Background
With the development of electromagnetic interference and anti-radiation weapon technology, the concealment of the electronic reconnaissance system is required to be higher, and the active detection and positioning system is being challenged seriously. Passive positioning differs from conventional active positioning systems in that it does not need to emit electromagnetic waves by itself, but rather relies on receiving signal energy emitted by the radiation source to position the radiation source. Due to the passive characteristic of the positioning method, the passive radar has the advantages of low power consumption, long action distance, good concealment, strong anti-interference capability and the like, so that the passive radar has a more important strategic position in modern combat. The common carrying platform of the passive positioning system comprises: airborne platform, carrier-borne platform, satellite-borne platform. The satellite-borne platform is located in the space, so that the satellite-borne passive positioning system has the excellent characteristics of wide detection range, high sensitivity, good maneuverability and no limitation by factors such as weather, national regions and the like, and along with the development of the aerospace technology, the satellite-borne passive positioning system is also paid more attention and is developed.
The satellite-borne passive positioning system comprises a single-satellite passive positioning system and a multi-satellite passive positioning system. Common positioning methods for multi-satellite passive positioning systems include time difference of arrival positioning, frequency difference of arrival positioning, and a combination of the two. Although the multi-satellite system has high positioning accuracy, the system is complex to implement and high in cost, and the problems of time synchronization and the like need to be considered, so that the practical application situation is complex. In contrast, although the volume and the load of a single satellite are limited by the erection of an antenna, which results in poor positioning accuracy of a single-satellite passive positioning system, the cost is low, the system is simple to erect, and the actual application situation is simple, so that the system is widely applied.
The single-satellite passive positioning method has many, the most common method is a two-dimensional phase interferometer direction-finding positioning method, the basic principle of the method is that a direction line pointing to a radiation source is obtained by using a phase method for direction finding, and the intersection point of the direction line and the ground is the position of the radiation source. The method can realize instantaneous positioning of the target, and the positioning precision is ideal. However, the positioning accuracy of the method depends on the length of the base line to a certain extent, the measurement accuracy becomes better along with the increase of the length of the base line, and the problem of phase ambiguity is caused by the overlong length of the base line. To solve the phase ambiguity problem, a more complicated antenna structure needs to be constructed or multiplexing needs to be performed. In practical engineering, the following two methods are generally adopted to solve the phase ambiguity problem: one method is to adopt a method of multi-baseline phase comparison angle measurement, form two long baselines and two short baselines in the same direction, the long baseline improves the precision, the short baseline solves the ambiguity, but in the actual system needs a plurality of antennas, because the satellite limits the antenna array shape and the array element number, the effect of solving the phase ambiguity by the method is limited. Another method for solving the phase ambiguity is to increase the number of receiving channels, one path of which can measure the angle without ambiguity by using the amplitude method although the angle measurement accuracy is low, and the other path of which can improve the angle measurement accuracy by using the direction measurement method, which obviously increases the complexity and cost of the device. Therefore, for a single-satellite passive positioning system, it is a worthy direction to reduce the complexity of the system as much as possible while ensuring the positioning accuracy.
Detailed Description
The technical solutions in the present invention will be described clearly and completely with reference to the accompanying drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the invention, rather than all embodiments, and all other embodiments obtained by those skilled in the art without any creative work based on the embodiments of the present invention belong to the protection scope of the present invention.
A satellite-borne passive positioning method based on a virtual aperture comprises the following steps:
the method comprises the following steps: performing Doppler processing of a slow time domain on the acquired signals to obtain time-Doppler data;
step two: performing short-time Fourier transform on the signal, and performing time-frequency estimation to obtain Doppler center frequency and Doppler modulation frequency;
step three: inverting the low-orbit equivalent squint distance model parameters through the obtained Doppler center frequency and the obtained Doppler frequency to obtain the satellite equivalent speed and the equivalent squint angle;
step four: carrying out azimuth matching filtering on the signals for multiple times according to the distance to obtain azimuth positioning information;
step five: rearranging the positioning results at different distances according to the distance, and obtaining positioning information in the distance direction through focusing;
step six: and selecting a smaller distance interval according to the distance positioning result, repeating the fourth step and the fifth step, and performing more accurate distance positioning to obtain the two-dimensional positioning information of the target.
