Pseudo-code resampling method for rapidly capturing navigation signal of GNSS (Global Navigation Satellite System) receiver

Abstract

The invention provides a pseudo code resampling method for rapidly capturing a navigation signal of a GNSS (Global Navigation Satellite System) receiver, which comprises the following steps of: receiving an intermediate frequency digital signal output by a front-section radio frequency chip by an FPGA (Field Programmable Gate Array), mixing the intermediate frequency digital signal with a locally generated carrier wave, designing different sampling rates according to different integral accumulation time to sample the same pseudo code, calculating RMS values of I / Q paths of the signal, and calculating the RMS values of the I / Q paths of the signal; and the calculated RMS value is read by the DSP and written into the rapid capturing module, and different thresholds are set for the RMS value to re-quantify the signal which needs to be rapidly captured so as to obtain a more accurate capturing result. Based on the high, medium and low orbit adaptive loop signal processing technology, different sampling rates are set according to different integral accumulation times, the obtained RMS result is utilized to assist in quantization of signals of the rapid capture module, and more accurate Doppler and code phase capture results are obtained for different integral accumulation times.

Description

Technical Field

[0001] This invention relates to the field of communication technology, and specifically to a pseudocode resampling method for rapid acquisition of navigation signals in a GNSS receiver. Background Technology

[0002] The development of spaceborne GNSS receivers in recent years has shown a significant trend of multi-technology integration, high precision, and enhanced anti-interference capabilities. Its application scenarios have expanded from low-orbit satellites to high-orbit satellites and even deep space exploration. At the same time, international cooperation and standardization processes have accelerated, promoting industrial synergy.

[0003] Low-Earth orbit (LEO) satellite receivers are widely used in communications, remote sensing, and other fields. The formulation of China's general specifications for LEO satellite receivers has promoted equipment interchangeability and large-scale networking. It is expected that the standard will evolve towards intelligence between 2025 and 2030, incorporating dynamic adaptation and autonomous diagnostic functions. High-Earth orbit (HEO) satellite receivers play a crucial role in disaster monitoring and resource exploration. Currently, spaceborne GNSS receivers are extending from Earth orbit to deep space. With the maturity of technologies such as quantum navigation and AI anti-interference, spaceborne receivers are expected to achieve higher accuracy and stronger reliability in extreme environments.

[0004] Besides traditional low Earth orbit (LEO) and high Earth orbit (HEO), GNSS receivers are also carried on some special orbital vehicles, such as highly elliptical orbit satellites. The orbits of highly elliptical orbit satellites have a perigee of approximately 200 km and an apogee of approximately 40,000 km, covering the characteristics of low, medium, and high orbits. Given the unique characteristics of this orbit, the navigation signal processing of spaceborne GNSS receivers needs to consider both the high dynamics of the low-Earth orbit segment and the weak signal reception and processing of the high-Earth orbit segment; current technical specifications cannot meet these requirements.

[0005] Therefore, a method is needed that can automatically switch the appropriate loop closing time according to different orbital altitudes, and use different sampling rates to resample the pseudocode for different loop closing times to assist in capture, thereby obtaining more accurate capture results. Summary of the Invention

[0006] This invention addresses the incompatibility between high-dynamic signals in low-Earth orbit and weak signals in high-Earth orbit by providing a pseudo-code resampling method for rapid acquisition of navigation signals by a GNSS receiver. This method is based on adaptive loop signal processing technology for high, medium, and low Earth orbits and uses GNSS receiver code resampling to assist in rapid acquisition of navigation signals.

[0007] This invention provides a pseudocode resampling method for rapid acquisition of navigation signals by a GNSS receiver, comprising the following steps: S1. Initialize the DSP control resampling module in the spaceborne GNSS receiver, and set different carrier NCO control words, pseudocode sampling rates and signal frequency directions for different navigation signals; The pseudo-code sampling rate of a spaceborne GNSS receiver in the high-Earth orbit segment is lower than that of a spaceborne GNSS receiver in the low-Earth orbit segment. S2. The resampling module outputs the carrier NCO control word to the local carrier generator in the GNSS receiver FPGA, outputs the signal frequency direction to the mixer in the FPGA, and outputs the pseudocode sampling rate to the RMS value calculation module in the FPGA. S3. The local carrier generator generates a local sine and cosine carrier based on the carrier NCO control word and outputs them to the mixer. The mixer selects whether to use the sum frequency or difference frequency mixing method according to the signal frequency direction, mixes with the input local carrier, calculates the average value of the mixing result, and outputs it to the RMS value calculation module. The RMS value calculation module calculates the RMS value of the I / Q signals according to the pseudocode sampling rate. The DSP reads the RMS value and outputs it to the DSP's fast acquisition control module. The fast acquisition control module writes the RMS value into the fast acquisition search module in the FPGA. The fast acquisition search module requantizes the signal to be acquired to obtain the quantized I / Q signal. S4. Perform an FFT search on the quantized I / Q signal to obtain the signal capture result, which is used for subsequent loop tracking.

