We analyzed the spatiotemporal variations of the imaging swath and region using the lunar-based synthetic aperture radar (LBSAR) for Earth observation. The ground and slant swaths following the Earth-observing geometry and synthetic aperture radar (SAR) configurations were formulated. The bounds of the imaging swath were highlighted for appropriately configuring the LBSAR. The results suggested that the imaging swath is determined by the coupling of orbit elements, near-look angle, antenna beamwidth, and to some extent by the Earth’s ellipsoidal shape and LBSARs site location on the Moon’s surface. The side-looking direction, despite its modest impact on the imaging swath, plays a dominant role in ascertaining LBSARs imaging region. Furthermore, spatiotemporal variations of ground swath in various epochs were analyzed to emphasize the challenge and importance of characterizing Earth observation capability in the LBSAR.

This letter examines how the principal component analysis (PCA) affects the performance of the Deep Forest 2021 (DF21) model for the hyperspectral inversion. To this end, the spectra contaminated by eight types of heavy metals are applied and processed by PCA. Subsequently, various retained principal components (PCs) are devoted to establishing the inversion model in line with the DF21. Two typical heavy metal elements, i.e., zinc (Zn) and chromium (Cr), are used for instance; it explores the accuracies of the DF21 model for inverting their concentrations under different PCs. The findings reveal that the DF21 model’s performance fluctuates with varying retained PCs. Furthermore, the fluctuations are influenced by the heavy metal’s type and its concentration distribution. As a result, the optimal performances for inverting the Zn and Cr concentrations appear at the first 9 PCs and first 8 PCs, respectively. Hence, it might not be feasible to improve the accuracy by retaining more PCs in hyperspectral inversion, at least not for the DF21 model.

In this paper, we proposed a modulated correlation function to characterize the multiscale property of the sea surface and adopted it in the AIEM (Advanced Integral Equation Model) to calculate the radar backscattering. Comparisons of NRBCS (Normalized Radar Backscattering Cross Section) with GMFs (Geophysical Model Functions) predictions and radar measurements are conducted in various wind conditions. Good consistency and accuracy confirm the proposed model’s accuracy and applicability in predicting radar backscattering. In addition, the relations between two modulation parameters and wind vectors are analyzed at C-band. The effective correlation lengths determined from the modulated correlation function show strong wind dependence, besides the incident angle and frequency in the context of radar backscattering.

This paper studies the orbital perturbation effects on the azimuthal resolution in lunar-based synthetic aperture radar (LBSAR). We derive explicit expressions for the Doppler frequency modulation rate (DFMR) and beam-crossing velocity using the antenna beam pointing and orbit models. Following that, the azimuthal resolution is expressed in line with orbital elements and SAR configurations. The results show that the long-term orbital variations caused by accumulated perturbation effects significantly affect the azimuthal resolution, which, in effect, produces aperiodic variations in the azimuthal resolution. Such a phenomenon is most distinguished for a large LBSAR look angle, leading to a fluctuation of over 30% or even larger in the azimuthal resolution across different cycles. Additionally, the errors give rise by short-term orbital perturbations could impact azimuthal resolution to a lesser extent, with corresponding fluctuations consistently below 3%. The findings reveal that it is imperative to consider the irregular variability of azimuthal resolution due to orbital perturbations in the LBSAR.

By expanding the first-order small slope approximation scattering model in the polynomial terms containing convolutions of the sea wave spectrum, this letter quantitatively investigated how the convolutions of the sea wave spectrum affect the normalized ocean backscattering cross-section (NBCS) under various wind speeds, radar frequencies, and incident angles. First, numerical results show that only a portion of convolution powers of the sea wave spectrum contribute significantly to the NBCS, and such a portion of convolution powers are defined as the “effective convolutions”. Based on this, an empirical model related to the radar frequency, incident angle, and wind speed is established to determine the effective convolutions to reduce the computation cost. Finally, the NBCSs using the effective convolutions by the empirical model and the reference data calculated by the direct numerical integration are compared. The comparison results indicate that compared to the direct numerical integration, implementing the first-order small slope approximation with the effective convolutions has high accuracy and relatively low computational cost, notably with small Rayleigh parameters.

The Lunar-Based SAR (LBSAR) has plenty of advantages, for example, large-scale mapping, high temporal resolution, and long-period operation, for earth’s observation. However, because of the extremely high orbit, there is a round-trip propagation time delay on the order of several seconds for the LBSAR signal. Consequently, the conventional “stop-and-go” approximation used for the synthetic aperture radar (SAR) signal modeling is no longer applicable in the LBSAR. Generally, such an approximation raises two groups of effects. One is the effect of the antenna displacement during the pulse duration, which gives rise to the center frequency shift and the frequency modulation (FM) rate variation in the chirp signal. The other one is correlated with the range history and manifests as the Doppler error. These two effects might, respectively, give rise to image distortions in LBSAR imaging. This letter quantitatively analyzes the effects of the “stop-and-go” approximation on the LBSAR imaging performance. Theoretical analysis shows that the impact of the antenna displacement during pulse duration contributes little to LBSAR imaging, and thus it can be reasonably ignored in signal processing. In contrast, the azimuth imaging is sensitive to the Doppler error owing to the “stop-and-go” approximation. Under this effect, there is a severe position deviation in the azimuth direction, although the focusing quality is almost uninfluenced. To compensate for the Doppler error adequately, an effective method is proposed to retrieve the range history of the LBSAR with “nonstop-and-go” configuration. Finally, point target responses are simulated for verifying the theoretical analysis and proposed method.

