CN117665813A - Method for improving space-borne SAR mapping bandwidth of middle earth orbit - Google Patents

Abstract

The invention discloses a method for improving the mapping bandwidth of a medium earth orbit space-borne SAR, which aims at solving the problems that the visibility under a MEO space-borne SAR system is difficult to meet, the sub-band separation is difficult, the sampling rate of an analog-to-digital converter (AD) is too high and the like in the existing SAR imaging technology.

Description

Method for improving space-borne SAR mapping bandwidth of middle earth orbit

Technical Field

The invention belongs to the technical field of spaceborne radars, and particularly relates to a method for improving the mapping bandwidth of a middle earth orbit spaceborne SAR.

Background

The current on-Orbit satellite-borne SAR systems are Low Earth Orbit (LEO) satellite-borne SAR systems, and have high resolution, but weak survivability, long revisit period and relatively smaller coverage range. On the other hand, with the development of technology, the downview visibility of the MEO spaceborne SAR system can be solved through reasonable system configuration, and the advantages of the MEO spaceborne SAR system are obvious compared with those of the LEO spaceborne SAR system in the aspects of revisitation period, viability, coverage range and the like.

Along with the improvement of coverage, the MEO spaceborne SAR system faces more serious distance ambiguity, and the traditional method for reducing the PRF of the system by the azimuth multichannel technology can not meet the requirement of solving the distance ambiguity of the MEO spaceborne SAR system, so that a SAR product without ambiguity in a wide mapping band can not be obtained.

Disclosure of Invention

In order to overcome the defects of the prior art, the invention provides a method for improving the mapping bandwidth of a medium earth orbit space-borne SAR, which aims at solving the problems that the visibility under a MEO space-borne SAR system is difficult to meet, the sub-band separation is difficult, the sampling rate of an analog-to-digital converter (AD) is too high and the like in the prior art.

The technical scheme adopted by the invention for solving the technical problems comprises the following steps:

step 1: designing a system for improving the MEO space-borne SAR mapping bandwidth;

step 1-1: according to parameters of the MEO spaceborne SAR and LEO spaceborne SAR systems, only considering power loss caused by propagation distance difference according to a power consistency principle, distributing the power loss to an antenna and average transmitting power of the MEO spaceborne SAR system, designing a depression linear array of the MEO spaceborne SAR system, peak power of a transmitter and time width of a transmitting signal, and determining the number of antenna array elements, a receiving and transmitting beam forming mode, the peak power and the time width parameters;

step 1-2: according to a formula of calculation of propagation loss of electromagnetic waves of L (dB) = 32.45 (dB) +20lg f (MHz) +20lg R (km), determining more propagation loss required by the MEO satellite-borne SAR relative to the LEO satellite-borne SAR under the condition of double-pass propagation, wherein L is the propagation loss, the unit is dB, f is carrier frequency, the unit is MHz, and R is propagation distance, and the unit is km;

step 1-3: according to Δl=l _MEO -L _LEO ＝20lg(R _MEO /R _LEO ) Calculating more propagation loss, R, required by MEO on-board SAR relative to LEO on-board SAR _MEO For the skew of MEO space-borne SAR, R _LEO Carrying out calculation for the slope distance of the LEO space-borne SAR to obtain delta L;

step 1-4: the method comprises the steps that delta L, which is lost by an MEO spaceborne SAR system due to electromagnetic wave space propagation, relative to an LEO spaceborne SAR system is distributed to peak power, transmission signal time width and antenna receiving and transmitting gain, so that multiple of the peak power and the transmission signal time width of the MEO spaceborne SAR system to be improved is obtained; the pitching receiving and transmitting antenna adopts a 1-dimensional linear array, and the array element number is a ₁ Adopts a during transmitting ₁ The array element DBF performs beam forming, and the receiving time is divided into 2The array element subarrays respectively receive DBF wave beams in a far area and a near area of the mapping zone to obtain a receiving and transmitting total gain;

step 2: MEO space-borne SAR distance ion band separation;

step 2-1: let the transmitting signal beConsists of m sub-pulses, a _m For transmitting steering vectors, S _m (t) is the mth sub-pulse signal, d is the antenna element spacing, θ _m 、λ _m 、T _m 、f _c，m 、K _m Pulse direction, wavelength, time width, carrier frequency and frequency modulation slope of the mth sub-pulse respectively;

