CN101206255A - Method for capturing GPS weak signal - Google Patents
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Abstract
The invention discloses a method for obtaining high signal-to-noise ratio signal to capture GPS weak signal by means of data bit jump information, including the following steps: (a) when a receiver receives a signal, the signal-to-noise ratio of the signal is estimated by means of a signal-to-noise ratio estimation module; (b) the detection threshold value and the bit jump detection threshold value of the signal are set by means of the estimated signal-to-noise ratio; (c) the detection judgment of bit jump is carried out; whether bit jump exists in the processed 20ms signal is detected by a bit jump detector according to the estimated signal-to-noise ratio; when bit jump exists, subsequent 20ms data series are converted according to the bit jump information and are not influenced by the bit jump; then, 40ms coherent integration is carried out; when bit jump does not exist, coherent integration is directly carried out to increase the signal-to-noise ratio; (d) longtime coherent integration result is compared with a set signal detection threshold value by a comparator so as to judge whether needed satellite signal exists in received signal.
Description
Technical Field
The invention relates to a method for capturing weak satellite signals in a GPS (global positioning system).
Background
The spread spectrum communication technology uses the code division multiple access coding technology, so that the system has good anti-interference and interception performance. Is widely used in modern communication systems, especially in the global positioning system which has been widely used, such as the GPS system in the united states, the galileo system in the european union, etc. For an environment with a high signal-to-noise ratio (such as outdoors or in a general open environment), for a GPS system, the code phase and the carrier frequency of the signal can be obtained by using only 1ms of data, i.e., the satellite signal can be acquired. But in weak signal environments (power spectral density below 30dB-Hz or even 20 dB-Hz), such as indoors, urban streets or narrow spaces, it is not sufficient to acquire satellite signals if only 1ms of data is used. This requires the energy of the satellite signal to be accumulated over multiple cycles to enable the correlation peak to be detected. James Bao-YenTsui in 2005 written "fundamental of Global Positioning systems receivers A software Approach" discussed coherent combination and non-coherent combination in capturing weak signals, it was concluded that using coherent combination can improve signal-to-noise ratio of signals when there is no data bit hopping. The GPS signal is a signal modulated by a navigation message, and the navigation message must not alternate between 0 and 1 because the alternate 0 and 1 signal has no information content. Therefore, the navigation data has the condition of connecting 1 and connecting 0, and the condition that one jump occurs only in 40ms (one data bit takes 20ms) occurs when the data bit jumps in the GPS signal. Then the possibility of 20ms and 40ms transitions must be considered in the signal processing. Therefore, in the process of processing the GPS signal, when the data bit jumps, coherent integration is used, which results in a rapid decrease in the signal-to-noise ratio. While the signal-to-noise ratio of the signal can be improved by using non-coherent integration, it results in "squaring loss" compared with coherent integration. A method called differential coherent integration can be used to improve the signal-to-noise ratio.
The three methods mentioned above can be expressed as follows:
the coherent integration method comprises the following steps:
non-coherent integration:
differential coherent integration method:
where N is the number of cycles of the cyclic pseudo-random code, i is the code phase of the random code, j is the code phase of the GPS signal, and y is the correlation operation of one random code cycle.
The three methods are characterized in that:
1) the coherent integration method does not cause square loss, has high success rate of detecting the peak value, but encounters the change of navigation data bit information in data of every 20ms, and the change causes that the coherent integration method is not suitable for being used in the correlation peak value detection with long integration time;
2) the non-coherent integration method takes the absolute value of the correlation result every 1ms before summing, which may alleviate the problems due to data bit variations, but may result in excessive squaring loss as the signal-to-noise ratio decreases. This method is also unsuitable for use in weak signal environments.
3) The differential coherent integration method multiplies the two adjacent correlation results, and as can be seen from equation 3, this method can alleviate the bit jump problem with reduced squaring loss compared to NCC [ non-coherent integration ]. This method has been shown in the above reference to be better than NCC in environments where the GPS signal is frequency offset and fading.
