CN110954926B - GNSS signal rapid capturing method with self-adaptive variable search range - Google Patents

GNSS signal rapid capturing method with self-adaptive variable search range Download PDF

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CN110954926B
CN110954926B CN201911270847.0A CN201911270847A CN110954926B CN 110954926 B CN110954926 B CN 110954926B CN 201911270847 A CN201911270847 A CN 201911270847A CN 110954926 B CN110954926 B CN 110954926B
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satellite
capturing
frequency
data
time
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CN110954926A (en
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毋蒙
许东欢
臧中原
董亮
何浩
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Shanghai Aerospace Control Technology Institute
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S19/00—Satellite radio beacon positioning systems; Determining position, velocity or attitude using signals transmitted by such systems
    • G01S19/01—Satellite radio beacon positioning systems transmitting time-stamped messages, e.g. GPS [Global Positioning System], GLONASS [Global Orbiting Navigation Satellite System] or GALILEO
    • G01S19/13—Receivers
    • G01S19/24—Acquisition or tracking or demodulation of signals transmitted by the system
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S19/00—Satellite radio beacon positioning systems; Determining position, velocity or attitude using signals transmitted by such systems
    • G01S19/01—Satellite radio beacon positioning systems transmitting time-stamped messages, e.g. GPS [Global Positioning System], GLONASS [Global Orbiting Navigation Satellite System] or GALILEO
    • G01S19/13—Receivers
    • G01S19/24—Acquisition or tracking or demodulation of signals transmitted by the system
    • G01S19/29—Acquisition or tracking or demodulation of signals transmitted by the system carrier including Doppler, related
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S19/00—Satellite radio beacon positioning systems; Determining position, velocity or attitude using signals transmitted by such systems
    • G01S19/01—Satellite radio beacon positioning systems transmitting time-stamped messages, e.g. GPS [Global Positioning System], GLONASS [Global Orbiting Navigation Satellite System] or GALILEO
    • G01S19/13—Receivers
    • G01S19/24—Acquisition or tracking or demodulation of signals transmitted by the system
    • G01S19/30—Acquisition or tracking or demodulation of signals transmitted by the system code related
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02D—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN INFORMATION AND COMMUNICATION TECHNOLOGIES [ICT], I.E. INFORMATION AND COMMUNICATION TECHNOLOGIES AIMING AT THE REDUCTION OF THEIR OWN ENERGY USE
    • Y02D30/00—Reducing energy consumption in communication networks
    • Y02D30/70—Reducing energy consumption in communication networks in wireless communication networks

Abstract

The invention provides a GNSS signal rapid capturing method with a self-adaptive variable search range, which is realized based on an FPGA and comprises the following steps: s1, dividing satellites to be captured into three states of out-of-lock heavy capturing, once capturing and not capturing, and estimating Doppler frequency shift in real time; s2, the satellite capturing in the unlocking and heavy capturing state is preferentially unfolded; s3, secondly, expanding the capturing of the satellite in the captured state; s4, finally unfolding the capturing of the satellite in the non-captured state; s5, according to given satellite signs and Doppler frequency shift, combining a preset low-frequency search range/high-frequency search range capturing mode, and selecting a strategy of low-first-high-second or low-first-high-second. Different from the traditional frequency parallel capturing method, the method breaks through the frame limit, expands the Doppler frequency shift searching range, saves logic resources and searching time, and achieves the purpose of self-adaptive capturing of GNSS signals under the condition of different speeds of the missile.

Description

GNSS signal rapid capturing method with self-adaptive variable search range
Technical Field
The invention relates to the technical field of satellite navigation, in particular to a method for quickly capturing a self-adaptive variable search range of a GNSS signal by a missile-borne receiver with extremely high highest speed.
Background
GNSS (Global Navigation Satellite System) is a global navigation satellite system for short, and its positioning principle is that signals of a plurality of satellites are received, navigation messages are obtained after carrier demodulation, pseudo code despreading and message decoding, ephemeris information is extracted from the navigation messages, observed quantities of satellite transmitting time, position, speed, pseudo range and the like are calculated, and then the position and speed of a receiver are calculated. Compared with inertial navigation, satellite navigation has the advantage of no divergence with time, so as to continuously increase the requirements of positioning precision along with the continuous increase of flight time and range, missiles increasingly depend on the combined navigation mode of satellites/inertia.
