Aerial Synthetic Aperture Radar Research Articles

Active on-orbit surface accuracy adjustment for spaceborne antennas based on multi-fidelity adaptive migration learning strategy

For the large spaceborne synthetic aperture radar antenna, its surface accuracy has a decisive impact on the on-orbit electrical performance, which is affected by manufacturing errors, assembly misalignments, and thermal deformations. Therefore, ensuring the surface accuracy on orbit and reducing its fluctuation are highly concerned issues for the large spaceborne antenna in practice. To this end, this study proposes an active adjustment method based on a multi-fidelity adaptive migration learning strategy to guarantee on-orbit surface accuracy. First, the theoretical surface accuracy model considering space thermal deformation is completely established. Then, the surrogate model of surface accuracy is developed to realize fast prediction and optimization, in which a new dynamic Kriging modeling method is proposed. Furthermore, the surrogate prediction model at other sites on the orbit is efficiently obtained by an innovative multi-fidelity migration learning strategy. On this basis, the active adjustment optimization model is constructed based on the surrogate prediction model, and the corresponding adjustment amounts can be obtained accordingly by solving this optimization problem. Finally, the effectiveness and advantages of the proposed method are comprehensively proven through case studies and experimental verification.

Estimation of the spatial resolution of a synthetic aperture radar antenna of the VHF frequency range placed on board an unmanned aerial vehicle along a real flight path

Background. Currently, there is an intensive development of radar systems for remote sensing of the Earth, including those placed on unmanned aerial vehicles. Synthetic aperture radars are an effective means of round-the-clock and all-weather observation of objects on the Earth’s surface. Radars operating in the VHF range allow you to observe hidden and subsurface objects. The need for practical implementation of VHF band radar with synthesizing the aperture on board unmanned aerial vehicles requires additional research on the spatial resolution of radars, taking into account the unstable trajectory of the carrier. Aim. The article considers the analysis of the influence of the flight path and its measurement errors on the quality of the radar image. Methods. The research method provides an analysis of the shape of the radar uncertainty function with the synthesis of an aperture, which was built along a real trajectory using semi-natural mathematical modeling. Results. As a result of the conducted research, it is shown that the quality of radar images is influenced not only by errors in the knowledge of motion parameters, but also by the very nature of the carrier’s movement and the duration of the flight. Conclusion. Based on the results of field tests, it turned out to be possible to analyze these factors and determine the requirements for the UAV navigation system. Thus, the article shows the possibility of quality control of VHF band radar with synthesized aperture placed on board a UAV.

Fast Reconfiguration Maneuvers of a Micro-satellite Constellation Based on a Hybrid Rocket Engine

AbstractIn this work, the formation flight of the CubeSat cluster RODiO (Radar for Earth Observation by synthetic aperture DIstributed on a cluster of CubeSats equipped with high-technology micro-propellers for new Operative services) with respect to a small satellite in LEO (Low Earth Orbit) has been analyzed. RODiO is an innovative mission concept funded by the Italian Space Agency (ASI) in the context of the Alcor program. The small satellite is equipped with an antenna that allows it to function as a transmitter, whereas RODiO functions as a receiver. The extension of the virtual SAR (Synthetic Aperture Radar) antenna can be achieved by establishing an along-track baseline performing an orbital coplanar maneuver. Another interesting scenario is the possibility to create a cross-track baseline performing an inclination change maneuver. Such formation reconfiguration maneuvers can be achieved in relatively short times only by use of a high-thrust propulsion system, i.e., based on conventional chemical technologies. From the study of maneuvers, it is possible to identify the required ∆V (order of magnitude of 10 m/s), which represents an input parameter for the design of propulsion system. Among the different kinds of propulsion systems, a Hybrid Rocket Engine was chosen. Based on the previous experience acquired by Department of Industrial Engineering (University of Naples Federico II), the preliminary design of the thrust chamber for a Hybrid Rocket Engine based on Hydrogen Peroxide (91 wt%) of the 10 N-class could be carried out, whose dimensions meet the compactness requirements of the CubeSat (1.5 U, 2 kg).