The signal propagation of the lunar-based synthetic aperture radar (LBSAR) is affected by perturbations of the lunar orbit, wherein the apsidal precession that exerts a significant impact on the LBSAR imaging performance of the LBSAR deserves special care. Accordingly, the orbital determination used to maintain well-focused quality and high geometric fidelity in the existing SAR system becomes critical for the LBSAR. In this article, through establishing criteria for the orbital determination of LBSAR based on its imaging performance under the influence of apsidal precession, we investigate the accuracy requirements for the LBSAR orbital determination in terms of the position and velocity determinations. Analysis results show that the required accuracy for the LBSAR position determination depends on the geometric fidelity in the range direction, while the accuracy requirement for the velocity determination is dominated by the azimuth positioning accuracy. The focusing quality is not a primary issue for the LBSAR orbital determination. In addition, the far look angle of LBSAR accounts for the highest accuracy requirement in the position and velocity determinations; thus, it can be treated as the optimum look angle for the LBSAR orbital determination. It is also found that both velocity and position determinations are challenging in the${z}$-direction for the LBSAR.

The imaging performance of the lunar-based SAR (LBSAR) is susceptible to the orbital perturbation effects. In particular, the perturbations of orbital inclination and right ascension of ascending node (RAAN) could give rise to the temporally varying orbit drift of LBSAR and further lead to Doppler errors in the radio signal. As a result, the LBSAR image performance might be influenced by such effects. This study comprehensively probes into the phase error induced by perturbations of orbital inclination and RAAN, and its effects on the LBSAR imaging performance are further explored. It is found the LBSAR imaging performance in terms of focusing quality and geometric fidelity are affected by the perturbations of orbital inclination and RAAN, wherein the deterioration of focusing quality is closely associated with the synthetic aperture time. In this regard, the azimuth resolution on a decameter level is optimum for Earth observation of LBSAR with satisfactory image quality. Regarding the geometric fidelity, the accuracy requirement for the orbit determination of the LBSAR under perturbations of the orbital inclination and RAAN is proposed. The analysis results show that the LBSAR orbit determination in terms of the position and velocity determinations are most strenuous in the z-direction. Finally, point target responses are simulated to illustrate the preceding analysis.

There have been considerable interests in the lunar-based synthetic aperture radar (LBSAR) for monitoring large-scale geoscience phenomena. However, the signal distortions given rise by lunar orbital perturbations, especially the apsidal precession, are particularly severe in the LBSAR. The apsidal precession effects can induce a coordinate drift of the LBSAR, which can further lead to the variation in the range history of the LBSAR. As a result, LBSAR’s image performance might be affected. In this letter, we thoroughly investigate whether the apsidal precession effects cause the phase decorrelation in the signal of the LBSAR, and how such effects impact the LBSAR imaging. The theoretical result shows that the impact of the lunar apsidal precession mainly results in the first-order and second-order Doppler errors, which further influence the geometric location and focusing quality along the azimuth direction. Numerical simulations using the point target response show good consistency with the theoretical analysis. To this end, the lunar apsidal precession effects deserve special care in the LBSAR for high imaging quality.

Due to the earth and moon's revolutions, the Doppler centroid of the moon-based synthetic aperture radar (moon-based SAR) varies dramatically along the orbit of the moon when the SAR system is looking broadside (nonsquinted). However, the Doppler centroid should be kept as small as possible to avoid range ambiguity and to alleviate difficulty with focus. This letter presents the Doppler properties along the orbit of the moon in accordance with the antenna coordinate system. Based on the Doppler analysis and the phase scan, we propose a method for Doppler centroid steering to minimize the Doppler centroid frequency without rotating the platform. The new method can accurately compensate the Doppler centroid to zero, because it considers the effects of the lunar orbit and the relative motion between earth's target and moon-based SAR. To validate the proposed method, we also derived the lower and upper bounds of the look angle. Subsequently, we performed simulations in accordance with Jet Propulsion Laboratory Development Ephemeris 430 (JPL DE430). Finally, the performance requirement of the phase scan is analyzed so as to validate the proposed method.

