A Ground Target Positioning Method Based on Doppler Effect

Abstract

The present invention provides a ground target positioning method based on the Doppler effect, including obtaining high-precision Doppler data; determining the satellite position information corresponding to the message capture time; performing ground target Doppler positioning according to the obtained message signal frequency and the solved satellite position, using the least squares algorithm constrained by the surface elevation to complete the target position positioning solution; determining the ground terminal signal transmission frequency according to the same ground terminal message captured by multiple satellites, correcting the inter-satellite measurement frequency according to the frequency difference solved by different satellites for the same target, and performing multi-satellite data fusion; using the multi-satellite fusion data to apply the Doppler positioning algorithm to perform secondary positioning of the target position. The present invention realizes ground target position positioning through high-precision time scale design, high-precision frequency measurement, and inter-satellite frequency difference correction, so that the method improves the timeliness and accuracy of positioning without relying on inter-satellite clock synchronization.

Description

Technical Field

[0001] The invention belongs to the technical field of satellite message collection, and in particular relates to a ground target positioning method based on the Doppler effect. Background Art

[0002] Nowadays, with the intensification of human activities, people's pursuit and cognition of data are becoming more and more popular, and more and more people are paying attention to it. However, the research on satellite data collection in special environments is not sufficient, especially in some large-connection, low-power data transmission, the satellite data collection system is still in its infancy. The traditional manual method of obtaining data has the characteristics of high cost, high risk and low efficiency. Therefore, data collection for satellite links (such as marine remote sensing information, animal migration information, etc.) has become a development trend in various countries.

[0003] At present, the detection of ocean and polar environments at home and abroad is mainly achieved through scientific research ships, and animal migration is mainly obtained by manual tracking. The current data acquisition has the defects of small range and high cost. The use of satellite-borne receivers to effectively receive data can make up for the above defects, but the existing data collection satellites are prone to interference for large-scale and wide-coverage signal collection, and the success rate is not high.

[0004] Since satellite-borne data collection is subject to many limitations, it is necessary to provide a method for locating ground targets and solve the signal interference problem caused by the large-scale and wide-coverage signal collection of data collection satellites. Summary of the invention

[0005] In order to overcome the deficiencies in the prior art, the inventors have conducted intensive research and provided a ground target identification and positioning method based on Doppler effect positioning. A frequency division multi-channel method is designed according to the wide coverage characteristics to achieve data collision separation. The method can provide different scene applications such as environmental protection, marine monitoring, agriculture, shipping, wildlife protection, etc., and provide global surveillance data and locations, which has broad promotion prospects and application value.

[0006] The technical solution provided by the present invention is as follows:

[0007] A ground target positioning method based on the Doppler effect comprises the following steps:

[0008] S1, the satellite receiver uses FFT to capture the carrier signal before the message, and then sends the signal to be demodulated to the back-end frequency-locked loop + phase-locked loop for convergence tracking to obtain the signal frequency emitted by the target to be located;

[0009] S2, using the whole satellite second pulse + receiver internal nanosecond count to obtain the time as the final output of the message capture time, and determining the satellite position information corresponding to the message capture time according to the message capture time and satellite orbit data;

[0010] S3, based on the obtained message signal frequency and the solved satellite position, the least squares algorithm with surface elevation constraint is used to perform ground target Doppler positioning, complete the target position positioning solution, and output the target position and transmission frequency;

[0011] S4, according to the same ground terminal message captured by multiple satellites, the ground terminal signal transmission frequency is determined respectively, and the inter-satellite measurement frequency is corrected according to the frequency difference calculated by different satellites of the same target, and multi-satellite data fusion is performed;

[0012] S5, uses multi-satellite fusion data to apply the Doppler positioning algorithm to perform secondary positioning of the target position.