[0008] The pseudocode resampling method for fast acquisition of navigation signals by a GNSS receiver according to the present invention, as a preferred embodiment, includes step S1, The carrier NCO control word is: ; Where N is the number of bits in the carrier NCO, This is the operating clock for the spaceborne GNSS receiver; This is the digital intermediate frequency signal after mixing.

[0009] The pseudo-code resampling method for fast acquisition of navigation signals by a GNSS receiver according to the present invention, as a preferred embodiment, in step S1, when the spaceborne GNSS receiver is located in the high orbit segment, the loop closing time is 10ms or 20ms, and the pseudo-code sampling rate is one sampling rate; When the onboard GNSS receiver is in the low Earth orbit segment, the loop closure time is 1~5ms, and the pseudocode sampling rate is twice the sampling rate.

[0010] The pseudocode resampling method for fast acquisition of navigation signals in a GNSS receiver according to the present invention, as a preferred embodiment, includes the following mixing process in step S3: ; Where ad is the input digital intermediate frequency signal, sin and cos are the local carrier, and I and Q are the same-direction and quadrature branch components of the signal: The mixing result is: ; in, This indicates the direction of the signal frequency.

[0011] The pseudocode resampling method for fast acquisition of navigation signals in a GNSS receiver according to the present invention, as a preferred method, in step S3, the quantization process is as follows: according to the different levels of the three thresholds th1, th2, and th3, the quantization result with better effect is selected during the debugging process to generate the quantized intermediate frequency digital signal; The quantitative correspondence is as follows: ; in, The sign bit is determined by the amplitude of the I / Q signals. Comparing with thresholds th1, th2, and th3 yields The value of is determined according to The quantization result is obtained by searching the signed quantization lookup table. .

[0012] The pseudocode resampling method for fast acquisition of navigation signals in a GNSS receiver, as described in this invention, is a preferred embodiment. ; Signed quantization lookup table, When both are zero, the two's complement is 1. Both are zero and When the value is 1, the two's complement is 3. =1 and When the value is zero, its two's complement is 5. When the value is 1, the two's complement is 7.

[0013] The pseudocode resampling method for fast acquisition of navigation signals in a GNSS receiver, as described in this invention, is a preferred embodiment. ; In the pseudocode resampling method for rapid acquisition of navigation signals by a GNSS receiver described in this invention, as a preferred method, K is a value with good effect selected during the debugging process, and K is greater than 64.

[0014] The pseudocode resampling method for rapid acquisition of navigation signals in a GNSS receiver according to the present invention, as a preferred embodiment, in step S4, satellite signal acquisition adopts a combination of coherent accumulation and non-coherent accumulation; When capturing weak signals in high orbit, increase the coherent accumulation length; when capturing high dynamic signals in low orbit, decrease the coherent accumulation length. In step S4, the code sampling rate is increased when capturing the low-orbit signal.

[0015] The pseudocode resampling method for fast acquisition of navigation signals in a GNSS receiver described in this invention, as a preferred method, is as follows: when the navigation signal is the BeiDou B1I frequency point, the number of bits of the carrier NCO control word is 32 bits, the local oscillator is 1575MHz, and the frequency of the digital intermediate frequency signal after mixing is -13.902MHz. The pseudo-code sampling rate for the high-orbit segment is 2.046MHz, and the pseudo-code sampling rate for the low-orbit segment is 4.098MHz.

