CN112904383B

CN112904383B - Self-adaptive switching method for tracking loop of single-frequency multi-frequency GNSS receiver

Info

Publication number: CN112904383B
Application number: CN202110066992.8A
Authority: CN
Inventors: 杨溶; 战兴群; 黄冀
Original assignee: Shanghai Jiaotong University
Current assignee: Shanghai Jiaotong University
Priority date: 2021-01-19
Filing date: 2021-01-19
Publication date: 2022-04-08
Anticipated expiration: 2041-01-19
Also published as: CN112904383A

Abstract

A self-adaptive switching method for a tracking loop of a single-frequency multi-frequency GNSS receiver is characterized in that a GNSS digital intermediate frequency signal is acquired through a radio frequency front end of a hardware platform of the GNSS receiver, an index reflecting environmental change is obtained through calculation and then is compared with a preset threshold so as to select the single-frequency tracking loop or the multi-frequency tracking loop to perform closed-loop tracking. The invention adjusts the switching state of the loop by selecting the index which can reflect the environmental change and setting the corresponding threshold for detection. When the signal quality is good, the independent tracking mode of each frequency point is adopted, and the interference of calculated quantity and other frequency point observed quantity is reduced. When the signal quality is reduced and the lock is easy to lose, a multi-frequency joint tracking mode is adopted, the observed quantity of normal signals of other frequency points is used for assisting, and the robustness and the anti-interference capability of the GNSS receiver are improved. Therefore, the tracking method of single-frequency independent and multi-frequency auxiliary self-adaptive switching can improve the positioning reliability, precision and continuity of the GNSS receiver and has great potential value.

Description

Self-adaptive switching method for tracking loop of single-frequency multi-frequency GNSS receiver

Technical Field

The invention relates to a technology in the field of satellite navigation, in particular to a self-adaptive switching method for a tracking loop of a single-frequency multi-frequency Global Navigation Satellite System (GNSS) receiver.

Background

The high-reliability satellite navigation receiver has wide application prospects in application scenes such as unmanned aerial vehicles, internet of vehicles, automatic driving and the like, and is widely concerned at home and abroad. The Global Navigation Satellite System (GNSS) can provide all-weather navigation position information for the scenes, so that the unmanned aerial vehicle or the vehicle can realize absolute positioning. However, the GNSS signal quality is very easy to deteriorate, and many difficulties are brought to the design of the GNSS receiver, which are specifically embodied as follows: 1. the GNSS signal is an open transmission channel from a satellite transmitting end to a user receiving end, and the ionospheric scintillation phenomenon occurring in the process of passing through the ionosphere can seriously weaken the signal quality, and is specifically represented as random fluctuation and scintillation of parameters such as a received signal phase, amplitude, an arrival angle and the like. Ionospheric scintillation can cause navigation satellite lock loss, receiver failure, and seriously affect signal measurement accuracy and navigation, positioning, and timing functions. 2. In urban canyon areas with tall buildings, standing trees, dense trees and serious electromagnetic radiation pollution, GNSS signals are easily influenced by shielding, multipath effect, interference and the like, so that signal energy attenuation and phase distortion are caused, tracking and measuring errors of a receiver are increased, and even positioning interruption is caused. 3. The GNSS signals are very weak after reaching the ground, and are very easy to be subjected to radio frequency interference caused by various communication or broadcast radio signals and other human factors, and the radio frequency interference can cause the accuracy of a code ring and a carrier ring of a GPS receiver to be reduced, the error rate to be increased, the navigation accuracy to be reduced or the tracking loss to be caused, thereby causing the positioning interruption.