Development of a deployable Synthetic Aperture Radar antenna for a nanosatellite conceptual design

For a real-time maritime surveillance application in the extended territorial waters of New Zealand, this work aims to develop a viable concept design for a deployable Synthetic Aperture Radar (SAR) antenna to fit in a CubeSat design space. To trade off a large antenna aperture, which is essential to enhance SAR performance, and a high packaging efficiency, potentially ≤ 12U, a 4 m x 0.3 m reflectarray with a stiff-and-flexible and novel passive deployable system is selected as the most promising solution for this application. A High Strain Composite (HSC) structure with a shallow “tape-measure” inspired shape is presented to support the SAR reflectarray and compactly stow it through a rollable deployment system. The release of the elastic energy stored in the coiled structure will enable the deployment, and the structural stiffness will be provided by a boundary condition that maintains the naturally curved cross-section during stowage. With this design constraint, the transition zone (ploy region) that develops between the stowed and deployed state may significantly impact the stowage capabilities. The elastic behaviour of the structure will be analytically studied in terms of the change of curvatures and bending stiffness as they dominate the deformed shape in the coiling process. Existing analytical models that predict the ploy regions are considered to estimate the natural ploy length and the equivalent coil diameter. Finite Element (FE) models are developed to compare the analytical models and explore the stress field generated within the ploy region when the length of this region is forced to be shorter than the natural ploy length. FE model observations and data are also used to improve the analytical model that describes the deformed shape by imposing non-uniform boundary conditions on the curvatures’ field distribution within the ploy region.

Practical Noise Model of an Active Electronically Scanning Array

Active electronically scanning arrays are now widely used in radar and communication systems. The noise characteristics are critical to the system signal-to-noise ratio, thus modeling of the antenna array noise is useful. In this paper, a two-port noise model of an active electronically scanning array is presented, which is more practical than the model in previous publications. In this model, an antenna array with a multistage combiner network is equivalent to a one-stage combiner network, thus the calculation of the noise figure is greatly simplified. Since the model is based on measurable parameters, it can be applied directly to the design of electronically scanning arrays. The noise model of a spaceborne synthetic aperture radar antenna system is displayed for example. Some useful conclusions are got about the noise figure for tapered antenna arrays. For a weighted antenna array with taper efficiency εT, if the gain of low noise amplifiers is sufficiently large, the noise figure of the antenna array is increased by 1/εT, compared to a uniformly weighted array. Besides, the deterioration of the signal-to-noise ratio for a tapered antenna array system originates from the increase of antenna noise temperature, rather than the decrease of antenna aperture gain.

Design of the connection mechanism between the basic units of a planar deployable antenna

Under the development trend of large-size antennas, the demand for modular expansion of synthetic aperture radar antennas is increasing, and the connection mechanism is an important part of the modular expansion. This paper designed and studied the connection mechanism between the basic units of the planar deployable antenna mechanism. Firstly, for the basic deployable units of a new type of planar deployable antenna spatial support mechanism proposed previously by the author, the connection mechanism between the basic units was synthesized, and a series of feasible planar deployable mechanism configurations were obtained. Meanwhile, a spatial support mechanism for planar deployable antennas with scissor was optimally constructed by analysis. Through finite element comparative analysis, it was found that the mechanism improves the stiffness and reduces the maximum deformation. Then, scale optimization was performed by using the control variable method. Finally, the prototype was developed, and experimental research was conducted to verify the feasibility of the mechanism. The spatial support mechanism of the planar deployable antenna with scissor proposed in this paper improves the reliability, stiffness, modal, and other related properties of the unfolding process. Therefore, it has broad application prospects in the field of aerospace deployable antennas.

SAR Antenna Pattern Measurement by Internal Calibration Method for GF-3 Satellite

GF-3 is the first C-band multipolarization synthetic aperture radar (SAR) satellite in China. The SAR system is equipped with an active phased array antenna to electronically generate various antenna beams. The accuracy of SAR antenna patterns is of main importance for precise SAR image processing. The traditional method for antenna pattern measurement is the near-field method but it is unable to satisfy the demand for fast measuring in-orbit. In order to measure the SAR antenna pattern quickly with high accuracy, we propose the internal calibration method. The internal calibration uses calibration signals which are routed along the nominal signal path in SAR systems, thus monitoring the gain and phase variations for each T/R channel is possible. This paper will focus on this method, including its principle, operation steps, and results, compared to the near-field method. The internal calibration method provides a valuable solution for SAR antenna pattern measurement.

