CN113671504A - Anti-echo interference difference frequency design method for distributed double-star imaging altimeter - Google Patents
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Abstract
The invention relates to an anti-echo interference difference frequency design method for a distributed double-star imaging altimeter, which comprises the following steps of: a. introducing a difference frequency between two satellite transmission signals; b. designing a transmission signal bandwidth according to the introduced difference frequency; c. the two satellites transmit pulses at the same time and receive echoes from the sea surface or the gentle terrain; d. the two satellites independently demodulate the echo according to the carrier frequency of each transmitted signal; e. the two satellites respectively carry out band-pass filtering on the demodulated baseband signals; f. the two satellites respectively carry out SAR imaging processing on the signals subjected to band-pass filtering to obtain single-view complex images; g. and carrying out interference data processing on the single-view complex image. The invention can ensure that the frequency spectrums of the two satellite transmitting signals are not overlapped, and the mapping bandwidth can be improved by at least one time compared with a time division multiplexing anti-interference method while the anti-interference capability of echo signals is improved.
Description
Technical Field
The invention relates to an anti-echo interference difference frequency design method for a distributed double-star imaging altimeter.
Background
The distributed double-star imaging altimeter can realize centimeter-level high-precision measurement of the sea surface or the gentle terrain based on a difference frequency interference technology. Two satellites simultaneously transmit and receive echo signals, and certain central carrier frequency difference, namely difference frequency, is introduced between the transmitted signals based on a baseline decorrelation compensation principle. The imaging altimeter is based on a trigonometric geometric measurement method, conjugates are carried out on single-vision complex images acquired by the two radar antennas respectively to obtain an interference phase, and height information of sea surface or slowly varying terrain is extracted from the interference phase based on system parameters such as base line length, base line inclination angle and the like. The imaging altimeter is carried on a distributed double-satellite platform, and a flexible interference baseline is formed between phase centers of transmitting/receiving antennas on two satellites, so that the length of the baseline is not limited by the size and weight of a single-satellite platform. The longer the baseline of the imaging altimeter, the higher the altimetry sensitivity and the greater the potential for improving the accuracy of the altimetry, but the overlong baseline introduces a series of problems. Firstly, the excessively long baseline can cause the coherence between two complex images to be seriously reduced, so that the potential of the imaging altimeter for further improving the measurement accuracy by increasing the length of the baseline is restricted. Secondly, the distributed double-satellite imaging altimeter is mainly used for sea surface altitude measurement, and two satellites simultaneously transmit and receive echo signals. However, because the distance between the satellites is short and the carrier frequency difference exists between the transmitted signals in the difference frequency mode, mutual interference will be generated between the echo signals received by the satellites, which will lead to reduced coherence and affect the final height measurement accuracy. Finally, in order to avoid or reduce mutual interference between echoes as much as possible, it is necessary to allow the two satellites to alternately transmit pulses and alternately receive echo signals, i.e., to operate in a time division multiplexing mode, so as to ensure that the receiving time windows of the two satellites do not overlap. However, the complexity of system design and algorithm processing of the imaging altimeter is inevitably increased, and the mapping bandwidth is reduced by at least one time, which is very disadvantageous for ensuring full sea height measurement coverage and quick revisit of the satellite altimeter.
Disclosure of Invention
The invention aims to provide an anti-echo interference difference frequency design method for a distributed double-star imaging altimeter.
In order to achieve the above object, the present invention provides an anti-echo interference difference frequency design method for a distributed two-star imaging altimeter, comprising the following steps:
a. introducing a difference frequency between two satellite transmission signals;
b. designing a transmission signal bandwidth according to the introduced difference frequency;
c. the two satellites transmit pulses at the same time and receive echoes from the sea surface or the gentle terrain;
d. the two satellites independently demodulate the echo according to the carrier frequency of each transmitted signal;
e. the two satellites respectively carry out band-pass filtering on the demodulated baseband signals;
f. the two satellites respectively carry out SAR imaging processing on the signals subjected to band-pass filtering to obtain single-view complex images;
g. and carrying out interference data processing on the single-view complex image.
According to one aspect of the invention, in the step (a), the difference frequency is introduced according to an interference baseline decoherence principle based on a distributed two-star imaging altimeter system design parameter.
According to one aspect of the invention, the design parameters of the distributed double-star imaging altimeter system comprise a base line length B, a radar down-view angle theta and a central carrier frequency f0And a flying height;
according toDesigning the central frequency f of another satellite transmitting signal according to the design parameters of the distributed double-satellite imaging altimeter system0+ Δ f, said difference frequency Δ f being introduced between the central frequencies.
