Synthetic Aperture Radar Payload Research Articles

Mission operations management of a regional coverage highly agile SAR satellite: Few key aspects

The regional coverage satellite constellations are in demand for fulfilling earth observation needs of specific user regions of interest as opposed to global coverage thus enhancing the revisit period for the area. In this paper, we present a few key mission aspects of the regional coverage constellation designed and operated by the Indian Space Research Organization (ISRO) in low earth orbit. ISRO developed three X-Band Synthetic Aperture Radar (SAR) Earth Observation satellites and placed them in three inclined orbit planes to enhance the Indian region's coverage. This is India's first inclined orbit constellation designed for remote sensing applications in low earth orbit. The satellites were built around a standard bus carrying a Synthetic Aperture Radar payload with a deployable Radial Rib Antenna (RRA). They were designed to achieve a high agility capability. As SAR sensors are capable of all-weather, day, and night imaging; thus, most suitable in inclined orbits where the illumination conditions vary continuously for imaging. Though the inclined orbits provide more revisit opportunities to the region of interest, their characteristics pose challenges in mission design and operations. Though the mission design and planning involved a host of elements, this paper brings out the details of a few key mission analysis elements, planning aspects and operations that were critical in the course of various mission phases. The mission analysis included mission-specific requirements like launch windows selection for the constellation and reference attitude generation specific to inclined orbit geometry. Mission planning had to take care of Radial Rib Antenna (RRA) deployment requirements which were very critical for the mission, apart from taking care of individual subsystem requirements. The SAR payload data product quality and targeting accuracy are very sensitive to the satellite image point range (vis-a-vis SAR operation data window). Extensive post-launch calibration activity was carried out to meet all these mission requirements and data product specifications. The challenge was in fixing the payload beam alignment and fixing the uncertainties in onboard and ground computations. The paper also describes the methods of fixing all possible error sources affecting the high-resolution SAR payload performance.

An analytic method for precisely determining the revisit time of SAR satellites and optimizing RGT orbit design

This paper addresses a particular analytic method for calculating the revisit time (RT) of Synthetic Aperture Radar (SAR) satellites in the circular repeating ground track orbit (RGTO) and its application to optimizing the orbital elements according to the payload characteristics. The most critical aspect of SAR payloads is sun-independent, allowing photos to be collected from ascending and descending phases (both-phase). Different from the traditional methods only considering single-phase, the proposed method is mapping every pass into its ascending and descending points on the Equator. A new concept of half-subcycle is presented in this paper to determine the distribution of the ascending and descending points. Through the combination of subcycle and half-subcycle, the both-phase ground track sequences can be fully obtained without resorting to complex computer programs. The minimum, maximum, and average RT, considered at the Equator, can be computed fast by establishing and analyzing the revisiting tracks in the both-side non-nadir instantaneous access area of the SAR payload. The step value of the RT can be a half-day instead of a full day as in previous researches. By studying both-phase acquisition and both-side non-nadir looking, the obtained results reflect better the actual characteristics of SAR payloads, comparing to traditional approaches. The method is also applied in studying SAR satellite’s orbital design process. With mission requirements and payload characteristics provided as inputs, the optimal orbit is rapidly determined.

Unified Classification Framework for Multipolarization and Dual-Frequency SAR

For synthetic aperture radar (SAR), multi-polarization and multi-frequency modes greatly enrich the acquired earth resource information and have been widely applied in remote sensing fields. In this paper, we compare the classification capabilities of multi-polarization and dual-frequency SAR. To meet the objective of selecting consistent and complete polarimetric information, a unified classification framework is proposed. In the framework, covariance matrices are used directly as inputs instead of polarimetric indicators. Additionally, the Wishart mixture model (WMM) is utilized to characterize the statistical distribution of polarimetric SAR data. Besides, the data log-likelihood function is utilized to mitigate the influence of the initial values on the expectation-maximization (EM) algorithm. Then, among the combinations of four sample-to-subclass distances and two schemes for obtaining sample-to-class distances, the one with the best classification performance is selected as the default for this framework. In the experiments, the classification capabilities of full polarization (FP), compact polarization (CP), and dual polarization (DP) modes are first compared through the proposed classification framework. Then, we compare the classification capabilities of dual-frequency SAR in FP, CP, and DP modes. For PolSAR system design, it is necessary to strike a balance between demand indexes (classification performance, coverage width, and so on) and cost (such as budget, weight, and so on). The comparison results provide a reference for the optimization of polarization modes and frequency bands of the existing multi-polarization and dual-frequency SAR payloads and the design of future SAR systems.