[0016] In this invention, the GNSS receiver navigation signal processing FPGA receives the intermediate frequency digital signal output from the front-end RF chip, mixes it with the locally generated carrier, and samples the same pseudocode at different sampling rates based on different integration and accumulation times. The RMS values ​​of the I / Q channels of the signal are calculated, and the calculated RMS values ​​are read by the DSP and written to the fast acquisition module. Different thresholds are set for the RMS values ​​to requantize the signals that need to be acquired quickly in order to obtain more accurate acquisition results.

[0017] This invention is based on adaptive loop signal processing technology for high, medium and low orbits. Different sampling rates are set according to different integration and accumulation times. The obtained RMS results are used to assist in the quantization of the signal of the fast acquisition module. For different integration and accumulation times, more accurate Doppler and code phase acquisition results are obtained.

[0018] Adaptive loop signal processing technology refers to the ability to automatically switch different loop closing times for different orbital altitudes of a spaceborne GNSS receiver.

[0019] The present invention has the following advantages: This invention provides a pseudo-code resampling method for rapid acquisition of navigation signals in a GNSS receiver with a DSP+FPGA architecture. Different parameters, such as sampling rate, can be set during the initialization of the code resampling module based on different pseudo-codes and different loop closing times. After the intermediate frequency digital signal is mixed with the local carrier, the signal RMS value is calculated according to preset parameters. The calculated RMS value is fed back to the fast acquisition module, where the RMS value is requantized to achieve more accurate acquisition. Attached Figure Description

[0020] Figure 1 A flowchart of a pseudocode resampling method for rapid acquisition of navigation signals by a GNSS receiver; Figure 2 This is a schematic diagram of the structural connections of a pseudocode resampling method for rapid acquisition of navigation signals in a GNSS receiver; Figure 3 A schematic diagram of the noncoherent accumulation algorithm structure for a pseudocode resampling method for fast acquisition of navigation signals in a GNSS receiver; Figure 4 Matching operation of a matched filter at one-time sampling for a pseudocode resampling method for fast acquisition of navigation signals in a GNSS receiver. Figure 1 ; Figure 5 Matching operation of a matched filter at one-time sampling for a pseudocode resampling method for fast acquisition of navigation signals in a GNSS receiver. Figure 2 ; Figure 6 Matching operation of a matched filter at double sampling time for a pseudocode resampling method for fast acquisition of navigation signals in a GNSS receiver. Figure 1 ; Figure 7 Matching operation of a matched filter at double sampling time for a pseudocode resampling method for fast acquisition of navigation signals in a GNSS receiver. Figure 2 . Detailed Implementation

[0021] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Example 1

[0022] like Figure 1 , Figure 2 As shown, a pseudocode resampling method for fast acquisition of navigation signals by a GNSS receiver includes the following steps: S1. The initialization process of the code sampling module is controlled by the DSP. Different carrier NCO control words, pseudo-code sampling rates, and signal frequency directions are set for different navigation signals.

[0023] Taking the BeiDou B1I frequency point (center frequency 1561.098MHz / code rate 2.046MHz) as an example, the working clock is 62MHz, the carrier NCO bit is 32bit, the local oscillator is 1575MHz, and the frequency of the digital intermediate frequency signal after mixing is -13.902MHz. The formula for calculating the carrier NCO control word is: ; Where N is the number of bits in the carrier NCO; This is the system's operating clock, with a frequency of 62MHz. The digital intermediate frequency signal after mixing is B1I in this embodiment, with a frequency value of 13.902MHz and a frequency direction of [missing information]. (Negative frequency direction); When the loop closure time is 10ms or 20ms (high-orbit segment), the pseudo-code sampling rate is set to one sampling rate of 2.046MHz. When the loop closure time is 1~5ms (low-orbit segment), the pseudo-code sampling rate is set to two sampling rates of 4.098MHz. The sampling rate can be flexibly configured through the DSP.

[0024] S2, the carrier NCO control word is input to the local carrier generator to generate local sine and cosine carriers, which are then input to the mixer. In the mixer, based on the set frequency direction, the sum-frequency or difference-frequency mixing method is selected, and the signal is mixed with the input local carriers. The mixing process is shown below, where ad is the input digital intermediate frequency signal, sin and cos are the local carriers, and I and Q are the in-direction and quadrature branch components of the signal: ; The mixing result is: ; After the mixing result is averaged, it is input into the RMS value calculation module. The RMS values ​​of the I / Q signals are calculated according to the preset pseudo-code sampling rate. The RMS value calculation result is read by the DSP through the EMIF and input into the fast acquisition control module. Then it is written into the fast acquisition search module to requantize the signal to be acquired. The quantization process is shown below, where K=75 is the calculated gain and th is the set threshold.