step 2-2: the carrier frequencies of the 1 st to M th sub-pulses are sequentially reduced, M distance sub-bands from far to near are sequentially illuminated according to the sequence of the 1 st to M th sub-pulses during transmission, and echoes of the M distance sub-bands are overlapped in time by controlling a transceiving time window;

step 2-3: at the receiving front end, the separation of a far-zone distance sub-band and a near-zone distance ion band is realized through a subarray beam forming technology;

step 2-4: in the digital domain, each distance ion band is separated through frequency domain filtering;

step 2-5: respectively carrying out imaging focusing on each distance ion band, and carrying out sub-band splicing in an image domain so as to obtain a synthesized distance wide mapping band;

step 3: the burden of the analog-to-digital converter is reduced;

step 3-1: according to priori information of global telemetry and remote sensing, the frequency bandwidth of each sub-band of a transmitting signal is adaptively adjusted, and the synthesized bandwidth of all sub-bands is reduced from the source;

step 3-2: at the analog front end, a receiving antenna subarray beam forming technology is adopted, far and near ion bands are separated in a space domain filtering mode, all the sub-bands are divided into two parts according to the irradiation distance, the synthesized bandwidth of each part is reduced to half of the original bandwidth, and the AD sampling burden is further reduced.

Preferably, said R _MEO The value is taken to be 8000km, R _LEO The value takes 800km, Δl=40db, a ₁ ＝48。

The beneficial effects of the invention are as follows:

the invention solves the problems that the visibility under the distance of the traditional spaceborne SAR system is difficult to meet, the separation of the distance sub-bands is difficult, the sampling rate of an analog-to-digital converter (AD) is too high, and the like.

Drawings

Fig. 1 is a schematic diagram of a transmit beam DBF of the present invention.

Fig. 2 is a schematic diagram of a received beam DBF according to the present invention.

Fig. 3 is a schematic diagram of a distance subband separation method according to an embodiment of the present invention.

Fig. 4 is a focused image of the distance subbands 1-8 according to an embodiment of the present invention.

Fig. 5 is a contour plot of the synthesized focused image from subbands 1-8 according to an embodiment of the present invention.

Detailed Description

The invention will be further described with reference to the drawings and examples.

Aiming at solving the problems that the existing SAR imaging technology is difficult to meet the requirement of visibility under an MEO satellite-borne SAR system, the sub-band separation is difficult, the sampling rate of an analog-to-digital converter (AD) is too high and the like, the invention aims to provide a method for improving the mapping bandwidth of the MEO satellite-borne SAR system.

A method for improving the mid-earth orbit space-borne SAR mapping bandwidth, comprising the steps of:

step 1: designing a system for improving the MEO space-borne SAR mapping bandwidth;

step 1-1: according to typical system parameters of the MEO spaceborne SAR and the LEO spaceborne SAR, under the same condition, only considering power loss caused by the difference of propagation distances according to a power consistency principle, distributing the power loss to an antenna and average transmitting power of the MEO spaceborne SAR system, designing a depression linear array of the MEO spaceborne SAR system, peak power of a transmitter and time width of a transmitting signal, and determining parameters such as the number of antenna array elements, a receiving and transmitting beam forming mode, the peak power, the time width and the like;

step 1-2: according to the classical electromagnetic wave propagation loss calculation empirical formula of L (dB) = 32.45 (dB) +20lgf (MHz) +20lgR (km), determining more propagation loss required by the MEO satellite-borne SAR relative to the LEO satellite-borne SAR under the double-pass propagation condition, wherein L is the propagation loss, the unit is dB, f is the carrier frequency, the unit is MHz, R is the propagation distance, and the unit is km;

step 1-3: according to Δl=l _MEO -L _LEO ＝20lg(R _MEO /R _LEO ) Calculating more propagation loss, R, required by MEO on-board SAR relative to LEO on-board SAR _MEO For the skew of the MEO space-borne SAR, a typical value is taken as 8000km, R _LEO Taking 800km for the slope distance of the LEO satellite-borne SAR, and carrying out calculation to obtain delta L=40 dB;

