CN113671504B - 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 of a distributed double-star imaging altimeter, which comprises the following steps: a. introducing a difference frequency between two satellite transmission signals; b. designing a transmitting signal bandwidth according to the introduced difference frequency; c. the two satellites simultaneously transmit pulses and receive echoes from the sea surface or gentle terrain; d. the two satellites independently demodulate the echo signals according to the carrier frequencies of the respective transmitted signals; e. each of the two satellites carries out band-pass filtering on the demodulated baseband signals; f. the two satellites respectively perform SAR imaging processing on the signals after band-pass filtering to obtain single-vision complex images; g. and carrying out interference data processing on the single-view complex image. The method can ensure that the frequency spectrums of the two satellite transmitting signals are not overlapped, and can improve the anti-interference capability of echo signals and at least one time compared with a time division multiplexing anti-interference method when the mapping bandwidth is improved.
Description
Technical Field
The invention relates to an anti-echo interference difference frequency design method of a distributed double-star imaging altimeter.
Background
The distributed double-star imaging altimeter can realize centimeter-level high-precision measurement of sea surfaces or gentle terrains based on a difference frequency interference technology. The 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 decoherence compensation principle. The imaging altimeter is based on a triangle geometry measurement method, the single-view complex images acquired by two radar antennas are conjugated to obtain an interference phase, and then the altitude information of the sea surface or the slowly-varying terrain is extracted from the interference phase based on system parameters such as a base line length, a base line dip angle and the like. The imaging altimeter is carried on the distributed double-star platform, and a flexible interference baseline is formed between the phase centers of the transmitting/receiving antennas on the two satellites, so that the length of the baseline is not limited by the size and the weight of the single-star platform. The longer the baseline of the imaging altimeter, the higher the altimetric sensitivity, and the greater the potential for improving altimetric accuracy, but an excessively long baseline will introduce a series of problems. First, too long a baseline can result in a severe decrease in coherence between the two complex images, thereby limiting the imaging altimeter's potential to further improve altimetry accuracy by increasing the baseline length. 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 satellites is relatively short, and carrier frequency difference exists between the transmitting signals in the difference frequency mode, mutual interference is generated between echo signals received by satellites, and the coherence is reduced and the final height measurement accuracy is affected. Finally, in order to avoid or minimize mutual interference between echoes, it is necessary to make the two satellites alternately transmit pulses and alternately receive echo signals, i.e. operate in a time division multiplexing mode, so as to ensure that the two satellite receiving time windows do not overlap. However, this tends to increase the complexity of the system design and algorithm processing of the imaging altimeter, and the mapping bandwidth is reduced by at least one time, which is very disadvantageous for ensuring full coverage of the sea surface height measurement of the satellite altimeter, quick revisit and the like.
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 purpose, the invention provides an anti-echo interference difference frequency design method of a distributed double-star imaging altimeter, which comprises the following steps:
a. introducing a difference frequency between two satellite transmission signals;
b. Designing a transmitting signal bandwidth according to the introduced difference frequency;
c. The two satellites simultaneously transmit pulses and receive echoes from the sea surface or gentle terrain;
d. the two satellites independently demodulate the echo signals according to the carrier frequencies of the respective transmitted signals;
e. each of the two satellites carries out band-pass filtering on the demodulated baseband signals;
f. the two satellites respectively perform SAR imaging processing on the signals after band-pass filtering to obtain single-vision complex images;
g. And carrying out interference data processing on the single-view complex image.
According to one aspect of the invention, in said step a, said difference frequency is introduced according to interference baseline decoherence principles based on distributed dual star imaging altimeter system design parameters.
According to one aspect of the invention, the design parameters of the distributed dual-star imaging altimeter system comprise a baseline length B, a radar lower view angle theta, a center carrier frequency f 0 and a flying height;
and designing the center frequency f 0 +delta f of another satellite transmitting signal according to the design parameters of the distributed double-satellite imaging altimeter system, and introducing the difference frequency delta f between the center frequencies.
According to one aspect of the invention, in the step b, the transmission signal bandwidths W b of the two satellites are the same and smaller than the difference frequency Δf;
If the signal sampling rate is greater than the transmit signal bandwidth W b, the signal sampling rate is less than the difference frequency Δf.
