Compared with directly estimating the stable attitude of space targets, estimating the hotspot area observed by an Earth imaging satellite using inverse synthetic aperture radar (ISAR) imaging is a more practical and challenging task. In this article, by parameterizing the time-varying attitude of an Earth imaging satellite in spotlight mode into a fixed hotspot area location in the geodetic coordinates, a novel method is proposed. First, the migrations of the scatterers on the rotating component with respect to reference scatterers on the satellite body in the range and Doppler dimensions (MRCRSRDs) are analyzed, which are used for recognizing the scatterers on the rotating component. Next, it is proven that for the same scatterer on the rotating component, the vector difference sequences of the projection vectors normalized using the range and Doppler coordinates are orthogonal to the position vector of the scatterer. Based on this characteristic, an objective function for estimating the hotspot area is constructed. Then, based on the error principle, expressions of the estimated hotspot area longitude and latitude are derived, along with expressions of the estimation accuracy. Finally, the effects of different factors on the MRCRSRD are analyzed by traversal, under the assumption that the scatterer coordinates obtained by radar are error-free, the feasibility of the proposed method is validated and the estimation error caused by the grid quantization is evaluated, and then the robustness of the proposed method is validated by comparative experiments using parameter search method (PSM) and indirect adjustment method (IAM) under different Gaussian noise conditions.

Main lobe interrupted sampling repeater jamming (ISRJ) is a typical form of coherent jamming that achieves both deception and suppression effects, significantly degrading the detection and tracking performance of radar systems. On the basis of the periodic sampling mechanism of the ISRJ scheme and the consistent frequencies of jamming slices with the same repeater order after dechirping, we construct a time-frequency (TF) plane to suppress the ISRJ signals in the radar echoes. First, the dechirped echo signals are segmented and reshaped on the basis of the jammer's sampling interval to form a signal matrix. The energies of jamming slices with the same repeater order in different sampling intervals are then accumulated by applying a discrete Fourier transform to generate a TF plane. This plane, characterized by high time and frequency resolution and high energy concentration, facilitates the separation of target echoes from jamming slices and an accurate estimation of jamming parameters. The theoretical resolutions are derived, and the distribution characteristics of the target echoes and jamming slices in the TF plane are analyzed. On the basis of these characteristics, jamming slices are distinguished from target echoes, and key parameters such as the slice location and width are estimated. By zeroing the elements containing jamming signals in the TF plane, the ISRJ signals are suppressed. The numerical simulation results verify the feasibility of the proposed method, which shows stable performance under different jamming-to-signal ratios, signal-to-noise ratios and jamming parameters.

The stripmap mode is a basic and important mode for spaceborne synthetic aperture radars (SARs). Estimating the time-varying area observed by spaceborne SARs operating in stripmap mode is a practical but challenging field of research. In this article, we propose a novel method that parameterizes the time-varying area observed by the spaceborne SAR operating in the boresight stripmap mode into a fixed antenna attitude. Based on the principle of minimizing the dihedral angle between the plane containing the ideal estimated scatterers and the plane containing the actual parabolic antenna edge of a spaceborne SAR, an objective function is established for estimating the area observed by a spaceborne SAR operating in the boresight stripmap mode. Then, simulation experiments are designed to validate the feasibility and the robustness of the proposed method. The experimental simulation results show that the proposed method is feasible, and even under low signal-to-noise ratio (SNR) conditions of 10 dB, the proposed method still has good robustness.

The problem of initial orbit determination (IOD) for Low Earth Orbit (LEO) objects using bistatic radar too-short arc (TSA) observations is addressed. For TSA observations, the traditional IOD methods suffer low accuracy. For LEO objects with stable attitude, the high order kinematic parameters can be obtained from the time derivatives of the radar echo phase. In this paper, an analytical IOD method is presented using bistatic radar TSA observations, which contain the position measurements (bistatic range, azimuth angle, and elevation angle) and the high order kinematic measurements (bistatic velocity, acceleration, and jerk). As the undetermined target state variables constitute a complex system of equations that can only be solved iteratively, an auxiliary coordinate system based on the bistatic geometry is defined to help reduce the equations to one unary quartic equation. Further, the closed-form expressions of the orbital state are derived. The performance of the proposed method is evaluated using linearization approximations. Numerical simulations are carried out for several typical LEO observation scenarios to demonstrate the performance of the proposed method.