Disclosure of Invention

The invention provides a self-adaptive switching method of a tracking loop of a single-frequency multi-frequency GNSS receiver, aiming at the problems that the tracking loss lock of the existing single-frequency GNSS receiver occurs in a challenging environment and the problems that the calculated amount of the existing multi-frequency GNSS receiver is too large in a normal environment and the observed amounts among different frequency points influence each other, and aims to select an index capable of reflecting the environmental change and set a corresponding threshold for detection so as to adjust the switching state of the loop. When the signal quality is good, the independent tracking mode of each frequency point is adopted, and the interference of calculated quantity and other frequency point observed quantity is reduced. When the signal quality is reduced and the lock is easy to lose, a multi-frequency joint tracking mode is adopted, the observed quantity of normal signals of other frequency points is used for assisting, and the robustness and the anti-interference capability of the GNSS receiver are improved. Therefore, the tracking method of single-frequency independent and multi-frequency auxiliary self-adaptive switching can improve the positioning reliability, precision and continuity of the GNSS receiver and has great potential value.

The challenging environment is as follows: buildings, trees and the like shield the GNSS signals; multipath effect caused by indirect signals generated by reflection of GNSS signals in areas such as building wall surfaces and the like; random fluctuation of parameters such as GNSS signal amplitude, phase and arrival angle caused by ionospheric scintillation; radio frequency interference caused by artifacts such as broadcast radio signals to GNSS signals, and the like.

The invention is realized by the following technical scheme:

the invention relates to a self-adaptive switching method for a tracking loop of a single-frequency multi-frequency GNSS receiver.

The single frequency is as follows: the carrier frequency of the signals transmitted by the GNSS satellites is of a single frequency band type.

The multi-frequency means that: the carrier frequencies of signals transmitted by GNSS satellites are of two or more frequency band types. Taking the beidou navigation system No. three as an example, the beidou navigation system provides five public service signals of B1I, B1C, B2a, B2B and B3I. Wherein: the center frequency of the B1I frequency band is 1561.098MHz, the center frequency of the B1C frequency band is 1575.420MHz, the center frequency of the B2a frequency band is 1176.450MHz, the center frequency of the B2B frequency band is 1207.14MHz, and the center frequency of the B3I frequency band is 1268.520 MHz.

The tracking loop refers to: and according to the rough estimated values of the carrier frequency and the code phase of the GNSS satellite signal, gradually and finely estimating the two signal parameters through a tracking loop, and simultaneously outputting a GNSS signal observed value. The tracking loop typically needs to be continuously operated periodically in the form of closed loop feedback to achieve a continuous lock on the satellite signal. The tracking loop consists of a carrier tracking loop and a code tracking loop and is used for tracking a carrier and a pseudo code in a GNSS signal respectively, and the tracking loop is switched in a self-adaptive mode aiming at the switching of the carrier tracking loop.

The hardware platform of the GNSS receiver refers to: the method is utilized to adjust various operations of a hardware platform baseband digital signal processor by, but not limited to, using an Application Specific Integrated Circuit (ASIC) chip for radio frequency front end processing and baseband digital signal processing.

The radio frequency front-end processing means: the method comprises the steps of receiving signals of all visible GNSS satellites through a GNSS antenna, filtering and amplifying the signals through a pre-filter and a pre-amplifier, mixing the signals with a sine wave local oscillator signal generated by a local oscillator to convert the signals into intermediate frequency signals in a down-conversion mode, and converting the intermediate frequency signals into digital intermediate frequency signals in discrete time through an analog-to-digital converter.

The baseband digital signal processing means: the method comprises the steps of copying a local carrier and a local pseudo code signal which are consistent with a received satellite signal by processing a digital intermediate frequency signal output by a radio frequency front end, thereby realizing the acquisition and tracking of the GNSS signal, obtaining measurement values of a GNSS pseudo range, a carrier phase and the like from the GNSS signal, and adjusting a navigation message.

The index reflecting the environmental change is as follows: the influence of environmental factors such as multipath, occlusion, and interference on the GNSS signal index includes, but is not limited to, parameters representing signal energy variation, such as carrier-to-noise ratio, and parameters representing receiver tracking loop stability, such as phase error.

The single-frequency tracking loop is as follows: aiming at a plurality of frequency band signals contained in the digital intermediate frequency signals, each frequency band has a respective independent tracking channel, and each channel has a respective correlator, a phase discriminator, a local reference signal generator, an environmental parameter estimator and a loop filter.