TanDEM-X Long-Term System Performance After 10 Years of Operation

The TanDEM-X mission, formed by the TanDEM-X satellite in cooperation with its almost identical twin TerraSAR-X (TSX), has mainly been designed to acquire bistatic synthetic aperture radar (SAR) images of the Earth. Initiated in 2010, the primary mission objectives were to generate a global digital elevation model (DEM) to perform scientific measurements and to explore novel SAR techniques. Up to the present day, all these objectives have been completed with outstanding results. In addition, the satellites maintained a stable image quality from the start throughout their entire lifetime. The ground segment, which was tailored to the capabilities of the space segment, as well as an excellent design and manufacturing of the satellites were the key elements to achieve such a performance. In addition, the elaborate calibration concept and a careful treatment of the on-board resources enabled full operability up to the present day and allowed additional mission objectives such as the generation of a Change-DEM layer as an update to the global DEM. The stable bus and SAR payload performance as revealed by long-term system monitoring provide the basis for further extension of the TanDEM-X mission for several more years. In the following paper, these long-term system monitoring results are presented to provide an overview of the evolution of the radar system. The results and experience gained with more than a decade of TanDEM-X mission operation are a great value for both the scientific and commercial community and serve as a forerunner for the development of future SAR missions.

FINITE ELEMENT MODELING OF LAPAN-A5/CHIBASAT DEPLOYABLE SOLAR PANEL COMPOSITE PLATE

LAPAN-A5/ChibaSat, that will carry synthetic aperture radar payload, requires a lot more power generation capacity, compared to its predecessor. Therefore, its solar panel will be deployed in orbit to ensure maximum sun exposure. Since the deployable system requires solar panel plate that lightweight and strong, honeycomb composite material is selected. The selection of such material requires special treatment in the satellite structural stiffness calculation. The objective of the research is to find simple stiffness model of the plate, so that later it can be integrated with the total satellite structural model and used by launch authority in its total vehicle modal analysis. The modeling used finite element software, and the simplified honeycomb model is validated using standar plate stiffnes problem. After model validation, the boundary condition as in the LAPAN-A5/ChibaSat deployable system is imposed. The result shows that the stiffness of the deployable solar panel plate has met the launch requirement of PSLV’s auxiliary payload. Therefore, the design mode can be used in the development of LAPAN-A5/ChibaSat.

Dual‐polarised X‐band patch sub‐array with hybrid feed network for integrated small‐satellite SAR payload

A low-profile wideband dual-polarised X-band patch sub-array fed by a hybrid series/parallel network is designed for an integrated small-satellite synthetic aperture radar payload. The experimental results demonstrate that the sub-array, with 1 × 16 patch radiators, fully covers the targeted 9.65 GHz band with 14 dB bandwidths of 9–10.3 GHz for V-polarisation (VP) and 9.3–9.95 GHz for H-polarisation (HP). The isolation between the different feed ports is higher than 36 dB with a peak value of 45 dB. The sub-array achieves cross-to-co-polarisation ratios of below −32 dB within half-power beamwidth scopes of 6° for both HP and VP. The sidelobes maintain −24 dB for HP and −20 dB for VP. The average realised gains are around 17 dBi for both polarisations.

The SAR Payload Design and Performance for the GF-3 Mission.

This paper describes the C-band multi-polarization Synthetic Aperture Radar (SAR) sensor for the Gaofen-3 (GF-3) mission. Based on the requirement analysis, the design of working modes and SAR payload are given. An accurate antenna model is introduced for the pattern optimization and SAR performance calculation. The paper concludes with an overview of predicted performance which was verified by in-orbit tests.

