Satellite-borne airborne integrated GNSS-R soil moisture inversion system

Abstract

The application discloses a kind of spaceborne airborne integrated GNSS-R soil moisture inversion systems, including global navigation satellite system constellation, airborne observation platform, spaceborne observation platform and inter-satellite communication link. Airborne and spaceborne observation platform extracts ionosphere-free direct wave phase, constructs complex exponential sequence and executes point-by-point multiplication operation with reflected wave baseband complex signal to complete phase conjugate reconstruction, offset local crystal oscillator drift. Airborne observation platform performs Doppler center frequency mapping and extracts dimensionality reduction reference sequence and sends to spaceborne observation platform. Spaceborne observation platform executes two-dimensional interference integral operation with dimensionality reduction reference sequence and spaceborne reconstructed reflection signal to complete spatial deconvolution, filters out background scattering noise, and combines radar equation and dielectric conversion model, and outputs soil volume water content data with airborne spatial resolution.

Description

Technical Field

[0001] This invention relates to the field of global navigation satellite system reflectance measurement and microwave remote sensing technology, specifically to a spaceborne and airborne integrated GNSS-R soil moisture inversion system. Background Technology

[0002] Global Navigation Satellite System Reflectance (GNSS-R) technology is a passive remote sensing technique that uses microwave forward scattering signals from navigation satellites to detect the Earth's surface environment. Soil moisture is a key physical quantity in the Earth's water cycle and energy exchange processes. GNSS-R technology, by receiving microwave signals scattered from the Earth's surface and retrieving soil moisture, has become a widely used detection method in the field of microwave remote sensing due to its advantages such as wide radiation source distribution, low detection cost, and all-weather observation capabilities.

[0003] In existing technologies, GNSS-R soil moisture observation systems are mainly divided into independent spaceborne observation platforms and airborne observation platforms. Spaceborne platforms operate in low Earth orbit, possessing large-scale wide-area coverage capabilities, but their spatial physical resolution is typically low due to limitations in observation altitude and antenna footprint. Airborne platforms operate within the troposphere, enabling the acquisition of high spatial resolution surface microwave scattering characteristics, but their observation swath and coverage area per flight are limited. To balance the engineering requirements of wide-area coverage and high spatial resolution detection, utilizing independently operating heterogeneous observation platforms to receive signals from the same microwave radiation source and performing cross-platform joint interferometry at the data level is a technical approach in the field of remote sensing to improve the overall detection performance of GNSS-R systems.

[0004] However, existing cross-platform joint interferometric observation systems face a technical deficiency in practical engineering applications due to the asynchronous spatiotemporal reference of the underlying hardware. The spaceborne and airborne observation platforms are heterogeneous platforms in terms of physical topology, without physical clock cable connections. Their RF front-ends and baseband signal processing hardware are driven by independent local physical crystal oscillators. Due to inherent frequency manufacturing deviations and phase noise in different crystal oscillators, independent crystal phase drift components are introduced during microwave signal downconversion and sampling. When the system performs cross-platform two-dimensional cross-correlation interferometric integration on the observation signals acquired from the airborne and spaceborne platforms, the independent crystal phase drift results in a lack of a unified spatial phase reference for the two signals. Fluctuations in the phase reference cannot meet the coherent alignment physical conditions necessary for interferometric integration processing, causing continuous changes in the direction of the complex vector within the integrator. This results in the interferometric correlator failing to output an effective coherent energy peak, preventing the system from completing the subsequent physical quantity inversion process. Existing post-processing software calibration or common-view time synchronization methods can only correct macroscopic clock errors at the data level and cannot eliminate instantaneous phase mismatch introduced by the short-stable phase noise of independent crystal oscillators within the coherent integration time window. Therefore, they are difficult to meet the engineering requirements of strict phase coherence for real-time cross-platform interferometric processing. Summary of the Invention

[0005] To address the problems mentioned in the background section, this invention provides a spaceborne and airborne integrated GNSS-R soil moisture retrieval system. This system includes a Global Navigation Satellite System constellation, an airborne observation platform, a spaceborne observation platform, and an inter-satellite communication link. The airborne observation platform operates in tropospheric airspace, while the spaceborne observation platform operates in near-Earth orbit. Both the airborne and spaceborne observation platforms are equipped with airborne and spaceborne zenith direct wave receiving antennas for synchronously receiving direct wave signals broadcast by the Global Navigation Satellite System constellation; and both are equipped with airborne and spaceborne bottom-look-ahead reflected wave receiving antennas for synchronously receiving surface reflected wave signals scattered by the target observation area.

[0006] Preferably, the airborne baseband processing module within the airborne observation platform separates independent radio frequency branches corresponding to the first and second carrier frequencies through a power allocation mechanism and a dual-channel bandpass filter network in its internal radio frequency front-end. Driven by a unified airborne local sampling clock, the airborne baseband processing module extracts the airborne first-frequency carrier phase observation value and the airborne second-frequency carrier phase observation value within the same sampling epoch. Utilizing the physical dispersion characteristic that the ionospheric phase delay is inversely proportional to the square of the corresponding carrier frequency, it performs a dual-frequency differential linear combination operation to cancel the first-order ionospheric delay term at the algebraic level, outputting the airborne ionospheric direct-reaching wave phase. Similarly, the spaceborne baseband processing module within the spaceborne observation platform, driven by a unified spaceborne local sampling clock, extracts the spaceborne carrier phase observation value within the same sampling epoch and performs a dual-frequency differential linear combination operation, outputting the spaceborne ionospheric direct-reaching wave phase.

[0007] Preferably, the airborne baseband processing module performs digital down-conversion on the reflected wave digital intermediate frequency signal output from the RF front-end, outputting an airborne reflected wave baseband complex signal; and constructs an airborne complex exponential sequence in the digital domain with the natural constant as the base and the negative value of the phase of the airborne ionosphere-free direct wave as the imaginary exponent. At the same clock cycle, the airborne baseband processing module performs time-domain pointwise complex multiplication on the airborne complex exponential sequence and the airborne reflected wave baseband complex signal to generate an airborne reconstructed reflected signal. The spaceborne baseband processing module performs the same spaceborne complex exponential sequence construction and pointwise complex multiplication operation to generate a spaceborne reconstructed reflected signal. The time-domain pointwise complex multiplication operation cancels out the phase drift components generated within the airborne reflected wave baseband complex signal and by the local physical crystal oscillator within the spaceborne reflected wave baseband complex signal.

[0008] Preferably, the manifold mapping and compression module within the airborne observation platform reads the airborne platform's position coordinates and velocity vector, calculates the satellite-borne platform's position coordinates and velocity vector based on orbital ephemeris data, and calculates the spatial coordinates of the Earth's surface specular reflection points within the target observation area using an elevation model. During system deployment, the airborne observation platform is configured to operate within the internal airspace of its current observation footprint to ensure that the airborne observation footprint is completely contained within its spatial geometric projection. The manifold mapping and compression module projects the airborne velocity vector to the airborne line-of-sight unit vector pointing to the Earth's surface specular reflection points, calculating the airborne Doppler shift; it also projects the satellite velocity vector to the satellite line-of-sight unit vector pointing to the Earth's surface specular reflection points, calculating the satellite Doppler shift; and performs a differential subtraction operation on the satellite Doppler shift and the airborne Doppler shift, outputting the differential Doppler shift amount that varies with the observation epoch. The manifold mapping and compression module uses the differential Doppler frequency shift to construct a phase rotation operator and applies it to the airborne reconstructed reflected signal to perform Doppler center frequency mapping operation, thus completing the data alignment of the airborne observation signal to the spaceborne delayed Doppler two-dimensional grid coordinate system.