According to an aspect of the present invention, in the step (b), the transmission signal bandwidths W of the two satellitesbIs the same and less than the difference frequency Δ f;
if the signal sampling rate is larger than the bandwidth W of the transmitting signalbThen the signal sampling rate is less than the difference frequency Δ f.
According to one aspect of the invention, the carrier frequencies f of the signals transmitted by the two satellites are not introduced before the difference frequency is introduced0And the transmission signal bandwidth WbSimilarly, the frequency of the ground object signal in the spatial domain is represented by wavenumber k:
wherein λ isgIs the spatial wavelength of the ground object;
the spectral shift of the earth feature observed by two satellites is expressed by the relative shift of the time signal spectrum:
wherein f is0A carrier frequency of a transmitted signal for a reference satellite; b is⊥Bcos (θ - α) is a component of the baseline perpendicular to the radar line of sight on the reference satellite, α is the baseline inclination; theta is the radar down-view angle of the reference satellite; beta is the average slope angle of the observation scene; h is the flight altitude of the reference satellite;
the relative shift of the frequency spectrum causes the signal coherence to be reduced when the signal coherence gammaBCan be expressed as:
wherein, WbFor two satellitesBandwidth.
According to one aspect of the invention, in step (d), the two satellites demodulate the respective received echoes to obtain baseband signals, and the demodulation uses the center frequency f corresponding to the carrier frequency of the respective transmitted signals0And f0+Δf。
According to an aspect of the present invention, in the step (e), a bandwidth W of a band pass filter for the band pass filteringsGreater than or equal to the transmission signal bandwidth WbOr sampling rate, but less than the difference frequency af.
According to one aspect of the invention, in step (g), the interferometric data processing includes complex image registration, flat-land phase removal, and phase unwrapping.
According to the concept of the invention, in order to avoid mutual interference between two satellite receiving echoes, a difference frequency compensation baseline decoherence principle and system parameters of an imaging altimeter can be based on, the purpose of improving the echo anti-interference capability of a double-satellite system is achieved, the difference frequency and the transmitting signal bandwidth are reasonably designed, the frequency spectrums of the two satellite transmitting signals are ensured not to be overlapped, and the mutual interference between the echo signals is avoided or reduced as much as possible based on a frequency division multiplexing principle, so that the echo signal anti-interference capability is improved, and simultaneously, the mapping bandwidth is improved by at least one time compared with a time division multiplexing anti-interference method for resisting the echo interference by shortening a receiving window, and the method has important significance for remote sensing detection requiring wide area coverage.
According to one scheme of the invention, in order to improve signal coherence, a certain difference is introduced between carrier frequencies of signals transmitted by two satellites, so that the imaging altimeter works in a difference frequency interference mode. The purpose of introducing the difference frequency is to reset the relative shift of the echo spectrum caused by the long base line, increase the overlapping part of the echo spectrum as much as possible and realize the improvement of the coherence of the complex image.
According to one scheme of the invention, based on the principle of frequency division multiplexing, the difference frequency and the bandwidth of the transmitted signal are reasonably designed, and the coherence between the complex images of the imaging altimeter is improved.
According to one scheme of the invention, the two satellite transmitting signals occupy different frequency spectrum ranges, coherent demodulation and band-pass filtering are carried out at the receiving end according to respective corresponding carrier frequencies, separation of echo signals is realized, and reduction of height measurement precision caused by mutual interference between the received echoes is avoided.
Drawings
FIG. 1 is a flow chart of an anti-echo interference difference frequency design method for a distributed two-star imaging altimeter according to an embodiment of the invention;
FIG. 2 is a graph schematically illustrating relative shifts in the spectrum of an echo signal caused by a baseline of the method of one embodiment of the invention;
FIG. 3 is a schematic representation of the relative shift of the spectrum of a difference frequency compensated echo signal of the method of one embodiment of the present invention;
FIG. 4 is a schematic diagram of the anti-echo interference of the time division multiplexing method of the method according to an embodiment of the present invention;
FIG. 5 is a schematic diagram of an echo receiving window of a frequency division multiplexing method according to the method of an embodiment of the present invention;
FIG. 6 is a schematic diagram illustrating that the difference frequency and signal bandwidth of the method of one embodiment of the present invention are not suitable for echo interference;
fig. 7 schematically shows a difference frequency and signal bandwidth adaptive anti-echo interference flow chart of the method according to an embodiment of the invention.
