An Improved Equivalent Squint Range Model and Imaging Approach for Sliding Spotlight SAR Based on Highly Elliptical Orbit

As an emerging orbital system with flexibility and brand application prospects, the highly elliptical orbit synthetic aperture radar (HEO SAR) can achieve both a low orbit detailed survey and continuous earth surface observation in high orbit, which could be applied to marine reconnaissance and surveillance. However, due to its large eccentricity, two challenges have been faced in the signal processing of HEO SAR at present. The first challenge is that the traditional equivalent squint range model (ESRM) fails to accurately describe the entire range for the whole orbit period including the perigee, the apogee, and the squint subduction section. The second one is to exploit an efficient HEO SAR imaging algorithm in the squinted case which solves the problem that traditional imaging algorithm fails to achieve the focused imaging processing of HEO SAR during the entire orbit period. In this paper, a novel imaging algorithm for HEO SAR is presented. Firstly, the signal model based on the geometric configuration of the large elliptical orbit is established and the Doppler parameter characteristics of SAR are analyzed. Secondly, due to the particularity of Doppler parameters variation in the whole period of HEO, the equivalent velocity and equivalent squint angle used in MESRM can no longer be applied, a refined fourth-order equivalent squint range model(R4-ESRM) that is suitable for HEO SAR is developed by introducing fourth-order Doppler parameter into Modified ESRM (MESRM), which accurately reconstructs the range history of HEO SAR. Finally, a novel imaging algorithm combining azimuth resampling and time-frequency domain hybrid correlation based on R4-ESRM is derived. Simulation is performed to demonstrate the feasibility and validity of the presented algorithm and range model, showing that it achieves the precise phase compensation and well focusing.

The Lunar Fossil Figure in a Cassini State

The present ellipsoidal figure of the Moon requires a deformation that is significantly larger than the hydrostatic deformation in response to the present rotational and tidal potentials. This has long been explained as due to a fossil rotational and tidal deformation from a time when the Moon was closer to Earth. Previous studies constraining the orbital parameters at the time the fossil deformation was established find that high orbit eccentricities (e ≳ 0.2) are required at this ancient time, which is difficult to reconcile with the freezing of a fossil figure owing to the expected large tidal heating. We extend previous fossil deformation studies in several ways. First, we consider the effect of removing South Pole−Aitken (SPA) contributions from the present observed deformation using a nonaxially symmetric SPA model. Second, we use the assumption of an equilibrium Cassini state as an additional constraint, which allows us to consider the fossil deformation due to nonzero obliquity self-consistently. A fossil deformation established during Cassini state 1, 2, or 4 is consistent with the SPA-corrected present deformation. However, a fossil deformation established during Cassini state 2 or 4 requires large obliquity and orbit eccentricity (ϵ ∼ 68° and e ∼ 0.65), which are difficult to reconcile with the corresponding strong tidal heating. The most likely explanation is a fossil deformation established during Cassini state 1, with a small obliquity (ϵ ∼ −0.2°) and an orbit eccentricity range that includes zero eccentricity (0 ≤ e ≲ 0.3).

Exploiting Potentialities of Dynamic Satellites for Future Space-based Quantum Network: Downlink Scheduling and Orbital Deployment

With the goal of a space-based quantum network is to have satellites distribute keys between any nodes on the ground, we consider an evolved quantum network from a near-term form, in which a space-based relay, Micius, executes a sequence of Satellite QKD (SatQKD) missions, allowing any two cities to have a shared key. Accordingly, we develop a comprehensive framework integrated with precise orbital modelling and a cloud statistics model to enable a preassessment of SatQKD. Using this framework, we consider three different scheduling strategies and estimate the keys that can be delivered to cities. By the assistance of using different optimizations on scheduling problem formulations, it is possible to allows for the possibility to consider strategies for different missions such as extending connection for distant nodes, prioritized delivery to nodes with higher privileges, and promoting keys utilization. We also provide a comparison of the total number of keys delivered using different-altitude satellites. It is demonstrated that the plan for constructing a low-Earth orbit (LEO) satellite constellation is more efficient than that for employing an expensive high-orbit satellite to an execution of scheduling SatQKD.

Integrated High-Accuracy Correction Technology of Radio-Wave Refraction for Deep-Space (High-Orbit) Targets

The radio-wave refraction error caused by the troposphere and ionosphere badly affects accuracy in terms of the navigation, positioning, measurement, and control of a target; it is the main source of errors in high-accuracy measurement and control systems. The high-accuracy technology needed for radio-wave refraction error correction (mainly in the troposphere and ionosphere) has been the focus of research for a long time. At present, the correction methods used for radio-wave refraction errors have a low accuracy. For an S-band radio-wave signal, the accuracy of refraction error correction can generally only reach m-level (elevation angle of 15° and above), and thus has difficulty meeting the requirements of dm-level accuracy refraction error correction for deep-space and high-orbit targets. To improve the accuracy of radio-wave refraction error correction for deep-space and high-orbit targets, a novel correction method for tropospheric and ionospheric range error due to refraction is proposed in this study, on the basis of the measured data from a water vapor radiometer and dual-frequency Global Navigation Satellite System (GNSS). The comprehensive calibration test is conducted in combination with the Chinese Area Positioning System (CAPS) in Kunming. Results show that this method can effectively correct the range error due to refraction that is caused by the troposphere and ionosphere. For an S-band radio-wave signal, the accuracy of refraction error correction can reach dm-level accuracy (elevation angle of 15° and above), which is 50% higher than that achieved with traditional methods. This work provides an effective support system for major projects, such as lunar exploration and Mars exploration.