[0009] Preferably, the manifold mapping and compression module separates the real and imaginary data streams of the spatially aligned reference waveform, performs one-bit quantization truncation, discards the amplitude information of the spatially aligned reference waveform, and extracts the reduced-dimensional reference sequence that retains the signal phase quadrant. The manifold mapping and compression module acquires the synchronization second pulse signal to generate an absolute timestamp, uses the absolute timestamp as the frame header identifier of the baseband transmission data frame, and places the reduced-dimensional reference sequence in the data payload area of ​​the baseband transmission data frame. The airborne communication module within the airborne observation platform performs channel coding and radio frequency carrier modulation on the baseband transmission data frame and transmits it to the onboard communication module within the spaceborne observation platform via the inter-satellite communication link.

[0010] Preferably, the cross-platform interferometry and inversion module within the spaceborne observation platform reads the absolute timestamps of the reduced-dimensional reference sequence and the reconstructed reflection signal from the spaceborne platform, constructs a dual-channel first-in-first-out buffer queue based on the absolute timestamps, performs data sliding window shift registration according to the absolute timestamps within the buffer queue, and synchronously extracts data from the same observation epoch. The cross-platform interferometry and inversion module performs time translation on the reduced-dimensional reference sequence according to the relative time delay search step size, performs phase rotation on the reduced-dimensional reference sequence according to the relative Doppler frequency search step size, and performs point-by-point complex multiplication and accumulation-to-zero operations with the reconstructed reflection signal to generate a complex matrix of the cross-platform interferometric delay-Doppler mapping.

[0011] Preferably, under the ideal condition that the delayed Doppler trajectories of the airborne and spaceborne observation footprints approximately coincide after the aforementioned mapping, within the coherent integration time window, the phase difference between the coherent scattering components originating from within the airborne observation footprint in the spaceborne reconstructed reflection signal and the complex vector output after point-by-point complex multiplication with the dimensionality-reduced reference sequence tends to be constant, and the coherent peak power is accumulated in the same direction within the integrator accumulator. The incoherent scattering components originating from outside the airborne observation footprint in the spaceborne reconstructed reflection signal and the broadband random phase sequence output after point-by-point complex multiplication with the dimensionality-reduced reference sequence causes continuous phase flips in the complex vectors, which cancel each other out within the integrator accumulator. The cross-platform interferometry and inversion module performs spatial deconvolution operations on the spaceborne observation footprint at the physical level through phase-matching selectivity. Under ideal geometric constraints and sufficient signal-to-noise ratio conditions, the spatial resolution of the spaceborne reconstructed reflection signal can be equivalently improved to the physical scale of the airborne observation footprint.

[0012] Preferably, the cross-platform interferometry and inversion module extracts absolute peak data from the complex matrix of the cross-platform interferometric delay-Doppler mapping to calculate the surface microwave coherent scattering power. The module uses the first line-of-sight propagation path length between the global navigation satellite system constellation and the surface specular reflection point, and the second line-of-sight propagation path length between the surface specular reflection point and the onboard observation platform, to calculate and compensate for free-space propagation path loss in the surface microwave coherent scattering power. After loss compensation, the module subtracts microwave transmission power, transmitting antenna directivity gain, and receiving antenna gain parameters from the power data to output the surface reflectivity. The module then inputs the surface reflectivity into a polarization reflection coefficient conversion model to calculate the relative permittivity of the target observation area, and inputs the relative permittivity into an internally configured soil dielectric empirical conversion model to calculate and output the soil volumetric water content data for the target observation area.

[0013] The present invention, by adopting the above technical solution, can bring the following beneficial effects:

[0014] 1. This invention extracts the phase of the ionosphere-free direct wave through airborne and spaceborne baseband processing modules and constructs a complex exponential sequence in the digital domain. The complex exponential sequence and the reflected wave baseband complex signal are then subjected to point-by-point complex multiplication in the time domain to complete virtual phase-locked conjugate reconstruction. This effectively cancels the phase drift component of the local physical crystal oscillator on the heterogeneous platform. A unified spatial phase reference is established for the spacecraft and airborne dual platforms without physical cable connection. This provides high-precision data input that meets the strict coherent alignment physical conditions for subsequent cross-platform two-dimensional cross-correlation interference integration.

[0015] 2. This invention calculates the differential Doppler frequency shift based on the projection of the velocity vector onto the line of sight using a manifold mapping and compression module to perform center frequency mapping, and performs one-bit quantization truncation on the spatially aligned reference waveform to extract a dimension-reduced reference sequence that retains only the phase quadrant. This achieves Doppler mismatch compensation caused by the relative motion of heterogeneous platforms and frequency domain alignment of airborne signals to the spaceborne coordinate system. While preserving the core phase evolution law, redundant amplitude envelopes are stripped away, effectively overcoming the bottleneck of physical channel transmission bandwidth in inter-satellite communication links.

[0016] 3. This invention performs data sliding window displacement registration based on absolute timestamps through a cross-platform interferometry and inversion module. It performs two-dimensional interferometric integral operation of point-by-point complex multiplication on the dimension-reduced reference sequence and the satellite-reconstructed reflection signal. This realizes the filtering of background incoherent scattering noise received by the satellite carrier beam by utilizing the coherent accumulation characteristics of the coherent components in the airborne footprint. At the physical level, it completes spatial deconvolution operation on the large-scale observation footprint of the satellite, and realizes the effect of directly and equivalently limiting the spatial physical resolution of the satellite-reconstructed reflection signal to the physical scale of the airborne observation footprint. Attached Figure Description

[0017] Figure 1 This is a diagram illustrating the overall architecture of the spaceborne and airborne integrated GNSS-R soil moisture inversion system of this invention.

[0018] Figure 2 This is a flowchart of the integrated spaceborne and airborne GNSS-R soil moisture inversion method of the present invention;

[0019] Figure 3 This is a schematic diagram of the virtual phase-locked loop conjugate reconstruction operation logic of the present invention;

[0020] Figure 4 This is a diagram illustrating the physical layer spatial deconvolution and cross-platform interference integration mechanism of the present invention.

[0021] Figure 5 This is a schematic diagram illustrating the logic of surface reflectance calculation and soil moisture inversion in this invention. Detailed Implementation

[0022] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0023] Example 1

[0024] Please refer to the appendix. Figure 1This invention provides a spaceborne and airborne integrated GNSS-R soil moisture inversion system, which adopts a collaborative observation architecture of spaceborne and airborne heterogeneous platforms, uses GNSS dual-frequency signals as a unified radiation source and clock reference, and achieves high-resolution, large-scale soil moisture inversion through cross-platform interferometric processing; the aforementioned spaceborne and airborne integrated GNSS-R soil moisture inversion system includes a GNSS satellite constellation 10, an airborne observation platform 20, a spaceborne observation platform 30, and an inter-satellite communication link 40;

[0025] The GNSS satellite constellation 10 is used to broadcast dual-frequency navigation signals, which serve as a space microwave radiation source for the system to perform surface feature detection on the one hand, and as a common clock synchronization physical reference for cross-platform heterogeneous nodes on the other hand.

[0026] The airborne observation platform 20 operates in the tropospheric airspace and is used to collect surface reflected wave signals with high spatial resolution. The airborne observation platform 20 includes an airborne zenith direct wave receiving antenna 201, an airborne bottom-view reflected wave receiving antenna 202, an airborne baseband processing module 203, an airborne inertial navigation module 204, a manifold mapping and compression module 205, and an airborne communication module 206.