The primary condition for realizing GNSS positioning is to complete the acquisition of satellite signals, namely, acquire the information such as pseudo code phase, doppler shift and the like of the satellite signals, initialize a tracking loop, and realize stable tracking and subsequent resolving steps of the signals. Parallel capture of multiple channels involves more parallel operations and is therefore typically implemented using FPGAs. For the missile-borne information processing platform with highly integrated functions, the FPGA not only realizes the related functions of satellite navigation, but also completes the functions of information calculation, signal receiving and transmitting, communication interface control and the like, and the logic resources allocated for the capture module are limited. Under the condition, the rapid capturing method based on the segmented accumulation and DFT is a suitable frequency domain parallel searching method, the addition involved in the DFT operation is realized through a time-division multiplexing adder and a multiplier, a large amount of FPGA resources can be saved, and meanwhile, compared with the traditional time domain serial searching method, the method is extremely short in time consumption and has the effect of missile acceleration.
The maximum flying speed of the missile is extremely high, and the Doppler frequency shift caused by the extremely high flying speed is very large, so that compared with a common low-speed receiver, the missile-borne receiver has the following difficulties in GNSS signal capture design:
(1) If the search range is improperly set, the maximum Doppler frequency shift generated by missile motion is not covered, and the acquisition module gives out wrong frequency information due to the spectrum aliasing effect, so that a loop cannot work;
(2) When the rapid capturing method of sectional accumulation and DFT is adopted, the frequency resolution is limited to be in excess of the traction capacity of the frequency locking ring, and the constraint on the data storage length is implied; when the DFT operation adopts a time-sharing multiplexing method, each time a point is accumulated, DFT addition/multiplication operation related to the point is performed, and the occupied addition/multiplication unit is released before the calculation of the next point is completed, so that the constraint of the data storage length and the accumulated point number is implied; two constraint conditions determine the maximum frequency search range, however, the high speed of the missile leads the maximum Doppler to exceed the range and needs to be solved by adopting a reasonable design method;
(3) When the missile flies, the direction of the antenna is changed due to rolling and pitching, so that temporary unlocking of certain satellites is caused, and a reasonable capturing strategy is required to be designed to capture the satellites preferentially.
The GNSS signal quick capture technology with the self-adaptive variable search range is mainly used for quick capture of GNSS signals under the condition that FPGA resources are relatively limited, capture time margin is small, doppler frequency shift is higher than the maximum search range under the existing frame, wherein the FPGA resources are caused by high integration of missile-borne information processing, and the capture time margin is small due to large missile dynamics. At present, most domestic researches are carried out on a simple satellite receiver, and the method is different from a missile-borne comprehensive information processing platform carried by the method, is not limited by FPGA resources, and is not applicable to the use scene of the method because the research direction is mainly focused on introducing various auxiliary information to shorten the capture time, avoid repeated searching and the like.
Disclosure of Invention
The technical problems to be solved by the invention are as follows: the GNSS signal rapid capturing method with the adaptive variable search range is provided, and has the characteristics of low FPGA resource consumption, wide capturing range and short capturing time.
In order to solve the above problems, the present invention provides a method for quickly capturing GNSS signals with a self-adaptive variable search range, comprising the following steps:
s1, dividing a satellite to be captured into three states of lock losing recapture, once capturing and non-capturing according to actual positioning resolving conditions, and estimating Doppler frequency shift of a satellite signal on a receiver in real time;
s2, the capturing of the satellite in the out-of-lock recapture state is preferentially unfolded, and the satellite signals and Doppler frequency shift are sent to a capturing module to enable capturing;
s3, secondly, expanding the capturing of the satellite in the captured state, and sending satellite signals and Doppler frequency shift into a capturing module to enable capturing;
s4, finally, the capturing of the satellite in the non-captured state is unfolded, and the satellite signals and Doppler frequency shift are sent to a capturing module to enable capturing;
s5, combining two preset capture modes according to given satellite signs and Doppler frequency shift: 1. in the range ofLow frequency acquisition mode of (a); 2. in the range of->Is a high frequency acquisition mode of (a). Wherein N is the data length after segmented accumulation and is the power of 2, f rl /f rh The frequency resolution in the low frequency/high frequency mode, respectively, decides to take a low-first-high-last or high-first-low-last strategy to start acquisition of the satellite.
