Short antenna spaceborne SAR concepts

This analysis concludes that, for a “swath/azimuth resolution” (SAR) ratio not exceeding 10⁴ (as is the case for the missions designed and imagined up until now), it is possible, thanks to new imaging modes based on timing diagram saturation, to reduce the length of the antenna by two (defined along the speed vector) under equivalent performance, antenna area, onboard energy and spectral occupancy conditions. One of the advantages is that the Spotlight mode is not needed except if one wishes to exceed the ratio of 10⁴, which gives, in addition to the elimination of the operational limitations of this mode, high cost reduction possibilities with very simplified antenna architectures. These principles are applicable to all types of satellite architectures provided that the rejection of the nadir echo is done other than in the temporal domain, here too saturated. They are especially interesting for the RADAR SAIL case, lowering the costs of the RADAR satellite concept invented by CNES, based on the use of a vertical antenna placed in the orbit plane. On the one hand, they get round the length limitation related to this concept and generalise its utilisation range. On the other hand, the absence of the nadir echo (physically rejected on account of the SAIL concept geometry) enables antenna simplification to be pushed as far as possible without having to preserve a minimum number of control points required for beam nulling toward nadir and, in addition, a single diagram, that of the pencil beam, enables all incidences and modes to be covered, which simplifies the characterisation, calibration and operation of the Radar.     

Range-Doppler Imaging of Rotating Objects

During the integration time required to obtain fine Dopplerfrequency resolution in a range-Doppler imaging radar, a point on a rotating object may move through several range and Doppler resolution cells and produce a smeared image. This motion can be compensated by storing the appropriately processed return pulse, and the angular coordinates are determined by the angular coordinates of the radar antenna. The resulting stored data represents the three-dimensional Fourier transform of the object reflectivity density, and hence can be processed by an inverse Fourier transformation. Also included is an analysis of the three-dimensional radar/object geometry with separate source and receiver locations. The effects of various system aberrations are investigated and experimental results from a microwave test range which demonstrate the image improvement are presented.     

单脉冲双极化天线阵列设计

文章研究了单脉冲双极化天线阵列。天线阵混合馈电微带单元由共面微带线馈电和缝隙耦合微带线馈电两者结合形成,"十"字形微带单脉冲比较器与天线阵同面,具有结构紧凑、加工方便和成本低的优点。经仿真验证,在工作频带内其辐射单元双端口隔离度高于34dB;并且2个极化端口在其对应主平面方向的副瓣电平均低于-20dB。天线可适用于新一代多极化雷达系统。     

Attitude and antenna pointing design of bistatic radar formations

Spaceborne bistatic radar observations allow original scientific applications to be carried out. Furthermore, assuming transmitting and receiving antennas operating on separated platforms, key design issues relevant to formation flying must be solved. Mathematical models are presented for computation of attitude and pointing angles. Main design constraint is the capability of maintaining swath overlap, but selected strategy also depends on the cost of spacecraft attitude maneuvering or antenna beam electronic steering. The model has been applied considering a large transmitting/receiving primary mission and a receiving-only small satellite. In this case antenna steering was preferred. Finally, if the passive antenna is smaller than the active one, overlap maintenance is simplified, obviating the need for yaw rotations.     

Design and calibration of the SeaWinds Scatterometer

The SeaWinds Scatterometer is a Ku-band Earth orbiting remote sensing radar. It has a 1 m dish antenna shared by two beams with respective nadir look angles of 40 and 46 deg, scanning azimuthally to provide greater than 90% daily coverage of the Earth at an altitude of 800 km. The first sensor was launched in 1999 and produces sea surface wind field to 2 m/s accuracy at 25 km resolution. The design and calibration of the SeaWinds radar is described here.     