According to one aspect of the present invention, before the difference frequency is not introduced, the carrier frequency f 0 of the two satellite transmission signals is the same as the transmission signal bandwidth W b, and the frequency of the ground object signal in the spatial domain is expressed by the wave number k as follows:
Wherein lambda g is the spatial wavelength of the ground object;
the spectral relative shift of the time signal for the ground object wavenumber spectral shift observed by the two satellites is expressed as:
Wherein f 0 is the carrier frequency of the transmitted signal of the reference satellite; b ⊥ = Bcos (θ - α) is the component of the baseline perpendicular to the radar line of sight direction on the reference satellite, α is the baseline tilt; θ is the radar down view angle of the reference satellite; beta is the average gradient angle of the observed scene; h is the altitude of the reference satellite;
the spectral relative shift results in a decrease in signal coherence, where signal coherence γ B can be expressed as:
Wherein W b is the transmission signal bandwidth of the two satellites.
According to one aspect of the present invention, in the step d, the two satellites demodulate the received echoes to obtain the baseband signal, and the center frequencies f 0 and f 0 +Δf corresponding to the carrier frequencies of the transmitted signals are adopted during demodulation.
According to an aspect of the present invention, in the step e, a bandwidth W s of the band-pass filter used for the band-pass filtering is greater than or equal to the transmission signal bandwidth W b or a sampling rate, but less than the difference frequency Δf.
According to one aspect of the invention, in said step g, said interferometric data processing includes complex image registration, land-level phase removal and phase unwrapping.
According to the conception of the invention, in order to avoid mutual interference between two satellite received echoes, the system parameters of the imaging altimeter and the baseline decoherence principle based on the difference frequency compensation can be used for improving the anti-echo anti-interference capability of the double-satellite system, the difference frequency and the bandwidth of the transmitted signals are reasonably designed, the frequency spectrum of the transmitted signals of the two satellites is ensured not to overlap, the mutual interference between the echo signals is avoided or reduced as much as possible based on the frequency division multiplexing principle, so that the mapping bandwidth is improved by at least one time compared with the time division multiplexing anti-interference method for resisting the echo interference by shortening the receiving window while the anti-interference capability of the echo signals is improved, and the invention has important significance for remote sensing requiring wide area coverage.
According to one aspect of the present invention, in order to improve signal coherence, a certain difference is introduced between the carrier frequencies of the two satellite transmitted signals, so that the imaging altimeter operates in a difference frequency interference mode. The purpose of introducing the difference frequency is to reset the relative offset of the echo spectrum caused by the long base line, so that the overlapping part of the echo spectrum is increased as much as possible, and the coherence of the complex image is improved.
According to one scheme of the invention, based on the principle of frequency division multiplexing, the difference frequency and the bandwidth of the transmitted signals are reasonably designed, so that the coherence between complex images of the imaging altimeter is improved.
According to one scheme of the invention, two satellite transmitting signals occupy different frequency spectrum ranges, and coherent demodulation and band-pass filtering are carried out at a receiving end according to respective corresponding carrier frequencies, so that echo signals are separated, and the reduction of height measurement accuracy caused by mutual interference between received echoes is avoided.
Drawings
FIG. 1 schematically illustrates a flow chart of a distributed dual-star imaging altimeter anti-echo interference difference frequency design method according to one embodiment of the present invention;
FIG. 2 schematically illustrates a schematic diagram of a baseline resulting echo signal spectrum relative shift of a method of one embodiment of the invention;
FIG. 3 schematically illustrates a schematic diagram of the relative shift in the spectrum of the difference frequency compensated echo signal of the method of one embodiment of the invention;
FIG. 4 schematically illustrates a time division multiplexing method anti-echo scheme of a method according to an embodiment of the invention;
Fig. 5 schematically shows a frequency division multiplexing method echo receive window diagram of a method according to an embodiment of the invention;
FIG. 6 schematically illustrates a schematic of differential frequency versus signal bandwidth mismatch echo interference for a method of one embodiment of the present invention;
fig. 7 schematically shows a flow chart of the difference frequency and signal bandwidth adaptation anti-echo interference of the method of one embodiment of the invention.