Further, in the step S1, the state of the satellite to be acquired is defined in detail as follows:
and (5) losing lock and heavy catching: the receiver is in a positioning state, the time information is effective, and the time of the last participation of the satellite in positioning is not more than 10s from the current time;
the once captured state: the receiver is in a positioning state, the time information is valid, the satellite does not belong to a lock-out recapture state, but the ephemeris is stored and still is in a valid period;
an uncaptured state: neither lock loss heavy acquisition nor satellite that was acquired.
Further, in the step S1, the method for estimating the doppler shift of the satellite signal at the receiver is as follows:
losing lock weight and capturing satellite:
wherein f e For Doppler shift, c is the speed of light, f G Is the carrier frequency, x, y, z and v x ,v y ,v z The position and the speed of the receiver at the current moment under the EFEC coordinate system are respectively; x is x r ,y r ,z r And v xr ,v yr ,v zr The position and the speed of the satellite under the EFEC coordinate system when the satellite participates in positioning last time are respectively;
satellite was acquired:
wherein x is t ,y t ,z t And v xt ,v yt ,v zt The estimated position and velocity of the satellite based on the stored ephemeris information and the current time t;
an uncaptured state: if the receiver is currently in a positioning state, then
Otherwise, f e ＝0 (4)
Further, the step S5 includes the following steps:
s51, judging f e If in the size ofIf the internal policy selection signal ST is set to 1, the policy of low before high is adopted, otherwise, ST is set to 0, and the policy of high before low is adopted. Simultaneously setting the acquisition time indication signal SD to 0;
s52, performing deceleration processing on the input I, Q zero intermediate frequency signals to obtain Ic and Qc zero intermediate frequency data consistent with the pseudo code rate, wherein the steps are always executed;
s53, storing Ic and Qc data in real time. If st=1, store T coh L length data, L is the number of chips of one pseudo code period (1 ms), T coh Is the data accumulation time in ms and the end is added with 0 to obtain the common valuePoint data Ica, qca, wherein ceil is an upward rounding operation; if st=0, store +.>Length data, end 0 is added to get total ∈0>Point data Ica, qca;
s54, initializing search code phase to 0, taking phase as initial phase, reading the satellite pseudo code stored locally, and then accumulating with Ica and Qca in a correlated way to obtain despread D orDot data Ip, qp. Then, carrying out sectional accumulation on Ip and Qp: if st=1, one data is accumulated per M points, < >>If ST=0, every +.>The dots accumulate a data. The accumulated result of N points can be obtained to form a complex signal to be analyzed
x(n)＝I acum (n)+j·Q acum (n) n＝0:1:N-1 (6)
S55, performing DFT required operation once every accumulated value x (n) is obtained in the step S54. After all DFT operations are finished, updating the maximum amplitude and related information;
s56, circularly executing the steps S53-S55 until the search code phase in the step S54 is traversed from 0 to L-1, and obtaining the DFT average amplitude. If SD is 0, the process proceeds to S57, otherwise, the process proceeds to S58;
s57, if the ratio of the maximum amplitude value to the average amplitude value exceeds the threshold of the mode, judging that the satellite capturing is completed, waiting for a next capturing instruction and sending out related information; if the threshold is not exceeded, the ST is reversed, the SD is set to be 1, and the steps S53-S56 are executed again;
s58, if the ratio of the maximum amplitude value to the average amplitude value exceeds the threshold of the mode, judging that the satellite capturing is completed, waiting for a next capturing instruction and sending out related information; if the threshold is not exceeded, the acquisition of the satellite fails, and the next acquisition instruction is waited.