Sidelobe blanking with integration and target fluctuation

In radar systems, sidelobe blanking (SLB) is used to mitigate impulsive interference that enters the radar through sidelobes of the main antenna. SLB employs an auxiliary antenna channel with the output being compared with that of the main antenna channel and a decision is then made as to whether or not to blank the main channel output. SLB performance determination involves the evaluation of several probability functions. Based on the classical Maisel SLB architecture, this work extends previous performance results, in which detection was limited to the case of a single radar pulse with either Marcum or Swerling I target fluctuation. Probability expressions have been generalized to include both an arbitrary number of integrated pulses and target fluctuation models based on the gamma distribution. The Swerling fluctuation models are all special cases of the gamma distribution. Results are derived in terms of two generalized probability functions, one for detection and the other for blanking. With these generalized probability functions, the SLB design and performance results can be determined. Examples are presented and discussed.     

Analysis and Design of Antennas for Air Traffic Collision Avoidance Systems

The analysis and design procedure of an antenna for a CW Doppler radar system being developed for pilot warning of midair collision hazards is presented. The antenna consists of two vertical arrays of half-wavelength dipoles mounted near a circular conducting cylinder. Each vertical array is composed of three vertical dipoles. Each array provides relatively uniform illumination (2-3 dB) in the forward 180 angular segment of the horizontal plane and approximately + 10-150 coverage in the vertical plane. The antenna could be used in a two-mode operation, either in a standard monopulse radar system (sum and difference amplitude patterns) or in a system where amplitude and phase are the measurable quantities.     

Ka波段高增益圆极化机载微带天线阵的设计与实现

设计、分析、制作了一种工作在Ka波段的4单元高增益、圆极化机载微带天线阵列。利用顺序旋转阵列和串并馈方式的微带线网络馈电,显著改善了天线的轴比带宽和驻波带宽,减少了馈线损耗,实现了高增益。采用L型微带调配枝节对贴片单元进行匹配,便于组阵,解决了阵列空间布局不合理的难题;馈电网络和天线单元集成在同一介质基板上,易于加工和制作。测试结果表明:该天线阵具有高增益和良好的圆极化特性,从而证实了该方法的正确性和实用性。本文所设计的天线由于具有体积小、质量轻、结构紧凑、易于集成等优点,在机载相控阵雷达以及卫星通信系统中具有很好的应用前景。     

Spiky sea clutter at high range resolutions and very low grazingangles

X-band (9.5-10.0 GHz) backscatter at near grazing incidence (0.2 deg) from the sea off the coast of Kauai, Hawaii, was measured with a radar characterized by a high spatial resolution in range (0.3 m) and a high temporal resolution (2000 Hz pulse repetition frequency (PRF)). Extensive amounts (over 20 min per measurement) of vertically and horizontally polarized sea clutter data were taken with upwind (UP) and crosswind (CR) transmit geometries during the collection campaign. Specific but representative examples of the clutter were statistically and phenomenologically analyzed over time scales varying from long (200 s), to intermediate (5 s), to short (50 ms), and over range swaths varying from full (160 m), to partial (30 m), to a single range cell (0.3 m). All analyses and results presented here are noncoherent, involving only the clutter amplitudes. Each type of clutter exhibited the characteristic spiky behavior which has come to be expected from microwave sea backscatter observed at low grazing angles and high range resolutions, while showing, between themselves, marked transmit geometry and polarization dependent contrasts, with the horizontally polarized clutter, measured with an UP transmit geometry, being especially notable for its frequently occurring, significant high frequency spectral content. Within the same clutter type, differences were observed in the probability distributions of radar cross sections (RCS) of spatially and temporally extended spiking events     

Radar beamwidth reduction techniques

It is essential and desirable to get a high resolution echo in a radar image without using a large antenna. On scanning a radar antenna, the target's distribution is smoothed by the antenna beam and a smeared echo appears on the radar display. This paper describes three different types of beam compression radars: subsurface SAR; correlation array radar; and inversion processed radar. The experimental results show that the beamwidth on display was effectively compressed by the signal processing     

Earth Footprints of Satellite Antennas

This paper develops the expressions for computing the footprint on Earth (i.e., power density contours) of a satellite antenna. The expression for determining the antenna pattern coordinate as a function of the Earth coordinates is derived. The antenna pattern operator which converts the pattern function into the available power density is obtained. In this case it is assumed that the Earth-based antenna is directed towards the satellite. The radar operator is also derived; this operator yields the power density intercepted by the Earth.     