However, both methods 2) and 3) are to avoid the signal-to-noise ratio loss caused by the data bit jump during the integration period, and although a certain effect is obtained, the signal-to-noise ratio is still smaller than that of coherent integration.
Disclosure of Invention
The invention provides a method for acquiring a high signal-to-noise ratio signal by using data bit hopping information to acquire a GPS weak signal, aiming at the problem that coherent integration cannot be used for a long time to improve the signal-to-noise ratio due to bit hopping.
In order to achieve the purpose, the invention adopts the following technical scheme to realize the purpose:
a method of acquiring a GPS weak signal, comprising the steps of:
a. utilizing a signal-to-noise ratio estimation module to estimate the signal-to-noise ratio of the signal after the signal is received by the receiver;
b. setting a detection threshold value and a bit jump detection threshold value of the signal by using the estimated signal-to-noise ratio; the size of the bit jump detection threshold is in direct proportion to the size of a signal-to-noise ratio;
c. carrying out bit jump detection judgment, detecting whether a bit exists in the processed 20ms signal to jump or not according to the estimated signal-to-noise ratio through a bit jump detector, when the bit exists to jump, converting the following 20ms data sequence according to the bit jump information so as not to be influenced by the bit jump, and then carrying out coherent integration with the length of 40ms instead of 20 ms; then, judging the next bit jump position, wherein the coherent integration time of 20ms can be prolonged every time of judgment until the set judgment times or the required signal-to-noise ratio is reached; when no bit jump exists, directly carrying out coherent integration to improve the signal-to-noise ratio;
d. the long-time coherent integration result is compared and judged with a set signal detection threshold value through a comparator, and whether a satellite signal required by positioning exists in a received signal or not is obtained;
e. when the satellite signal required for positioning is found to exist, the satellite signal can be continuously and normally received through the GPS receiver to obtain a navigation message, so that positioning is realized; if the required satellite signal is not found, replacing the satellite and repeating the steps a-d.
In the above scheme, the following equation is used for determining bit jump, and bit jump can be automatically detected under different signal-to-noise ratios:
wherein,
where p is the number of data points used, q is the number of frequency partitions, where the entire time-frequency space to be searched is partitioned into multiple identical subspaces,
l is the number of splits, ε represents the portion currently participating in the operation, N ∈ {1, 2, … 20 }. The above equation indicates that if the ratio of two adjacent peak values is greater than the predetermined bit jump detection threshold BTT or SNR, it is determined that the position of the bit jump is detected, otherwise, the coherent integration time can be continued to be extended.
In step c, the number of said judgments is 3, and the required signal-to-noise ratio is 130 db.
Compared with the prior art, the method has the advantages that after the position and jump position information is obtained by the method, the jump signal is processed to be a jump-free signal, and then the coherent integration operation is carried out on the jump signal, so that the coherent integration time can be greatly increased, and the required signal-to-noise ratio is achieved. When the method is used for capturing the weak signal, the hardware is more convenient to realize, less hardware resources are utilized, and the time for searching the weak signal can be shortened.
Drawings
FIG. 1 is a block diagram of the steps of the method of the present invention.
Fig. 2 is a logic block diagram of a specific bit transition detection and determination method in fig. 1 according to the present invention.
Detailed Description
The invention is described in further detail below with reference to the figures and the embodiments.
As shown in fig. 1, a method for capturing GP weak signal includes the following steps:
a. according to the signal-to-noise ratio characteristics of the received signals, a signal-to-noise ratio estimation module is used for estimating the signal-to-noise ratio of the signals after the signals are received by the receiver;
b. after the signal-to-noise ratio estimated value of the signal is obtained, the detection threshold value and the bit jump detection threshold value of the signal are set according to the signal-to-noise ratio,
c. the detection and judgment of bit jump are carried out, whether the existing bit jumps in the processed 20ms signal is detected by a bit jump detector according to the estimated signal-to-noise ratio, when the existing bit jumps, the following 20ms data sequence is converted (the inverted signal is multiplied by-1) according to the bit jump information so as to ensure that the existing bit jumps are not influenced by the bit jump, then the coherent integration with the length of 40ms is carried out instead of 20ms, then the judgment of the next bit jump position can be carried out, and the coherent integration time of 20ms can be prolonged until the set judgment times or the required signal-to-noise ratio is reached once judgment; the number of determinations may be determined to be 3. The required signal-to-noise ratio is 130 db; when there is no bit jump, coherent integration is directly performed to improve the signal-to-noise ratio.
