CN102141627A - Burst type navigation signal system and receiving method - Google Patents
Burst type navigation signal system and receiving method Download PDFInfo
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Abstract
The invention discloses a burst type navigation signal system and a receiving method, relating to the space technology. A signal broadcast carrier can be a navigation satellite, a space balloon or an airship and the like; a navigation signal of the burst type navigation signal system comes from a burst type satellite navigation beacon broadcasted by a straight hair type satellite navigation system or a forwarding type satellite navigation system through a satellite or an intermittent satellite navigation signal; a user terminal uses a high-gain antenna to carry out the time-sharing sky coverage in manners of pitching and azimuth 2-dimensional scanning; and a satellite within the direction scope of the high-gain antenna has the additional signal to interference ratio advantage endowed by the high-gain antenna, thereby the anti-interference effect is reached. With the utilization of the burst signal system and the receiving method, the problem of a large-area phased array antenna in the traditional airspace anti-interference measure is effectively avoided, and the time-frequency domain anti-interference measure is also convenient to carry out simultaneously; and the burst type navigation signal system is easily arranged on a smaller or high-mobility weapon platform and has low cost and high reliability.
Description
Technical Field
The invention relates to the technical field of space, in particular to a burst type navigation signal system and a burst type navigation signal receiving method.
Background
The widespread use of satellite navigation has led to the creation of the concept of "navigational confrontation". With the continuous upgrade of the navigation countermeasure technology, various interference means for dealing with the satellite navigation receiver emerge endlessly; the interference signal body is divided into pulse interference, decoy interference, broadband noise suppression interference, broadband frequency modulation suppression interference, narrow-band interference and the like; the carrier of the interference machine is divided into a ground vehicle-mounted interference machine, a balloon-mounted high-altitude interference machine, an airplane-mounted interference machine and the like. Various interference countermeasure have been taken as a countermeasure against interference, and from the viewpoint of the navigation receiver, there are an interference countermeasure by various time domain filtering, an interference countermeasure by frequency domain filtering, an interference countermeasure by spatial domain filtering by an antenna array, and the like. The interference and the anti-interference are an endless relationship between a spear and a shield, and it is a very significant matter from the perspective of the shield to continuously develop an anti-interference means with better performance and lower cost.
The design of the whole satellite navigation system is designed based on the starting point of the receiver for continuously receiving the satellite navigation signals, so that most of the existing satellite navigation receivers are designed based on the idea of continuously tracking the satellite navigation signals, and no report about a satellite navigation method for navigation and positioning by using burst signals is found in various documents at present.
Disclosure of Invention
The invention aims to disclose a burst type navigation signal system and an anti-interference satellite navigation receiving method based on burst signals.
In order to achieve the purpose, the technical scheme of the invention is as follows:
a "burst" navigation signal regime, comprising:
A) the signal broadcasting carrier can be a navigation satellite, a space balloon or an airship;
B) the navigation signals originate from:
a) burst navigation beacons broadcast by the direct navigation satellite;
b) a ground station of the forwarding type satellite navigation system goes up to a satellite and then transmits a burst type satellite navigation beacon through the satellite;
c) when an enemy carries out intermittent interference on a continuous navigation signal transmitted by a navigation satellite of the enemy, the satellite navigation signal with clean fragments received by a receiver during the intermittent period is interfered;
d) when continuous interference is carried out on a continuous navigation signal transmitted by the navigation satellite of the enemy, the receiver receives a burst satellite navigation signal with better signal-to-interference ratio at the moment when the antenna is aligned with the navigation satellite by rotating the satellite navigation signal antenna;
C) a user terminal:
the high-gain antenna is used for covering the sky in a time-sharing manner in a pitching and azimuth two-dimensional scanning manner, and the satellite within the pointing range of the high-gain antenna has the additional signal-to-interference ratio advantage given by the antenna gain, so that the anti-interference effect is achieved; the anti-interference mode avoids the problem of a large-area phased array antenna in the traditional airspace anti-interference measure, is convenient for simultaneously implementing the time-frequency domain anti-interference measure, is easy to arrange on small and high-mobility weapon platforms, and has low manufacturing cost and high reliability.
The burst navigation signal system is a burst navigation signal, or a pulse navigation signal, and the duration of a signal pulse is between 10 milliseconds and 1 second or so; for a direct-type satellite navigation system, the signal system provides another possible approach for realizing navigation signal area enhancement on a navigation satellite: the satellite navigation signal adopts burst pulse form, the power amplifier on the satellite only needs to increase the radiation power during the pulse duration, and when the duty ratio of the burst signal is lower, the implementation difficulty of navigation signal area enhancement can be greatly reduced.
In the burst navigation signal system, when the duty ratio of the burst signal is relatively low, the duty ratio is about 10%.
The burst navigation signal system adopts navigation messages with short length and high speed, and because the satellite navigation signals adopt a burst pulse form, a power amplifier on a satellite increases the radiation power during the pulse duration, and the navigation messages are transmitted at a higher transmission speed after the satellite signal transmission power is improved.