The multi-frequency tracking loop is characterized in that: aiming at a plurality of frequency band signals contained in the digital intermediate frequency signals, each frequency band has a respective independent tracking channel, each channel has a respective correlator, a phase discriminator, a local reference signal generator and an environmental parameter estimator, and each channel shares a loop filter.

The correlator is as follows: after the digital intermediate frequency signal is mixed with the local copy carrier, the mixing result is related to the local copy pseudo code, so that the pseudo code on the received signal is stripped.

The phase discriminator is characterized in that: including but not limited to a multiplier, for discriminating phase differences of the tracking loop input signal and the output signal.

The local reference signal generator is as follows: the local replica signal is consistent with the input digital intermediate frequency signal by adjusting the frequency of the output signal.

The environment parameter estimator comprises: the correlator and phase detector based results output indicators reflecting environmental changes including, but not limited to, carrier-to-noise ratio, phase error, etc.

The loop filter is as follows: including but not limited to low pass filters, kalman filters, etc., for reducing noise in the tracking loop.

By taking Kalman filter, Beidou third generation B1, B2 and B3 three frequency band signals, and taking the environmental index parameter as the carrier-to-noise ratio as an example, the specific steps of multi-frequency tracking comprise: in the baseband signal processing stage of the GNSS receiver, a phase-locked loop structure is used for initializing carrier tracking; taking the phase error as a common Kalman filter input parameter of each frequency band, taking the outputs of the signal carrier-to-noise ratio estimator and the phase interference estimation module as filter gain adjustment parameters of the Kalman filter, and inputting the filter result into a three-frequency carrier state estimator; the three-frequency carrier state estimator is used for independently estimating three frequency phases respectively based on phase errors on three frequency bands and filter gains of a Kalman filter, effectively reserving phase change characteristics caused by distortion on each signal by realizing three-frequency joint estimation aiming at frequency tracking, and extracting scene characteristic parameters and monitoring interference; and respectively inputting the state estimators into a local reference signal generator and a phase interference estimation module to complete closed-loop tracking of the three-frequency carrier signal.

The initialization comprises the following specific steps: the digital intermediate frequency signal is input into a correlator to perform correlation operation with a reproduced carrier signal generated by a local reference signal generator, an accumulation result of the same-direction and orthogonal branches is generated and further input into a phase discriminator, and the phase discriminator acquires phase errors on three frequency bands according to the accumulation result of the same-direction and orthogonal branches of each channel.

Technical effects

The invention integrally solves the problems of tracking loop lock losing caused by serious signal attenuation and rapid phase change caused by ionospheric flicker, multipath effect or radio frequency interference in the existing single-frequency GNSS receiver, and the problems of large calculation amount and easy influence of weak signals on the tracking precision of strong signals due to mutual coupling of tracking of various frequency points in the existing multi-frequency tracking technology.

Compared with the prior art, the method has the advantages that the index reflecting the environmental change is used as the judgment basis, so that the GNSS receiver can switch the single-frequency tracking loop and the multi-frequency tracking loop in real time, the estimation of the Doppler frequency of the interfered signal can be assisted by the normal signal observation quantity by means of the frequency correlation of the multi-frequency signal transmitted by the same satellite and received by the same receiver, the amplitude attenuation caused by the interference is compensated, the single-frequency independent tracking loop can be switched, and the mutual influence of strong and weak signal tracking is effectively eliminated while the calculated quantity is reduced.

Drawings

FIG. 1 is a functional diagram of a GNSS receiver hardware platform according to the present invention.

FIG. 2 is a diagram of a single frequency tracking loop of a GNSS receiver.

Fig. 3 is a schematic diagram of a multi-frequency tracking loop of a GNSS receiver.

FIG. 4 is a flowchart of a GNSS receiver tracking loop adaptive handoff method.