[0025] ; The quantization gain K is a non-fixed value, selected during the debugging process to achieve good results. However, K must be greater than the minimum threshold th1 by division, i.e., right shift. Otherwise, the th1 threshold may be zero, causing the th1 threshold to fail and the quantization result to be inaccurate.

[0026] The amplitude of the I / Q signals Compared to the three thresholds: ; The signal is requantized based on the comparison results, whereby... For the signal sign bit, For the quantization results (the I / Q quantization processes are the same): The 4-bit signed quantization lookup table is as follows. Based on the three thresholds th1, th2, and th3, the quantization result with the best effect was selected during the debugging process to generate the quantized intermediate frequency digital signal (two's complement). The highest bit is the sign bit, where 0 represents a positive value and 1 represents a negative value. The quantization correspondence is as follows: ; ; Performing an FFT search on the quantized I / Q signal can yield more accurate signal capture results, which are then used for subsequent loop tracking.

[0027] In this invention, satellite signal acquisition is achieved using the PMF+FFT algorithm. The acquisition process employs a combination of coherent and incoherent accumulation to improve the final acquisition sensitivity. The structure combining coherent and incoherent accumulation is as follows: Figure 3 As shown; The coherent accumulation length is set to N (ms), the number of incoherent accumulations is set to M, and the gain generated by the coherent accumulation is... Incoherent accumulation generates gain As shown in the following formula: ; ; in, This is the squared loss of incoherent accumulation.

[0028] Taking L1CA signal capture as an example, the lowest signal level that can be captured is: ; In the formula: , where is Boltzmann's constant; As the reference thermal noise temperature, take ;at this time: ; B represents the signal pre-correlation bandwidth, which is 4.092MHz. The receiver noise figure is approximately 2.0 dB. For incoherent cumulative gain, M is 10, and the squared loss L is 8.3 dB. The calculation yields... The squared loss of incoherent accumulation is very large, so improving signal acquisition sensitivity mainly relies on coherent accumulation gain.

[0029] For coherent accumulation gain, For the number of matched filter points, Since the code rate of B1I is relatively low, increasing the coherent accumulation gain mainly relies on increasing the coherent accumulation length. Because the message bit length of a B1I signal is 20ms, the coherent accumulation length of the B1I signal can be set to 1ms-20ms. A longer coherent accumulation length results in higher coherent gain, suitable for capturing weak signals in high-orbit orbits; a shorter coherent accumulation length results in a larger dynamic range, suitable for capturing high-dynamic signals in low-orbit orbits.

[0030] For the design of the matched filter, the number of matching points is fixed in the FPGA as 1023 points, taking a 10ms coherent accumulation length as an example.

[0031] When the code sampling rate is one sampling rate, the matching operation of the matched filter is as follows: Figure 4 , Figure 5 As shown; When the code sampling rate is double sampling, the matching operation of the matched filter is as follows: Figure 6 , Figure 7 As shown; Theoretically, the lower the code sampling rate (minimum 1x code rate), the longer the integration length into a matched filter, and the greater the coherent accumulation gain. Therefore, a low sampling rate can be used for weak signals. However, in practice, because the initial code phase is unknown, adjacent matched filters generally will not form a 1ms chip. For 1x code sampling, the maximum loss is 0.5ms*2=1ms, and for 2x code sampling, the maximum loss is 0.25ms*2=0.5ms. Therefore, the higher the code sampling rate, the smaller the loss. Since the acquisition sensitivity requirements for low-orbit signals are not high, a high code sampling rate can be used to ensure the lower limit of the acquisition probability.