Synthetic-Aperture-Radar–Based Spacecraft Terrain Relative Navigation

A spacecraft terrain relative navigation system for low elevation orbit satellites is presented relying on measurements obtained from a synthetic aperture radar (SAR) that are fused with inertial measurements and attitude determination system data in a multiplicative extended Kalman filter. Techniques such as autofocusing and geocoding are employed to retrieve information from SAR images for incorporation into the navigation filter. Monte Carlo simulation results are presented in which actual filter performance is compared to predicted filter performance. Specifically, two test cases, with varied initial SAR antenna misalignment uncertainty, show that the SAR-based terrain relative navigation system is a valid approach to GPS-denied navigation.

Design and optimization of new space modular planar antenna

The increasing interest in large-scale antennas has heightened the need for a deployable antenna mechanism with a high folding ratio and stiffness for SAR (synthetic aperture radar) antennas. This paper designed and studied a new type of planar antenna support mechanism. First, this article completed the design of the new antenna mechanism. In contrast, this mechanism has the advantages of good geometric stability and modular expansion. Then, an equivalent model was established to verify the working performance of the mechanism. According to the mechanism design plan of Section 2, this article completes the design and optimization of mechanism parameters, which contain strut section parameters and height parameters. Finally, a scaled-down prototype has been developed, which was used to test deployment performance.

The Influence of the Azimuth RCS Pattern of Calibrator on SAR Absolute Calibration

The realization of radiometric calibration relies on the known artificial point targets as the reference, such as corner reflectors. One of the criteria of the calibrator is that the 3 dB-beamwidth of the azimuth radar cross section (RCS) pattern needs to be larger than the synthetic aperture radar (SAR) antenna 3-dB beamwidth to ensure the SAR antenna, which has a narrow azimuth beamwidth, can align to the calibrator while also making the nominal RCS value of the calibrator approximately constant during the radiometric calibration. However, the RCS of the calibrator is actually inconstant within a synthetic aperture period, especially when the SAR antenna beamwidth is wide. The simplified treatment will introduce errors to calibration. In this letter, the azimuth RCS pattern is taken into account. The error caused by the constant RCS is modeled. Based on the error model, an improved criterion of the calibrator is discussed. The experiments use the known SAR system parameters for numerical simulation based on dihedral corner reflectors (DCRs) with different sizes. The results verify the error model and indicate that when the SAR beamwidth is larger than the beamwidth of the calibrator, and the introduced error cannot be ignored. The relationship between the size of the calibrator and the SAR beamwidth is further analyzed through the experiments.

Study on Design of Novel Function/Structure Integration Mounting Plate for Space borne SAR Antenna for Wireless Communication

A novel lightweight and high-precision synthetic aperture radar (SAR) antenna mounting plate scheme based on functional structure integration technology is proposed. The thermal control function, high and low frequency blind insertion signal transmission function of the system are combined with the antenna structure to greatly reduce the size and weight of SAR antenna. The design verification of a high-density integrated SAR antenna mounting plate breaks through the key process technologies such as accurate splicing of multiple interfaces of the mounting plate, highly reliable cementation of non-uniform honeycomb and large-scale warpage control, and solves the key technical difficulties of high precision, high temperature and high reliability. The results show that the antenna mounting plate (size 649mm × 430mm) the overall flatness is better than 0.28mm, the thickness limit dimension deviation is less than 0.1mm, and the temperature consistency is less than 1.8°C. It can meet the requirements of lightweight, structural stiffness and strength, RF blind plug connection and heat dissipation for space borne SAR antenna.

Hybrid Analog-Digital SAR Instrument With Reflector Antenna and Overlapped Subarray Feed for Earth Observation

As part of the European Space Agency (ESA) studies of new instrument for Earth observation, the design of a synthetic aperture radar (SAR) architecture based on overlapped array antennas was launched last year. The objective of the activity is to develop a test unit of a reflector based SAR antenna fed by an array organized in overlapped subarrays, supporting novel multichannel wide-swath SAR architectures, and exploiting multiple beams in both azimuth and range/elevation directions. In the letter, a hybrid analog/digital beamforming on reception with analog beamforming approach on transmission is analyzed. The antenna system consists of a large oversize reflector with a feedarray capable of generating multiple beams on receive for digital processing. In the letter, a description of the hybrid SAR Instrument is presented, describing the characteristics of the system, the antenna, the considered feedarray options and the main characteristics and system performances.

