David Hounam

TanDEM-X: a TerraSAR-X add-on satellite for single-pass SAR interferometry

TanDEM-X is a mission proposal for a TerraSAR-X add-on satellite for high-resolution single-pass SAR interferometry. This mission proposal has been selected for a Phase A study within the scope of a Call for Proposals for a next German Earth Observation Mission to be launched in 2008/2009. The mission has the goal of generating a global Digital Elevation Model (DEM) with an accuracy corresponding to the DTED-3 specifications (12 m posting, 2 m relative height accuracy for flat terrain). This goal will be achieved by means of a second, TerraSAR-X like satellite (TanDEM-X) flying in a close orbit configuration with TerraSAR-X. This paper describes the mission concept and requirements, including several innovative aspects like operation modes, orbit selection and maintenance as well as PRF and phase synchronization. Results from a detailed performance estimation show the achievable DEM accuracy. Finally, an overview of the potential of the TanDEM-X mission for several scientific applications is presented.

A technique for the identification and localization of SAR targets using encoding transponders

Targets in SAR images can be tagged for unambiguous identification and localization by equipping them with transponders that encode the signal received from the SAR before it is retransmitted. The encoding technique described below requires no modification of the SAR. The SAR data can be processed to image the transponders with the background suppressed. The transponder response is suppressed in a conventionally processed SAR image. The encoding technique enables the transponder signal to be automatically detected in the SAR data.

The calibration concept of TerraSAR-X: a multiple-mode, high-resolution SAR

TerraSAR-X is a high-resolution X-band synthetic aperture radar (SAR) satellite due for launch in 2006. The sensor has a spatial resolution down to 1 m and operates in Stripmap, Spotlight, and ScanSAR modes with selectable or dual polarization. It can image on the left or right side of the subsatellite track, which is achieved by rolling the satellite. There are also experimental modes for wide bandwidth, providing even higher resolution, and for full polarization and along-track interferometry (ATI), the latter two being achieved by splitting the receive antenna into two halves. Owing to this high degree of flexibility of the instrument and the tight performance requirements, the calibration of the sensor is a major challenge, and new concepts are needed to keep the costs affordable. For this purpose a novel internal calibration concept was developed, the so-called PN-gating method. In using this method, individual transmit and receive modules can be characterized under the most realistic conditions. Furthermore, cost-effect concepts for external calibration are mandatory because of the large number of modes and possible antenna patterns. The characterization of the antenna is based on a precise antenna pattern model, which provides reference patterns required for the relative radiometric correction of the SAR data. The paper describes the calibration measures planned for the TerraSAR-X instrument and discusses their implementation. The concept is applicable to other advanced SAR sensors.

TerraSAR-X: calibration concept of a multiple mode high resolution SAR

Due to the high degree of flexibility of the TerraSAR-X instrument and the tight performance requirements, the calibration of TerraSAR-X becomes a major challenge and new concepts are needed to keep the costs affordable. The paper decsribes the Calibration Concept of TerraSAR-X and provides an overview of the Calibration Segment as part of the TerraSARX Ground Segment.

An autonomous, non-cooperative, wide-area traffic monitoring system using space-based radar (TRAMRAD)

To meet the challenges of ever increasing road traffic and the associated economic and sociological impacts, new techniques and technologies for better traffic management are needed. The TRAMRAD project (Traffic Monitoring with space-based Radar) aims to profit from research and development in earth observation and advances in radar remote sensing techniques to define a future space-based sensor system for the wide-area monitoring of road traffic. The paper describes the requirements for the system and the concepts being investigated. In particular, it discusses the complex detection conditions, the requirements on the radar instrument and the methods for processing the data. Possible system concepts are described and their capabilities are discussed.

An encoding SAR-transponder for target identification

A coded transponder was developed for operation in C-band and verified using the SAR sensor of the Active Microwave Instruments (AMI) on the ERS-1 and ERS-2 satellites. The coded transponder uses signal modulation techniques which affect the SAR signal in both range and azimuth and decouples the transponder signal from the background signal. Encoding adds additional features without losing any of the advantages transponders have over passive targets.

Techniques for reducing SAR antenna size

A major driver for the cost of a SAR satellite is the size of the antenna, which largely determines the dimensions of the payload, and, hence, indirectly of the whole satellite. As part of the SAFARI study to develop strategies for future space-borne SAR systems, techniques were investigated for suppressing ambiguities during processing, in order to relax the requirements on the antenna. The techniques are also valid for airborne SAR systems. The paper summarises the methods and discusses the impact on sensor design and the applications.

Soil parameter estimation and analysis of bistatic scattering X-band controlled measurements

In this paper, we will present well controlled experimental bistatic X-band measurements of rough surfaces, which have been recorded in the Bistatic Measurement Facility (BMF) at the DLR Oberpfaffenhofen, Microwaves and Radar Institute. The bistatic measurement sets are composed of soils with different statistical roughness and different moistures controlled by a TDR (Time Domain Reflectivity) system. The BMF has been calibrated using the Isolated Antenna Calibration Technique (IACT). The validation of the calibration was achieved by measuring the reflectivity of fresh water. In the second part, the first validation of the specular algorithm by estimating the soil moisture of two surfaces with different roughness scales will be reported. Additionally, a new technique using the coherent term of the Integral Equation Method (IEM) to estimate the soil roughness will be presented, as well as evaluation of the sensitivity of phase and reflectivity with regard to moisture variation in the specular direction.

Analysis of multi-frequency polarimetric data for assessment of bare soil roughness

The aim of this study is to assess the bare soil surface roughness parameter, i.e. the root mean square height (h/sub rms/) when the moisture is constant, using anechoic chamber measurements based on fully polarimetric scatterometer data. An incidence angle based algorithm has been proposed to assess the bare soil height h/sub rms/. For this purpose, sets of experimental backscattering data have been evaluated on different types of rough surfaces (Rough Gaussian, h/sub rms/ = 2.5 cm, Smooth Gaussian, h/sub rms/ = 0.4 cm and Medium Mixed surface, h/sub rms/ = 0.9 cm) with known geometrical and dielectric properties. The scattering matrix of those three surfaces under test was measured in monostatic mode vs. frequency ( 1 - 19 GHz) and incidence angles ( /spl theta/ = 10/spl deg/ to 50/spl deg/ in steps of 10/spl deg/ for h/sub rms/ = 2.5 cm and 0.9 cm and 5/spl deg/ for h/sub rms/ = 0.4 cm) data. An empirical relationship has been developed between backscattering, h/sub rms/ and incidence angle independently for L-, C-, X- and Ka-band for all polarizations (i.e. HH, VV and HV). This relationship provides the calculated backscattering values, which is helpful in the inversion process. A good agreement has been obtained between the observed and calculated h/sub rms/. The analyses show the strong dependence of h/sub rms/ on incidence angle, polarization and frequency. This type of work is also helpful in the near future to predict the optimum sensor parameters (i.e. incidence angle, polarization and frequency) for measuring the bare soil roughness.