UAVSAR: New NASA Airborne SAR System for Research

NASA's Jet Propulsion Laboratory is currently building a reconfigurable, polarimetric L-band synthetic aperture radar (SAR), specifically designed to acquire airborne repeat track SAR data for differential interferometric measurements. Differential interferometry can provide key deformation measurements, important for studies of earthquakes, volcanoes, and other dynamically changing phenomena. Using precision real-time GPS and a sensor controlled flight management system, the system will be able to fly pre-defined paths with great precision. The expected performance of the flight control system will constrain the flight path to be within a 10 m diameter tube about the desired flight track. The radar will be designed to be operable on a Unpiloted Arial Vehicle (UAV) but will initially be demonstrated on a NASA Gulfstream III. The radar will be fully polarimetric, with a range bandwidth of 80 MHz (2 m range resolution), and will support a 16 km range swath. The antenna will be electronically steered along track to assure that the antenna beam can be directed independently, regardless of the wind direction and speed. Other features supported by the antenna include elevation monopulse and pulse-to-pulse re-steering capabilities that will enable some novel modes of operation. The system will nominally operate at 45,000 feet (13,800 m). The program began as an Instrument Incubator Project (IIP) funded by NASA Earth Science and Technology Office (ESTO).     

Zenith Pass Problem of Inter-satellite Linkage Antenna Based on Program Guidance Method

While adopting an elevation-over-azimuth architecture by an inter-satellite linkage antenna of a user satellite, a zenith pass problem always occurs when the antenna is tracing the tracking and data relay satellite （TDRS）. This paper deals with this problem by way of, firstly, introducing movement laws of the inter-satellite linkage to predict the movement of the user satellite antenna followed by analyzing the potential pass moment and the actual one of the zenith pass in detail. A number of specific orbit altitudes for the user satellite that can remove the blindness zone are obtained. Finally, on the base of the predicted results from the movement laws of the inter-satellite linkage, the zenith pass tracing strategies for the user satellite antenna are designed under the program guidance using a trajectory preprocessor. Simulations have confirmed the reasonability and feasibility of the strategies in dealing with the zenith pass problem.     

用于SAR运动补偿的DGPS/SINS组合系统研究

使用考虑位置误差相关项的伪距率观测模型 ,研究了用于合成孔径雷达运动补偿的差分 GPS/ SINS伪距率组合系统。结果表明 ,组合系统的长期位置精度能达到 1 m左右。 GPS数据更新率低于 INS,在 GPS测量时间间隔内 ,组合系统的性能仅由 INS决定。虽然 INS误差随时间积累 ,在 GPS数据更新率为 1 s的情况下 ,即使采用中等精度的惯性仪表 ,其相对位置精度为厘米级 (这里相对位置精度指组合系统在 GPS测量时间间隔内位置误差的变化范围)。     

Space-time-frequency processing of synthetic aperture radar signals

The subject of this work is the detection and high resolution microwave imaging of objects moving on the ground and observed by an airborne radar. The proposed approach is based on a combined space-time and time-frequency processing. The space-time processing makes use of a linear array antenna and exploits the radar motion for filtering the received echoes in order to improve as much as possible the signal-to-disturbance ratio. The signal is then mapped onto the time-frequency domain, by computing its Wigner-Ville distribution, for a further filtering and for estimating its instantaneous frequency, necessary for the formation of a high resolution image of the moving object     

The Lunar Reconnaissance Orbiter Miniature Radio Frequency (Mini-RF) Technology Demonstration