[0027] The airborne zenith direct wave receiving antenna 201 points towards the zenith and is polarized in a right-hand circular polarization manner. It is used to synchronously receive the direct wave signal broadcast by GNSS satellite constellation 10.

[0028] The airborne bottom-view reflected wave receiving antenna 202 is pointed towards the ground surface and is polarized in a left-hand circular polarization manner. It is used to receive the ground reflected wave signal scattered by the target observation area.

[0029] The airborne baseband processing module 203 is connected to the airborne zenith direct wave receiving antenna 201 and the airborne bottom-view reflected wave receiving antenna 202, respectively. It is used to downconvert and sample the received direct wave signal and the ground reflected wave signal. By tracking the dual-frequency direct wave signal, it extracts the ionospheric-free direct wave phase to eliminate ionospheric dispersion error, and uses the ionospheric-free direct wave phase to perform phase conjugate compensation on the ground reflected wave signal to generate the airborne reconstructed reflection signal.

[0030] The airborne inertial navigation module 204 is used to acquire and output the attitude parameters and velocity vectors of the airborne observation platform 20 in real time during its operation.

[0031] The manifold mapping and compression module 205 is connected to the airborne baseband processing module 203 and the airborne inertial navigation module 204, respectively. It is used to perform dynamic compensation mapping of the Doppler center frequency of the airborne reconstructed reflection signal according to the attitude parameters and velocity vector, and to perform low-bit-width quantization to extract the signal phase sequence and generate a dimension-reduced reference sequence.

[0032] The airborne communication module 206 is connected to the manifold mapping and compression module 205 and is used to modulate and transmit the dimension-reduced reference sequence to the inter-satellite communication link 40.

[0033] The spaceborne observation platform 30 operates in near-Earth orbit and is used to acquire surface reflected wave signals on a large scale and perform low-level interferometric calculations. The spaceborne observation platform 30 includes a spaceborne zenith direct wave receiving antenna 301, a spaceborne bottom-view reflected wave receiving antenna 302, a spaceborne communication module 303, a spaceborne baseband processing module 304, and a cross-platform interferometry and inversion module 305.

[0034] The spaceborne zenith direct wave receiving antenna 301 points towards the zenith and is polarized in a right-hand circular polarization manner. It is used to receive direct wave signals broadcast by GNSS satellite constellation 10.

[0035] The spaceborne bottom-view reflected wave receiving antenna 302 points towards the Earth's surface and is polarized in a left-hand circular polarization manner. It is used to receive surface reflected wave signals scattered by a large area of ​​the Earth's surface.

[0036] The onboard communication module 303 is used to receive the dimension-reduced reference sequence sent by the airborne observation platform 20 via the inter-satellite communication link 40;

[0037] The spaceborne baseband processing module 304 is connected to the spaceborne zenith direct wave receiving antenna 301 and the spaceborne bottom-view reflected wave receiving antenna 302, respectively. It is used to downconvert and sample the direct wave signal and the ground reflected wave signal, extract the corresponding ionospheric-free direct wave phase, and perform phase conjugate compensation on the ground reflected wave signal to generate the spaceborne reconstructed reflection signal.

[0038] The cross-platform interferometry and inversion module 305 is connected to the spaceborne baseband processing module 304 and the spaceborne communication module 303, respectively. It takes the spaceborne reconstructed reflection signal as the input observation data and the dimension-reduced reference sequence as the local sliding reference template. It performs two-dimensional integral operation to generate a cross-platform interferometric delayed Doppler map and extracts energy parameters based on the cross-platform interferometric delayed Doppler map to invert the soil moisture of the target observation area.

[0039] Inter-satellite communication link 40 is used to realize bidirectional data interaction between airborne observation platform 20 and satellite observation platform 30, carrying the transmission of data such as dimensionality reduction reference sequence and orbital ephemeris, and providing a data channel for cross-platform collaborative processing.

[0040] Example 2

[0041] Please refer to the appendix. Figure 2-5Based on Example 1, to achieve high-resolution, large-scale soil moisture inversion, the working method of the spaceborne and airborne integrated GNSS-R soil moisture inversion system of this invention is divided into five steps according to the time-series control logic: step S100 synchronous observation start-up, step S200 dual-frequency phase extraction and conjugate reconstruction, step S300 Doppler mapping and dimensionality reduction transmission, step S400 cross-platform interferometric integration and spatial deconvolution, and step S500 surface reflectance calculation and soil moisture inversion. The specific implementation process of each step is as follows:

[0042] Step S100: Simultaneous observation and signal acquisition on dual platforms

[0043] Under the GNSS system time reference, the airborne observation platform 20 and the spaceborne observation platform 30 synchronously start the observation mission. The airborne zenith direct wave receiving antenna 201 and the spaceborne zenith direct wave receiving antenna 301 synchronously receive the dual-frequency direct wave signal broadcast by the GNSS satellite constellation 10. At the same time, the airborne bottom-view reflected wave receiving antenna 202 and the spaceborne bottom-view reflected wave receiving antenna 302 respectively receive the surface reflected wave signal scattered by the target observation area. The airborne baseband processing module 203 and the spaceborne baseband processing module 304 respectively extract the synchronization second pulse signal from the dual-frequency direct wave signal, and use the synchronization second pulse signal to add an absolute timestamp to the surface reflected wave signal to complete the time reference calibration of the observation data of the two platforms.

[0044] In step S100, the signal separation and sampling are achieved using a multi-platform dual-frequency cooperative receiving mechanism, including the following steps:

[0045] Step S111, GNSS satellite constellation 10 broadcasts the first carrier frequency. Second carrier frequency The dual-frequency direct wave signal is radiated to the Earth's surface, serving as a space microwave radiation source and clock synchronization reference for the airborne observation platform 20 and the spaceborne observation platform 30 to detect surface features.

[0046] In step S112, within the tropospheric airspace, the airborne zenith direct wave receiving antenna 201 receives dual-frequency direct wave signals, and the airborne bottom-view reflected wave receiving antenna 202 receives dual-frequency reflected wave signals scattered by the observation area of ​​the ground target. After receiving the two radio frequency signals, the airborne baseband processing module 203 separates independent radio frequency branches corresponding to the first carrier frequency and the second carrier frequency through a power distribution mechanism and a dual-channel bandpass filter network. The independent radio frequency branches are subjected to analog mixing and analog-to-digital conversion operations, and the airborne direct wave digital intermediate frequency signal and the airborne reflected wave digital intermediate frequency signal are output. The digital carrier tracking loop inside the airborne baseband processing module 203 performs carrier stripping on the airborne direct wave digital intermediate frequency signal, locks the first carrier frequency and the second carrier frequency respectively, and outputs the airborne first frequency point carrier phase observation value and the airborne second frequency point carrier phase observation value.

[0047] In step S113, within the near-Earth orbit, the onboard zenith direct wave receiving antenna 301 receives dual-frequency direct wave signals, and the onboard bottom-view reflected wave receiving antenna 302 receives dual-frequency reflected wave signals scattered by the Earth's surface. The onboard baseband processing module 304 uses the same power distribution mechanism and dual-channel bandpass filter network to perform mixing and sampling operations on the input RF signals, outputting the onboard direct wave digital intermediate frequency signal and the onboard reflected wave digital intermediate frequency signal; its internal digital carrier tracking loop extracts the onboard first frequency point carrier phase observation value and the onboard second frequency point carrier phase observation value.