Detailed Description
In order to more clearly describe the embodiments of the present invention or the prior art, the following will be described
The drawings that are required for the embodiments will be briefly described. It is apparent that the drawings in the following description are only some embodiments of the present invention, and that other drawings may be obtained from these drawings without inventive effort for a person of ordinary skill in the art.
In describing embodiments of the present invention, the terms "longitudinal," "transverse," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," and the like are used in terms of orientation or positional relationship shown in the drawings for convenience of description and simplicity of description only, and do not denote or imply that the devices or elements in question must have a particular orientation, be constructed and operated in a particular orientation, so that the above terms are not to be construed as limiting the invention.
The present invention will be described in detail below with reference to the drawings and the specific embodiments, which are not described in detail 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 double-star imaging altimeter, which belongs to the technical field of microwave remote sensing and is provided by water depth detection based on multi-source data and new satellite system research. According to the method, a difference frequency is introduced between two satellite transmitting signals, then the bandwidth of the transmitting signals is reasonably designed according to the introduced difference frequency, the two satellites simultaneously transmit pulses and receive echoes from sea surfaces or gentle terrains, the received echoes are independently demodulated according to carrier frequencies of the respective transmitting signals, then band-pass filtering is carried out on the demodulated two baseband signals, SAR imaging processing is carried out on the band-pass filtered signals by the two satellites, two single-view complex images are obtained, and finally subsequent interference data processing is carried out on the single-view complex images.
The principle of introducing the difference frequency to improve the coherence of the echo signals by the distributed double-star imaging altimeter is that the baseline length of the imaging altimeter, namely the physical distance between the phase centers of two antennas, determines the height measurement sensitivity to a great extent, and the higher the height measurement sensitivity is, the higher the height measurement precision improvement potential is. The baseline length of the imaging altimeter is greatly improved by the distributed double-star platform, but the longer the baseline is, the more serious the signal decorrelation is, and the further improvement of the height measurement accuracy is restricted. Before the difference frequency is not introduced, the carrier frequency f 0 and the bandwidth W b of the two satellite transmission signals are identical, which is equivalent to the observation of the ground object in the identical frequency spectrum window, as shown in fig. 2. The frequency of the clutter signal in the spatial domain is denoted by wavenumber k:
Wherein lambda g is the spatial wavelength of the ground object. Due to the small difference between the two satellite observation angles, the same spectrum window is mapped to the space wave number domain, so that the relative offset occurs, and finally, the observed ground object wave number spectrum components are different.
The frequency and wavenumber are different representations of the signal in the time and space domains, so that the spectral shift of the ground object wavenumber observed by two satellites can be expressed as the spectral relative shift of the time signal:
Wherein f 0 is the carrier frequency of the transmitted signal of the reference satellite (one of the two satellites); b ⊥ = Bcos (θ - α) is the component of the baseline perpendicular to the radar line of sight direction on the reference satellite, α is the baseline tilt; θ 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 angle of incidence of the electromagnetic wave; h is the altitude of the reference satellite;
the spectral relative shift results in a decrease in signal coherence, where signal coherence γ B can be expressed as:
Wherein W b is the transmission signal bandwidth of the two satellites. According to the principle of decoherence of the distributed double-star imaging altimeter signals, a certain carrier frequency difference, namely a difference frequency delta f, can be introduced between the signals transmitted by the two antennas. The difference frequency is equal to the frequency spectrum offset to ensure that the frequency spectrums of the received signals overlap as much as possible, so that the signal coherence is improved, as shown in fig. 3, and the frequency spectrum components of the ground objects observed in the frequency spectrum windows of the two satellites are completely consistent.