Preferably, the method for obtaining the time-doppler data in the step one comprises:
the dimension of the fast time domain of the acquired signals is R, Fourier transform is carried out along the slow time axis, and the dimension of the Doppler domain of the data is D, so that the obtained time-Doppler two-dimensional data is { data }, the dimension is R multiplied by D, the repetition frequency of signal pulses is PRF, and the synthetic aperture time is D/PRF.
Preferably, the method for obtaining the doppler center frequency and the doppler modulation frequency in the step two comprises:
doppler parameter estimation is carried out, in order to ensure the accuracy of parameter estimation, short-time Fourier transform of the azimuth direction is carried out on the acquired signals, time-frequency estimation is carried out, the window length of the short-time Fourier transform is L, the step length is M, and the number of data segments is
Performing linear fitting on each section of data center to obtain Doppler center frequency f
dcAnd Doppler frequency f
1r。
Preferably, the method for obtaining the satellite equivalent velocity and the equivalent squint angle in the third step is as follows:
for the satellite-borne situation, an equivalent strabismus distance model is selected, and the model expression is as follows:
wherein R iscIs the distance, V, of the target at the time of the beam centerrIs the equivalent velocity of the satellite, θrIs the equivalent squint angle, taIs the azimuth slow time;
the azimuth frequency of the echo signal can be obtained as follows:
according to the Doppler center frequency and the Doppler modulation frequency obtained in the step two, a parameter VrAnd thetarThe inversion can be obtained according to the following formula:
however, due to the one-way nature of passive positioning, the distance R cannot be obtained by the distance-wise matched filtering before the azimuth-wise matched filteringcAnd thus the equivalent speed VrThe estimation of (2) adopts geometric estimation, and the final distance model parameter estimation formula is as follows:
wherein, VgIs the speed of movement of the radar beam on the earth's surface, VsIs the velocity of the satellite.
Preferably, the method for obtaining the azimuth positioning information in the fourth step is as follows:
discretizing distance search range R1,…Rk,…RRAnd (4) performing azimuth matched filtering according to discrete distance, wherein the dimensionality is R, the obtained data dimensionality is still R multiplied by D, and the constructed matched filter formula is as follows:
h(ta,Rk)=exp(jπγkta 2)
wherein, taFor azimuthal slow time, gammakIn order to match the frequency of the tuning of the filter,
the equivalent filter constructed in the doppler domain according to the stationary phase principle is:
wherein f is
aIn the form of an azimuth frequency, the azimuth frequency,
λ is the wavelength of the signal and,
the matched filtering result is:
wherein, w (t)r,ta) The signal is the signal after the signal passes through the carrier frequency.
Preferably, the method for obtaining the location information of the distance direction by focusing in the step five comprises the following steps:
rearranging the positioning results at different distances according to the distances when matching the filter Rk=RcWhen the target is located at a distance, the target is focused along the azimuth direction, and the target is defocused at other distances, so that the distance direction of the target can be located.
Preferably, the method for obtaining the two-dimensional positioning information of the target in the sixth step is as follows:
selecting a distance { R 'with smaller interval according to the obtained distance direction positioning information'1,…R'k,…R'RAnd fourthly, performing azimuth matched filtering and distance dimension rearrangement according to the methods in the fourth step and the fifth step to obtain a new more accurate distance positioning result.
The examples are described in connection with the principle shown in fig. 1, where sr(tr,ta) Representing the acquired signal at a frequency fcUsing exp { -j2 π fctaFrequency-off-loading the signal, taThe azimuth slow time.