Detailed Description

As shown in fig. 1, the hardware platform of a GNSS receiver according to this embodiment includes: GNSS radio frequency antenna 101, radio frequency front end processing module 111, baseband digital signal processing module 121, positioning resolving module 131, and environmental index threshold discriminator 141, wherein: the GNSS radio frequency antenna 101 receives GNSS satellite signals 102 of all visible GNSS satellites, inputs the GNSS satellite signals 102 into a radio frequency front-end processing module 111, filters and amplifies the signals, mixes the signals with a sine wave local oscillator signal generated by a local oscillator to convert the signals into intermediate frequency signals, then converts the intermediate frequency signals into digital intermediate frequency signals 112 of discrete time through an analog-to-digital converter, inputs the digital intermediate frequency signals 112 into a baseband digital signal processing module 121, and copies local carrier waves and local pseudo code signals consistent with the received satellite signals, so as to capture and track the GNSS signals, obtain measured values of GNSS pseudo-range phases and the like from the signals and adjust navigation messages 122, and inputs the measured values of GNSS pseudo-range phases and the like and the adjusted navigation messages 122 into a positioning module 131 to realize positioning; the environment change index 123 is output by the baseband digital signal processing module 121, and the environment change index 123 is input to the environment index threshold discriminator 141, compared with the threshold, the type of the tracking loop adopted in the baseband digital signal processing module 121, i.e., single frequency or multiple frequency, is determined, and the tracking loop selection result 142 is input to the baseband digital signal processing module 121.

According to the judgment of the environment index threshold discriminator 141, when the environment index is lower than the threshold value, then, the digital

intermediate frequency signals

201, 221 and 241 of the frequency bands of B1, B2 and B3 enter three first to

third tracking loops

200, 220 and 240 with the same structure respectively, taking the beidou third generation navigation system B1, B2 and B3 as an example, and taking the selected environmental index as the carrier-to-noise ratio, taking 200 as an example, the digital intermediate frequency signal 201 is input into the correlator 202, and is subjected to correlation operation with locally copied pseudo codes, correlation results are respectively taken as the input of the phase detector 205 and the carrier-to-noise ratio estimator 206, the results of the phase detector 205 and the carrier-to-noise ratio estimator 206 are taken as the input of the joint filter 207, loop noise is removed, the output result of the joint filter 207 is taken as the input of the carrier state estimator 208, the carrier state is estimated, and the estimation result can further adjust the copying of the local signal in the reference signal generator 210. The estimation results of the first to third carrier-to-noise ratio estimators 206, 226 and 246 may be used as the input of the environment index threshold discriminator 141, and compared with the threshold preset in the environment index threshold discriminator 141 to determine the type of the tracking loop used in the next step.

According to the judgment of the environment index threshold discriminator 141, when the environment index is higher than the threshold value, the multi-frequency tracking loop shown in fig. 3 is entered.

Taking the frequency bands of the Beidou third generation navigation system B1, B2 and B3, taking the environment index as the carrier-to-noise ratio as an example, the specific operation is as follows:

the method comprises the following steps: the digital

intermediate frequency signals

301, 321, and 341 are input to the first to

third correlators

302, 322, and 342 and correlated with the reproduced

carrier signals

315, 335, and 355 generated by the first to third local reference signal generators 314, 334, and 354 to generate the

accumulation results

303, 304, 323, 324, 343, and 344 of the in-phase and quadrature branches. The accumulated

results

303, 323 and 343 are input to the first to

third phase detectors

306, 326 and 346 and

phase errors

308, 328 and 348 in three frequency bands are obtained according to the accumulated results of the in-phase and quadrature branches of each channel. In addition, the correlator

outputs correlation values

304, 324 and 344 as inputs to the first to third signal-to-carrier-to-noise ratio estimators 307, 327 and 347 for monitoring GNSS signal quality and calculating observed noise in real time, and

outputs index results

309, 329 and 349 as inputs to the environment index threshold discriminator 141.