[0048] The airborne first frequency carrier phase observation and the airborne second frequency carrier phase observation both physically include the actual geometric distance, the local clock error of the airborne receiver, the ionospheric phase delay error on the airborne direct wave path, and the tropospheric delay error. The spaceborne first frequency carrier phase observation and the spaceborne second frequency carrier phase observation both physically include the actual geometric distance, the local clock error of the spaceborne receiver, and the ionospheric phase delay error on the spaceborne direct wave path. Due to the different spatial heights and observation geometry of the spaceborne observation platform 30 and the airborne observation platform 20, the ionospheric puncture point positions on the spaceborne direct wave path and the airborne direct wave path are spatially separated, resulting in a non-uniform distribution of ionospheric phase delay errors on the spaceborne and airborne direct wave paths. The two platforms acquire carrier phase data including non-common-mode ionospheric delay error and independent receiver local clock error through a parallel acquisition process, forming an independent and synchronous multi-frequency observation sequence.

[0049] Step S200: Ionosphere-free phase extraction and signal conjugate reconstruction

[0050] The airborne baseband processing module 203 and the spaceborne baseband processing module 304 perform carrier phase tracking on the received dual-frequency direct wave signal according to the coherent integration time window, and respectively calculate and extract the ionospheric direct wave phase within the current time window; then, they use the ionospheric direct wave phase to perform phase conjugation operation on the surface reflected wave signal to generate airborne reconstructed reflection signal and spaceborne reconstructed reflection signal with absolute timestamps, respectively.

[0051] Step S200 is implemented based on the ionosphere-free combined phase extraction method and the virtual phase-locked loop conjugate reconstruction mechanism. The specific process is as follows:

[0052] (2.1) Dual-frequency ionosphere-free phase extraction

[0053] In step S211, when the GNSS microwave signal passes through the ionosphere, it generates a dispersion effect and introduces a carrier phase lead error. The spaceborne observation platform 30 is in a near-Earth orbit, and the airborne observation platform 20 is in the troposphere. The ionospheric penetration points corresponding to the direct wave paths of the two are separated from each other in spatial coordinates, resulting in the ionospheric phase delay error accumulated by the two direct wave paths exhibiting a non-uniform non-common mode distribution.

[0054] In step S212, the airborne baseband processing module 203, driven by a unified airborne local sampling clock, extracts the airborne first frequency point carrier phase observation value and the airborne second frequency point carrier phase observation value within the same sampling epoch; utilizing the physical dispersion characteristic that the ionospheric phase delay is inversely proportional to the square of the corresponding carrier frequency, it performs a dual-frequency differential linear combination operation on the carrier phase observation values ​​of the two frequency points, cancels the first-order ionospheric delay term at the algebraic level, and outputs the airborne ionospheric direct wave phase.

[0055] In step S213, synchronously, the spaceborne baseband processing module 304, driven by a unified spaceborne local sampling clock, extracts the spaceborne first frequency carrier phase observation value and the spaceborne second frequency carrier phase observation value within the same sampling epoch; and performs dual-frequency differential linear combination operation using the same physical dispersion characteristics to cancel the ionospheric phase delay error on the spaceborne direct wave path and output the spaceborne ionospheric-free direct wave phase.

[0056] In step S214, the output airborne ionosphere-free direct wave phase and spaceborne ionosphere-free direct wave phase have both been freed from the dispersion interference caused by the space propagation medium and stored in their respective registers as the absolute physical reference for subsequent phase conjugate reconstruction operations.

[0057] (2.2) Error mechanism of heterogeneous platform crystal oscillator

[0058] In step S221, the airborne observation platform 20 and the spaceborne observation platform 30 are heterogeneous platforms without physical clock cable connections in terms of physical topology: the airborne baseband processing module 203 is driven by an airborne local physical crystal oscillator, providing the airborne baseband sampling clock and RF down-conversion reference frequency; the spaceborne baseband processing module 304 is driven by an independent spaceborne local physical crystal oscillator, providing the spaceborne baseband sampling clock and RF down-conversion reference frequency.

[0059] In step S222, at the hardware physical level of microwave signal processing, there are hardware manufacturing frequency deviations and phase noise in the two local crystal oscillators. In the analog mixing operation of microwave signal downconversion, the instantaneous phase of the local oscillation signal will be directly coupled into the phase component of the input radio frequency signal. Within the coherent integration time window, the independently existing hardware manufacturing frequency deviation will accumulate over time, forming airborne crystal oscillator phase drift and spaceborne crystal oscillator phase drift, respectively.

[0060] In step S223, after the ground reflected wave signal received by the airborne bottom-view reflected wave receiving antenna 202 is down-converted, the phase component of the baseband digital signal is superimposed with the phase drift of the airborne crystal oscillator; after the ground reflected wave signal received by the spaceborne bottom-view reflected wave receiving antenna 302 is down-converted, the phase component of the baseband digital signal is superimposed with the phase drift of the spaceborne crystal oscillator.

[0061] In step S224, when the cross-platform interferometry and inversion module 305 performs cross-platform cross-correlation interferometric integration, the input data stream is required to have a strictly consistent phase spatiotemporal reference. Since the phase drift of the two crystal oscillators is in an independent and irregular state of change, the relative differential phase between them fluctuates nonlinearly with time. If the baseband digital signals obtained by the two independent down-conversions are directly input into the interferometric integration operation, the nonlinear fluctuation of the relative differential phase will cause the signal vector direction in the integrator to change continuously, which cannot meet the physical conditions for coherent accumulation. As a result, the cross-platform interferometric delay Doppler map cannot output an effective physical correlation peak. Therefore, before performing cross-platform cross-correlation interferometric integration, the system needs to remove the phase drift of the two crystal oscillators from the baseband digital signal in the digital domain and establish a unified phase reference that is not constrained by heterogeneous hardware.

[0062] (2.3) Virtual PLL conjugate reconstruction operation

[0063] To address the crystal oscillator error issue in heterogeneous platforms, a phase reference unification is achieved through a virtual phase-locked loop conjugate reconstruction operation. The specific process is as follows:

[0064] Step S231: The airborne baseband processing module 203 acquires the airborne reflected wave digital intermediate frequency signal output from the RF front-end, performs orthogonal digital down-conversion and low-pass filtering on it, and outputs the airborne reflected wave baseband complex signal. Subsequently, an airborne complex exponential sequence is constructed in the digital domain with the natural constant as the base and the negative value of the phase of the airborne ionosphere-free direct wave as the imaginary exponent. Based on the airborne baseband sampling clock, the airborne complex exponential sequence and the airborne reflected wave baseband complex signal are aligned in discrete time. The two are then subjected to point-by-point complex multiplication in the time domain under the same clock cycle. The mathematical model of the airborne virtual phase-locked loop conjugate reconstruction is expressed as:

[0065] Where t is the baseband fast time variable; j is the imaginary unit; The airborne reflected wave baseband complex signal output by the airborne baseband processing module 203; The phase of the airborne ionosphere-free direct wave extracted by the airborne baseband processing module 203; It is an exponential function with the natural constant as its base; This is the airborne reconstructed reflection signal after phase conjugate compensation;

[0066] In step S232, synchronously, the spaceborne baseband processing module 304 acquires the spaceborne reflected wave digital intermediate frequency signal output from the RF front-end, performs orthogonal digital down-conversion and low-pass filtering on it, and outputs the spaceborne reflected wave baseband complex signal; constructs a spaceborne complex exponential sequence with the natural constant as the base and the negative value of the phase of the spaceborne ionosphere-free direct wave as the imaginary exponent; based on the spaceborne baseband sampling clock, the spaceborne complex exponential sequence and the spaceborne reflected wave baseband complex signal are aligned in discrete time; and the two are subjected to point-by-point complex multiplication in the time domain under the same clock cycle. The mathematical model of the spaceborne virtual phase-locked loop conjugate reconstruction is expressed as:

[0067] in, This is the spaceborne reflected wave baseband complex signal output by the spaceborne baseband processing module 304; The phase of the direct wave without ionosphere on the spaceborne baseband processing module 304 is extracted. This is the reconstructed spaceborne reflection signal after phase conjugate compensation;

[0068] Step S233: The above pointwise complex multiplication operation in the time domain cancels out the phase drift components of the airborne local physical crystal oscillator within the airborne reflected wave baseband complex signal and the satellite-borne local physical crystal oscillator within the satellite-borne reflected wave baseband complex signal at the phase level. By extracting the direct wave phase of the same-source GNSS satellite as a reference input, the system uses a unified microwave radiation source outside the hardware to establish a virtual phase-locked tracking reference in the digital domain. This enables the airborne reconstructed reflection signal and the satellite-borne reconstructed reflection signal to have a unified GNSS spatiotemporal phase reference at the physical level. Their phase evolution is no longer affected by the hardware errors of independent heterogeneous platforms, providing data input that meets the coherent physical conditions for subsequent cross-platform interferometric integration.

[0069] Step S300: Spatiotemporal Doppler manifold dynamic mapping preprocessing and reference signal dimensionality reduction quantization and link transmission

[0070] During the data transmission time period, the manifold mapping and compression module 205 reads the attitude parameters and velocity vector output by the airborne inertial navigation module 204, and calculates the differential Doppler frequency shift in the line-of-sight direction between the two platforms by combining the orbital ephemeris of the spaceborne observation platform 30. Based on the differential Doppler frequency shift, the airborne reconstructed reflection signal is mapped to the Doppler center frequency, and low-bit-width quantization is performed to reduce the dimension, generating a dimension-reduced reference sequence. Finally, the airborne communication module 206 sends the dimension-reduced reference sequence with an absolute timestamp to the inter-satellite communication link 40.

[0071] Step S300 includes two core components: spatiotemporal Doppler center frequency mapping preprocessing and reference signal dimensionality reduction quantization and link transmission. The specific implementation process is as follows:

[0072] (3.1) Spatiotemporal Doppler manifold dynamic mapping preprocessing

[0073] In step S311, the airborne observation platform 20 and the spaceborne observation platform 30 are in relative motion in space, which causes the Doppler center frequency of the surface reflected wave signal scattered by the target observation area to differ from the perspective of the two platforms. The input signal without Doppler compensation will cause the subsequent correlation integration to fail to accumulate the effective energy peak due to frequency mismatch. Therefore, the Doppler center frequency mapping operation needs to be performed first.

[0074] In step S312, the manifold mapping and compression module 205 reads the airborne platform position coordinates and airborne velocity vector output in real time by the airborne inertial navigation module 204, synchronously acquires the orbital ephemeris data of the spaceborne observation platform 30, and calculates the spaceborne platform position coordinates and spaceborne velocity vector at the corresponding observation time based on the orbital ephemeris data; combined with the geographic elevation model of the target observation area, it calculates the spatial coordinates of the surface mirror reflection points within the target observation area.

[0075] Step S313: Based on the spatial coordinates of the surface mirror reflection point, construct the airborne line-of-sight unit vector pointing from the airborne observation platform 20 to the surface mirror reflection point, and the spaceborne line-of-sight unit vector pointing from the spaceborne observation platform 30 to the surface mirror reflection point; project the airborne velocity vector onto the airborne line-of-sight unit vector to calculate the airborne Doppler frequency shift of the microwave signal reaching the airborne observation platform 20; project the spaceborne velocity vector onto the spaceborne line-of-sight unit vector to calculate the spaceborne Doppler frequency shift of the microwave signal reaching the spaceborne observation platform 30.

[0076] Step S314: Perform a differential subtraction operation on the spaceborne Doppler frequency shift and the airborne Doppler frequency shift, outputting the differential Doppler frequency shift amount that varies with the observation epoch; construct a phase rotation operator using the differential Doppler frequency shift amount, apply the phase rotation operator to the airborne reconstructed reflected signal, and perform a Doppler center frequency mapping operation. The mathematical model of the Doppler center frequency mapping operation is expressed as follows:

[0077] Where t is the baseband fast time variable; j is the imaginary unit; The airborne reconfiguration reflection signal output by the airborne baseband processing module 203; For observation epochs The varying differential Doppler frequency shift; the integral term represents the cumulative phase rotation introduced by differential Doppler during the baseband fast time t; It is an exponential function with the natural constant as its base; This is the spatially aligned reference waveform output after Doppler center frequency mapping;

[0078] Step S315: Through the Doppler center frequency mapping operation, the airborne reconstructed reflection signal is translated in the frequency domain dimension to the observation view reference of the spaceborne observation platform 30, eliminating the Doppler frequency offset introduced by the relative motion of the heterogeneous platforms, and completing the data alignment of the airborne observation signal to the spaceborne delayed Doppler two-dimensional grid coordinate system.

[0079] (3.2) Dimensionality reduction and quantization of reference signal and link transmission

[0080] Because the physical channel bandwidth of inter-satellite communication link 40 has a transmission threshold, high-bit-width complex baseband data sequences will exceed the link's carrying capacity. Therefore, digital signal dimensionality reduction processing needs to be performed at the airborne observation platform 20, which is achieved through the following steps:

[0081] Step S321: The manifold mapping and compression module 205 receives the spatial alignment reference waveform and starts the airborne digital signal dimensionality reduction processing flow.

[0082] Step S322: The manifold mapping and compression module 205 separates the real and imaginary data streams of the spatially aligned reference waveform, performs one-bit quantization truncation on the real and imaginary data streams, discards the amplitude information of the spatially aligned reference waveform, and extracts the dimension-reduced reference sequence that retains the signal phase quadrant. The mathematical operation model of one-bit quantization truncation is expressed as follows:

[0083] Where t is the baseband fast time variable; j is the imaginary unit; The spatial alignment reference waveform is input to the manifold mapping and compression module 205; This is a sign extraction function used to determine the positive or negative state of the input value and output the binary mapping value; This indicates the extraction of the real part of a complex signal; This indicates the extraction of the imaginary part of a complex signal; This is the dimension-reduced reference sequence output after dimensionality reduction and quantization.

[0084] In step S323, the manifold mapping and compression module 205 acquires the GNSS synchronization second pulse signal extracted by the airborne baseband processing module 203, and uses the GNSS synchronization second pulse signal to generate an absolute timestamp; the absolute timestamp is used as the frame header identifier of the baseband transmission data frame, and the dimension reduction reference sequence is placed in the data payload area of ​​the baseband transmission data frame to complete the encapsulation of the baseband transmission data frame.

[0085] In step S324, the airborne communication module 206 receives the baseband transmission data frame output by the manifold mapping and compression module 205, performs channel coding and radio frequency carrier modulation on the baseband transmission data frame, outputs the airborne uplink radio frequency signal, and transmits the airborne uplink radio frequency signal to the inter-satellite communication link 40 through the airborne radio frequency transmitting antenna.

[0086] In step S325, the onboard communication module 303 in the onboard observation platform 30 captures the airborne uplink radio frequency signal transmitted by the inter-satellite communication link 40 through the onboard microwave receiving antenna, performs radio frequency demodulation and physical layer decoding operations on the airborne uplink radio frequency signal, and recovers the baseband transmission data frame; reads the frame header identifier of the baseband transmission data frame, separates the absolute timestamp and the dimension reduction reference sequence, and transmits them to the cross-platform interferometry and inversion module 305 as a local reference template for subsequent deconvolution operations of space physical quantities.