Further, in the step S5, the frequency resolution calculating method includes:
further, in the step S54, a data accumulation time T coh The selection method of N is as follows:
known missile maximum velocity v d And may be considered approximately flying at the earth's surface (relative to satellite orbit altitude), the maximum doppler shift may be estimated as follows:
wherein v is G R is the estimated value of the average speed of the satellite e Taking 6378km, R for the earth radius s An estimated distance from the earth center for the satellite orbit;
known frequency-locking ring traction range is + -f ll The method comprises the following steps: "the high frequency search range must be greater than the maximum Doppler; maximum degree of ambiguity of frequency (f rh Half of (f) rl ) The pulling range of the frequency locking ring is smaller than that of the frequency locking ring; t (T) coh The bit flip probability should be made smaller than a predetermined value sigma; and M is not less than N' four aspects to carry out constraint:
f rh ＜f ll (9)
f(T coh )＜σ (10)
M≥N (11)
wherein f (T) coh ) Is formed by T coh The bit-flipping probability function is selected to be proper T according to the situation under the four constraints coh And N.
Further, the step S55 includes the steps of:
s551, setting a ROM with depth of N, storing two paths of signals of sin and cos with period of N, and forming fundamental wave signals of each frequency required for DFT operation:
meanwhile, setting the dual-port RAMr and RAMi with the depth of N, and storing real and imaginary parts in DFT calculation.
S552. initializing ROM read address addr to 0, initializing address step index to 0 if st=1, initializing index to 0 if st=0
S553, temporarily accumulating the value x (n), and starting operation: read corresponding data ROM in ROM cos (addr)/ROM sin (addr) and x (n) are complex multiplied.
S554, when the next clock rising edge comes, the ROM read address is updated: addr= (addr+index) mod (N), mod being a remainder operation.
S555, circularly executing the steps S553 to S554. If st=1, then N times are performed, and after N clocks, N complex products are obtained:
if st=0, then executeSecondary, experience->Clock, get->The complex product of:
S556. reading accumulated results stored in the dual-port RAMr and RAMiRespectively with the real part Re (X (k) n ) And imaginary part Im (X (k)) n ) Adding to obtain +.>Writing into the dual-port RAMr and RAMi for updating; at the same time, the address index index=index+1 is updated, and the process goes to step S553.
S557, circularly executing the steps S553 to S556 for N times to obtain a final DFT calculation result X (k) k＝0:1:N-1 (st=1)/(ST.)(st=0).
S558, updating DFT average amplitude A mean Calculating the maximum amplitude of the DFT result, and if the maximum amplitude is larger than the recorded value, updating the maximum amplitude A max Doppler shift (corresponding to dual port RAM address), search code phase.
Drawings
FIG. 1 is a schematic diagram of a frequency search range according to an embodiment of the present invention;
FIG. 2 is a DFT calculation flow diagram in a low frequency search mode of an embodiment of the invention;
fig. 3 is a DFT computation flow chart in a high frequency search mode according to an embodiment of the present invention.
Detailed Description
The following describes the embodiments of the present invention in more detail with reference to the accompanying drawings.
As shown in fig. 1, the present invention presets two capture modes: a low frequency search range mode and a high frequency search range mode. The low-frequency mode finally gives out N-point frequency information; high frequency mode presentationDot frequency information, and uncomputed +.>The dot frequency information is substantially identical to the N dot frequency information range of the low frequency mode. Therefore, the search ranges of the two are complementary, all possible Doppler frequency shift points are covered, and a capture strategy of firstly low and then high or firstly high and then low is adopted according to actual conditions, so that the capture time can be shortened;
when the current tracking channel is detected to be idle, starting a capturing process, and arranging the priority of the satellites to be captured from high to low according to the states of losing lock and re-capturing, capturing once and not capturing, wherein the satellite numbers and Doppler frequency shift estimated values of the satellites are sent to a capturing module;
if the frequency shift estimation value is within the low frequency search range, ST is set to 1, otherwise ST is set to 0, and the SD signal is set to 0.
The input zero intermediate frequency data I, Q is processed by deceleration, firstly, the pseudo code NCO with fixed step length is set, and the step length is thatWherein f s For the system clock frequency, f c For the pseudo code rate, m is the accumulator bit width, thereby generating a pulse enable signal f consistent with the pseudo code rate c_en ；
Next, at f c_en Accumulate I, Q when invalid, f c_en And when the pseudo code rate is effective, outputting an accumulation result, and clearing an accumulator to obtain the Ic and Qc data after the speed reduction consistent with the pseudo code rate.