Step two:

phase errors

308, 328 and 348 are used as input parameters of a joint filter 360 common to three frequency bands,

outputs

309, 329 and 349 of signal carrier-to-noise ratio estimators 307, 327 and 347 are used for adjusting filtering gain of a loop filter, a filtering result 361 is input to a three-frequency carrier state estimator 362, and an estimated state quantity 363 comprises five components which are carrier phases of the three frequency bands respectively and Doppler change rate on a reference frequency common to the three frequency bands.

Step three: when the state quantity 363 output by the three-frequency carrier state estimator 362 is multiplied by the

frequency multiplication coefficients

364, 365 and 366 of B1, B2 and B3, respectively, the state quantity is input to the first to third carrier state estimators 310, 330 and 350, thereby completing the closed-loop tracking of the three-frequency carrier signal.

As shown in fig. 4, a flow chart of a method for adaptively switching a tracking loop of a single-frequency multi-frequency global GNSS receiver is shown, taking a carrier-to-noise ratio as an example, in a correlator 402, a local replica signal is correlated with a received digital intermediate frequency signal through the correlator, correlation results are respectively input into a phase detector and a carrier-to-noise ratio estimator, a carrier error 404 and an output signal carrier-to-noise ratio 406 are determined, and the carrier errors 404 and 406 are input into an environment index threshold discriminator 141, and compared with a preset threshold:

when the carrier state estimation value is lower than the threshold value, selecting a left multi-frequency tracking loop, inputting carrier errors 404 and 406 into a multi-frequency combined filter, determining a gain matrix 408, and expanding the carrier state estimation to each carrier frequency band 412 according to the frequency after determining an optimal multi-frequency carrier estimation value 410;

otherwise, selecting a right single-frequency tracking loop, inputting the carrier errors 404 and 406 into the filters of the respective frequency bands respectively, determining respective gain matrixes 409, and determining respective single-frequency carrier estimation values 411.

After determining the carrier estimation values 411 or 412 of each frequency point, the carrier state estimation is used as the input 414 of the local reference signal generator in each channel, and the local reference signal generator updates the local replica signal 416 and inputs the local replica signal to the next stage 402, thereby forming a closed-loop tracking process.

When the signal quality is good, the independent tracking mode of each frequency point is adopted, and the interference of calculated quantity and other frequency point observed quantity is reduced. When the signal quality is reduced and the lock is easy to lose, a multi-frequency joint tracking mode is adopted, the observed quantity of normal signals of other frequency points is used for assisting, and the robustness and the anti-interference capability of the GNSS receiver are improved. Therefore, compared with the existing tracking method, the tracking method of single-frequency independent and multi-frequency auxiliary self-adaptive switching can improve the positioning reliability, precision and continuity of the GNSS receiver, and has great potential value.

The foregoing embodiments may be modified in many different ways by those skilled in the art without departing from the spirit and scope of the invention, which is defined by the appended claims and all changes that come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

Claims

1. A self-adaptive switching method for a tracking loop of a single-frequency multi-frequency GNSS receiver is characterized in that GNSS digital intermediate frequency signals are collected through a radio frequency front end of a hardware platform of the GNSS receiver, indexes reflecting environmental changes are obtained through calculation and then are compared with a preset threshold so as to select the single-frequency tracking loop or the multi-frequency tracking loop to carry out closed-loop tracking;

the tracking loop refers to: according to the rough estimated values of the carrier frequency and the code phase of the GNSS satellite signals, estimating the two signal parameters step by step through a tracking loop, and outputting a GNSS signal observed value at the same time;

the index reflecting the environmental change is as follows: the influence of multipath, shielding and interference environmental factors on GNSS signal indexes;