In the burst navigation signal system, the satellite navigation signal antenna is a high-gain antenna, the gain of the high-gain antenna is more than or equal to 5dBic, and the satellite navigation signal antenna covers the sky in a time-sharing manner in a pitching and azimuth two-dimensional scanning manner.
A user terminal in the navigation signal system is based on the navigation positioning receiving method of the burst signal satellite, no matter what kind of signal source, from the perspective of the user terminal, the satellite navigation signal received by the burst type satellite navigation terminal is 'burst'; the burst type signal navigation receiver can only measure pseudo range and Doppler frequency according to discontinuous intermittent signals to carry out navigation positioning calculation; the method comprises the following steps:
1) the pseudo-range extraction method comprises the steps of firstly, extracting a spread spectrum code from a received burst satellite navigation signal, and then obtaining a coarse estimation value of a code phase and a Doppler frequency by adopting a traditional serial or parallel search algorithm; on the basis of the obtained coarse estimation value, carrying out fine refinement processing on the code phase to obtain a fine code phase;
2) the Doppler frequency extraction method is used for stripping spread spectrum codes from burst satellite navigation signals from a coarse code phase or a refined code phase, and then calculating the Doppler frequency of the navigation signals by using a classical spectrum estimation or modern spectrum estimation method;
3) the navigation parameter resolving method combines inertial navigation unit (IMU) observed quantity with satellite navigation pseudo-range and Doppler observed quantity to jointly resolve the navigation parameters of the receiver: the position and the speed of the receiver are restrained to a certain degree by utilizing auxiliary information of an inertial navigation IMU assembly, meanwhile, a high-stability frequency source is adopted to maintain time between burst satellite navigation signal pulses received at different moments, and a Kalman filtering method is adopted to resolve the position, the speed and the time information from pseudo-range and Doppler information obtained from different burst satellite navigation signals.
The navigation positioning receiving method has the advantages that the code phase refining processing is realized by accurately solving the code phase by using a finer searching step length and a method of curve fitting of the shape of a relevant peak on the basis of roughly estimating the code phase and the Doppler frequency, the refining searching adopts a Zoom FFT algorithm, and meanwhile, the position integral and instantaneous speed information given by inertial navigation assistance are used for further reducing the calculated amount of the refining code phase searching
In the navigation positioning receiving method, under the condition of short code period, the code period is short, and an integer fuzzy distance corresponding to the code period exists between the obtained refined code phase and the actual pseudo range, and for GPS and GLONASS civil codes, the fuzzy distance is an integral multiple of 300 kilometers, and the integer multiple must be correctly solved, wherein the solution of the integer multiple is to reconstruct the emission time of signals by taking the code period as a unit.
The navigation positioning receiving method is characterized in that the speed in the navigation parameters is measured, the Doppler frequency shift of burst GPS signals is estimated by using a direct periodogram method, the line-of-sight change rate is calculated, a line-of-sight change rate equation is established, and the speed of a receiver is further obtained.
In the navigation positioning receiving method, the navigation parameter calculating method is characterized in that because the clock error of the satellite navigation receiver drifts in time, relatively accurate time keeping is carried out between each burst of satellite navigation signal pulse, the accuracy of the crystal oscillator frequency is calibrated, and the arrival time of each burst of satellite navigation signal pulse is kept within 10 nanoseconds within a few seconds; in Kalman filtering, receiver local clock error drift is used as a parameter to be estimated for resolving, so that the influence of insufficient observation quantity of pseudo range and Doppler frequency in single burst satellite navigation signals is overcome, and navigation parameters are reliably resolved by using multi-burst satellite navigation signals.
The navigation positioning receiving method, the doppler frequency estimation and velocity calculation process thereof, includes:
s1, performing frequency domain FFT capture on the original intermediate frequency sampling signal, and roughly capturing the code phase and Doppler frequency shift of the signal by using a parallel code phase search capture method;
s2, capturing an initial code phase and Doppler frequency shift, and further refining the Doppler frequency shift;
s3, thinning the initially captured code phase to obtain an ideal thinned code phase;
and S4, readjusting the local code by using the code phase refined in the step S3, performing modulo two addition operation on the local code and the original signal, and stripping the C/A code. For the high dynamic condition, the frequency generated by the local code generator is readjusted according to the Doppler frequency shift attached to the C/A code;
s5, performing periodic spectrum estimation on the intermediate frequency carrier plus noise signal obtained after C/A code stripping, wherein the frequency corresponding to the maximum value of the obtained spectrum peak is the estimated carrier Doppler frequency shift;
s6, solving the line-of-sight change rate equation of n (n is more than or equal to 4) satellites by using a least square method according to the Doppler frequency shift and the line-of-sight change rate equation between the receiver and the GPS satellite to obtain the user speed and the clock error change rate of the user receiver.