Storing data to be analyzed, setting depth asL is the number of chips of one pseudo code period (1 ms), T coh For the data accumulation time in ms, N is the number of points after data accumulation, ceil is the rounding-up operation. If ST is 1, then at f c_en When effective, ic and Qc are circularly written into the front T of the RAM in real time coh L-segment interval; if ST is 0, write before +.>Segment areaA compartment;
at f s The data are read out from the RAM again at the speed, and if ST=1, all the data of the D point are sequentially read out; if ST is 0, the data are read out sequentiallyPoint data;
meanwhile, according to satellite signals, the search code phase is initialized to 0, and the satellite pseudo codes stored in the local ROM are circularly read out by taking the search code phase as an initial phase (ROM read address);
and carrying out sectional correlation accumulation on the local pseudo code and the read Ic and Qc data. If st=1, then everyAccumulating a data by the points; if ST=0, every +.>The dots accumulate a data. The accumulated results of N points can be obtained to form a complex signal to be analyzed:
x(n)＝I acum (n)+j·Q acum (n) n＝0:1:N-1 (1)
performing DFT operation on x (N), analyzing frequency information, and storing sin and cos signals with a period of N in a ROM with a depth of N to form fundamental wave signals of each frequency required during operation; setting a dual-port RAMr and a dual-port RAMi with the depth of N at the same time, and storing real part and imaginary part results in calculation;
the ROM read address addr is initialized to 0, if ST is 1, the address step index is initialized to 0, and if ST is 0, the index is initialized to
For example, the n+1th accumulated value x (n) is currently calculated, the ROM data ROM is read cos (addr)/ROM sin (addr) performing a complex multiplication operation with x (n);
when the next clock rising edge comes, the ROM read address is updated: addr= (addr+index) mod (N), mod being a remainder operation;
the last step of loop calculation is executed for N times if ST is 1; if ST is 0, then executeSecondary, in->After a clock period, get ∈>The complex product of:
reading accumulated results stored in RAMr and RAMiRespectively with the real part Re (X (k) n ) And imaginary part Im (X (k)) n ) Adding to obtain +.>Writing the address corresponding to the dual-port RAMr and the RAMi again to finish updating; updating the address index at the same time, index=index+1;
after the above six steps are circularly executed for N times, the whole accumulated result x (N) is obtained n＝0:1:N-1 N-point DFT calculation result X (k) k＝0:1:N-1 (st=1);point calculation results->(st=0);
calculate allThe X (k) amplitude of the point is obtained to obtain the maximum amplitude of the result (when the search code phase is phase), if the maximum amplitude is larger than the previously stored maximum amplitude, the maximum amplitude A is obtained max The Doppler shift point and the search code phase (phase) are updated. At the same time, the average amplitude A is recalculated mean The method comprises the steps of carrying out a first treatment on the surface of the The search code phase update phase=phase+1.
After the above twelve steps are circularly executed for L times, all code phases 0:1:L-1 are traversed to obtain all N.L code phasesMaximum amplitude A of DFT result max And average amplitude A mean ；
St=1, if a max ＞Td l ·A mean Wherein Td is l If the satellite is the capture discrimination threshold of the low-frequency range mode, the satellite is declared to be successfully captured, and a next capture command is waited, otherwise, ST is inverted, SD is taken to be 1, and a high-frequency capture flow is started; st=0, dividing the threshold Td h The other steps are the same;
if sd=1, the acquisition is still unsuccessful, and the acquisition of the satellite is declared to be failed. And starting the capturing process of the next satellite to be captured until all channels are in a non-idle state.
Parts of the invention not set forth in detail are well known in the art. Although embodiments of the present invention have been described above with reference to the accompanying drawings, variations and modifications may be effected by one skilled in the art within the scope of the claims.