The Miniature Radio Frequency (Mini-RF) system is manifested on the Lunar Reconnaissance Orbiter (LRO) as a technology demonstration and an extended mission science instrument. Mini-RF represents a significant step forward in spaceborne RF technology and architecture. It combines synthetic aperture radar (SAR) at two wavelengths (S-band and X-band) and two resolutions (150 m and 30 m) with interferometric and communications functionality in one lightweight (16 kg) package. Previous radar observations (Earth-based, and one bistatic data set from Clementine) of the permanently shadowed regions of the lunar poles seem to indicate areas of high circular polarization ratio (CPR) consistent with volume scattering from volatile deposits (e.g. water ice) buried at shallow (0.1–1 m) depth, but only at unfavorable viewing geometries, and with inconclusive results. The LRO Mini-RF utilizes new wideband hybrid polarization architecture to measure the Stokes parameters of the reflected signal. These data will help to differentiate “true” volumetric ice reflections from “false” returns due to angular surface regolith. Additional lunar science investigations (e.g. pyroclastic deposit characterization) will also be attempted during the LRO extended mission. LRO’s lunar operations will be contemporaneous with India’s Chandrayaan-1, which carries the Forerunner Mini-SAR (S-band wavelength and 150-m resolution), and bistatic radar (S-Band) measurements may be possible. On orbit calibration, procedures for LRO Mini-RF have been validated using Chandrayaan 1 and ground-based facilities (Arecibo and Greenbank Radio Observatories).     

Differential Lobing in Sidelobe Suppression Systems

The sidelobe suppression function in the secondary surveillance radar system is analyzed when separate main and control antenna are used. Their vertical lobing structures are not matched with each other, so a differential lobing pattern comes out whose maxima and minima are enveloped by smooth curves depending on vertical displacement of phase centers and horizon cutoff rates. The developed analysis provides a method for the evaluation of the system performance for any antenna configuration and for the determination of design guidelines.     

MMW scanning antenna

Cost reduction is one of the most important considerations in millimeter-wave (MMW) radar development. Hardware, especially the phase-shifters, for electronic beam scanning in the MMW band presents very complex fabrication challenges that dramatically raise the device cost. A typical mechanically-scanning antenna contains one or more hinged parts (lenses, mirrors or feeds). In operation they experience strong mechanical accelerations and other forces that sharply limit the scanning speed. We propose a solution to this problem, using a leaky-wave periodically loaded antenna design and a new architecture. This paper presents the basic design, and discusses related issues     

Coherent Doppler tomography-a technique for narrow band SAR

Some concerns regarding a technique of narrowband synthetic aperture radar (N-SAR) imaging called coherent Doppler tomography (CDT), which may be a good candidate for spaceborne applications, are addressed. Using a single-frequency signal, are addressed. Using a single-frequency signal, resolution of two tenths of a wavelength can be achieved in the point spread function if the radar platform circles the ground path to be imaged. However, the high sidelobe level of -8-dB in the point spread function results in an unacceptable dynamic range. To reduce the sidelobe level, two approaches are presented: coherent processing using multiple discrete frequencies and noncoherent subaperture processing. Simulation results demonstrate that the sidelobe level is substantially reduced by both methods. However, the resolution is degraded and the computational overhead is greatly increased for noncoherent subaperture processing. Also presented is a possible satellite geometry configuration that could utilize N-SAR processing to provide high-resolution global mapping capability     

极化滤波反射面隐身天线研究

介绍了一种极化滤波反射面天线,将天线工程中的周期结构反射面技术应用于解决飞行器天线的RCS问题,采用由金属丝栅网构成的周期结构作为天线主反射面,使其对正交极化的来波具有良好的透射性,从而降低了天线的RCS。给出了此天线辐射方向图和雷达散射截面(RCS)的实测结果,并与普通金属反射面天线的结果进行了比较。