The burst type satellite navigation signal system provides another possible approach for realizing navigation signal area enhancement on the navigation satellite. The satellite navigation signal adopts burst pulse form, the power amplifier on the satellite only needs to increase the radiation power during the pulse duration, and when the duty ratio of the burst signal is relatively low (such as about 10%), the implementation difficulty of the navigation signal area enhancement can be greatly reduced. The regional enhancement of the navigation signals is realized on the navigation satellite without adopting two types of means. The method is characterized in that limited navigation signal power is concentrated in a smaller local area by increasing the power of a navigation signal transmitted by a navigation satellite and increasing the aperture of a navigation signal radiation antenna on the satellite. The regional enhancement of the navigation signal is realized by the first means, if the satellite navigation signal is a continuous wave signal, the power consumption of a power amplification effective load on the satellite is increased sharply, and the difficulty of thermal control on the satellite is brought; the second approach to implementing the regional augmentation of the navigation signal means that an antenna with a large aperture needs to be configured on the satellite, which brings difficulty to the satellite manufacturing.
The burst type satellite navigation signal is also very convenient to broadcast in a forwarding type satellite navigation system, and the burst type signal broadcast of the satellite to a user can be realized only by changing the navigation signal which is uplinked to the satellite by the ground control station into the burst type. Under the condition of adopting burst signals, the navigation signals can be hidden in normal broadcast signals of the communication satellite conveniently, and particularly, the implementation of navigation war is facilitated by intentionally reducing the transmission frequency (reducing duty ratio) of the burst signals.
For a weapon platform (such as a medium-long range weapon striking system and the like) with low cost, small volume and high maneuverability, in order to resist electromagnetic interference of an enemy, a high-gain satellite navigation antenna can be arranged on the weapon platform, and the sky is covered in a time-sharing manner in a pitching and azimuth two-dimensional scanning manner. In this case, each satellite navigation beacon signal always arrives in a time-sharing manner, and has a burst characteristic. Similarly, enemy jamming signals are also time-sharing and fragmented and are likely not to overlap in time with the navigation beacon signals (unless the source of the jamming is in the same direction as the satellite navigation satellites). In this case, for satellites within the pointing range of the high-gain antenna, the antenna gain has an additional signal-to-interference ratio advantage, so that the effect of interference resistance is achieved.
The invention can also be applied to the application occasions such as sea surface sonar buoy positioning, forest areas and the like, and the received signals are burst due to the reasons that sea waves flap the receiver antenna and the trees and leaves are shielded; in severe dynamic environments with high dynamics and high maneuverability (such as application occasions of spinning shells, spinning missiles, fighters and the like), because the receiver antenna is not omnidirectional, the received navigation signal is fragmented and has the characteristic of burst property.
Drawings
FIG. 1 is a schematic diagram of a burst-based navigation signal structure according to the present invention;
FIG. 2 is a flow chart of a refined code phase of the navigation positioning receiving method of the present invention;
FIG. 3 is a diagram illustrating a signal length and an estimated code phase accuracy curve of the navigation positioning receiving method according to the present invention;
FIG. 4 is a diagram illustrating the constraint boundary of the initial position and time error under the condition of GPS CA code;
FIG. 5 is a flow chart of the navigation positioning receiving method for calculating Doppler and velocity according to the present invention;
FIG. 6 is a contour diagram of mean square error of Doppler shift estimation under different carrier-to-noise ratios and different signal lengths, wherein the unit of the contour is Hz;
FIG. 7 is a graph of receiver speed settings for a simulation using a GPS high dynamic simulator in accordance with the present invention;
FIG. 8 is a signal acquisition flow chart of the present invention using GSS7700 simulator to simulate signal and acquisition flow;
FIG. 9 is C/N0Velocity measurement error plots for a burst length of 43dBHz of 10ms and 15ms, respectively.
Detailed Description
The invention relates to a burst type navigation signal system and a receiving method, wherein the concept of burst signal navigation is as follows:
the main sign of burst-type satellite navigation is that from the perspective of the user terminal, the satellite navigation signal received by the burst-type satellite navigation terminal is "burst" or "pulse", which is the distinctive feature of burst-type satellite navigation and conventional continuous signal satellite navigation.
In a burst-oriented satellite navigation system, a burst-oriented satellite navigation signal seen from a receiving terminal may originate from the following situations:
● burst navigation beacons broadcast by the direct navigation satellites themselves;
● the ground station of the repeating satellite navigation system goes upward to the satellite and then transmits the burst satellite navigation beacon by the satellite;
● when the enemy carries out intermittent interference to the continuous navigation signal sent by the navigation satellite of our party, the receiver receives the satellite navigation signal with clean fragments during the interference interval;
● continuous interference of enemy to the continuous navigation signal transmitted by our navigation satellite, the receiver receives burst satellite navigation signal with better signal-to-interference ratio at the moment when the antenna is aligned with the navigation satellite by rotating the high-gain satellite navigation signal antenna.
The invention relates to a burst type navigation signal system and a receiving method, which comprises the following steps:
1. system architecture and principles
The system-level structure of the satellite navigation system based on the burst signal is not essentially different from that of the traditional satellite navigation system, and the satellite navigation system based on the burst signal also comprises a navigation satellite constellation, ground measurement and control, a user receiving terminal and the like.