[0013] A ground target positioning method based on the Doppler effect provided by the present invention has the following beneficial effects:

[0014] The present invention provides a ground target positioning method based on the Doppler effect, satellite Doppler ground target positioning, which has high-precision time scale and high-precision frequency measurement, completes multi-satellite data fusion processing without being based on inter-satellite clock unification, can support shorter time and higher-precision positioning, and improve data application efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 A flow chart of a ground target positioning method based on the Doppler effect of the present invention;

[0016] Figure 2 Design schematic diagrams for system time scales;

[0017] Figure 3 This is the least squares Doppler positioning solution flow chart;

[0018] Figure 4 This is the flow chart of multi-satellite network positioning solution. DETAILED DESCRIPTION

[0019] The following detailed description of the present invention will make the features and advantages of the present invention more clear and explicit.

[0020] The word "exemplary" is used exclusively herein to mean "serving as an example, embodiment, or illustration." Any embodiment described herein as "exemplary" is not necessarily to be construed as preferred or advantageous over other embodiments. Although various aspects of the embodiments are shown in the drawings, the drawings are not necessarily drawn to scale unless otherwise noted.

[0021] The present invention provides a ground target positioning method based on the Doppler effect, such as Figure 1 As shown, the following steps are included:

[0022] (1) Obtain the signal frequency emitted by the target to be located: Use FFT (Fast Fourier Transform) to capture the pre-message carrier signal, and then send the demodulated signal to the back-end loop for convergence tracking. The loop uses a frequency-locked loop + phase-locked loop. Due to the large initial frequency deviation, the loop first works in the frequency-locked loop to quickly complete the frequency lock, and then switches to the phase-locked loop to complete precise phase synchronization and tracking. This architecture can significantly shorten the convergence time.

[0023] During the message capture phase, a 256-point FFT is used to capture the carrier frequency. The data sampling rate is designed to be 80kHz, the number of FFT sampling points is 256, and the frequency resolution is 80kHz / 256=312.5Hz (±156.25Hz). A frequency-locked loop is used to assist a phase-locked loop in frequency tracking to achieve rapid convergence.

[0024] To reduce measurement errors, a frequency-locked loop + phase-locked loop is used to achieve carrier synchronization and measure the signal frequency at least three times in succession. The measurement interval is tens of milliseconds. Multiple measurements reduce the error of a single measurement, and the loop output error does not exceed 5Hz, thereby improving measurement accuracy.

[0025] Since the signal frame header is a specific fixed sequence, the satellite receiver uses the local training sequence to perform correlation calculations with the received signal. The calculation formula is as follows:

[0026] Δ total =∑(treSeq(nT) treSeq * ((n+1)T))·(data(nT)·data * ((n+1)T))

[0027] Where treSeq(nT) represents the local training sequence; treSeq * ((n+1)T) represents the conjugate of the training sequence delayed by 1 code element; data(nT) represents the signal to be demodulated; data * ((n+1)T) represents the signal to be demodulated which is delayed by one symbol; n=1…31; T represents the symbol period.

[0028] The above formula is used to calculate the correlation of the matched received signal. When the signal energy and the matching peak value reach the specified threshold, the signal capture is considered to be completed.

[0029] (2) The time obtained by using the whole satellite second pulse + the receiver's internal FPGA nanosecond count is used as the final output of the message capture time. According to the message capture time and the satellite orbit data, the satellite position information corresponding to the message capture time is determined.

[0030] Figure 2A schematic diagram of the time scale design of the data collection system inside the receiver is given. The whole satellite second pulse interface is used inside the system. The data collection system receives the second pulse signal in the whole second. After the baseband board (on which FPGA+DSP chips are arranged) receives the hardware second pulse signal, the baseband FPGA clears the local nanosecond count Tns (accuracy ±19.8 nanoseconds). The whole satellite directionally outputs the second pulse time code within 100ms of the second pulse signal and is received by the data collection system. After receiving the second pulse, the interface board of the data collection system communicates with the baseband board at a fixed time of 300ms (T1 time) to transmit the second pulse time to the baseband board.

