjusting the epoch count in the receiver according to the offset of the frame header in the original data to enable the epoch count of the initial position of one frame of the navigation message to be equal to 0. The method has simple algorithm flow, improves the storage space and the operation speed, approaches the misjudgment rate to 0, and can meet the engineering requirements.

Description

Rapid frame synchronization method for satellite-based enhanced L5 signal

Technical Field

The invention belongs to the field of satellite navigation, and relates to a frame synchronization method of signals.

Background

At present, the satellite navigation technology is widely applied in various fields and becomes an important link in national development, but due to the limitations of technology and systems, the requirement cannot be met in certain high-precision application occasions. Satellite-Based Augmentation System (SBAS) has the characteristics of high precision, wide range and low cost, and becomes an important technical direction for Satellite navigation development in various countries.

The SBAS is to monitor the navigation satellite by a large number of widely distributed monitoring stations (with known positions), obtain raw measurement data and send the raw measurement data to the master control station. The main control station calculates various correction information such as ephemeris error, satellite clock error, ionospheric delay and the like, and finally the correction information is broadcasted to a large number of users through an uplink injection station to a geosynchronous Orbit (GEO) satellite, so that the positioning accuracy is improved.

In 2016, the United states issues 'draft of interface control document (SBAS ICD) of IWG satellite-based augmentation system L5', L5 signals are added, double-frequency tracking of L1 and L5 is achieved, ionospheric delay errors can be effectively corrected, corresponding integrity protection is provided, and availability and continuity of the system are guaranteed.

One frame of the signal text of the SBAS L5 is 250 bits long. The first 4 bits of a frame are the frame identification, followed by a 6-bit information type number and a 216-bit data field, and finally a 24-bit CRC check. The navigation message frame format of the SBAS L5 signal is shown in fig. 1.

The frame identification of the SBAS L5 signal message adopts a distributed arrangement. A complete 24-bit frame id is sequentially distributed in the first four bits of the 6 frames according to the 010111000110100100111010 order.

A Forward Error Correction (FEC) code rate of 1/2 is applied to the original navigation message at 250bps to generate a data stream at 500 bps. The specific circuit is shown in fig. 2. And then, the data is converted into a data stream with a rate of 1000bps through Manchester encoding (Manchester Encoded), and the data stream is synthesized with a spread spectrum code and then transmitted through carrier modulation.

However, the traditional characteristic character matching method has the defects of large data bit demand and low recognition rate in the face of the distributed characteristic of the SBAS L5 message frame identification.

Disclosure of Invention

In order to overcome the defects of the prior art, the invention provides a frame header searching method based on CRC (cyclic redundancy check), which can quickly and accurately complete frame synchronization.

The technical scheme adopted by the invention for solving the technical problem comprises the following steps:

1) data acquired by an SBAS L5 receiver through a carrier wave is subjected to Manchester decoding to generate first-time decoded data;

2) decoding the FEC by using a Viterbi decoding algorithm to generate original navigation message data after Manchester decoding;

3) finding the position or offset of the frame identifier in the original navigation message data;

4) and adjusting the epoch count in the receiver according to the offset of the frame header in the original data to enable the epoch count of the initial position of one frame of the navigation message to be equal to 0.

And 3) finding the position or offset of the frame identifier by adopting a characteristic character matching method.

The step 3) comprises the following steps:

step 3.1) acquiring 500 bit original data sequence after Viterbi decoding;

step 3.2) matching the nth 4-bit frame identification in the 500-bit original data sequence, and ending the step 3 if n is more than 6);

step 3.3) if the nth frame identification is found and the offset is less than 250, jumping to step 3.4), otherwise, adding 1 to the value of n and jumping to step 3.2);

step 3.4) performing CRC check judgment;

step 3.5) finding a frame header if the CRC is correct, otherwise, skipping to the step 3.3) to continue searching the frame header in the rest original data;

step 3.6) calculates the position of the frame header in the 500-bit original data.

The step 4) comprises the following steps:

step 4.1) obtaining 2400 bit data, and performing Manchester decoding to obtain 1200 bit data;

step 4.2) carrying out Viterbi decoding on the 1200-bit data;

step 4.3) searching frame headers in the 600-bit data based on a CRC method;

step 4.4) if the frame header is found, jumping to step 4.8), otherwise, continuing searching;

step 4.5) processing 1200 dislocation, and carrying out VCP decoding;

step 4.6) jump to step 4.8) if the frame header is found, otherwise jump to step 4.7); if the data bit search is the second time of negation, ending the step 4);

step 4.7) inverting the 2400-bit data, and repeating the steps 4.1) to 4.6);

step 4.8) completes the epoch adjustment to make the epoch count of the position where a frame starts 0.

The invention has the beneficial effects that: the algorithm flow is simple, the storage space and the operation speed are improved, the misjudgment rate approaches to 0, and the engineering requirements can be met.