1) In a space segment, a navigation signal emitted by a satellite navigation constellation has the characteristic of burst, the duration of burst signal pulse is about 10 milliseconds to 1 second, the modulation of the burst navigation signal is similar to the signal modulation of a traditional satellite navigation system, firstly, pseudo code spreading is carried out on a navigation message, and secondly, a combined code after the spreading is modulated on a carrier wave. However, the burst navigation signal system can adopt navigation messages with shorter length and higher speed, and because the satellite navigation signals adopt a burst pulse form, a power amplifier on a satellite can increase the radiation power during the pulse duration and improve the satellite signal transmission power, thereby being capable of transmitting the navigation messages at a faster transmission speed. The navigation message can be broadcast in a basic format with the length of 300 bits and the transmission time of 12s, and the navigation message forms different types of data blocks according to contents, and the time intervals of different data content broadcasts are different.
2) User segment
The operating principle of the receiving system based on the burst navigation signal is as follows:
the main difference between burst-signal satellite navigation receivers and conventional continuous-signal receivers is in terms of the functionality that the receiver itself must have to perform to accomplish the navigation task: the burst type signal satellite navigation receiver is based on the basic observed quantities such as the segment pulse type signal extraction pseudo range, the Doppler frequency and the like, so that the pseudo range and the Doppler extraction algorithm are different from the conventional receiver essentially; because the burst signal satellite navigation receiver can not necessarily receive more than 4 satellite navigation signals at the same time, and the navigation message can not be obtained by a continuous signal tracking mode, the algorithm for resolving the navigation parameters from the pseudo range and the Doppler is also greatly different from the traditional receiver.
The anti-interference satellite navigation receiving technology based on burst signals is used in the situation of low-cost anti-interference receivers of microminiature and high maneuvering platforms in the following description in combination with a typical battlefield environment.
In general, for a weapon platform which is sensitive in manufacturing cost, small in size and high in maneuverability, a high-gain antenna is used, and the sky is covered in a time-sharing mode in a pitching and azimuth two-dimensional scanning mode. In this case, each satellite navigation beacon signal always arrives in a time-sharing manner and is burst. Similarly, the interfering signals are also time-shared and intermittent and do not overlap in time with the navigation beacon signals (unless the interferer is in the same spatial direction as the navigation satellites). In this case, for satellites within the pointing range of the high-gain antenna, the antenna gain has an additional signal-to-interference ratio advantage, so that the effect of interference resistance is achieved. In another aspect, by adopting the method, the traditional airspace anti-interference problem is converted into a time domain anti-interference problem, then the time-frequency domain signal processing method is adopted in a targeted manner to further suppress interference, and the burst signal is utilized to solve basic observed quantities such as pseudo range, Doppler frequency and the like, so that navigation positioning calculation is completed.
The anti-interference method of the invention avoids the problem of large-area phased array antenna in the traditional airspace anti-interference measure, is convenient for simultaneously implementing the time-frequency domain anti-interference measure, is easy to be arranged on small and high-mobility weapon platforms, and has low manufacturing cost and high reliability.
The structure of a burst-mode satellite navigation receiver may be similar to a conventional receiver in terms of receiver structure. As shown in fig. 1, it is a structure diagram of an anti-interference satellite navigation receiving method demonstration system based on burst signal of the present invention, which is a low-cost space-time-frequency anti-interference receiver of a microminiature, high maneuvering platform. In the figure, an antenna 1 has higher gain and has the capability of two-dimensional space scanning in pitch and azimuth; the radio frequency front end 2 and the A/D converter 3 follow the popular design of the traditional continuous signal receiver; the inertial navigation assistant 4 adopts a micro inertial navigation MIMU component; the digital signal processing platform 5 is composed of a large-scale FPGA and a high-performance DSP, so that the requirement of extracting pseudo range and Doppler frequency from burst signals and calculating navigation parameters is met.
The anti-interference satellite navigation receiver based on burst signals is different from the traditional continuous signal receiver mainly by the following three points:
● in terms of the selection of the antenna 1, since the anti-jamming satellite navigation receiver based on burst signals of the present invention does not require to receive navigation signals of at least more than 4 navigation satellites at the same time, the antenna 1 can be selected to be high-gain or common according to different application requirements. This is different from a conventional continuous signal receiver;
● the anti-interference satellite navigation receiver based on burst signal of the invention needs inertial navigation module IMU or navigation message auxiliary and crystal oscillator with certain stability and accuracy requirement; when the method is applied to a severe battlefield environment, because continuous satellite navigation signals cannot be obtained, navigation messages such as a navigation satellite ephemeris and the like may need to be input from the outside. It is worth pointing out that, for the traditional receiver, under the severe battlefield environment, the complete navigation message is probably not received;
● the anti-interference satellite navigation receiver based on burst signal of the invention is based on the basic observed quantity of the segment pulse signal extraction pseudo-range, Doppler frequency, etc., therefore, the algorithm of pseudo-range and Doppler extraction is different from the traditional receiver essentially; in addition, because the anti-interference satellite navigation receiver based on the burst signal is likely not to observe the pseudo range and the Doppler frequency of different navigation satellites at the same time, an algorithm for solving the navigation parameters from the pseudo range and the Doppler is also greatly different from the traditional receiver.