The inter-satellite distance of the distributed double satellites is usually between hundreds of meters and thousands of meters, and signals are transmitted and received almost simultaneously, and due to the difference of signal carrier frequencies, echo signals received by two satellites are inevitably interfered by echo signals transmitted by the other satellite. In order to solve this problem, if means for adjusting the position and width of the reception window of the echo signal, that is, the time division multiplexing method is adopted, as shown in fig. 4, the two satellites alternately transmit pulse signals in time sequence, and the echo signals of the respective transmission pulses are alternately received within the reception time window. Satellites are typically hundreds of kilometers or more from the target, so there may be delays of N pulse repetition periods in the echo receive time window. Since the two satellites are alternately received, the respective reception time window length is reduced by at least one time compared with the non-alternating reception mode in fig. 5. Although the echo receiving windows in fig. 5 overlap in time, the respective echoes of the two satellites can still be separated in the frequency domain on the premise of ensuring that the difference frequency is greater than the signal bandwidth due to the different carrier frequencies of the echo signals. The time width T b of the receiving window determines the mapping bandwidth of the imaging altimeter, and since the pulse transmitting frequency of the two antennas is fixed, 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.
Based on the system design parameters and principles, the invention adopts the frequency division multiplexing anti-echo interference difference frequency design method, and the mapping bandwidth can be improved by at least one time compared with the time division multiplexing method. When the frequency difference is introduced, according to the basic principle formula of the coherence of the difference frequency compensation signal (namely the difference frequency delta f expression), the difference frequency (namely the carrier frequency difference) is introduced according to the interference baseline decoherence principle based on the design parameters of the distributed double-star imaging altimeter system so as to improve the coherence between echo signals. In the invention, design parameters of the distributed double-star imaging altimeter system comprise system parameters such as a base line length B, a radar lower view angle theta, a carrier frequency f 0 (namely the center frequency of a reference satellite transmitting signal), flying height and the like. Thus, the center frequency f 0 +Δf of another satellite transmitting signal is reasonably designed according to the parameters, namely, a difference frequency Δf is introduced between the center frequencies, so as to compensate the spectrum relative offset caused by a long base line. One specific set of system parameter values is shown in table 1 below:
table 1 (distributed double star imaging altimeter main system parameters)
According to the formula of the difference frequency Deltaf, the relative shift of the frequency spectrum between the received echoes of the imaging altimeter is 83MHz, and the magnitude of the introduced difference frequency is also the value.
Then, according to the set difference frequency, the bandwidths W b of the two satellite transmission signals are designed to ensure that the bandwidths W b of the two satellite transmission signals are the same and smaller than the difference frequency Δf, otherwise, the spectrums of the received echoes will still generate mutual interference due to mutual aliasing, as shown in fig. 6. If the signal sampling rate is greater than the bandwidth W b, it is necessary to ensure that the signal sampling rate is less than the difference frequency Δf. For example, the parameters of table 1 above, the transmit signal bandwidth W b should be less than 83MHz. Besides the design of the signal bandwidth of the imaging altimeter, 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 adaptation of the difference frequency to prevent echo interference, so that the design of the difference frequency and the signal bandwidth, as well as the design of other system parameters such as the baseline length, the radar carrier frequency and the like, is an iterative optimization design process on the premise of ensuring main performance indexes such as height measurement precision and the like, as shown by a dotted arrow in fig. 1.
Then, the two satellites simultaneously transmit signals of the same bandwidth W b according to the respective carrier frequencies, and simultaneously receive and demodulate echo signals. The two satellites demodulate the received echoes to obtain baseband signals, and center frequencies f 0 and f 0 +Δf corresponding to the carrier frequencies of the transmitted signals are adopted during demodulation, namely, the local oscillation frequencies adopted during demodulation are consistent with the carrier frequencies of the transmitted signals of the two satellites. The received echoes of the two satellites are identical, but the baseband signals obtained by the two satellites are different because the center frequencies f 0 and f 0 +Δf corresponding to the carrier frequencies of the respective transmitted signals are used for demodulation, as shown in fig. 7.
The two satellites then each bandpass filter the demodulated two baseband signals, and the bandwidth W s of the bandpass filter used for bandpass filtering should be greater than or equal to the transmit signal bandwidth W b or the sampling rate, but less than the difference frequency Δf to avoid spectral aliasing.