Feasibility study: highly integrated chipset design for compact synthetic aperture radar payload on micro-satellite

Micro- and nano-satellites are attractive due to the low development and launching costs. Carried by micro- and nano-satellites, synthetic aperture radars (SARs) have great potential for urban, oceanography, land use, and agriculture usages. Different than conventional satellites, the payload of micro- and nano-satellites is limited. This imposes great challenges on the SAR system design. Traditional SARs adopt thousands of millimeter-wave integrated circuit (MMIC) components; they are bulky and power hungry. This paper investigates the feasibility to deploy the emerging technologies to shrink the volume, reduce the cost, and improve the power efficiency of the SAR system. Highly integrated radar transceiver integrated circuits (ICs) are reviewed, and an SAR transceiver IC is developed. Design technologies of low-noise amplifiers (LNAs) and high-power amplifiers (HPAs) are compared, and Gallium Nitride (GaN) technology is proposed. A novel microelectromechanical system-based delay line is also proposed for satellite SAR to reduce the system size. Existing issues and expected improvements of these technologies are also elaborated. This work shows a clear route map for the future shrinkage of SAR system, and would be a useful guideline to the development of compact SARs for micro-satellites.

Low-Profile Broadband Dual-Polarized Integrated Patch Subarray for X-Band Synthetic Aperture Radar Payload on Small Satellite

A low-profile broadband dual-polarized patch subarray is designed in this letter for a highly integrated X-band synthetic aperture radar payload on a small satellite. The proposed subarray is lightweight and has a low profile due to its tile structure realized by a multilayer printed circuit board process. The measured results confirm that the subarray yields 14-dB bandwidths from 9.15 to 10.3 GHz for H-pol and from 9.35 to 10.2 GHz for V-pol. The isolation remains better than 40 dB. The average realized gains are approximately 13 dBi for both polarizations. The sidelobe levels are 25 dB for H-pol and 20 dB for V-pol. The relative cross-polarization levels are 30 dB within the half-power beamwidth range.

Mission scheduling optimization of SAR satellite constellation for minimizing system response time

A Synthetic Aperture Radar (SAR) satellite constellation can be used in Intelligence, Surveillance and Reconnaissance (ISR) missions such as the detection of abnormal activities by repeatedly observing an area of interest (AoI). In order to cover an AoI more efficiently, coverage overlap should be avoided. Mission scheduling optimization can be greatly beneficial in increasing the observation efficiency of a constellation of satellites equipped with SAR payloads. This paper describes an optimal scheduling algorithm developed for such missions and the proposed Concept of Operation (CONOP). The optimization of constellation operation was performed by minimizing the system response time which is defined as the time it takes from the user image request to final distribution of the data. An optimizing algorithm based on a genetic algorithm is introduced in order to reduce the system response time. In this paper, the modeling and mission scheduling algorithm for this type of mission has been developed and described. The results show that the mission planning using the optimal scheduler can reduce the system response time, and this reduction is more pronounced when the coverage criterion (required area coverage %) is higher.

다목적실용위성 5호의 SAR 영상 검·보정을 위한 이동형 능동 트랜스폰더 개발

다목적실용위성 5호는 영상 레이더(SAR: Synthetic Aperture Radar)을 탑재하여 야간이나 구름이 있는 지역에 대해서도 지속적인 영상촬영이 가능하다. 발사 후 초기 운영 시험에서는 SAR 탑재체에 의해 관측된 영상에 대한 기하 및 복사 보정을 수행한다. 이러한 보정은 지표상의 위치 및 RCS(Radar Cross Section)을 알고 있는 보정장치에서 반사된 SAR 신호에 의해 나타난 영상 픽셀에서의 위치 및 값을 이용하여 처리된다. 이러한 보정을 위해 기존 반사기와 달리 능동 트랜스폰더는 RCS 값을 변경할 수 있으며, 내부 시간 지연 기능에 의해 가상의 위치로 이동하는 효과를 영상에서 나타낼 수 있다. 본 논문에서는 능동 트랜스폰더의 개발에 필요한 요구사항을 도출하여 이로부터 설계, 조립 및 시험결과를 정리하였다. KOMPSAT-5(KOrea Multi-Purpose SATellite-5) has a benefit of continuously conducting its mission in all weather and even night by loading SAR(Synthetic Aperture Radar) payload, which is different from optical sensor of KOMPSAT-2 satellite. During IOT(In-Orbit Test) periods, SAR image calibration should be conducted through ground target of which location and RCS is pre-determined. Differently from the conventional corner reflector, active transponder has a capability to change its internal transfer gain and delay, which allows active transponder to be shown in a pixel of SAR image with very high radiance and virtual location. In this paper, the development of active transponder is presented from design to I&T(Integration and Test).