Detailed Description
To more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following description will be made
The drawings that are required to be used in the embodiments will be briefly described. It is obvious that the drawings in the following description are only some embodiments of the invention, and that for a person skilled in the art, other drawings can be derived from them without inventive effort.
In describing embodiments of the present invention, the terms "longitudinal," "lateral," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," and the like are used in an orientation or positional relationship that is based on the orientation or positional relationship shown in the associated drawings, which is for convenience and simplicity of description only, and does not indicate or imply that the referenced device or element must have a particular orientation, be constructed and operated in a particular orientation, and thus, the above-described terms should not be construed as limiting the present invention.
The present invention is described in detail below with reference to the drawings and the specific embodiments, which are not repeated herein, but the embodiments of the present invention are not limited to the following embodiments.
Referring to fig. 1, the invention discloses an anti-echo interference difference frequency design method for a distributed two-satellite imaging altimeter, belongs to the technical field of microwave remote sensing, and provides water depth detection and new satellite system research based on multi-source data. The method comprises the steps of firstly introducing a difference frequency between two satellite transmitting signals, then reasonably designing a transmitting signal bandwidth according to the introduced difference frequency, simultaneously transmitting pulses by the two satellites and receiving echoes from sea or gentle terrain, independently demodulating the received echoes according to carrier frequencies of the respective transmitting signals, respectively carrying out band-pass filtering on two demodulated baseband signals, respectively carrying out SAR imaging processing on the signals subjected to band-pass filtering by the two satellites, obtaining two single-view complex images, and finally carrying out subsequent interference data processing on the single-view complex images.
The distributed double-star imaging altimeter introduces the principle that the difference frequency improves the coherence of echo signals, namely, the length of a base line of the imaging altimeter, namely, the physical distance between the phase centers of two antennas determines the height measurement sensitivity of the imaging altimeter to a great extent, and the higher the height measurement sensitivity is, the higher the height measurement precision improvement potential is. The distributed double-star platform greatly improves the length of a base line of the imaging altimeter, but the longer the base line is, the more serious the signal decoherence is, and the further improvement of the high measurement precision is restricted. As shown in FIG. 2, the carrier frequencies f of the signals transmitted by the two satellites are not introduced before the difference frequency is introduced0Sum bandwidth WbIdentical, this is equivalent to observing the feature with the same frequency spectrum window. The frequency of the ground object signal in the spatial domain is represented by wavenumber k as:
wherein λ isgIs the spatial wavelength of the ground object. Due to the slight difference between the observation angles of the two satellites, the relative shift occurs when the same spectral window is mapped to the spatial wavenumber domain, and finally the observed wavenumber spectral components of the ground objects are different.
The frequency and wavenumber are different representations of the signal in the time domain and the space domain, so the spectral shift of the earth object observed by two satellites can be equivalently expressed by the relative shift of the spectrum of the time signal:
wherein f is0A carrier frequency of a transmission signal of a reference satellite (one of the two satellites); b is⊥Bcos (θ - α) is a component of the baseline perpendicular to the radar line of sight on the reference satellite, α is the baseline inclination; theta is the radar down-view angle of the reference satellite; beta is the average slope angle of the observed scene, which actually changes the local incident angle of the electromagnetic wave; h is the flight altitude of the reference satellite;
the relative shift of the frequency spectrum causes the signal coherence to be reduced when the signal coherence gammaBCan be expressed as:
wherein, WbThe transmission signal bandwidth of two satellites. According to the principle of signal decoherence of the distributed two-star imaging altimeter, a certain carrier frequency difference, namely difference frequency delta f, can be introduced between the signals transmitted by the two antennas. The difference frequency is equal to the spectrum offset to ensure that the spectrums of the received signals overlap as much as possible, so as to improve the signal coherence, as shown in fig. 3, at this time, the spectrum components of the ground objects observed in the respective spectrum windows of the two satellites are completely consistent.
The distance between two distributed stars is usually between hundreds of meters and thousands of meters, and the two stars are almost simultaneously transmittedThe signals are transmitted and received, and due to the difference of the carrier frequencies of the signals, the echo signals received by the two satellites are inevitably interfered by the echo of the signal transmitted by the other satellite. In order to solve this problem, if a means for adjusting the position and width of the reception window of the echo signal, that is, a time division multiplexing method is adopted, as shown in fig. 4, the two satellites alternately transmit pulse signals in time sequence, and alternately receive the echo signals of the respective transmission pulses within the reception time window. The satellite is typically several hundred kilometers away from the target, so the echo receive time window may be delayed by N pulse repetition periods. Since the two satellites are alternately received, the length of the receiving time window of each satellite is reduced by at least one time compared with the non-alternate receiving mode in fig. 5. Although the echo receiving windows in fig. 5 overlap in time, due to the different carrier frequencies of the echo signals, the respective echoes of the two satellites can be separated in the frequency domain on the premise of ensuring that the difference frequency is greater than the signal bandwidth. Time width T of receiving windowbThe mapping bandwidth of the imaging altimeter is determined, and the time division multiplexing receiving mode can reduce the mapping bandwidth by at least one time compared with the frequency division multiplexing mode adopted by the method because the pulse transmitting frequency of the two antennas is fixed.