step 1-4: the 40dB of the multi-loss of the MEO space-borne SAR system due to the electromagnetic wave space propagation relative to the LEO space-borne SAR system is distributed as follows: the peak power is 4dB, the time width of a transmitting signal is 6dB, the receiving and transmitting gain of an antenna is 30dB, the peak power of the MEO satellite-borne SAR system is required to be improved by about 2.5 times, the time width of the transmitting signal is required to be improved by about 4 times, a 1-dimensional linear array is adopted as a pitching receiving and transmitting antenna, the number of array elements is 48, the beam forming is carried out by adopting 48 array element DBF during transmitting, the receiving time is divided into 2 24 array element subarrays, the receiving DBF beam forming is carried out on two areas far and near a mapping zone respectively, and the receiving and transmitting total gain obtained in the mode is about 31dB; as shown in fig. 1 and 2.

Step 1-5: the observed time width may be suitably reduced for the closer ion bands, distributing more system power to the farther-away sub-bands where propagation losses are greater.

Step 2: MEO space-borne SAR distance ion band separation;

step 2-1: adopting a signal pattern of an intra-pulse multi-frequency sub-pulse (MFSP) and a receiving antenna sub-array beam forming technology, and realizing the separation of the distance sub-bands of the MEO satellite-borne SAR by a space-frequency combination method;

step 2-2: the transmitting signal isConsists of m sub-pulses, a _m For transmitting steering vectors, S _m (t) is the mth sub-pulse signal, d is the antenna element spacing, θ _m 、λ _m 、T _m 、f _c，m 、K _m Pulse direction, wavelength, time width, carrier frequency and frequency modulation slope of the mth sub-pulse respectively;

step 2-3: the carrier frequencies of the 1 st to M th sub-pulses are sequentially reduced, and M distance sub-bands far and near are sequentially illuminated according to the sequence of the 1 st to M th sub-pulses during transmission, so that echoes of the M distance sub-bands are overlapped in time to the greatest extent by reasonably controlling a transceiving time window;

step 2-4: at the receiving front end, the separation of a far-zone distance sub-band and a near-zone distance ion band is realized through a subarray beam forming technology;

step 2-5: in the digital domain, each distance ion band is separated by the frequency domain filtering step by step;

step 2-6: and respectively carrying out imaging focusing on each distance ion band, and carrying out sub-band splicing in an image domain, thereby obtaining a synthesized distance wide mapping band.

Step 3: the burden of an analog-to-digital converter (AD) is reduced;

step 3-1: according to priori information of global telemetry and remote sensing, the frequency bandwidth of each sub-band of a transmitting signal can be adaptively adjusted, for example, for wide areas such as ocean, desert, mountain, farmland and the like, the frequency bandwidth can be properly reduced according to application requirements, and the synthesized bandwidth of all sub-bands is reduced from the source;

step 3-2: at the analog front end, a receiving antenna subarray beam forming technology is adopted, a far-distance ion band and a near-distance ion band are separated in a space domain filtering mode, all the sub-bands are divided into two parts according to the irradiation distance, the synthesized bandwidth of each part is reduced to be approximately half of the original bandwidth, and the AD sampling burden is further reduced.

Examples:

to verify the effectiveness of the method of the present invention, the simulation parameters in Table 1 are presented herein.

The frequency interval of sub pulses corresponding to the intermediate ion bands in simulation is 10MHz, the synthesized bandwidth of 8 intermediate ion bands is 400MHz according to the distance resolution setting in table 1, and after the pitch dimension antenna sub-array DBF is carried out, the spatial filtering of the near area (1-4 sub bands) and the far area (5-8 sub bands) is realized. At this time, the synthesized bandwidth of the near zone and the far zone is 200MHz, and when AD sampling is performed, the sampling frequency only needs to be more than 200MHz, and the AD sampling frequency in simulation is set to 240MHz; after the near zone and the far zone are separated, frequency domain filtering is carried out in a digital domain respectively, and finally separation of each sub-band is realized. The simulation flow is shown in fig. 3.

Table 1 simulation data parameters

FIG. 4 shows a focused image of each distance self-mapping band obtained after distance azimuth two-dimensional focusing by using the distance sub-band separation method proposed by the present invention; fig. 5 shows a contour map of a spliced full swath obtained using the method of the present invention.