Drawings

FIG. 1 is a SBAS L5 text format;

fig. 2 is an L5 FEC register value;

FIG. 3 is a block diagram of a frame synchronization process of a navigation receiver;

FIG. 4 is a flow chart of a search frame header algorithm based on CRC check;

fig. 5 is a flow chart of SBAS L5 frame synchronization engineering implementation.

Detailed Description

The present invention will be further described with reference to the following drawings and examples, which include, but are not limited to, the following examples.

The frame synchronization process of the SBAS L5 provided by the present invention is as shown in fig. 3, and data acquired by a receiver through a carrier needs to undergo 4 processes of manchester decoding, viterbi decoding, frame header search, and epoch adjustment.

a) Manchester decoding

The SBAS L5 receiver first performs manchester decoding on the data acquired by the carrier to generate first-time decoded data. The manchester decoding rule is executed according to the general rule of 0 being 10 and 1 being 01.

b) Viterbi decoding

And decoding the FEC by using a Viterbi decoding algorithm to generate original navigation message data after Manchester decoding. The viterbi decoding uses the parameters of code rate 1/2, generator polynomial G1(171) G2(133) to complete the software decoding.

c) Search frame header

The position or offset of the frame identifier is found in the original navigation message data string, and a characteristic character matching method is generally adopted.

The conventional signature method aims at the distributed frame header of the SBAS L5, and needs to acquire 6 frames of original data, namely 6 seconds of 1500-bit data, at least. Considering the boundary of the frame id, at least 1504 bits of data are needed to contain the complete 6 frame ids. Due to the distributed 4-bit frame identification, a large number of misjudgments can be encountered in the 1500-bit original navigation message data sequence, and the complexity of the algorithm is greatly improved.

The invention adopts a frame identification searching method based on CRC check, and the frame identification can be accurately found as long as a complete frame exists in the navigation message data sequence. Considering the boundary case, the minimum required data bit is 500 bits.

In order to ensure that the CRC check can accurately identify a whole frame of data, the missing rate of the CRC check, that is, different data have the same check, is analyzed.

For a general CRC code of (n, k) type, k is a length of a binary sequence to be coded, n is a length of the coded binary sequence, and r-n-k is a degree of generating a polynomial.

Given lengthInformation sequence m of k _i (i ═ 1 to k) can be expressed by the following polynomial expressions:

m(x)＝m _k +m _k-1 x+m _k-2 x ² +…m ₁ x ^k-1 (1)

the generator polynomial g (x) can be expressed as the following polynomial:

polynomial m (x) x ^r Divided by the generator polynomial g (x) to yield the remainder R (x) as follows:

R(x)＝p _r +p _r-1 x+p _r-2 x ² +…p ₁ x ^r-1 (3)

then p is ₁ p ₂ …p _r A CRC check code sequence is constructed.

For k data sequences to be encoded, there will be 2 ^k Different expressions, and the generator polynomial for degree r represents at most 2 ^r A different residue. Then theoretically there would be 2 ^k /2 ^r ＝2 ^k-r Different data sequences share a residue, and the probability of misjudgment exists. The larger the value of k-r, the more misjudged data sequences occur, but the probability of misjudgment is only 1/2 relative to the value of r ^r 。

The L5 navigation message takes the form of (250,226), which generates a polynomial of:

wherein, the first and the second end of the pipe are connected with each other,

therefore, the probability of generating misjudgment by the L5 frame header search is 1/2 ²⁴ And approaches to 0, thereby completely meeting the actual requirements of engineering.

The specific flow of searching for the frame header is shown in fig. 4.

Step 1: the 500-bit original data sequence after viterbi decoding is obtained.

Step 2: and matching the n-th 4-bit frame identification in the 500-bit data sequence, and jumping to the step 7 if n is more than 6.

And 3, step 3: if the nth frame identification is found and the offset is less than 250, jumping to step 4, otherwise n +1 and jumping to step 2.

And 4, step 4: and performing CRC check judgment.

And 5: if CRC check is correct, finding frame head, otherwise, skipping to step 3, and continuously searching frame head in the rest original data

Step 6: the position of the frame header in the 500 bits of original data is calculated.

And 7: and (6) ending.

d) And adjusting the epoch.

And adjusting the epoch count in the receiver according to the offset of the frame header in the original data. Make the epoch count of the start position of one frame of the navigation message equal to 0.

Navigation receiver tracking loop algorithms typically employ a costas phase discriminator. Because it is not sensitive to 180 degree phase inversion of the I and Q signals, inversion of the navigation data bits may occur. The bit stream input to the Viterbi decoder has a dislocation phenomenon caused by uncertain positions of the start G0G1, which results in incorrect decoding result.

In view of two kinds of uncertainty of navigation message data in practical engineering application, four cases are required to search frame headers in sequence. (1) The data bit does not reverse Viterbi decoding G0G1 and no dislocation is generated; (2) the data bit does not reverse the Viterbi decoding G0G1 to generate a dislocation; (3) the data bit reversal Viterbi decoding G0G1 does not generate dislocation; (4) the data bit reversal viterbi decoding G0G1 produces a misalignment.