NAVIGATION AND REMOTE SENSING PAYLOADS AND METHODS OF THE SARVANT UNMANNED AERIAL SYSTEM

Abstract. In a large number of scenarios and missions, the technical, operational and economical advantages of UAS-based photogrammetry and remote sensing over traditional airborne and satellite platforms are apparent. Airborne Synthetic Aperture Radar (SAR) or combined optical/SAR operation in remote areas might be a case of a typical "dull, dirty, dangerous" mission suitable for unmanned operation – in harsh environments such as for example rain forest areas in Brazil, topographic mapping of small to medium sparsely inhabited remote areas with UAS-based photogrammetry and remote sensing seems to be a reasonable paradigm. An example of such a system is the SARVANT platform, a fixed-wing aerial vehicle with a six-meter wingspan and a maximumtake- of-weight of 140 kilograms, able to carry a fifty-kilogram payload. SARVANT includes a multi-band (X and P) interferometric SAR payload, as the P-band enables the topographic mapping of densely tree-covered areas, providing terrain profile information. Moreover, the combination of X- and P-band measurements can be used to extract biomass estimations. Finally, long-term plan entails to incorporate surveying capabilities also at optical bands and deliver real-time imagery to a control station. This paper focuses on the remote-sensing concept in SARVANT, composed by the aforementioned SAR sensor and envisioning a double optical camera configuration to cover the visible and the near-infrared spectrum. The flexibility on the optical payload election, ranging from professional, medium-format cameras to mass-market, small-format cameras, is discussed as a driver in the SARVANT development. The paper also focuses on the navigation and orientation payloads, including the sensors (IMU and GNSS), the measurement acquisition system and the proposed navigation and orientation methods. The latter includes the Fast AT procedure, which performs close to traditional Integrated Sensor Orientation (ISO) and better than Direct Sensor Orientation (DiSO), and features the advantage of not requiring the massive image processing load for the generation of tie points, although it does require some Ground Control Points (GCPs). This technique is further supported by the availability of a high quality INS/GNSS trajectory, motivated by single-pass and repeat-pass SAR interferometry requirements.

Electrical performance verification methodology for large reflector antennas: based on the P-band SAR payload of the ESA BIOMASS candidate mission

In this paper, an electrical performance verifi- cation methodology for large reflector antennas is pro- posed. The verification methodology was developed for the BIOMASS P-band (435 MHz) synthetic aperture radar (SAR), but can be applied to other large deployable or fixed reflector antennas for which the verification of the entire antenna or payload is impossible. The two-step methodology is based on accurate measurement of the feed structure characteristics, such as complex radiation pattern and radiation efficiency, with an appropriate measurement technique, and then accurate calculation of the radiation pattern and gain of the entire antenna including support and satellite structure with an appropriate computational soft- ware. A preliminary investigation of the proposed meth- odology was carried out by performing extensive simulations of different verification approaches. The experimental validation of the methodology included measurements of the prototype BIOMASS feed in several structural configurations with spherical, cylindrical, and planar near-field techniques. The measured characteristics for the feed structure were then used in the calculation of the radiation pattern and gain of the entire reflector antenna. The main emphasis of the work was on the assessment of the achievable pattern and gain uncertainty for the entire antenna and its compliance with the BIO- MASS SAR requirements.