Based on the system design parameters and principles, the invention adopts an anti-echo interference difference frequency design method of frequency division multiplexing, and compared with a time division multiplexing method, the mapping bandwidth can be improved by at least more than one time. When frequency difference is introduced, according to a basic principle formula (namely the difference frequency delta f expression) of the difference frequency compensation signal coherence, based on design parameters of a distributed double-star imaging altimeter system, difference frequency (namely carrier frequency difference) is introduced according to an interference baseline decoherence principle so as to improve the coherence between echo signals. In the invention, the system design parameters of the distributed double-star imaging altimeter comprise a base length B, a radar lower visual angle theta and a carrier frequency f0(i.e., reference satellite transmission signal center frequency) and fly height. Thus, the center frequency f of the other satellite transmission signal is reasonably designed according to the parameters0+ Δ f, i.e., a difference frequency Δ f is introduced between the center frequencies to compensate for the relative shift in the spectrum caused by the long baseline. One set of specific system parameter values is shown in table 1 below:
table 1 (distributed double star imaging altimeter main system parameters)
According to the formula calculation of the difference frequency deltaf, the relative shift of the frequency spectrum between the receiving echoes of the imaging altimeter is 83MHz, and the magnitude of the introduced difference frequency also needs to be the value.
Then, according to the set difference frequency, designing the bandwidth W of the two satellite transmission signalsbGuarantee the bandwidth W of two satellite transmission signalsbEqual and smaller than the difference frequency af, otherwise the frequency spectra of the received echoes will still interfere with each other due to mutual aliasing, as shown in fig. 6. If the signal sampling rate is greater than the bandwidth WbThen it is necessary to ensure that the signal sampling rate is less than the difference frequency Δ f. Taking the parameters in Table 1 above as examples, the bandwidth W of the transmitted signal is nowbShould be less than 83 MHz. The design of the signal bandwidth of the imaging altimeter needs to consider factors such as resolution, signal to noise ratio and the like in addition to the consideration of adaptation with difference frequency to prevent echo interference, so that the design of the difference frequency and the signal bandwidth, the length of a base line, radar carrier frequency and other system parameters is an iterative optimization design process on the premise of ensuring main performance indexes such as height measurement accuracy and the like, as shown by a dotted arrow in fig. 1.
Then, two satellites transmit the same bandwidth W according to their respective carrier frequenciesbAnd simultaneously receiving and demodulating the echo signal. Two satellites demodulate the received echoes to obtain baseband signals, and the central frequency f corresponding to the carrier frequency of the transmitted signals is adopted during demodulation0And f0+ Δ f, i.e. the local oscillator frequency used during demodulation is the same as the carrier frequency of the signal transmitted by each of the two satellites. The received echoes of the two satellites are identical, but the demodulation adopts the central frequency f corresponding to the carrier frequency of the respective transmitted signals0And f0+ Δ f, so the baseband signals obtained by the two satellites are different, as shown in fig. 7.
Then two satellites respectively carry out band-pass filtering on the two demodulated baseband signals, and the band-pass filter is used for band-pass filteringBandwidth W ofsShould be greater than or equal to the transmission signal bandwidth WbOr sampling rate, but less than the difference frequency af to avoid spectral aliasing.
And finally, the two satellites respectively carry out SAR imaging processing on the two paths of signals subjected to respective band-pass filtering to obtain corresponding (two) single-view complex images, and on the basis, subsequent interference data processing is carried out on the single-view complex images, such as basic steps of a high-range inversion algorithm of complex image registration, flat ground phase removal, phase unwrapping and the like. Due to the fact that the distributed double-star imaging altimeter introduces the difference frequency, the relative frequency spectrum offset of the echo signals is corrected, and the frequency spectrums are completely overlapped, as shown in fig. 7. The frequency spectrums from respective transmitted pulses in the echo signals are not interfered with each other, so that the decrease of the coherence of complex images caused by the mutual interference of the echoes is avoided, and the subsequent elevation inversion accuracy is ensured.