[0087] Step S400: Cross-platform two-dimensional interferometric integral operation and physical layer spatial deconvolution mechanism

[0088] After receiving the dimension-reduced reference sequence through the spaceborne communication module 303, the cross-platform interferometry and inversion module 305 extracts the absolute timestamps inside the dimension-reduced reference sequence and the spaceborne reconstructed reflection signal, and performs time axis alignment operation on the data stream; the aligned dimension-reduced reference sequence is used as a matched filter template and two-dimensional sliding cross-correlation integral is performed with the spaceborne reconstructed reflection signal to complete the spatial deconvolution of the spaceborne observation footprint and output the cross-platform interferometric delay Doppler map.

[0089] In step S400, the resolution improvement is specifically achieved based on the physical layer spatial deconvolution mechanism. The specific implementation process is as follows:

[0090] (4.1) Cross-platform two-dimensional interferometric integral calculation

[0091] In step S411, the onboard communication module 303 in the onboard observation platform 30 extracts the dimension-reduced reference sequence containing the absolute timestamp and sends it to the cross-platform interferometry and inversion module 305; simultaneously, the onboard baseband processing module 304 transmits the onboard reconstructed reflection signal with the absolute timestamp to the cross-platform interferometry and inversion module 305.

[0092] In step S412, the cross-platform interferometry and inversion module 305 reads the absolute timestamp of the dimension-reduced reference sequence and the absolute timestamp of the satellite-reconstructed reflection signal, and constructs a dual-channel first-in-first-out buffer queue based on the absolute timestamp; within the dual-channel first-in-first-out buffer queue, data sliding window displacement registration is performed according to the absolute timestamp to ensure that the digital sequences extracted synchronously for the operation are in the same GNSS observation epoch.

[0093] In step S413, the cross-platform interferometry and inversion module 305 configures the extracted dimension-reduced reference sequence as a local sliding matching template inside the two-dimensional interferometric integrator, using the onboard reconstructed reflection signal as the observation input data stream; sets the relative time delay search step size and the relative Doppler frequency search step size, and constructs a two-dimensional search grid; performs time translation on the dimension-reduced reference sequence according to the relative time delay search step size, performs phase rotation on the dimension-reduced reference sequence according to the relative Doppler frequency search step size, and performs point-by-point complex multiplication and accumulation-clearing operations with the onboard reconstructed reflection signal;

[0094] Step S411: The cross-platform interferometry and inversion module 305 outputs a complex matrix based on the two-dimensional cross-correlation integral operation. The mathematical model of the cross-platform two-dimensional cross-correlation integral operation is expressed as follows:

[0095] in, The complex matrix of the cross-platform interferometric delay-Doppler mapping output by the cross-platform interferometry and inversion module 305; The length of the coherent integration time window of the system; t is the baseband fast time variable; The satellite-borne reconstructed reflection signal output by the satellite-borne baseband processing module 304; This is the dimension-reduced reference sequence after time-shift registration; Indicates the complex conjugate operation; is the relative time delay search step size variable inside the correlator; v is the relative Doppler frequency search step size variable; is an exponential function with the natural constant as its base; j is the imaginary unit;

[0096] The cross-platform interferometry and inversion module 305 completes signal accumulation within the coherent integration time window and outputs a complex matrix of cross-platform interferometric delay Doppler mapping. This complex matrix records the three-dimensional spatial distribution characteristics of surface microwave coherent scattering energy in the target observation area under the joint observation geometry of the satellite and the aircraft, providing the underlying raw mapping data for extracting reflectivity physical parameters.

[0097] Since the amplitude information of the dimension-reduced reference sequence is truncated after one-bit quantization, the peak power output of the cross-correlation integral has a signal-to-noise ratio loss of about 1.96dB compared with the full-precision correlation, which needs to be compensated for in the subsequent selection of coherent integration time window or peak detection threshold design.

[0098] (4.2) Physical layer spatial deconvolution mechanism

[0099] In step S421, in the observation geometric spatial distribution, the envelope range of the spaceborne observation footprint corresponding to the spaceborne observation platform 30 is larger than the envelope range of the airborne observation footprint corresponding to the airborne observation platform 20. The airborne observation footprint is completely contained within the spaceborne observation footprint in the spatial geometric projection. The spaceborne reconstructed reflection signal output by the spaceborne baseband processing module 304 is superimposed with the coherent scattering component inside the airborne observation footprint and the incoherent scattering component outside the airborne observation footprint from the space observation perspective.

[0100] In step S422, the dimension-reduced reference sequence output by the manifold mapping and compression module 205 only records the coherent carrier phase and Doppler characteristics generated by the surface medium inside the airborne observation footprint. The cross-platform interferometry and inversion module 305 introduces the dimension-reduced reference sequence into the internal physical correlator as a sliding matched filter template for the spaceborne reconstructed reflection signal. Spatial deconvolution is achieved through cross-platform two-dimensional cross-correlation integral operation.

[0101] Step S423: Under the ideal condition that the delayed Doppler trajectories of the airborne observation footprint and the spaceborne observation footprint approximately coincide after the aforementioned mapping, within the coherent integration time window, the coherent scattering component originating from the airborne observation footprint in the spaceborne reconstructed reflection signal has approximately the same Doppler frequency shift evolution law and carrier phase change law as the dimension-reduced reference sequence; after the two are multiplied point by point, the phase difference of the output complex vector tends to a constant, and in the integrator, they are accumulated in the same direction to output the coherent peak power;

[0102] In step S424, the incoherent scattering components in the reconstructed reflection signal originating from outside the airborne observation footprint are multiplied pointwise by the reduced-dimensional reference sequence to output a broadband random phase sequence, causing the complex vectors to undergo continuous phase flips and cancel each other out in the integrator accumulator; the cross-platform interferometry and inversion module 305 performs spatial deconvolution operation on the airborne observation footprint at the physical level through phase matching selectivity. Under ideal geometric constraints and sufficient signal-to-noise ratio, the spatial resolution of the reconstructed reflection signal can be equivalently improved to the physical scale of the airborne observation footprint.

[0103] Step S425: Through the phase-matching selectivity at the signal level, the system filters out the background spatial scattering noise of the spaceborne wide-angle beam reception and performs spatial deconvolution operation on the spaceborne observation footprint at the physical level. Under ideal geometric constraints and sufficient signal-to-noise ratio, the spatial resolution of the reconstructed reflection signal of the spaceborne satellite can be equivalently improved to the physical scale of the airborne observation footprint.

[0104] Step S500: Surface reflectance calculation and soil moisture inversion

[0105] The cross-platform interferometry and inversion module 305 extracts the peak power from the cross-platform interferometric delay Doppler map, strips the hardware gain parameters according to the radar equation, calculates the surface reflectivity of the target observation area, inputs the surface reflectivity into the dielectric conversion model, and finally outputs the surface soil moisture data.

[0106] In step S500, the method based on the solution of surface reflectivity of bistatic radar equations, the conversion of dielectric model, and the soil moisture output is implemented. The specific implementation process is as follows:

[0107] (5.1) Surface reflectivity solution of bistatic radar equations

[0108] Step S511: The cross-platform interferometry and inversion module 305 reads the complex matrix of the cross-platform interferometric delay Doppler mapping output by the two-dimensional interferometric integral operation, extracts the two-dimensional absolute value peak data, and calculates the surface microwave coherent scattering power in the target observation area.