In the following, the pseudo range and doppler frequency extraction principle and the navigation parameter (position, velocity, time) solution principle of the burst signal based anti-interference satellite navigation receiving method according to the present invention are described in detail.
It is conceivable that if the navigation satellite transmits a navigation signal specifically designed for the burst satellite navigation regime, the time of transmission of the navigation signal at the satellite may be relatively easily extracted from the signal. The difficulty in recovering the code phase is much greater for the case of a burst of segments in the continuous pilot signal received by the receiver. Therefore, the following description will be made of a case where the burst content of the continuous satellite navigation signal is received.
(1) Pseudorange extraction principle
Pseudoranges are the basis for the navigation receiver to compute position, time information. The pseudorange corresponding to a certain navigation satellite is obtained from the burst navigation signal, and most importantly, the following two quantities must be obtained: firstly, the code phase of the navigation satellite spread spectrum code corresponding to the measurement time of the receiver, and secondly, the transmitting time of the code phase received by the receiver on the satellite.
A burst fragment signal output by an A/D converter received by a digital signal processing unit of a receiver may contain a navigation signal corresponding to a certain satellite. The spreading code sequences of different satellite navigation signals correspond to different spreading code periods, for example, the spreading code period of the L1 CA code of the GPS is 1ms, the spreading code period of the L2C code of the GPS is divided into two types, the code period of the L2CM code is 20ms, and the code period of the L2CL code is 1.5 s; the code period of the GLONASS min-code is 1 ms; the code period of the civil code of the Beidou second system is 2 ms; the military code periods of GPS, GLONASS and Beidou systems are all very long.
The first step in extracting pseudorange is to extract the fine code phase of the spreading code from the received signal, and to achieve this, a conventional serial or parallel search algorithm may first be employed to obtain a coarse estimate of the code phase and doppler frequency. On the basis, the code phase is finely refined, and as shown in fig. 2, a fine code phase is obtained.
The core idea of code phase refinement processing is to accurately solve the code phase by using a finer search step length and a correlation peak shape curve fitting method on the basis of roughly estimating the code phase and the Doppler frequency, wherein the refined search can adopt a Zoom FFT algorithm to accelerate the search speed, and meanwhile, the position integral and instantaneous speed information given by inertial navigation assistance can be used for further reducing the search calculation amount of the refined code phase. The specific details are well known in the art and will not be described herein. Fig. 3 shows the relationship between the code phase refinement accuracy obtained by monte carlo simulation and the effective length of the burst signal and the signal-to-noise ratio, where the preset true code phase is 0.982 chips of the CA code, and it can be seen from fig. 3 that better code phase estimation accuracy can be achieved by using the method of refining the code phase.
In the case of a short code period (e.g., GPS, GLONASS, and beidou system min code), an integer ambiguity distance corresponding to the code period still exists between the refined code phase obtained above and the actual pseudorange due to the short code period. For GPS and GLONASS civilian codes, the ambiguity distance is an integer multiple of 300 kilometers, and this integer multiple must be solved correctly. In other words, the integer multiple is solved by using the code period as a unit to reconstruct the emission time of the signal.
For ambiguity distances of integer multiples of 300 km, the signal transmission time can be reconstructed relatively easily if the location of the receiver and the local time of the receiver are known roughly, since 300 km is already a not small number. Fig. 4 shows the error bound constraints of the initial approximate position and time of the receiver needed to correctly reconstruct the signal transmission time for the GPS civilian signal, which we have analyzed. As can be seen from fig. 4, even in the case of short code periods, such as GPS and GLONASS, the requirement for a priori knowledge of the position and time of the receiver is relaxed.
For various military code signals and GPS L2 civil code signals, because the period of a spread spectrum code is long, the reconstruction of the emission time of the signals on a navigation satellite is much easier compared with the GPS civil code and the GLONASS civil code.
(2) Doppler frequency extraction principle
In the case of a continuous signal, the doppler shift of the carrier frequency is usually measured by continuously tracking the received GPS signal, and adjusting the frequencies of the local carrier and the code generator by the carrier tracking loop and the code tracking loop to synchronize the local carrier and the code with the input signal, thereby obtaining the doppler shift. This method is essentially based on the fact that it works for continuous signals of nearly infinite length, and is clearly not suitable for burst signal applications. After analysis and comparison, the frequency of the signal with limited length is estimated by applying the periodogram method of classical spectrum estimation, and the method has the characteristics of simple realization and strong real-time performance, and can obtain higher resolution particularly when the signal-to-noise ratio is lower.
The Doppler frequency is the basis of the digital signal processing unit for calculating the movement speed of the receiver, the core of solving the Doppler frequency is that a spread spectrum code is stripped from a burst navigation signal from a coarse acquisition code phase or a refined code phase, and then the Doppler frequency of the navigation signal is calculated by using a classical spectrum estimation method or a modern spectrum estimation method.