[0031] The baseband FPGA successfully captures a message at time T2. The internal nanosecond count Tns is latched at the time of message capture (synchronously outputting the first Doppler signal). The message duration varies from 360ms to 920ms. Therefore, after the message is captured at time T2, the baseband FPGA completes message demodulation at time T3 (T3') and the demodulated data is read by the baseband DSP. The message capture time T is recorded while reading the message. The working conditions are as follows:

[0032] (a) If the second pulse signal and inter-board communication data are not received in sequence during the period from T2 message capture to T3 message framing, it is considered that there is no cross-second output. At this time, the Tmark signal is invalid and is 0. The message capture time is the baseband board maintenance time + nanosecond count, that is, T2 = 2 + Tns-Tmark = 2 + Tns, and the time mark is accurate;

[0033] (b) During the period from T2 message capture to T3' message framing, if the second pulse signal and inter-board communication data are received in sequence, it is considered to be a cross-second output. At this time, the Tmark signal is 1, and the message capture time is the baseband board maintenance time + nanosecond count, that is, T2=3+Tns-Tmark=2+Tns, and the time mark is accurate.

[0034] (3) Based on the obtained message signal frequency and the solved satellite position, the least squares algorithm with surface elevation constraint is used to perform ground target Doppler positioning, complete the target position positioning solution, and output the target position and transmission frequency.

[0035] The satellite Doppler signal calculation formula is:

[0036]

[0037] Where F is the frequency measured by the satellite, f is the transmission frequency, and v s is the satellite velocity, v b is the speed of the target (negligible compared to the satellite speed), c is the speed of light, r s is the satellite position, r b is the target position, the coordinate system is the inertial system, and ε is the measurement noise. The satellite position and speed can be calculated based on the time code, and the unknown quantity is X = (rbx 、r by 、r bz , f), a total of 4 unknown quantities, r b =(r bx ,r by ,r bz ) is the target location.

[0038] Theoretically, by demodulating the same target message N times (N≥4) and combining the Doppler calculation formula N times, the unknown quantity can be solved. Since three consecutive measurements of the received signal can effectively reduce the measurement error, the measurement noise is considered to be 0; the satellite moving speed is greater than 7km / s, and the ground target moves at a low rate relative to the satellite. The ground target defaults to v b Ignore and set to 0, the target will not move position r in a short period of time b constant.

[0039] In the above formula, v s 、r s , F is known; the ground target position r b =(r bx ,r by ,r bz ) is unknown, the transmission frequency f is unknown, a total of 4 unknown variables. Let X = (r bx 、r by 、r bz , f), the equation contains 4 unknown variables. Theoretically, 4 measurements are performed to form a system of equations to solve the unknown variable values:

[0040]

[0041] Due to the existence of measurement errors, the above formula cannot find a unique solution, so the least squares algorithm is used to achieve iterative convergence of the position variables, such as Figure 3 As shown, the method is as follows:

[0042] The Doppler calculation formula F is derived from X to obtain the partial derivative matrix Then the observed value and the calculated theoretical value satisfy the following linear equation:

[0043]

[0044] where F 观测 is the observed value, F 理论 is the theoretical value calculated based on the relative position.

[0045] Due to the measurement errors in position measurement, the small displacement of ground targets during multiple measurements, and the influence of the spatial distribution of the satellite position of the received message on positioning, in order to make the measurement results more accurate, the surface constraint equation about the target position is added. The target position can be limited to the surface of the earth, so that the solution results are further converged. The constraint equation is added to approximate the surface of the earth as an ellipsoid, and the relationship is as follows:

[0046]

[0047] Among them, a=b=6378.140km, c=6356.755km.

[0048] Linearizing the above constraint equations yields

[0049]

[0050] in, remember

[0051] Given the transmission frequency f, the initial value of ground target information X = (r bx , r by , r bz , f), then according to the Doppler calculation formula, F 理论 Known, It is known that F 观测 If the satellite measurement value is known, the correction value of the parameter to be solved under the constraint condition is:

[0052] dX=ΔX-(G T G) -1 *H T [H(G T G) -1 H T )] -1 *(HΔX-a) (5)

[0053] Update X after calculating dX k =X k-1 +dX continues to iterate until dX is smaller than a certain threshold.