[0109] Step S512: The cross-platform interferometry and inversion module 305 acquires the microwave transmission power and directivity gain of the transmitting antenna of the GNSS satellite constellation 10 at the current observation epoch, and reads the receiving antenna gain parameters of the satellite bottom-view reflected wave receiving antenna 302 configured inside the satellite observation platform 30 and the system's carrier wavelength data.

[0110] Step S513: The cross-platform interferometry and inversion module 305 combines the spatial coordinates of the surface mirror reflection point calculated by the preceding Doppler mapping preprocessing process to calculate the first line-of-sight propagation path length from GNSS satellite constellation 10 to the surface mirror reflection point, and the second line-of-sight propagation path length from the surface mirror reflection point to the spaceborne observation platform 30.

[0111] Step S514: The cross-platform interferometry and inversion module 305 performs hardware and spatial parameter separation operation on the surface microwave coherent scattering power based on the bistatic radar equation operation logic, calculates the free space propagation path loss of the microwave signal using the first line-of-sight propagation path length and the second line-of-sight propagation path length, and compensates for the free space propagation path loss in the surface microwave coherent scattering power.

[0112] In step S515, the cross-platform interferometry and inversion module 305 subtracts the microwave transmission power, the directivity gain of the transmitting antenna, and the gain parameters of the receiving antenna from the power data after loss compensation. After spatial loss compensation and hardware gain parameter subtraction, the calculated surface reflectance physical quantity is output. Since the spatial deconvolution operation of the preceding physical layer filters out the incoherent volume scattering interference other than the airborne observation footprint, the surface reflectance output here directly characterizes the Fresnel reflection characteristics of the surface of the target observation area.

[0113] In step S516, the cross-platform interferometry and inversion module 305 establishes a nonlinear mapping relationship between the physical quantity of surface reflectivity and the relative permittivity of the surface based on the polarization reflectance coefficient model, and calculates the relative permittivity of the surface in the target observation area; the relative permittivity of the surface is input into the soil dielectric empirical conversion model, and the soil volumetric water content data of the target observation area are calculated and output.

[0114] (5.2) Dielectric model conversion and soil moisture output

[0115] Step S521: The cross-platform interferometry and inversion module 305 obtains the surface reflectivity calculated from the previous steps and extracts the microwave signal observation incident angle from the spaceborne observation platform 30 to the surface specular reflection point of the target observation area. Since both the airborne bottom-view reflection wave receiving antenna 202 and the spaceborne bottom-view reflection wave receiving antenna 302 use left-hand circular polarization to receive microwave signals, the obtained surface reflectivity is the left-hand circular polarization reflectivity.

[0116] In step S522, the cross-platform interferometry and inversion module 305 establishes a polarization reflection coefficient conversion model. Based on Fresnel's law of reflection of microwave electromagnetic scattering, a nonlinear mapping relationship is established between the left-hand circular polarization reflectivity and the horizontal and vertical polarization reflection coefficients. The incident angle of the microwave signal observation is used as a geometric constraint condition and input into the polarization reflection coefficient conversion model. The relative permittivity of the surface of the target observation area is calculated by a nonlinear algebraic equation root-finding algorithm.

[0117] Step S523: The cross-platform interferometry and inversion module 305 calls the internally configured soil dielectric empirical conversion model, which includes a polynomial function that characterizes the relationship between the relative permittivity of the land surface and the soil volumetric water content. The relative permittivity of the land surface is input into the soil dielectric empirical conversion model, and the soil volumetric water content data of the target observation area is obtained through algebraic polynomial operations.

[0118] In step S524, the cross-platform interferometry and inversion module 305 combines and encapsulates the soil volumetric water content data with the corresponding spatial coordinates of the surface specular reflection points; after physical layer spatial deconvolution operation, the spatial physical resolution of the soil volumetric water content data is equivalent to the airborne observation footprint boundary of the airborne observation platform 20; finally, according to the observation time series, a two-dimensional mapping map of soil moisture in the target observation area with airborne spatial resolution is output, completing the GNSS-R soil moisture inversion process.

Claims

1. A spaceborne and airborne integrated GNSS-R soil moisture inversion system, characterized in that, include: GNSS satellite constellation (10), airborne observation platform (20), spaceborne observation platform (30) and inter-satellite communication link (40); The GNSS satellite constellation (10) is used to broadcast dual-frequency navigation signals; The airborne observation platform (20) operates in the tropospheric airspace and includes an airborne zenith direct wave receiving antenna (201), an airborne bottom-view reflected wave receiving antenna (202), an airborne baseband processing module (203), an airborne inertial navigation module (204), a manifold mapping and compression module (205), and an airborne communication module (206). The airborne baseband processing module (203) performs down-conversion and baseband sampling on the received direct wave signal and the ground reflection wave signal, extracts the ionospheric-free direct wave phase to eliminate ionospheric dispersion error, and performs phase conjugate compensation on the ground reflection wave signal to generate an airborne reconstructed reflection signal. The manifold mapping and compression module (205) performs dynamic compensation mapping of the Doppler center frequency on the airborne reconstructed reflection signal according to the attitude parameters and velocity vector, and extracts the signal phase sequence to generate a dimension-reduced reference sequence. The spaceborne observation platform (30) operates in near-Earth orbit and includes a spaceborne zenith direct wave receiving antenna (301), a spaceborne bottom-view reflected wave receiving antenna (302), a spaceborne communication module (303), a spaceborne baseband processing module (304), and a cross-platform interferometry and inversion module (305). The spaceborne baseband processing module (304) extracts the corresponding ionospherically free direct wave phase and performs phase conjugate compensation on the surface reflected wave signal to generate a spaceborne reconstructed reflection signal. The cross-platform interferometry and inversion module (305) takes the satellite-borne reconstructed reflection signal as input observation data and the dimension-reduced reference sequence as local sliding reference template. Under the geometric constraint that the airborne observation footprint is covered by the space of the satellite-borne observation footprint, it performs two-dimensional cross-correlation integral operation to generate a cross-platform interferometric delayed Doppler map and inverts the soil moisture of the target observation area based on the cross-platform interferometric delayed Doppler map.

2. The space-borne airborne integrated GNSS-R soil moisture inversion system according to claim 1, characterized in that, Both the airborne zenith direct wave receiving antenna (201) and the satellite-borne zenith direct wave receiving antenna (301) point towards the zenith and are both right-hand circularly polarized, used to synchronously receive the direct wave signal broadcast by the GNSS satellite constellation (10); Both the airborne bottom-view reflected wave receiving antenna (202) and the spaceborne bottom-view reflected wave receiving antenna (302) point towards the ground surface and are polarized in a left-hand circular polarization manner, used to receive the ground reflected wave signal scattered by the target observation area.

3. The space-borne on-board integrated GNSS-R soil moisture inversion system according to claim 1, characterized in that, The internal radio frequency front-end of the airborne baseband processing module (203) separates independent radio frequency branches corresponding to the first carrier frequency and the second carrier frequency through a power allocation mechanism and a dual-channel bandpass filter network; The airborne baseband processing module (203) extracts the airborne first frequency point carrier phase observation value and the airborne second frequency point carrier phase observation value within the same sampling epoch under the unified airborne local sampling clock. It then uses the physical dispersion characteristic that the ionospheric phase delay is inversely proportional to the square of the corresponding carrier frequency to perform dual-frequency differential linear combination operation to cancel the first-order ionospheric delay term at the algebraic level and output the airborne ionospheric direct wave phase. The onboard baseband processing module (304) extracts the onboard first frequency carrier phase observation value and the onboard second frequency carrier phase observation value within the same sampling epoch under the unified onboard local sampling clock, and performs dual-frequency differential linear combination operation to output the onboard ionosphere-free direct wave phase.