Claims (6)

1. The GNSS signal rapid acquisition method with the adaptive variable search range is characterized by comprising the following steps of:
s1, dividing a satellite to be captured into three states of lock losing recapture, once capturing and non-capturing according to actual positioning resolving conditions, and estimating Doppler frequency shift of a satellite signal on a receiver in real time;
s2, the capturing of the satellite in the out-of-lock recapture state is preferentially unfolded, and the satellite signals and Doppler frequency shift are sent to a capturing module to enable capturing;
s3, secondly, expanding the capturing of the satellite in the captured state, and sending satellite signals and Doppler frequency shift into a capturing module to enable capturing;
s4, finally, the capturing of the satellite in the non-captured state is unfolded, and the satellite signals and Doppler frequency shift are sent to a capturing module to enable capturing;
s5, according to given satellite signs and Doppler frequency shift, two capturing modes are preset in combination: 1. in the range ofLow frequency acquisition mode of (a); 2. in the range of->And (3) withA high frequency acquisition mode of (2); wherein N is the data length after segmented accumulation and is the power of 2, f rl /f rh The frequency resolution in the low frequency/high frequency mode, respectively, decides to take a low-first-high or high-first-low strategy, starts to acquire the satellite,
the step S5 is realized in the FPGA, and the switching of the two modes can be realized by changing the accumulation length and the index address, the logic resource is multiplexed, the hardware cost is reduced, and the method comprises the following steps:
s51, judging f e Size f (f) e For estimating Doppler shift of satellite signals at receiver, if atIf the internal policy selection signal ST is 1, the policy selection signal ST indicates that a low-first-high policy is adopted, otherwise, the policy selection signal ST is 0, and the policy selection signal ST indicates that a high-first-low policy is adopted; simultaneously setting the acquisition time indication signal SD to 0;
s52, performing deceleration processing on the input I, Q zero intermediate frequency signals to obtain Ic and Qc zero intermediate frequency data consistent with the pseudo code rate, wherein the steps are always executed;
s53, storing Ic and Qc data in real time; if st=1, store T coh L length data, L is the number of chips of one pseudo code period, T coh Is the data accumulation time in ms and the end is added with 0 to obtain the common valuePoint data Ica, qca, wherein ceil is an upward rounding operation; if st=0, store +.>Length data, end 0 is added to get total ∈0>Point data Ica, qca;
s54, initializing search code phase to 0, taking phase as initial phase, reading the satellite pseudo code stored locally, and then accumulating with Ica and Qca in a correlated way to obtain despread D orDot data Ip, qp; then, carrying out sectional accumulation on Ip and Qp: if st=1, one data is accumulated per M points, < >>If ST=0, every +.>Accumulating a data by the points; therefore, the accumulated result of N points can be obtained to form a complex signal to be analyzed
x(n)＝I acum (n)+j·Q acum (n) n＝0:1:N-1 (5)
S55, each time an accumulated value x (n) is obtained in step S54, M or M is consumedDuring the period, the DFT operation is performed, and the calculation is completed before the next accumulated value x (n+1) comes, so as to realize the time division multiplexing of the adder/multiplier, thus M is larger than or equal to N; after the DFT operation is finished, updating the maximum amplitude and related information;
s56, circularly executing the steps S53-S55 until the search code phase in the step S54 is traversed from 0 to L-1, and obtaining the DFT average amplitude; if SD is 0, the process proceeds to S57, otherwise, the process proceeds to S58;
s57, if the ratio of the maximum amplitude value to the average amplitude value exceeds the threshold of the mode, judging that the satellite capturing is completed, waiting for a next capturing instruction and sending out related information; if the threshold is not exceeded, the ST is reversed, the SD is set to be 1, and the steps S53-S56 are executed again;
s58, if the ratio of the maximum amplitude value to the average amplitude value exceeds the threshold of the mode, judging that the satellite capturing is completed, waiting for a next capturing instruction and sending out related information; if the threshold is not exceeded, the acquisition of the satellite fails, and the next acquisition instruction is waited.
2. The method for quickly acquiring GNSS signals with adaptive variable search range according to claim 1, wherein in step S1, the states of the satellites to be acquired are defined in detail as follows:
and (5) losing lock and heavy catching: the receiver is in a positioning state, the time information is effective, and the time of the last participation of the satellite in positioning is not more than 10s from the current time;
the once captured state: the receiver is in a positioning state, the time information is valid, the satellite does not belong to a lock-out recapture state, but the ephemeris is stored and still is in a valid period;
an uncaptured state: neither lock loss heavy acquisition nor satellite that was acquired.