The method comprises the following steps: the collected signals are processed by the Doppler processing in the slow time domain to obtain time-Doppler data
The dimension of a fast time domain of acquired data is 512, Fourier transform is carried out along a slow time axis, the dimension of a Doppler domain of the data is 4096, so that the obtained time-Doppler two-dimensional data is { data }, the dimension of the time-Doppler two-dimensional data is 512 multiplied by 4096, the signal pulse repetition frequency is 5000Hz, the synthetic aperture time is 0.8192s, and a time-Doppler graph is drawn as shown in FIG. 2;
step two: performing short-time Fourier transform on the signal, and performing time-frequency estimation to obtain Doppler center frequency and Doppler frequency modulation
Performing Doppler parameter estimation, performing short-time Fourier transform of the azimuth direction on the acquired signals to ensure the accuracy of the parameter estimation, performing time-frequency estimation, wherein the window length of the short-time Fourier transform is 128, the step length is 1, the number of data segments is 3969, and performing linear fitting on each segment of data center to obtain the Doppler center frequency fdcAnd Doppler frequency f1r;
Step three: and (3) inverting the low-orbit equivalent squint distance model parameters through the obtained Doppler center frequency and Doppler modulation frequency to obtain the satellite equivalent speed and the equivalent squint angle
Due to the one-way nature of passive positioning, the distance R cannot be obtained by distance-wise matched filtering before the azimuth-wise matched filteringcAnd thus the equivalent speed VrThe estimation adopts geometric estimation, and the final parameter estimation formula of the equivalent squint angle is as follows:
wherein, VgIs the speed of movement of the radar beam on the earth's surface, VsIs the velocity of the satellite;
step four: carrying out azimuth matching filtering on the signals for multiple times according to the distance to obtain azimuth positioning information
Discretizing distance search range R1,…Rk,…R512}∈[500km,1000km]And the dimensionality is 512, azimuth matched filtering is carried out according to the discrete distance, the dimensionality of the obtained data after matched filtering is 512 multiplied by 4096, and the constructed matched filter formula is as follows:
h(ta,Rk)=exp(jπγkta 2)
wherein, taFor azimuthal slow time, gammakIn order to match the frequency of the tuning of the filter,
the equivalent filter constructed in the doppler domain according to the stationary phase principle is:
wherein f is
aIn the form of an azimuth frequency, the azimuth frequency,
λ is the wavelength of the signal and,
the matched filtering result is:
wherein, w (t)r,ta) The signal is the signal after the signal passes through the carrier frequency;
step five: rearranging the positioning results at different distances according to the distance, and obtaining the positioning information of the distance direction by focusing
Rearranging the positioning results at different distances according to the distances when matching the filter Rk=RcThen, focusing the distance along the azimuth direction, defocusing other distances, drawing a positioning result graph as shown in fig. 3, obtaining two-dimensional positioning information of the target, and obtaining a positioning error which is about 0.5km and is smaller than the passive positioning error of the single-satellite two-dimensional quadrature phase interferometer through error calculation;
step six: selecting a smaller distance interval according to the distance positioning result, repeating the fourth step and the fifth step, and performing more accurate distance positioning so as to obtain two-dimensional positioning information of the target
Selecting a smaller distance interval according to the distance positioning result, repeating the fourth step and the fifth step, and performing more accurate distance positioning, { R'1,…R'k,…R'512}∈[R0-5km,R0+5km],R0And obtaining a new more accurate distance direction positioning result for the distance direction information obtained in the step five.
It will be evident to those skilled in the art that the invention is not limited to the details of the foregoing illustrative embodiments, and that the present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein. Any reference sign in a claim should not be construed as limiting the claim concerned.
Furthermore, it should be understood that although the present description refers to embodiments, not every embodiment may contain only a single embodiment, and such description is for clarity only, and those skilled in the art should integrate the description, and the embodiments may be combined as appropriate to form other embodiments understood by those skilled in the art.