In a simulation test, a better doppler frequency estimation accuracy can be obtained by using a classical spectrum estimation method, the calculation time is relatively short, and a flow of doppler frequency estimation and velocity calculation is given as shown in fig. 5.
S1, the original intermediate frequency sampling signal is expressed by the following equation (1):
wherein, yjIs the jth satellite at τjSampling an intermediate frequency signal received at a moment; p is a radical ofrIs the power of the received signal; d (τ) is the navigation message modulation at 50 Hz; t is ts(τ) is the signal transmission delay; c (tau) is a 1.023MHz C/A code spreading sequence; omegaIFIs the received signal carrier intermediate frequency angular frequency; omegadIs the doppler shift between the receiver and the satellite; phi is a0Is the initial value of the carrier phase; n isjIs a variance of δn 2Zero mean gaussian white noise;
firstly, performing frequency domain FFT acquisition, and roughly acquiring the code phase and Doppler frequency shift of a signal by using a parallel code phase search acquisition method.
And S2, capturing the initial code phase and Doppler frequency shift, and further refining the Doppler frequency shift according to the carrier phase difference of n and m moments in a short time (as shown in formula 2).
And S3, thinning the initially captured code phase to obtain an ideal thinned code phase.
And S4, readjusting the local code by using the refined code phase, performing modulo two addition operation on the local code and the original signal, and stripping the C/A code. For high dynamics, the frequency generated by the local code generator is readjusted according to the Doppler shift added to the C/A code, the Doppler shift f added to the L1 carrierdrWith Doppler shift f superimposed on the C/A codedCAThe proportional relationship is as follows:
and S5, performing periodic spectrum estimation (formula 5) on the intermediate frequency carrier plus noise signal (formula 4) obtained after the C/A code is stripped, wherein the frequency corresponding to the maximum value of the obtained spectrum peak is the estimated carrier Doppler frequency shift.
S6, the relationship between the doppler shift and the rate of change of the distance (line of sight LOS) between the receiver and the GPS satellite (i.e., radial velocity) is:
wherein f isrIs the carrier frequency of the satellite signal. As can be seen from the equation (6), the line-of-sight change rate can be obtained by measuring the Doppler shift of the carrier frequencyNamely the observed value of the velocity measurement. According to the line-of-sight change rate equation (shown in formula 7), the least square method is used to solve the line-of-sight change rate equation of n (n is more than or equal to 4) satellites, so as to solve the user speed and the user receiver clock error change rate
Wherein,
is the rate of change of the user receiver clock difference,
is the delay change rate caused by the radio propagation delay error of the ith satellite, etc., because the time interval of the ranging process is very short, which can be ignored.
Simulation experiment
In order to objectively analyze the speed measurement performance of the method, two groups of simulation tests are designed:
in the first group, in order to verify and analyze the accuracy of estimating the Doppler frequency shift and measuring the speed by using a periodogram method under different signal-to-noise ratios and different instantaneous signal lengths, a section of static intermediate-frequency signals are simulated by using Matlab simulation, and for the group of signals, the real code phase and the Doppler frequency shift are exactly known, so that the group of signals can be conveniently compared and analyzed with the measured and estimated values.
The sampling rate of the signal being 5 x 106Hz, an intermediate frequency of 1.25MHz, respectively simulating C/N0If the intermediate frequency sampling signal is from 35dBHz to 49dBHz, when the refined code phase precision reaches 0.01 chip, the Doppler frequency shift is estimated by the method by respectively using the intercepted instantaneous signals with different lengths of 10-30 ms. Monte Carlo simulations are performed 1000 times respectively on signals with different lengths and different signal-to-noise ratios, and the mean square deviations of the Doppler frequency shifts and the true values are shown in FIG. 6: with the increase of the carrier-to-noise ratio, the Doppler frequency shift with higher precision can be estimated by using shorter burst satellite navigation signals.
Second group, simulation experiment is carried out on burst satellite navigation signal speed measurement precision under high dynamic condition by GSS7700GPS simulator of Spirent company in UK
A) Firstly, setting a speed change curve of a receiver on a simulator, as shown in fig. 7;
B) as shown in fig. 8, the original signal simulated by the simulator is accessed to a radio frequency front end, the radio frequency front end uses a GPS receiver OEM board, the radio frequency front end of the OEM board mainly adopts a chip dedicated to GP2015 by Zarlink corporation, and it down-converts the GPS L1 radio frequency signal of 1575.42MHz simulated by the simulator to an analog intermediate frequency of 4.3MHz by 3 times, then samples it with a sampling frequency of 5.714MHz, and quantizes the symbol amplitude of the sampled signal with the precision of 2-bit respectively, and finally obtains two digitized intermediate frequency signals of 1.414MHz including symbol and amplitude. The data acquisition unit adopts a DSP processing system, and the specific chip model is TMS320C6713 (floating point, 200Hz dominant frequency, 1800MIPS) of TI company, and the configuration is to transplant the algorithm to a hardware platform running in real time after the algorithm is successfully simulated in a software simulation environment. And finally, storing the real-time GPS intermediate frequency signals acquired by the DSP to the PC in a data file form by utilizing the simulator through a JTAG test port.