3. The method for fast acquisition of GNSS signals with adaptive variable search range according to claim 1, wherein in step S1, the method for estimating doppler shift of satellite signals at the receiver is as follows:
satellite in lock-out heavy-catching state:
wherein f e For Doppler shift, c is the speed of light, f G Is the carrier frequency, x, y, z and v x ，v y ，v z The position and the speed of the receiver at the current moment under the EFEC coordinate system are respectively; x is x r ，y r ，z r And v xr ，v yr ，v zr The position and the speed of the satellite under the EFEC coordinate system when the satellite participates in positioning last time are respectively;
satellite that acquired state:
wherein xt, y t ，z t And v xt ，v yt ，v zt The estimated position and velocity of the satellite based on the stored ephemeris information and the current time t;
an uncaptured state: if the receiver is currently in a positioning state, then
Otherwise, f e ＝0 (4)。
4. The method for fast capturing GNSS signals with adaptive variable search range according to claim 1, wherein the frequency resolution calculation method in step S5 is as follows:
wherein f c Is the pseudo code rate.
5. The method for fast acquisition of GNSS signals with adaptive variable search range according to claim 1, wherein in said step S54, the data accumulation time T coh The selection method of N is as follows:
known missile maximum velocity v d And can be approximately considered to fly on the earth, the maximum doppler shift is estimated as follows:
wherein v is G R is the estimated value of the average speed of the satellite e Taking 6378km, R for the earth radius s An estimated distance from the earth center for the satellite orbit; known frequency-locking ring traction range is + -f ll The method comprises the following steps: "the high frequency search range must be greater than the maximum Doppler; the maximum degree of ambiguity of frequency is f rh Half of (f) rl The traction range of the frequency locking ring is smaller than that of the frequency locking ring; t (T) coh The bit flip probability should be made smaller than a predetermined value sigma; and M is not less than N' four aspects to carry out constraint:
f rl <f ll (9)
f(T coh )<σ (10)
M≥N (11)
wherein f (T) coh ) Is formed by T coh The bit-flipping probability function is selected to be proper T according to the situation under the four constraints coh And N.
6. An adaptive variable search range as claimed in claim 1In step S55, in the low frequency mode, complete DFT operation is performed to obtain N-point frequency information; in the high frequency mode, only half DFT operation is performed to obtainThe upper half frequency information includes the following steps:
s551, setting a ROM with depth of N, storing sin and cos signals with period of N to form fundamental wave signals of various frequencies required for DFT operation
Setting a dual-port RAMr and a dual-port RAMi with the depth of N at the same time, and storing real and imaginary values in DFT calculation;
s552. initializing ROM read address addr to 0, initializing address step index to 0 if st=1, initializing index to 0 if st=0
S553, temporarily accumulating the value x (n), and starting operation: read corresponding data ROM in ROM cos (addr)/ROM sin (addr) performing a complex multiplication operation with x (n);
s554, when the next clock rising edge comes, the ROM read address is updated: addr= (addr+index) mod (N), mod being a remainder operation;
s555, circularly executing the steps S553 to S554; if st=1, then N times are performed, and after N clocks, N complex products are obtained:
if st=0, then executeSecondary, experience->Clock, get->The complex product of:
s556, reading accumulated results stored in RAMr and RAMiRespectively with the real part Re (X (k) n ) And imaginary part Im (X (k)) n ) Adding to obtain +.>Writing into the dual-port RAMr and RAMi for updating; updating the address index index=index+1 at the same time, and jumping to step S553;
s557, circularly executing the steps S553 to S556 for N times to obtain a final DFT calculation result
When st=1, X (k) k＝0:1:N-1 When st=0, the first time period, when st=0,
s558, updating DFT average amplitude A mean Calculating the maximum amplitude of the DFT result, and if the maximum amplitude is larger than the recorded value, updating the maximum amplitude A max Doppler shift.
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