[0054] The above process steps are summarized as follows:

[0055] Step 1, given the initial value condition X0 = (r bx0 、r by0 、r bz0 , f0);

[0056] Step 2, the theoretical value of the receiving frequency is given by the Doppler formula;

[0057] Step 3, calculate the difference dX between the observed value and the theoretical value;

[0058] Step 4, calculate the correction value X k =X k-1 +dX, k is the number of iterations;

[0059] Step 5: If the difference between the new and old positions is less than the specified value ε, the iteration is completed, otherwise return to Step 2;

[0060] The iterative calculation process can be set according to the actual situation (such as 200 times). If the iteration does not converge, it will exit and the current positioning solution will fail.

[0061] (4) Based on the same ground terminal message captured by multiple satellites, the ground terminal signal transmission frequency is determined respectively, and the inter-satellite measurement frequency is corrected according to the frequency difference calculated by different satellites of the same target, and multi-satellite data fusion is performed.

[0062] Figure 4 The multi-satellite network Doppler positioning solution process is given, in which the single-satellite data positioning solution process is called:

[0063] According to the input satellite orbit data (PVT data, updated every second / time) and the data collection system measurement data (measurement frequency and receiving time code), the satellite position at the time of message capture is fitted. After the data fitting is completed, the target information is solved according to the Doppler positioning algorithm (including the target position r b and transmission frequency f), and finally determine the output level of the positioning solution according to the true value of the ground target or the approximate true value of the target statistically in the system library, and output the software operation report (see steps 1 to 3).

[0064] The positioning accuracy depends on the displacement of the ground target during N measurements. For ground targets moving at low speed, the shorter the time, the smaller the displacement. Therefore, multi-satellite fusion data positioning can be used to improve positioning accuracy. However, the inter-satellite receiving system has no clock error calibration mechanism, so the clocks of different satellites are not unified, resulting in inconsistent measurement frequencies of the same signal. Therefore, a ground calibration target can be used to calibrate the inter-satellite frequency difference.

[0065] After multiple satellites are networked, different satellites solve the same target to obtain the transmission frequency f. Since the transmission frequency of the same target will not change significantly in a short period of time after configuration, the transmission frequency deviation is caused by measurement error and system clock inconsistency. The measurement error is caused by using the average transmission frequency f obtained by the positioning of each satellite as the measurement result. The average of multiple measurements can eliminate the error of a single measurement and obtain a more accurate result.

[0066] Different satellite positioning solutions can obtain multiple target transmission frequencies. Due to slight differences in satellite clocks, the transmission frequencies of the same ground target measured by different satellites are inconsistent. This frequency difference is considered to be caused by system clock errors. This error needs to be compensated when multi-satellite data fusion is performed. The specific implementation is as follows:

[0067] The inter-satellite solution error is:

[0068]

[0069] In the formula, is the average frequency of signal transmission from the Mth satellite to the same target, is the frequency difference between the Mth satellite and the first satellite for the same target, where M is the number of networked satellites;

[0070] During data fusion, the satellite clock needs to be normalized to the satellite 1 clock and the satellite measurement frequency needs to be corrected. After correction, the receiving frequency of each satellite is:

[0071]

[0072] In the formula, F M修正前 is the mean frequency received by the Mth satellite for the same target before correction, F M修正后 is the corrected mean frequency received by the Mth satellite for the same target;

[0073] After completing the frequency correction, the multi-star fusion data is completed.

[0074] (5) Use multi-satellite fusion data to apply the Doppler positioning algorithm to perform secondary positioning of the target position.

[0075] The fused data is subjected to secondary signal Doppler positioning solution, and the data from M satellites are fused. The interval between receiving equivalent signal messages is reduced to 1 / M of the original, which greatly improves the signal density. In this process, the target displacement is smaller and the positioning accuracy is higher.