d. The long-time coherent integration result is compared and judged with the signal detection threshold value set in advance through a comparator to obtain whether the satellite signal required by positioning exists in the received signal or not.
e. When the satellite signal required for positioning is found to exist, the satellite signal can be continuously and normally received through the GPS receiver to obtain a navigation message, so that positioning is realized; if the required satellite signal is not found, replacing the satellite and repeating the steps a-d.
The exact signal-to-noise ratio of the GPS signal is generally unknown, but the signal and ambient noise criteria can be estimated from the received 1ms data or a few ms data. Therefore, a signal detection threshold value and a data bit jump detection threshold value can be set according to an estimated value of the signal-to-noise ratio, the signal detection threshold value is used for determining whether a satellite signal required by the detected positioning exists in a received signal, and the data bit jump detection threshold value is used for judging whether the received signal is 1ms or a plurality of ms (for convenience of explanation, the data of 1ms is used for explanation below, and if the signal-to-noise ratio is very weak, the data of a plurality of ms can be collected) to judge whether the jump of the data bit exists.
The invention utilizes a bit jump detector to obtain bit jump information and realize bit jump detection and judgment. The detector needs to consider the related data every 20ms, the bit jump threshold depends on the value of the signal-to-noise ratio, and the bit jump detection threshold is used for verifying the occurrence of bit jump. The bit jump detection (BTT) and the signal-to-noise ratio (SNR) have a predetermined relationship (the size of a bit jump detection threshold is proportional to the size of the SNR), and the bit jump can be automatically detected under different SNR. Determining bit hopping utilizes the following equation:
wherein,
wherein, PCC NRepresenting the peak of coherent detection, N is the number of cycles of the cyclic pseudo-random code, i is the random code phase, j is the code phase of the GPS signal, and y is the correlation of one random code periodThe operation is carried out according to the operation parameters,(epsilon-1) x p/L is less than or equal to i epsilon x p/L, and j is more than or equal to 1 and less than or equal to q; n ∈ (1, 2, … 20); the above equation shows that all search spaces are divided into a plurality of identical subspaces, L is the number of divisions, epsilon indicates the part currently participating in the calculation, p is the number of data points used, and q is the number of frequency divisions.
The above equation indicates that if the ratio of two adjacent peaks is greater than a predetermined threshold btt (snr), then it is determined where a bit transition was detected, otherwise the coherent integration time may continue to be extended. If the occurrence of a transition in the data bit is detected, the subsequent 20ms data sequence is transformed (inverted, equivalent to multiplied by-1) and the coherent integration time is continued to be extended for coherent integration. The data bit transition is detected again by the end of the next 20 ms.
As shown in fig. 2, we first set two counters M and N to record the accumulated number of coherent integrations, M is the number of bit transitions, and then set the values of P and Q, where P is a detection criterion of N and Q is a detection criterion of M. These two values are affected by the signal-to-noise ratio of the 1 st MS signal. Coherent integration is then performed and compared to a threshold value simultaneously with the coherent integration. The purpose of the comparison is to detect from time to time whether the satellite signals required for positioning are detected. At the same time, each time the coherent accumulation is completed, the counter N is added with 1(N + +). When the accumulation times are too small, or the coherent integration time is not long enough (the number of milliseconds of coherent integration is too small), the signal-to-noise ratio is too small, and if a jump of the navigation message is encountered, the data after the jump is changed, and then the coherent integration is continued. Meanwhile, the counter M is also added with 1(M + +).
The above process is continued until the satellite signal required for positioning is detected, or the detection is stopped when the times M and N of detection are reached; if the signal-to-noise ratio of the long-time coherent integration signal still cannot be larger than the signal detection threshold value, that is, the required satellite signal is not found, the operation of replacing the satellite is performed.