In conclusion, the method is based on the principle of frequency division multiplexing, the difference frequency and the bandwidth of the transmitting signal are reasonably designed, and the coherence between the imaging altimeter complex images is improved. The two satellite transmitting signals occupy different frequency spectrum ranges, coherent demodulation and band-pass filtering are carried out at the receiving end according to respective corresponding carrier frequencies, separation of echo signals is achieved, and reduction of height measurement precision caused by mutual interference between the received echoes is avoided. Compared with a time division multiplexing method for resisting echo interference by shortening a receiving window, the method can at least double the mapping bandwidth of the distributed two-star imaging altimeter, and has important significance for remote sensing detection requiring wide-area coverage. Therefore, the method is suitable for high-precision measurement of the distributed double-star imaging altimeter with the difference frequency system in the ocean surface, the terrain slowly-changing land, the south pole ice cover and the north pole ice cover.
The above description is only one embodiment of the present invention, and is not intended to limit the present invention, and it is apparent to those skilled in the art that various modifications and variations can be made in the present invention. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.
Claims (8)
1. An anti-echo interference difference frequency design method for a distributed double-star imaging altimeter comprises the following steps:
a. introducing a difference frequency between two satellite transmission signals;
b. designing a transmission signal bandwidth according to the introduced difference frequency;
c. the two satellites transmit pulses at the same time and receive echoes from the sea surface or the gentle terrain;
d. the two satellites independently demodulate the echo according to the carrier frequency of each transmitted signal;
e. the two satellites respectively carry out band-pass filtering on the demodulated baseband signals;
f. the two satellites respectively carry out SAR imaging processing on the signals subjected to band-pass filtering to obtain single-view complex images;
g. and carrying out interference data processing on the single-view complex image.
2. The method of claim 1, wherein in step (a), the difference frequency is introduced according to an interference baseline decoherence principle based on distributed two-star imaging altimeter system design parameters.
3. The method of claim 2, wherein the distributed two-star imaging altimeter system design parameters include a baseline length B, a radar down-view angle θ, a center carrier frequency f0And a flying height;
designing the central frequency f of another satellite transmitting signal according to the design parameters of the distributed double-satellite imaging altimeter system0+ Δ f, said difference frequency Δ f being introduced between the central frequencies.
4. The method of claim 1, wherein in step (b), the transmission signal bandwidths W of the two satellitesbIs the same and less than the difference frequency Δ f;
if the signal sampling rate is larger than the bandwidth W of the transmitting signalbThen the signal sampling rate is less than the difference frequency Δ f.
5. According to claim 1The method being characterized in that the carrier frequencies f of the signals transmitted by the two satellites are not equal to the carrier frequencies f of the signals transmitted by the two satellites before the introduction of the difference frequency0And the transmission signal bandwidth WbSimilarly, the frequency of the ground object signal in the spatial domain is represented by wavenumber k:
wherein λ isgIs the spatial wavelength of the ground object;
the spectral shift of the earth feature observed by two satellites is expressed by the relative shift of the time signal spectrum:
wherein f is0A carrier frequency of a transmitted signal for a reference satellite; b is⊥Bcos (θ - α) is a component of the baseline perpendicular to the radar line of sight on the reference satellite, α is the baseline inclination; theta is the radar down-view angle of the reference satellite; beta is the average slope angle of the observation scene; h is the flight altitude of the reference satellite;
the relative shift of the frequency spectrum causes the signal coherence to be reduced when the signal coherence gammaBCan be expressed as:
wherein, WbThe transmission signal bandwidth of two satellites.
6. The method of claim 1, wherein in step (d), the two satellites demodulate the respective received echoes to obtain baseband signals, and the demodulation is performed using a center frequency f corresponding to a carrier frequency of the respective transmitted signals0And f0+Δf。
7. The method of claim 1, wherein the step of removing the metal layer is performed in a batch processIn the step (e), a bandwidth W of a band-pass filter used for the band-pass filteringsGreater than or equal to the transmission signal bandwidth WbOr sampling rate, but less than the difference frequency af.
8. The method of claim 1, wherein in step (g), the interferometric data processing comprises complex image registration, flat-ground phase removal, and phase unwrapping.
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| CN115113204A (en) * | 2022-07-26 | 2022-09-27 | 中国科学院空天信息创新研究院 | Satellite-borne InSAR (interferometric synthetic Aperture Radar) implementation method for distributed satellite dual-band split emission |
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