And selecting 2400-bit data to complete frame synchronization by considering the data bit loss in decoding and the boundary alignment characteristic of data storage in engineering. After Manchester and Viterbi decoding, 600 bits of data are ideally required, and the minimum data requirement is more than 500 bits.

The specific process of engineering implementation is shown in fig. 5, and the steps are as follows:

step 1: 2400 bit data are acquired, and Manchester decoding is carried out to acquire 1200 bit data;

step 2: performing Viterbi decoding on the 1200-bit data;

and 3, step 3: searching a frame header in the 600-bit data based on a CRC method;

and 4, step 4: jumping to step 8 if the frame header is found, otherwise, continuing searching;

and 5: carrying out VCP decoding on the 1200-bit dislocation;

step 6: and jumping to the step 8 if the frame header is found, otherwise, jumping to the step 7. If the data bit is searched for in the second negation, jumping to the step 9;

and 7: negating 2400-bit data, and repeating the steps 1-6;

and 8: completing epoch adjustment to make the epoch count of the position where one frame starts be 0;

and step 9: and (6) ending.

The implementation effect is as follows:

the frame synchronization method described in the present invention has been implemented by programming in the TI DSP 6657 platform in C language. The following table measures key module times in the frame synchronization process.

The frame sync was run 6 times and the time statistics are shown in table 1 below:

TABLE 16 time consumed for frame synchronization (in nanoseconds)

The time statistics of the key functions or functional modules in frame synchronization are shown in table 2:

TABLE 2 Key function and Module time measurement (in nanoseconds)

The time statistics for the decoding function after frame synchronization are shown in table 3:

TABLE 3 frame-synchronized post-decoding function time measurement (in nanoseconds)

By comparative analysis of tables 1, 2, 3:

(1) the text data of the analog source SBAS L5 signal is received, the frame synchronization is all successful, and the accuracy and the realizability of the frame synchronization are verified.

(2) The 1200 bit manchester decoding is approximately half the time of the 2400 bit manchester decoding. However, when 2400-bit manchester decoding occurs with a first bit discard, the time will extend from 280US to 425 US.

(3) In the six frame synchronization process, it is best to find the frame header twice, which takes about 30 MS. The worst is 4 times to find the frame header, which takes about 60 MS.

(4) The 600 bit viterbi is approximately half of the 1200 bit viterbi time.

(5) Viterbi decoding is the most time consuming process in frame synchronization.

Through the comparative analysis, the advantages of the invention can be seen:

(1) the frame synchronization method based on CRC provided by the invention can complete frame synchronization only by 2400-bit text, and realizes frame synchronization 3.5 seconds faster than the traditional 6000-bit original data.

(2) Compared with a frame header matching algorithm of 6 characters, the frame synchronization method of the SBAS L5 signal provided by the invention has the advantages that the algorithm flow is simpler, and the storage space and the operation speed are improved by about 2 times.

(3) The invention provides a frame synchronization method of SBAS L5 signals, the probability of misjudgment is 1/224 and approaches to 0, and the engineering requirements are met.

Claims (2)

1. A fast frame synchronization method for a satellite-based enhanced L5 signal, comprising the steps of:

1) data acquired by an SBAS L5 receiver through a carrier wave is subjected to Manchester decoding to generate first-time decoded data;

2) decoding the FEC by using a Viterbi decoding algorithm to generate original navigation message data after Manchester decoding;

3) finding the position or offset of the frame identifier in the original navigation message data; the method comprises the following steps:

step 3.1) obtaining 500 bit original data sequence after Viterbi decoding;

step 3.2) matching the nth 4-bit frame identification in the 500-bit original data sequence, and ending the step 3 if n is more than 6);

step 3.3) if the nth frame identification is found and the offset is less than 250, jumping to step 3.4), otherwise, adding 1 to the value of n and jumping to step 3.2);

step 3.4) performing CRC check judgment;

step 3.5) finding out a frame header if the CRC is correct, otherwise, skipping to the step 3.3) continuing to search the frame header in the rest original data;

step 3.6) calculating the position of the frame header in the 500-bit original data;

4) and adjusting the epoch count in the receiver according to the offset of the frame header in the original data to enable the epoch count of the initial position of one frame of the navigation message to be equal to 0.

2. The fast frame synchronization method for a satellite-based enhanced L5 signal according to claim 1, wherein: the step 4) comprises the following steps:

step 4.1) 2400 bit data is obtained, and Manchester decoding is carried out to obtain 1200 bit data;

step 4.2) carrying out Viterbi decoding on the 1200-bit data;

step 4.3) searching frame headers in the 600-bit data based on a CRC method;

step 4.4) if the frame header is found, jumping to step 4.8), otherwise, continuing searching;

step 4.5) processing 1200 dislocation, and carrying out VCP decoding;

step 4.6), jumping to step 4.8) if the frame header is found, otherwise, jumping to step 4.7); if the data bit search is the second time of negation, ending the step 4);

step 4.7) inverting the 2400-bit data, and repeating the steps 4.1) to 4.6);

step 4.8) completes the epoch adjustment to make the epoch count of the position where a frame starts 0.

Images