Claims (3)
1. A method of acquiring a weak GPS signal, comprising the steps of:
a. utilizing a signal-to-noise ratio estimation module to estimate the signal-to-noise ratio of the signal after the signal is received by the receiver;
b. setting a detection threshold value and a bit jump detection threshold value of the signal by using the estimated signal-to-noise ratio; the size of the bit jump detection threshold is in direct proportion to the size of a signal-to-noise ratio;
c. detecting and judging bit jump, detecting whether the processed 20ms signal has bit jump or not according to the estimated SNR by a bit jump detector, when the bit jumps, converting the following 20ms data sequence according to the bit jump information to prevent the data sequence from being influenced by the bit jump, and then performing coherent integration with the length of 40ms instead of 20 ms; then, judging the next bit jump position, wherein the coherent integration time of 20ms can be prolonged every time of judgment until the set judgment times or the required signal-to-noise ratio is reached; when no bit jump exists, directly carrying out coherent integration to improve the signal-to-noise ratio;
d. the long-time coherent integration result is compared and judged with a set signal detection threshold value through a comparator, and whether a satellite signal required by positioning exists in a received signal or not is obtained;
e. when the satellite signal required for positioning is found to exist, the satellite signal can be continuously and normally received through the GPS receiver to obtain a navigation message, so that positioning is realized; if the required satellite signal is not found, replacing the satellite and repeating the steps a-d.
2. The method of claim 1, wherein in step c, the determination of the bit jump uses the following equation and the bit jump can be automatically detected at different snr:
wherein,
wherein p is the number usedThe number of data points, q, is the number of frequency partitions, where the entire time-frequency space to be searched is partitioned into a number of identical subspaces,
l is the number of splits, ε represents the portion currently participating in the operation, N ∈ {1, 2, … 20 }. The above equation indicates that if the ratio of two adjacent peak values is greater than the predetermined bit transition detection threshold BTT or SNR, the position of the detected bit transition is determined, otherwise, the coherent integration time can be continued to be extended.
A method of acquiring weak GPS signals according to claim 1, wherein in step c, the number of determinations is 3 and the required signal to noise ratio is 130 db.
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| CN101359044B (en) * | 2008-09-19 | 2010-04-14 | 太原科技大学 | Method for calculating and eliminating interfere of GNSS strong signal to weak signal |
| CN101324658B (en) * | 2008-08-04 | 2011-01-26 | 哈尔滨工业大学 | Method for capturing full bit of GPS weak signal |
| CN102331579A (en) * | 2011-07-26 | 2012-01-25 | 中国商用飞机有限责任公司 | Airborne navigation positioning system and satellite navigation positioning method applying same |
| CN102510299A (en) * | 2011-11-17 | 2012-06-20 | 中国航天科工信息技术研究院 | Variable integration time SSC bit alignment method and system adopting same |
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| CN101359044B (en) * | 2008-09-19 | 2010-04-14 | 太原科技大学 | Method for calculating and eliminating interfere of GNSS strong signal to weak signal |
| CN102156288B (en) * | 2008-12-17 | 2013-04-03 | 北京航空航天大学 | Method for acquiring navigation satellite signal by using chaotic system |
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| CN102510299A (en) * | 2011-11-17 | 2012-06-20 | 中国航天科工信息技术研究院 | Variable integration time SSC bit alignment method and system adopting same |
| CN102510299B (en) * | 2011-11-17 | 2014-06-04 | 中国航天科工信息技术研究院 | Variable integration time SSC bit alignment method and system adopting same |
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| CN104360358A (en) * | 2014-09-19 | 2015-02-18 | 成都海亨利科技有限公司 | Tracking method for eliminating navigation bit uncertainty of Beidou second generation navigation receiver |
| CN106896380B (en) * | 2015-12-18 | 2019-06-28 | 大唐半导体设计有限公司 | A kind of adaptive navigation signal acquisition methods and device |
| CN106896380A (en) * | 2015-12-18 | 2017-06-27 | 大唐半导体设计有限公司 | A kind of adaptive navigation signal acquisition methods and device |
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