Optical design of an integrated imaging system of optical camera and synthetic aperture radar.

This paper presents an integrated imaging system of optical camera and synthetic aperture radar (SAR). The system can realize 400 nm-900 nm visible and near infrared band and 35 GHz microwave Ka band dual-band imaging. Compared with the single band imaging system, the observation ability and environmental adaptability of the integrated imaging system have been significantly improved. The optical camera shares a common front system with the synthetic aperture radar. After simulation, the average modulation transfer function (MTF) of 50 line pairs per millimeter (lp/mm) of the optical subsystem is 0.47. In addition, a principle prototype with a pupil diameter of 210 mm was developed to verify the performance of synthetic aperture radar antennas. After the experimental test, the SAR radiation pattern simulation results are in good conformity with the measured results, which are in line with the expected results.

Link Adjustment for Assembly Deviation Control of Extendible Support Structures via Sparse Optimization

The assembly accuracy of the extendible support structure is of importance for the imaging capability of synthetic aperture radar antennas. In general, due to manufacturing imperfections and installation variations, its assembly accuracy will be inevitably degraded. Therefore, controlling the assembly deviation is highly concerned in practice. To meet the accuracy requirement and make “the control” more efficient, this study proposes a novel method to quantitatively conduct dimensional adjustment of links for extendible support structures of synthetic aperture radar antennas. Since the extendible support structure is generally over-constrained in the deployed configuration, the relationship between the assembly deviation and the variation sources is first derived by means of equivalent transformation. Based on the mathematical expression of assembly deviation, an inequality constrained sparse optimization model for quantitatively resizing links is formulated. Then, an efficient algorithm integrating the alternating direction method of multipliers and binary search is developed to solve the above optimization model, thereby acquiring the optimal combination of link adjustment. Finally, numerical case studies are carried out to demonstrate the effectiveness of the proposed method in Matlab, which show that it can not only achieve satisfactory performance in prediction but also significantly improve the assembly efficiency.

Thermal Design and Analysis of Unfurlable CFRP Skin-Based Parabolic Reflector for Spaceborne SAR Antenna

To ensure high-resolution image acquisition for a spaceborne parabolic synthetic aperture radar (SAR) antenna, an appropriate thermal design is important to minimize the thermal distortion of the antenna reflector in severe on-orbit thermal environments. This paper describes the results of a preliminary thermal design and the analysis of a carbon fiber reinforced plastic skin-based unfurlable parabolic reflector for use in a spaceborne SAR antenna. The effectiveness of passive thermal designs for an antenna reflector using different thermal coatings was investigated with on-orbit thermal analyses according to the various antenna look angles to derive the most suitable design for minimizing the thermal gradient of the reflector. This contributes to minimize loss in antenna gain to ensure the SAR performance. In addition, the influence of the solar panel on the thermal gradient of the reflector was also analyzed because it is also important in affecting the thermal distortion.

Along-Track Interferometric SAR Systems for Ground-Moving Target Indication: Achievements, Potentials, and Outlook

Along-track interferometric (ATI) synthetic aperture radar (SAR) has proven a powerful tool for ground-moving target indication (GMTI). ATI-SAR systems use more than one antenna (at least two), mounted on a moving platform and displaced along the moving direction. Each SAR antenna provides a complex image of the same ground scene acquired with a very small time delay, depending on the separation (baseline) between the antennas. Starting from these interferometric images, ground-moving targets can be detected and their radial velocity estimated.