C) Data was continuously acquired for about 40 seconds for 5 satellites. And ephemeris is pre-stored for a period of time.
D) And measuring the speed of the receiver by using a Doppler frequency solving method.
As can be seen from fig. 9, even in the case of high dynamics, a certain accuracy can be achieved by using the burst satellite navigation signal, and if the signal-to-noise ratio of the signal is stronger (which is entirely possible) or the length of the effective signal is longer, the velocity measurement accuracy can be significantly improved.
(3) Navigation parameter calculation principle
The navigation parameter refers to information such as receiver position, speed, and time.
Since a single burst of signals may not contain enough signals (4) for a sufficient number of navigation satellites, in some cases, it may not be sufficient to resolve receiver position, velocity, and time information independently using a single burst of signals. Therefore, the basic ideas and principles of navigation parameter solution are: the position and the speed of the receiver are restrained to a certain degree by utilizing auxiliary information of an inertial navigation IMU assembly, meanwhile, a high-stability frequency source is adopted to maintain time between burst signal pulses received at different moments, and a Kalman filtering method is adopted to solve position, speed and time information from pseudo-ranges and Doppler information obtained from different burst signals.
In the existing research at home and abroad, the inertial navigation unit IMU observation quantity, the satellite navigation pseudo range and the Doppler observation quantity are combined to calculate the navigation parameters of the receiver in a combined manner, so that a plurality of mature results are obtained, and the inertial navigation unit IMU observation quantity can be borrowed in a targeted manner.
Since the clock error (referred to as clock bias) of a satellite navigation receiver drifts in time, it is important to have a relatively accurate time hold between each burst of signal pulses. Considering that the time interval of the arrival of different burst signal pulses at the receiver is generally not too large, and is generally in the order of seconds at most, a better crystal oscillator is adopted, the accuracy of the frequency of the crystal oscillator is calibrated, and it is fully possible to keep the arrival time of each burst signal pulse within 10 nanoseconds within a few seconds (the clock error within 10 nanoseconds does not have obvious influence on the navigation positioning result with general precision requirement). In Kalman filtering, the local clock error drift of the receiver can be used as a parameter to be estimated for resolving, the influence of insufficient observation quantity of pseudo range and Doppler frequency in a single burst signal can be completely overcome, and the navigation parameter can be reliably resolved by using multiple burst signals.
The anti-interference satellite navigation receiver based on the burst signal can meet the application occasion of high dynamics, and can process the fragment signals of CA codes of a GPS L1 frequency band, GLONASS L1 frequency band civil codes and L1 frequency band civil codes of a Beidou second system, so that the requirement of 1Hz real-time positioning frequency is met, the positioning precision is 30m (1 sigma), and the speed measurement precision is superior to the level of 1-2 m/s (1 sigma).
In the aspect of anti-interference, the influence of pulse interference can be basically overcome; for continuous wave interference, the suppression capability for each broadband noise interference source is better than 20dB under the condition of multiple interference sources.
Claims (11)
1. A burst-based navigation signal system, comprising:
A) the signal broadcasting carrier is a navigation satellite, a space balloon or an airship;
B) the navigation signals originate from:
a) burst navigation beacons broadcast by the direct navigation satellite;
b) a ground station of the forwarding type satellite navigation system goes up to a satellite and then transmits a burst type satellite navigation beacon through the satellite;
c) when an enemy carries out intermittent interference on a continuous navigation signal transmitted by a navigation satellite of the enemy, the satellite navigation signal with clean fragments received by a receiver during the intermittent period is interfered;
d) when continuous interference is carried out on a continuous navigation signal transmitted by the navigation satellite of the enemy, the receiver receives a burst satellite navigation signal with better signal-to-interference ratio at the moment when the antenna is aligned with the navigation satellite by rotating the satellite navigation signal antenna;
C) a user terminal:
the high-gain antenna is used for covering the sky in a time-sharing manner in a pitching and azimuth two-dimensional scanning manner, and the satellite within the pointing range of the high-gain antenna has the additional signal-to-interference ratio advantage given by the antenna gain, so that the anti-interference effect is achieved; the anti-interference mode avoids the problem of a large-area phased array antenna in the traditional airspace anti-interference measure, is convenient for simultaneously implementing the time-frequency domain anti-interference measure, is easy to arrange on small and high-mobility weapon platforms, and has low manufacturing cost and high reliability.
2. The burst navigation signal system of claim 1, wherein the burst navigation signal, or "pulse" type, has signal pulses of a duration between about 10 milliseconds and about 1 second; for a direct-type satellite navigation system, the signal system provides another possible approach for realizing navigation signal area enhancement on a navigation satellite: the satellite navigation signal is in the form of bursts and the power amplifier on the satellite only needs to increase the radiated power for the duration of the pulse when the duty cycle of the burst signal is low.
3. The burst navigation signal system of claim 1, wherein a relatively low duty cycle of said burst signal means a duty cycle of about 10%.