Powerful high-orbit gyro-TWT

This paper presents the results of a search for the optimal design of a high-orbit gyro-TWT, which would make it possible to reduce the magnetostatic field when operating at high frequencies close to the millimeter wavelength range, increase the gain and gain bandwidth, and increase the efficiency of the gyro-TWT. To search for the optimal configuration of the high-orbit gyro-TWT, the Gyro-K program was used, in which the equations for the excitation of an irregular waveguide by an electron beam are constructed on the basis of the coordinate transformation method of A.G. Sveshnikov, which is based on replacing the problem of exciting an irregular waveguide with the problem of exciting a regular waveguide with a unit radius. This method allows one to search for the solution of wave equations in the form of expansions in terms of the system of basis functions of a regular cylindrical waveguide. To solve Maxwell's equations, the Galerkin method was used, which is also called the orthogonalization method. The coefficients of the expansion of the field in terms of eigenbasic functions are determined in this method from the condition of the orthogonality of the residuals of the equations for the eigenbasis functions of a regular waveguide. The boundary conditions at the open ends of the waveguide are determined for each mode of the regular waveguide separately, which eliminates the incorrectness of setting the boundary conditions for the full field, as is the case when using the “picˮ technology. As a result, we obtain a system of ordinary differential equations for the expansion coefficients, which now depend only on the longitudinal coordinate. This approach makes it possible to transform the threedimensional problem of excitation of an irregular waveguide into a one-dimensional problem. Ohmic losses in the walls of the waveguide are taken into account on the basis of the Shchukin – Leontovich boundary conditions. For a self-consistent solution of the problem of excitation of an irregular waveguide by an electron beam, the iterative method of sequential lower relaxation was used. An optimized version of a high-orbit gyroTWT has been obtained, which has an electronic efficiency of 28 %, a wave efficiency of 23 %, a gain of 34 dB and a gain band of 11 % at an operating frequency of more than 30 GHz. This was achieved by introducing an additional conducting section of the waveguide into the absorbing part of the waveguide, which led to an improvement in the azimuthal grouping of electrons in the Larmor orbit and, as a consequence, to an increase in the lamp efficiency. A twofold increase in the waveguide length made it possible to increase the lamp gain. Ohmic energy losses in the walls of the waveguide reach 5 % of the power of the electron beam. The implementation of such a powerful gyro-TWT (2 MW) in the millimeter wavelength range will significantly increase the capabilities of radar at long distances and increase the resolution of the radar.

Three-dimensional inhomogeneous thermal fields of the “Photon-Amur 2.0” payload electronic board developed for nanosatellites

The thermal fields of the Photon-Amur 2.0 payload electronic board developed for nanosatellites were studied. The Photon-Amur 2.0 payload consists of an electronic control board with a casing mounted in a nanosatellite and a remote panel with experimental photovoltaic converters. A modified heat balance method was used for numerical simulation of the thermal fields of the control board and the casing. The constructed model and the obtained results of the numerical simulation were verified by comparison with the thermal diagrams obtained for the Photon-Amur 2.0 electronic board under normal operating conditions. For modeling the outer space operating conditions, it was assumed that there is a vacuum outside and inside the Photon-Amur 2.0 casing, and the thermal effect is transmitted from the nanosatellite racks to the payload electronic board through the fastenings. The thermal effect is of a periodic nature with amplitude of 45 to +80○C and a period of 96 min, which approximately corresponds to the motion of a nanosatellite in a 575 km-high orbit. It was demonstrated that with such composition of the payload module, its casing can work as a passive thermoregulator of thermal fields on the electronic board of Photon-Amur 2.0. The simulation showed that the casing helps to keep the temperature on the control board in the interval of 15C to +85C, which is acceptable for the electronic components used on the payload control board.

Elevation Spatial Variation Analysis and Compensation in GEO SAR Imaging

Due to geosynchronous synthetic aperture radar’s (GEO SAR) high orbit and low relative speed, the integration time reaches up to hundreds of seconds for a fine resolution. The short revisit cycle is essential for remote sensing applications such as disaster monitoring and vegetation measurements. Three-dimensional (3D) scene imaging mode is crucial for long-term observation using GEO SAR. However, this mode will bring a new kind of space-variant error in elevation. In this paper, we focus on the analysis of the elevation space-variant error. First, the decorrelation problems caused by the spatial variation are presented. Second, by combining with the SAR imaging geometry, the elevation spatial variation is decomposed into two-dimensional (2D) space variation of range and azimuth. Third, an imaging algorithm is proposed to solve the 3D space variation and improve the focusing depth. Finally, simulations with dot-matrix targets and distributed targets are performed to validate the imaging method.