Optimal Design of the Reflector Antenna to Improve Performance of C-Band Quad-Pol ScanSAR Systems

The performance of synthetic aperture radar (SAR) antenna determines the quality of images obtained from the spaceborne SAR system, and thus, SAR antenna should be designed to satisfy the target performance of SAR system. The performance indicators of a SAR system, such as ambiguity-to-signal ratio and resolution, also depend on the SAR operation mode. Therefore, it is important to consider the operation mode when designing an SAR antenna. In this paper, we analyzed the SAR antenna’s effect on ScanSAR (scanning synthetic aperture radar) which is a representative wide-swath mode and its quadrature-polarimetric (quad-pol) mode. In addition, we confirmed that the optimal reflector antenna for the C-band quad-pol ScanSAR system can be designed by analyzing the relation between the antenna size and the system performance.

Copernicus Sentinel-1 satellites: sensitivity of antenna offset estimation to orbit and observation modelling

Abstract. The SAR (Synthetic Aperture Radar) Copernicus Sentinel-1 satellites require a high orbit accuracy of 5 cm in 3D in comparison to external processing facilities. The official orbit products delivered by the Copernicus POD (Precise Orbit Determination) Service fulfil this requirement. Nevertheless, analyses have shown discrepancies in the orbit results for the two satellites Sentinel-1A and Sentinel-1B. Since the satellites are identical in construction estimated orbit parameters like the scale factor for the radiation pressure are expected to be at the same magnitude, which is not the case. Estimation of GPS antenna offsets leads to differences between the two satellites, which might explain the discrepancies in the estimated orbit parameters. Such offset estimations are, however, very sensitive to orbit and observation modelling. It has to be assured that the results are not biased by insufficient models. First of all, stabilisation of the antenna offset estimation is achieved by improving the observation modelling by applying single receiver ambiguity resolution. The Copernicus Sentinel-1 satellites have a very complex shape with the long SAR antenna and the two large solar arrays. Antenna offset estimation based on different satellite models may give results which differ by up to 1.5 cm. The dispersion of the estimates is quite large depending also on eclipse and non-eclipse periods. Consideration of simple assumptions on satellite self-shadowing effects improves the satellite model and also the results of the antenna offset estimation. Finally, more consistent results for the two Sentinel-1 satellites are achieved by applying the antenna offset estimates.

연료 슬로싱 효과를 고려한 위성 기동 간의 SAR 안테나 반사판의 진동 분석

Sloshing refers to the movement of a liquid free surface caused by disturbances of a liquid partially filled in a container. Sloshing generates forces and moments owing to the pressure distribution on the liquid free surface, which affect the dynamic stability of the vessel. An equivalent mechanical model that exerts the same amount of force and moment on the vessel wall as sloshing is used to analyze the sloshing effect on the vessel. This phenomenon also occurs in the fuel tank of a satellite structure that controls the attitude using a propulsion system with a liquid propellant. A satellite with a deployable synthetic-aperture radar (SAR) antenna, which is made of a thinner and lighter material, is likely to be more susceptible to various disturbances, including liquid sloshing. In this study, an analysis of a deployable SAR antenna reflector vibrating from the liquid fuel sloshing between the changes in the satellite’s attitude was conducted. The equivalent mechanical model is used to simulate the sloshing effect, and the vibrations of the reflector are compared and analyzed with and without the sloshing effect for the specified torque input.

Development and Performance Evaluation of Small SAR System for 100-kg Class Satellite

This article shows development and performance evaluation of a MicroX-SAR demonstration satellite, scheduled to be launched in 2020 for synthetic aperture radar (SAR) application. This compact SAR system and its instruments are briefly described. The SAR antenna consists of deployable seven panels with a slot array and the antenna size is 4.9 × 0.7 m. This article proposes a simple evaluation method of the SAR antenna. The seven-panel antenna array performance is estimated from isolated single-panel antenna measurement results and synthesized scattering matrix. It is validated by comparing with near-field measurement of a seven-panel antenna array. The on-board SAR system is combined with a single-point source simulator and the point source SAR image is evaluated. The ambiguity to the signal ratio is computed for the bandwidth-integrated seven-panel antenna radiation patterns, and finally the pulse repetition frequency selection is computed along with noise equivalent sigma zero to estimate the SAR image quality based on various requirements of swath-width and ground-range resolution as per the application of interest. The prototype system is capable of 1-m ground-range resolution SAR image acquisition for 28-km swath-width within acceptable limits of signal-to-noise ratio and ambiguity levels.