Finally, the two satellites respectively perform SAR imaging processing on the two paths of signals subjected to band-pass filtering to obtain corresponding (two) single-view complex images, and subsequent interference data processing is performed on the single-view complex images on the basis of the corresponding single-view complex images, such as complex image registration, land leveling phase removal, phase unwrapping and other elevation inversion algorithm basic steps. Since the distributed dual-star imaging altimeter introduces a difference frequency, the relative shift of the frequency spectrum of the echo signal is corrected, and the frequency spectrums are completely coincident, as shown in fig. 7. The frequency spectrums from the respective transmitting pulses in the echo signals are not mutually interfered, so that the reduction of complex image coherence caused by mutual interference of echoes is avoided, and the subsequent elevation inversion precision is ensured.
In summary, the method reasonably designs the difference frequency and the bandwidth of the transmitting signal based on the frequency division multiplexing principle, and improves the coherence between the imaging altimeter complex images. The two satellite transmitting signals occupy different frequency spectrum ranges, and the receiving end carries out coherent demodulation and band-pass filtering according to the carrier frequencies corresponding to the two satellite transmitting signals, so that the separation of echo signals is realized, and the reduction of height measurement accuracy caused by mutual interference between 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 double-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 in the areas of ocean surface, landform slowly-varying land, north-south pole ice cover and the like in a difference frequency system.
The above description is only one embodiment of the present invention and is not intended to limit the present invention, and various modifications and variations of the present invention will be apparent to those skilled in the art. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.
Claims (7)
1. An anti-echo interference difference frequency design method of a distributed double-star imaging altimeter comprises the following steps:
a. introducing a difference frequency between two satellite transmission signals;
b. Designing a transmitting signal bandwidth according to the introduced difference frequency;
c. The two satellites simultaneously transmit pulses and receive echoes from the sea surface or gentle terrain;
d. the two satellites independently demodulate the echo signals according to the carrier frequencies of the respective transmitted signals;
e. each of the two satellites carries out band-pass filtering on the demodulated baseband signals;
f. the two satellites respectively perform SAR imaging processing on the signals after band-pass filtering to obtain single-vision complex images;
g. Carrying out interference data processing on the single-view complex image;
In the step b, the transmission signal bandwidths W b of the two satellites are the same and smaller than the difference frequency Δf;
If the signal sampling rate is greater than the transmit signal bandwidth W b, the signal sampling rate is less than the difference frequency Δf.
2. The method according to claim 1, wherein in step a the difference frequency is introduced according to interference baseline decoherence principles based on distributed dual star imaging altimeter system design parameters.
3. The method of claim 2, wherein the distributed dual-star imaging altimeter system design parameters include a baseline length B, a radar down-view angle θ, a center carrier frequency f 0, and a flying height;
and designing the center frequency f 0 +delta f of another satellite transmitting signal according to the design parameters of the distributed double-satellite imaging altimeter system, and introducing the difference frequency delta f between the center frequencies.
4. The method according to claim 1, wherein before the difference frequency is not introduced, the carrier frequency f 0 of the two satellite transmission signals is the same as the transmission signal bandwidth W b, and the frequency of the ground object signal in the spatial domain is expressed by the wave number k as:
Wherein lambda g is the spatial wavelength of the ground object;
the spectral relative shift of the time signal for the ground object wavenumber spectral shift observed by the two satellites is expressed as:
Wherein f 0 is the carrier frequency of the transmitted signal of the reference satellite; b ⊥ = Bcos (θ - α) is the component of the baseline perpendicular to the radar line of sight direction on the reference satellite, α is the baseline tilt; θ is the radar down view angle of the reference satellite; beta is the average gradient angle of the observed scene; h is the altitude of the reference satellite;
the spectral relative shift results in a decrease in signal coherence, where signal coherence γ B can be expressed as:
Wherein W b is the transmission signal bandwidth of the two satellites.
5. The method according to claim 1, wherein in the step d, the two satellites demodulate the respective received echoes to obtain baseband signals, and the central frequencies f 0 and f 0 +Δf corresponding to the carrier frequencies of the respective transmitted signals are used for demodulation.
6. The method according to claim 1, wherein in said step e, the bandwidth W s of the band pass filter used for said band pass filtering is larger than or equal to said transmit signal bandwidth W b or sample rate, but smaller than said difference frequency Δf.
7. The method of claim 1, wherein in step g the interferometric data processing comprises complex image registration, land level phase removal and phase unwrapping.
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