[0032] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A pseudocode resampling method for fast acquisition of navigation signals in a GNSS receiver, characterized in that: Includes the following steps: S1. Initialize the DSP control resampling module in the spaceborne GNSS receiver, and set different carrier NCO control words, pseudocode sampling rates and signal frequency directions for different navigation signals; The pseudo-code sampling rate of a spaceborne GNSS receiver in the high-Earth orbit segment is lower than that of a spaceborne GNSS receiver in the low-Earth orbit segment. S2. The resampling module outputs the carrier NCO control word to the local carrier generator in the GNSS receiver FPGA, outputs the signal frequency direction to the mixer in the FPGA, and outputs the pseudocode sampling rate to the RMS value calculation module in the FPGA. S3. The local carrier generator generates a local sine carrier and a cosine carrier according to the carrier NCO control word and outputs them to the mixer. The mixer selects whether to use a sum-frequency or difference-frequency mixing method according to the signal frequency direction, mixes with the input local carrier, calculates the average value of the mixing result, and outputs it to the RMS value calculation module. The RMS value calculation module calculates the RMS value of the I / Q signals according to the pseudo-code sampling rate. The DSP reads the RMS value and outputs it to the fast acquisition control module of the DSP. The fast acquisition control module writes the RMS value into the fast acquisition search module in the FPGA. The fast acquisition search module requantizes the signal to be acquired to obtain the quantized I / Q signal. S4. Perform an FFT search on the quantized I / Q signal to obtain the signal capture result, which is used for subsequent loop tracking.

2. The pseudocode resampling method for fast acquisition of navigation signals in a GNSS receiver according to claim 1, characterized in that: In step S1, The carrier NCO control word is: ； Where N is the number of bits in the carrier NCO, This is the operating clock for the spaceborne GNSS receiver; This is the digital intermediate frequency signal after mixing.

3. The pseudocode resampling method for fast acquisition of navigation signals in a GNSS receiver according to claim 1, characterized in that: In step S1, when the onboard GNSS receiver is in the high orbit segment, the loop closing time is 10ms or 20ms, and the pseudocode sampling rate is one sampling rate. When the onboard GNSS receiver is in the low Earth orbit segment, the loop closure time is 1~5ms, and the pseudocode sampling rate is twice the sampling rate.

4. The pseudocode resampling method for fast acquisition of navigation signals in a GNSS receiver according to claim 1, characterized in that: In step S3, the mixing process is as follows: ； Where ad is the input digital intermediate frequency signal, sin and cos are the local carrier, and I and Q are the same-direction and quadrature branch components of the signal: The mixing result is: ； in, This indicates the direction of the signal frequency.

5. The pseudocode resampling method for fast acquisition of navigation signals in a GNSS receiver according to claim 1, characterized in that: In step S3, the quantization process is as follows: according to the different levels of the three thresholds th1, th2, and th3, the quantization result with better effect is selected during the debugging process to generate the quantized intermediate frequency digital signal. The quantitative correspondence is as follows: ； in, The sign bit is determined by the amplitude of the I / Q signals. Comparing with thresholds th1, th2, and th3 yields The value of is determined according to The quantization result is obtained by searching the signed quantization lookup table. .

6. The pseudocode resampling method for fast acquisition of navigation signals in a GNSS receiver according to claim 5, characterized in that: ； The signed quantization lookup table, When both are zero, the two's complement is 1. Both are zero and When the value is 1, the two's complement is 3. =1 and When the value is zero, its two's complement is 5. When the value is 1, the two's complement is 7.

7. The pseudocode resampling method for fast acquisition of navigation signals in a GNSS receiver according to claim 5, characterized in that: ； Where K is the quantization gain.

8. The pseudocode resampling method for fast acquisition of navigation signals in a GNSS receiver according to claim 7, characterized in that: K is a value selected during the debugging process that yields better results, and K is greater than 64.

9. The pseudocode resampling method for fast acquisition of navigation signals in a GNSS receiver according to claim 1, characterized in that: In step S4, satellite signal acquisition employs a combination of coherent and incoherent accumulation. When capturing weak signals from high orbits, increase the coherent accumulation length; When capturing low-orbit high-dynamic signals, reduce the coherent accumulation length; In step S4, the code sampling rate is increased when capturing the low-orbit signal.

10. The pseudocode resampling method for fast acquisition of navigation signals in a GNSS receiver according to claim 9, characterized in that: When the navigation signal is the BeiDou B1I frequency, the number of bits in the carrier NCO control word is 32 bits, the local oscillator is 1575MHz, and the frequency of the mixed digital intermediate frequency signal is -13.902MHz. The pseudo-code sampling rate for the high-orbit segment is 2.046MHz, and the pseudo-code sampling rate for the low-orbit segment is 4.098MHz.

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