4. The burst navigation signal system of claim 1 wherein shorter length, faster rate navigation messages are used, and wherein the satellite navigation signals are in the form of bursts, and wherein the power amplifier on the satellite increases the radiated power for the duration of the pulse, and wherein the navigation messages are transmitted at a faster transmission rate after the satellite signal transmit power is increased.
5. The burst navigation signal system of claim 1, wherein the satellite navigation signal antenna is a high gain antenna with a gain ≧ 5dBic time-shared to cover the sky in elevation and azimuth two-dimensional scanning.
6. A user terminal satellite-based navigation positioning receiving method according to claim 1, wherein the satellite navigation signals received by the burst-type satellite navigation terminal are "bursty" from the perspective of the user terminal, regardless of the signal source; the burst type signal navigation receiver can only measure pseudo range and Doppler frequency according to discontinuous intermittent signals to carry out navigation positioning calculation; the method comprises the following steps:
1) the pseudo-range extraction method comprises the steps of firstly, extracting a spread spectrum code from a received burst satellite navigation signal, and then obtaining a coarse estimation value of a code phase and a Doppler frequency by adopting a traditional serial or parallel search algorithm; on the basis of the obtained coarse estimation value, carrying out fine refinement processing on the code phase to obtain a fine code phase;
2) the Doppler frequency extraction method is used for stripping spread spectrum codes from burst satellite navigation signals from a coarse code phase or a refined code phase, and then calculating the Doppler frequency of the navigation signals by using a classical spectrum estimation or modern spectrum estimation method;
3) the navigation parameter resolving method combines inertial navigation unit (IMU) observed quantity with satellite navigation pseudo-range and Doppler observed quantity to jointly resolve the navigation parameters of the receiver: the position and the speed of the receiver are restrained to a certain degree by utilizing auxiliary information of an inertial navigation IMU assembly, meanwhile, a high-stability frequency source is adopted to maintain time between burst satellite navigation signal pulses received at different moments, and a Kalman filtering method is adopted to resolve the position, the speed and the time information from pseudo-range and Doppler information obtained from different burst satellite navigation signals.
7. The navigation positioning receiving method as claimed in claim 6, wherein the code phase refinement processing is to accurately solve the code phase by using a finer search step and a correlation peak shape curve fitting method based on the coarse estimated code phase and the doppler frequency, the refinement search adopts a ZoomFFT algorithm, and meanwhile, the position integral and instantaneous velocity information given by inertial navigation assistance are used to further reduce the calculation amount of the refinement code phase search
8. The navigation positioning and receiving method of claim 6, wherein the code phase, in case of short code period, due to short code period, there is an integer ambiguity distance corresponding to the code period between the obtained refined code phase and the actual pseudorange, and for GPS and GLONASS civilian code, the ambiguity distance is an integer multiple of 300 km, and the integer multiple must be correctly solved, and the integer multiple is solved by using the code period as a unit to reconstruct the transmission time of the signal.
9. The navigation positioning receiving method according to claim 6, wherein the determination of velocity in the navigation parameters is performed by estimating doppler shift of burst GPS signals using direct periodogram method, calculating line-of-sight change rate, establishing line-of-sight change rate equation, and determining velocity of the receiver.
10. The navigation positioning receiving method according to claim 6, wherein the navigation parameter solution method performs relatively precise time keeping between each burst of satellite navigation signal pulses due to clock errors of the satellite navigation receiver drifting in time, calibrates crystal oscillator frequency accuracy, and keeps each burst of satellite navigation signal pulse arrival time within 10 nanoseconds within a few seconds; in Kalman filtering, receiver local clock error drift is used as a parameter to be estimated for resolving, so that the influence of insufficient observation quantity of pseudo range and Doppler frequency in single burst satellite navigation signals is overcome, and navigation parameters are reliably resolved by using multi-burst satellite navigation signals.
11. The navigation positioning receiving method of claim 6, wherein the doppler frequency estimation and velocity calculation procedure comprises:
s1, performing frequency domain FFT capture on the original intermediate frequency sampling signal, and roughly capturing the code phase and Doppler frequency shift of the signal by using a parallel code phase search capture method;
s2, capturing an initial code phase and Doppler frequency shift, and further refining the Doppler frequency shift;
s3, thinning the initially captured code phase to obtain an ideal thinned code phase;
s4, readjusting the local code by using the code phase refined in the step S3, performing modulo-two addition operation on the local code and the original signal, stripping the C/A code, and readjusting the frequency generated by the local code generator according to the Doppler frequency shift added to the C/A code under the high dynamic condition;
s5, performing periodic spectrum estimation on the intermediate frequency carrier plus noise signal obtained after C/A code stripping, wherein the frequency corresponding to the maximum value of the obtained spectrum peak is the estimated carrier Doppler frequency shift;
s6, solving the line-of-sight change rate equation of n (n is more than or equal to 4) satellites by using a least square method according to the Doppler frequency shift and the line-of-sight change rate equation between the receiver and the GPS satellite to obtain the user speed and the clock error change rate of the user receiver.
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