4. The space-borne on-board integrated GNSS-R soil moisture inversion system according to claim 3, characterized in that, The airborne baseband processing module (203) performs digital down-conversion on the reflected wave digital intermediate frequency signal output from the RF front end, and outputs an airborne reflected wave baseband complex signal; the airborne baseband processing module (203) constructs an airborne complex exponential sequence in the digital domain with the natural constant as the base and the negative value of the phase of the airborne ionosphere-free direct wave as the imaginary exponent, and performs time-domain point-by-point complex multiplication operation between the airborne complex exponential sequence and the airborne reflected wave baseband complex signal under the same clock cycle; the mathematical model of the airborne virtual phase-locked loop conjugate reconstruction is expressed as: in, For baseband fast time variables; The imaginary unit; The baseband complex signal of the airborne reflected wave; The phase of the airborne ionosphere-free direct wave; It is an exponential function with the natural constant as its base; The above time-domain pointwise complex multiplication operation is used to cancel the slowly varying phase drift component introduced by the local crystal oscillator in the baseband signal, while retaining the signal phase modulation information caused by the ground scattering path. The spaceborne baseband processing module (304) constructs a spaceborne complex exponential sequence and performs time-domain pointwise complex multiplication with the spaceborne reflected wave baseband complex signal; the mathematical model for spaceborne virtual phase-locked loop conjugate reconstruction is expressed as: in, This refers to the baseband complex signal of the spaceborne reflected wave; The phase of the direct-reaching wave without ionosphere on the spacecraft; This is the reconstructed reflection signal from the satellite after phase conjugate compensation.

5. The space-borne on-board integrated GNSS-R soil moisture inversion system according to claim 1, wherein, The manifold mapping and compression module (205) reads the airborne platform position coordinates and airborne velocity vector, calculates the spaceborne platform position coordinates and spaceborne velocity vector based on the orbital ephemeris data, and calculates the spatial coordinates of the surface mirror reflection points within the target observation area in conjunction with the elevation model. The manifold mapping and compression module (205) projects the airborne velocity vector to the airborne line-of-sight unit vector pointing to the surface mirror reflection point to calculate the airborne Doppler frequency shift, projects the satellite velocity vector to the satellite line-of-sight unit vector pointing to the surface mirror reflection point to calculate the satellite Doppler frequency shift, and performs differential subtraction operation on the satellite Doppler frequency shift and the airborne Doppler frequency shift to output the differential Doppler frequency shift amount that varies with the observation epoch; The manifold mapping and compression module (205) constructs a phase rotation operator using the differential Doppler frequency shift, applies the phase rotation operator to the airborne reconstructed reflection signal to perform a Doppler center frequency mapping operation, and completes the data alignment of the airborne observation signal to the spaceborne delayed Doppler two-dimensional grid coordinate system.

6. The space-borne on-board integrated GNSS-R soil moisture inversion system according to claim 5, characterized in that, The manifold mapping and compression module (205) separates the real part data stream and the imaginary part data stream of the spatially aligned reference waveform, and performs one-bit quantization truncation on the real part data stream and the imaginary part data stream, discarding the amplitude information of the spatially aligned reference waveform and extracting the dimension-reduced reference sequence that retains the signal phase quadrant. The manifold mapping and compression module (205) acquires the GNSS synchronization second pulse signal to generate an absolute timestamp, and uses the absolute timestamp as the frame header identifier of the baseband transmission data frame, and places the dimension reduction reference sequence in the data payload area of ​​the baseband transmission data frame; The airborne communication module (206) performs channel coding and radio frequency carrier modulation on the baseband transmission data frame and sends it to the spaceborne communication module (303) through the inter-satellite communication link (40).

7. The space-borne on-board integrated GNSS-R soil moisture inversion system according to claim 6, characterized in that, The cross-platform interferometry and inversion module (305) reads the absolute timestamp of the dimension-reduced reference sequence and the absolute timestamp of the spaceborne reconstructed reflection signal, and constructs a dual-channel first-in-first-out buffer queue based on the absolute timestamp. The cross-platform interferometry and inversion module (305) performs data sliding window displacement registration according to the absolute timestamp within the dual-channel first-in-first-out buffer queue, so as to synchronously extract data in the same GNSS observation epoch for calculation.

8. The space-borne on-board integrated GNSS-R soil moisture inversion system according to claim 7, characterized in that, The cross-platform interferometry and inversion module (305) performs time translation on the reduced-dimensional reference sequence according to the relative time delay search step size, performs phase rotation on the reduced-dimensional reference sequence according to the relative Doppler frequency search step size, and performs point-by-point complex multiplication and accumulation-to-zero operation with the spaceborne reconstructed reflection signal. The mathematical model of the cross-platform two-dimensional cross-correlation integral operation is expressed as follows: in, The complex matrix of the cross-platform interferometric delay-Doppler map; The length of the coherent integration time window of the system; For baseband fast time variables; To reconstruct the reflected signal from the spacecraft; This is the dimension-reduced reference sequence after time-shift registration; Indicates the complex conjugate operation; The relative time delay search step size variable; The relative Doppler frequency search step size variable; It is an exponential function with the natural constant as its base; It is the imaginary unit.

9. The space-borne on-board integrated GNSS-R soil moisture inversion system according to claim 8, characterized in that, Under the ideal condition that the delayed Doppler trajectories of the airborne observation footprint and the spaceborne observation footprint approximately coincide after the aforementioned mapping, within the coherent integration time window, the phase difference of the complex vector output by the point-by-point complex multiplication of the coherent scattering component originating from the airborne observation footprint in the spaceborne reconstructed reflection signal with the dimension-reduced reference sequence tends to be constant, and the coherent peak power is accumulated in the same direction in the integrator. The incoherent scattering components originating from outside the airborne observation footprint in the reconstructed reflection signal are multiplied pointwise by the reduced-dimensional reference sequence to output a broadband random phase sequence, causing the complex vectors to undergo continuous phase flips and cancel each other out in the integrator accumulator; the cross-platform interferometry and inversion module (305) performs spatial deconvolution operation on the airborne observation footprint at the physical level through phase matching selectivity. Under ideal geometric constraints and sufficient signal-to-noise ratio, the spatial resolution of the reconstructed reflection signal can be equivalently improved to the physical scale of the airborne observation footprint.

10. A spaceborne and airborne integrated GNSS-R soil moisture inversion system according to claim 5, characterized in that, The cross-platform interferometry and inversion module (305) extracts the peak power from the cross-platform interferometric delay Doppler map and calculates the surface microwave coherent scattering power; The cross-platform interferometry and inversion module (305) uses the first line-of-sight propagation path length between the GNSS satellite constellation and the surface mirror reflection point, and the second line-of-sight propagation path length between the surface mirror reflection point and the spaceborne observation platform to calculate and compensate for free space propagation path loss in the surface microwave coherent scattering power; The cross-platform interferometry and inversion module (305) subtracts microwave transmission power, transmitting antenna directional gain, and receiving antenna gain parameters from the power data after loss compensation to output the surface reflectivity; the surface reflectivity is input into the polarization reflection coefficient conversion model to calculate the relative permittivity of the surface of the target observation area, and the relative permittivity of the surface is input into the internally configured soil dielectric empirical conversion model containing polynomial functions to calculate and output the soil volumetric water content data of the target observation area.

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