the hardware platform of the GNSS receiver comprises: GNSS radio frequency antenna, radio frequency front-end processing module, baseband digital signal processing module, location are solved module and environmental index threshold arbiter, wherein: the GNSS radio frequency antenna receives GNSS satellite signals of all visible GNSS satellites, the GNSS satellite signals are input into the radio frequency front-end processing module, the GNSS satellite signals are filtered, amplified, mixed with sine wave local oscillator signals generated by the local oscillator to be converted into intermediate frequency signals in a down-conversion mode, then the intermediate frequency signals are converted into discrete time digital intermediate frequency signals through the analog-to-digital converter, the digital intermediate frequency signals are input into the baseband digital signal processing module, and local carrier waves and local pseudo code signals consistent with the received satellite signals are copied, so that the GNSS signals are captured and tracked, GNSS pseudo ranges and carrier phases are obtained from the GNSS pseudo ranges and carrier phases and navigation messages are adjusted, and the GNSS pseudo ranges and carrier phases and the navigation messages are input into the positioning resolving module to achieve positioning; outputting an environment change index through a baseband digital signal processing module, inputting the environment change index into an environment index threshold discriminator, comparing the environment change index with a threshold, judging the type of a tracking loop adopted in the baseband digital signal processing module, namely single frequency or multi-frequency, and inputting a tracking loop selection result into the baseband digital signal processing module;

the single-frequency tracking loop is as follows: aiming at a plurality of frequency band signals contained in the digital intermediate frequency signals, each frequency band has a respective independent tracking channel, and each channel has a respective correlator, a phase discriminator, a local reference signal generator, an environmental parameter estimator and a loop filter;

the multi-frequency tracking loop is characterized in that: aiming at a plurality of frequency band signals contained in the digital intermediate frequency signals, each frequency band has a respective independent tracking channel, each channel has a respective correlator, a phase discriminator, a local reference signal generator and an environmental parameter estimator, and each channel shares a loop filter;

the correlator is as follows: after the digital intermediate frequency signal is mixed with the local copy carrier wave, the mixing result is related to the local copy pseudo code, so as to strip the pseudo code on the received signal;

the phase detector comprises: a multiplier for discriminating a phase difference between an input signal and an output signal of the tracking loop;

the local reference signal generator includes: the voltage-controlled oscillator generates a periodic oscillation signal with a certain frequency based on the result of the phase discriminator, and achieves the purpose of keeping the local replica signal consistent with the input digital intermediate-frequency signal by adjusting the frequency of the output signal;

the environment parameter estimator outputs indexes reflecting environment changes based on the results of the correlator and the phase discriminator, and comprises the following steps: carrier-to-noise ratio, phase error;

the loop filter includes: low pass filter, kalman filter, used to reduce noise in the tracking loop.

2. The adaptive handover method for the tracking loop of the single-frequency multi-frequency GNSS receiver of claim 1, wherein the rf front-end processing means: the method comprises the steps of receiving signals of all visible GNSS satellites through a GNSS antenna, filtering and amplifying the signals through a pre-filter and a pre-amplifier, mixing the signals with a sine wave local oscillator signal generated by a local oscillator to convert the signals into intermediate frequency signals in a down-conversion mode, and converting the intermediate frequency signals into digital intermediate frequency signals in discrete time through an analog-to-digital converter.

3. The adaptive single-frequency multi-frequency GNSS receiver tracking loop switching method of claim 1, wherein said baseband digital signal processing means: the method comprises the steps of copying a local carrier and a local pseudo code signal which are consistent with a received satellite signal by processing a digital intermediate frequency signal output by a radio frequency front end, thereby realizing the acquisition and tracking of the GNSS signal, obtaining a GNSS pseudo range and a carrier phase measurement value from the GNSS signal and adjusting a navigation message.

4. The single-frequency multi-frequency GNSS receiver tracking loop adaptive handover method of claim 1, wherein the index reflecting environmental changes comprises: carrier-to-noise ratio, phase error.

5. The adaptive switching method of tracking loops of single-frequency multi-frequency GNSS receiver as claimed in claim 1, wherein the initialization is implemented by inputting digital intermediate frequency signals into a correlator to perform correlation operation with the reproduced carrier signals generated by the local reference signal generator to generate the accumulation results of the homodromous and quadrature branches and further inputting the accumulation results into a phase discriminator, and the phase discriminator obtains phase errors in three frequency bands according to the accumulation results of the homodromous and quadrature branches of each channel.