In conclusion, the simulation experiment verifies the correctness, the effectiveness and the reliability of the invention.

Claims (2)

1. A method for improving the mid-earth orbit space-borne SAR mapping bandwidth, comprising the steps of:

step 1: designing a system for improving the MEO space-borne SAR mapping bandwidth;

step 1-1: according to parameters of the MEO spaceborne SAR and LEO spaceborne SAR systems, only considering power loss caused by propagation distance difference according to a power consistency principle, distributing the power loss to an antenna and average transmitting power of the MEO spaceborne SAR system, designing a depression linear array of the MEO spaceborne SAR system, peak power of a transmitter and time width of a transmitting signal, and determining the number of antenna array elements, a receiving and transmitting beam forming mode, the peak power and the time width parameters;

step 1-2: according to a formula of calculation of propagation loss of electromagnetic waves of L (dB) = 32.45 (dB) +20lgf (MHz) +20lgR (km), determining more propagation loss required by the MEO satellite-borne SAR relative to the LEO satellite-borne SAR under the condition of double-pass propagation, wherein L is the propagation loss, the unit is dB, f is carrier frequency, the unit is MHz, R is propagation distance, and the unit is km;

step 1-3: according to Δl=l _MEO -L _LEO ＝20lg(R _MEO /R _LEO ) Calculating more propagation loss, R, required by MEO on-board SAR relative to LEO on-board SAR _MEO For the skew of MEO space-borne SAR, R _LEO Carrying out calculation for the slope distance of the LEO space-borne SAR to obtain delta L;

step 1-4: the delta L of the MEO spaceborne SAR system, which is lost due to electromagnetic wave space propagation relative to the LEO spaceborne SAR system, is distributed to peak power, transmitting signal time width and antenna receiving and transmitting gain, so that the peak power and transmitting signal time width of the MEO spaceborne SAR system are obtainedAn increased multiple; the pitching receiving and transmitting antenna adopts a 1-dimensional linear array, and the array element number is a ₁ Adopts a during transmitting ₁ The array element DBF performs beam forming, and the receiving time is divided into 2The array element subarrays respectively receive DBF wave beams in a far area and a near area of the mapping zone to obtain a receiving and transmitting total gain;

step 2: MEO space-borne SAR distance ion band separation;

step 2-1: let the transmitting signal beConsists of m sub-pulses, a _m For transmitting steering vectors, S _m (t) is the mth sub-pulse signal, d is the antenna element spacing, θ _m 、λ _m 、T _m 、f _c，m 、K _m Pulse direction, wavelength, time width, carrier frequency and frequency modulation slope of the mth sub-pulse respectively;

step 2-2: the carrier frequencies of the 1 st to M th sub-pulses are sequentially reduced, M distance sub-bands from far to near are sequentially illuminated according to the sequence of the 1 st to M th sub-pulses during transmission, and echoes of the M distance sub-bands are overlapped in time by controlling a transceiving time window;

step 2-3: at the receiving front end, the separation of a far-zone distance sub-band and a near-zone distance ion band is realized through a subarray beam forming technology;

step 2-4: in the digital domain, each distance ion band is separated through frequency domain filtering;

step 2-5: respectively carrying out imaging focusing on each distance ion band, and carrying out sub-band splicing in an image domain so as to obtain a synthesized distance wide mapping band;

step 3: the burden of the analog-to-digital converter is reduced;

step 3-1: according to priori information of global telemetry and remote sensing, the frequency bandwidth of each sub-band of a transmitting signal is adaptively adjusted, and the synthesized bandwidth of all sub-bands is reduced from the source;

step 3-2: at the analog front end, a receiving antenna subarray beam forming technology is adopted, far and near ion bands are separated in a space domain filtering mode, all the sub-bands are divided into two parts according to the irradiation distance, the synthesized bandwidth of each part is reduced to half of the original bandwidth, and the AD sampling burden is further reduced.

2. A method for increasing mid-earth orbit space-borne SAR mapping bandwidth according to claim 1, wherein R _MEO The value is taken to be 8000km, R _LEO The value takes 800km, Δl=40db, a ₁ ＝48。