위성체 지향 및 안정화 오차로 인한 영상레이더 위성 영상 품질 저하 해석

레이더 영상의 품질은 영상레이더 탑재체 뿐만 아니라 위성 본체의 성능에 크게 좌우되기 때문에, 영상레이더 위성의 영상형성구조분석(Image Chain Analysis)은 위성체 설계단계에서 가장 중요한 해석들 중 하나이다. 특히, 위성체 지향 및 안정화 오차는 탑재체 고유 설계에서 유도되는 영상 품질에 가장 나쁜 영향을 끼친다. 이 연구에서는 위성체 지향 및 안정화 오차로 인한 레이더 영상 품질 저하를 분석하기 위한 영상형성구조분석 알고리즘이 개발되었다. 개발된 소프트웨어는 궤도 모델, 자세제어 모델, 영상레이더 탑재체 모델, 클러터 모델, 영상처리 모델로 구성된다. 이 해석을 위해, <TEX>$5km{\times}5km$</TEX>의 관측폭을 가지는 1m고해상도 모드 레이더 영상의 원시 데이터를 시뮬레이션하고, 영상처리를 수행하였다 영상 품질을 분석하기 위해 영상 내에 총 25개 점표적들을 생성하였다. 이 연구의 결과, 레이더 영상의 방사 정밀도는 위성체 지향오차에 가장 많은 영향을 받는 것으로 해석되었으며, 최고의 영상 품질을 만족시키기 위해서는 위성체 자세제어 시스템의 성공적인 설계가 가장 중요하다. Image chain analysis of synthetic aperture radar (SAR) satellite is one of the primary activities for satellite design because SAR image quality depends on spacecraft bus performance as well as SAR payload. Especially, satellite pointing and stability error make worst effect on the original SAR image quality which is implemented by SAR payload design. In this research, Image chain analysis S/W was developed in order to analyze the SAR image quality degradation due to satellite pointing and stability error. This S/W consists of orbit model, attitude control model, SAR payload model, clutter model, and SAR processor. SAR raw data, which includes total 25 point targets in the scene of <TEX>$5km{\times}5km$</TEX> swath width, was generated and then processed for analysis. High resolution mode (spotlight), of which resolution is 1m, was applied. The results of image chain analysis show that radiometric accuracy is the most degraded due to the pointing error. Therefore, the successful design of attitude control subsystem in spacecraft bus for enhancing the pointing accuracy is most important for image quality.

FEASIBILITY STUDY OF SYNTHETIC APERTURE RADAR - ADAPTABILITY OF THE PAYLOAD TO KOMPSAT PLATFORM

Synthetic Aperture Radar (SAR) has been used for mapping the surface geomorphology of cloudy planets like Venus as well as the Earth. The cloud-free Mars is also going to be scanned by SAR in order to detect buried water channels and other features under the very shallow subsurface af the ground. According to the 'Mid and Long-term National Space Development Plan' of Korea, SAR satellites, in addition to the EO (Electro-Optical) satellites, are supposed to be developed in the frame of the KOMPSAT (Korean Multi-Purpose Satellite) program. Feasibility of utilizing a SAR payload on KOMPSAT platform has been studied by KARI in collaboration with Astrium U.K. The purpose of the ShR program is Scientific and Civil applications on the Earth. The study showed that KOMPSAT-2 platform can accommodate a small SAR like Astrium’s MicroSAR. In this paper, system aspects of the satellite design are presented, such as mission scenario, operation concept, and capabilities. The spacecraft design is also discussed and conclusion is followed.

RADARSAT-1 mission planning: Meeting customer needs over 5 years of evolving operations

The RADARSAT-1 Mission Management Office (MMO) has been planning and scheduling the SAR payload, ground reception, and processing facilities that supply the World's synthetic aperture radar (SAR) user community with timely RADARSAT-1 data since 1995. The goal of the MMO is to provide users with timely, high-quality data in a manner continuously being refined to meet changing needs. As the RADARSAT-1 client base and network of stations expanded, the demand for data increased, new applications were found, and new requirements were outlined. Over 5 years the number of ground stations grew from three reception facilities (two in Canada, and one in the United States) to 16 worldwide. To support new facilities, higher data demands, and new data applications, the MMO system required changes to support time-critical information transfer between facilities and improve data delivery timelines and image quality. This paper has two main purposes: describe the current RADARSAT-1 acquisition planning operations system, and summarize the operational improvements made over the last 5 years with the focus being how changes affected SAR users.

The RADARSAT Synthetic Aperture Radar payload

The Canadian RADARSAT Remote Sensing Satellite will employ a Synthetic Aperture Radar (SAR) instrument with a very flexible set of operating modes that include selectable swath widths, ground incidence angle and resolution. This SAR instrument will be able to alter its beam pointing angle to anywhere within a 500 km Earth ground track, with either a wide or narrow swath, at ground incidence angles from 20 to 49°. Swath widths of 50, 100, 150 and 500 km will be available as standard configurations but the antenna pattern is reconfigurable in flight to enable other swath widths to be considered. In addition, the SAR instrument will be able to alter the across-track resolution. This paper describes the many design features of the baseline design of the RADARSAT SAR payload that provide the operating flexibility. The projected performance over the mission life will be described including the breadboard model development and performance predictions.