[0076] The present invention has been described in detail above in conjunction with specific implementations and exemplary examples, but these descriptions cannot be understood as limiting the present invention. Those skilled in the art understand that, without departing from the spirit and scope of the present invention, a variety of equivalent substitutions, modifications or improvements may be made to the technical solution of the present invention and its implementation methods, all of which fall within the scope of the present invention. The scope of protection of the present invention shall be subject to the attached claims.

[0077] The contents not described in detail in the specification of the present invention belong to the common knowledge of those skilled in the art.

Claims

1. A ground target positioning method based on the Doppler effect, characterized in that: The steps include: S1, the satellite receiver uses FFT to capture the carrier signal before the message, and then sends the demodulated signal to the back-end frequency-locked loop + phase-locked loop for convergence tracking to obtain the signal frequency emitted by the target to be located; the signal frequency is obtained three times in succession after the carrier synchronization moment is achieved by using the frequency-locked loop + phase-locked loop, and the signal frequencies measured three times in succession are used as sampling samples instead of single measurement data to increase the observation sample; S2, using the whole satellite second pulse + receiver internal nanosecond count to obtain the time as the final output of the message capture time, and determining the satellite position information corresponding to the message capture time according to the message capture time and satellite orbit data; S3, based on the obtained message signal frequency and the solved satellite position, the least squares algorithm with surface elevation constraint is used to perform ground target Doppler positioning, complete the target position positioning solution, and output the target position and transmission frequency; S4, according to the same ground terminal message captured by multiple satellites, the ground terminal signal transmission frequency is determined respectively, and the inter-satellite measurement frequency is corrected according to the frequency difference calculated by different satellites of the same target, and multi-satellite data fusion is performed; S5, uses multi-satellite fusion data to apply the Doppler positioning algorithm to perform secondary positioning of the target position.

2. The ground target positioning method based on the Doppler effect according to claim 1, characterized in that: In step S3, the step of performing ground target Doppler positioning using a least squares algorithm constrained by surface elevation based on the obtained message signal frequency and the solved satellite position, completing the target position positioning solution, and outputting the target position and the transmission frequency is specifically implemented in the following manner: Step 1, given the initial value condition X0 = (r bx0 、r by0 、r bz0 , f0); where f0 is the given initial value of the transmitting frequency, r b0 =(r bx0 ,r by0 ,r bz0 ) is the given initial value of the target position, r b0 The following surface constraint equations are satisfied: Among them, a = b = 6378.140 km, c = 6356.755 km; Step 2, the theoretical value of the receiving frequency is given by the Doppler formula; Step 3, calculate the difference dX between the observed value and the theoretical value: dX=ΔX-(G T G) -1 *H T [H(G T G) -1 H T )] -1 *(HΔX-a) Where ΔX is the initial deviation correction, G is the partial derivative of the Doppler formula with respect to position X, H is the partial derivative of the surface constraint equation with respect to X, Step 4, calculate the correction value X k =X k-1 +dX, k is the number of iterations; Step 5: If the difference between the new and old positions is less than the specified amount, the iteration is completed, otherwise return to Step 2.

3. The ground target positioning method based on the Doppler effect according to claim 1, characterized in that: In step S4, the step of determining the ground terminal signal transmission frequency respectively according to the same ground terminal message captured by multiple satellites, correcting the inter-satellite measurement frequency according to the frequency difference calculated by different satellites of the same target, and performing multi-satellite data fusion is specifically implemented in the following manner: The inter-satellite solution error is determined as: In the formula, is the average frequency of signal transmission from the Mth satellite to the same target, is the frequency difference between the Mth satellite and the first satellite for the same target, where M is the number of networked satellites; During data fusion, the satellite clock is normalized to the satellite 1 clock, and the satellite measurement frequency is corrected. After correction, the receiving frequency of each satellite is: In the formula, F M修正前 is the mean frequency received by the Mth satellite for the same target before correction, F M修正后 is the corrected mean frequency received by the Mth satellite for the same target; After completing the frequency correction, the multi-star fusion data is completed.

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