A new polarimetric synthetic aperture radar (SAR) images classification method based on residual network (ResNet) and deep autoencoder (DAE) is proposed in this letter. The patch-based classification and pixel-based classification are well integrated to achieve better classification accuracy and clearer contour features. The patch-based classification results with ResNet and pixel-based classification results with DAE are obtained respectively. According to the results, a hybrid method combining the patch-based and the pixel-based classification is developed to determine the category label of each pixel. The attractive feature of the combined method is to take full use of the polarization scattering characteristics in each pixel and spatial information of the polarimetric SAR data. To verify the proposed method, SAR images from Chinese GaoFen 3 (GF-3) space-borne SAR systems are used and experiments are performed, which shows the proposed method can achieve high accuracy and maintain contour features simultaneously. Compared with existing classification methods, the new method has a better performance in classification accuracy and false alarm probability (FAP).

The spatiotemporal coverage of a Moon-based synthetic aperture radar (SAR) is analyzed based on the imaging geometry, upon which the spatial coverage and image formulation rely. The distance from the Earth to the Moon-based SAR and bounds of the grazing and azimuthal angles jointly determine the coverage area on the Earth's surface. Meanwhile, the ground coverage of the Moon-based SAR is determined by the bounds of the grazing and azimuthal angles and geographic coordinates of the nadir point at a specified time. Moreover, the temporal variation in the spatial coverage is pertinent to the temporally varying nadir point of the Moon-based SAR on the Earth's surface. Furthermore, numerical simulations using the lunar ephemeris data are carried out to complement the analysis and to illustrate the spatiotemporal coverage. Finally, a guideline for the optimal site selection of a Moon-based SAR is proposed. In conclusion, a Moon-based SAR has the potential to perform long-term, continuous Earth observations on a global scale to enhance our capability to understand the planet.

In this paper, effects of the earth's curvature and lunar revolution on the performance of the moon-based synthetic aperture radar (SAR) are examined by a comprehensive analysis of the motion-induced Doppler frequency and Doppler rate on which azimuthal imaging relies. The motion effects include the earth's self-rotation related to the earth's curvature and lunar revolution around the earth. An extended hyperbolic range equation (EHRE) is proposed in line of the equivalent velocity and equivalent squint angle, and then the signal model based on the EHRE is established to simulate the moon-based SAR image from which the imaging performance is analyzed. Theoretical analyses show that the earth's curvature is a dominant factor in determining the moon-based SAR's Doppler and azimuthal resolution. Furthermore, the earth's curvature distorts the SAR image by way of rotating the azimuth imaging from the cross-range direction within a certain skewed angle. The overall effects of lunar revolution generate a velocity correction factor and a deviate squint angle, which subsequently deteriorate the azimuthal resolution and image focusing. Results also show that effects of the earth's curvature and lunar revolution are in connection with relative positions of the ground target and moon-based SAR. To this end, numerical simulations using point target response is carried out to accentuate the necessary for taking account of the Doppler error induced by the lunar revolution.

We analyze the error caused by `stop-and-go' assumption on Moon-based SAR image focusing. In Moon-based SAR imaging of earth, the extremely long propagation time delay and curved trajectory violate the `stop-and-go' assumption. The separation of transmitting and receiving antenna in Moon-based SAR is not regarded as monostatic, but quasi-bistatic mode. The Doppler parameters should be estimated under this condition. Preliminary analysis shows that error caused by stop-and-go assumption can seriously affect the focusing of Moon-based SAR imaging, and should be corrected or compensated.

We simulate to perform Earth radiation budget on the lunar surface. This simulation is to support the study of the Earth radiation budget from a new observation strategy. There are two instruments deployed on the lunar surface for monitoring the Earth outgoing radiation and detecting changes at global scale. As opposed to small instantaneous spatial coverage of the space-borne instruments, the instruments equipped on the lunar surface will have the observational scope of whole Moon-facing hemisphere to allow for a better quantification of the Earth outgoing radiation on the planetary scale. In this paper, we make simulations of equipping multispectral camera and active cavity radiometer on the lunar surface and analyze the characteristics of the Earth outgoing radiation acquired by the Moon-based platform.

We investigate the ionospheric effect on the global change observation lunar-based synthetic aperture radar (GCOLB-SAR), which offers an unprecedented temporal resolution and spatial coverage. However, the GCOLB-SAR is confronted with a great challenge: an extra-long distance electromagnetic wave propagating through atmospheric and ionospheric effects. The ionospheric effect for the satellite-borne SAR is no longer applicable for the GCOLB-SAR due to its ultra-long synthetic aperture time. In this paper, we consider the temporal-spatial variation effects of the ionosphere on the imaging of GCOLB-SAR. For the purpose of the study, an L-band GCOLB-SAR observation is considered. The signal model based on curved trajectory for GCOLB-SAR is derived and the temporal-spatial variation background ionosphere is analyzed. Preliminary analysis shows that the range shift is on the order of hundreds of meters, while the azimuth shift is of tens of meters. Such geometric deviations apparently should be corrected or compensated for GCOLB-SAR.