Kim Woelders

EMISAR: an absolutely calibrated polarimetric L- and C-band SAR

EMISAR is a high-resolution (2/spl times/2 m), fully polarimetric, dual-frequency (L- and C-band) synthetic aperture radar (SAR) system designed for remote-sensing applications. The SAR is operated at high altitudes on a Gulfstream G-3 jet aircraft. The system is very well calibrated and has low sidelobes and low cross-polar contamination. Digital technology has been utilized to realize a flexible and highly stable radar with variable resolution, swath width, and imaging geometry. Thermal control and several calibration loops have been built into the system to ensure system stability and absolute calibration. Accurately measured antenna gains and radiation patterns are included in the calibration. The processing system is developed to support data calibration, which is the key to most of the current applications. Recent interferometric enhancements are important for many scientific applications.

Dual polarization stacked microstrip patch antenna array with very low cross-polarization

This paper describes the development and performance of a wideband dual linear polarization microstrip antenna array used in the Danish high-resolution airborne multifrequency polarimetric synthetic aperture radar, EMISAR. The antenna was designed for an operating frequency of 1.25 GHz/spl plusmn/50 MHz and was built as an array of 8/spl times/2 probe-fed stacked microstrip patches. The feeding network is constructed in microstrip and is capable of handling 6 kW of peak input-power at an altitude of 45000 ft (unpressurized). The impedance bandwidth (return loss better than -14 dB) of the antenna is 10%, the isolation between the horizontal and the vertical ports of the array is 50 dB and the cross-polarization suppression is 40 dB. A new design principle for simultaneously achieving very low cross-polarization and low side lobes in dual linear polarization antenna arrays has been applied.

A low-cost glacier-mapping system

An old portable 60 MHz radar has been upgraded with a new digital data-processing and -acquisition system and a new antenna construction enabling a fast and low-cost installation on aTwin Otter aircraft. Augmented by a laser altimeter and kinematic global positioning system (GPS), the system has the capability of acquiring accurate data on location and ice-surface elevation, and adequate-quality data on ice thickness. The system has been applied successfully in mapping the Nioghalvfjerdsfjorden glacier, northeast Greenland, in spite of the difficult conditions with melting water on the glacier surface. The measurements from the floating part of the glacier have been evaluated by comparison of radar data with laser-altimeter and in situ measurements.

Microstrip antenna for polarimetric C-band SAR

The paper outlines the design and the measured performance of a 224-element dual-linearly polarized microstrip array antenna with low cross-polarization. The array is currently being flown on the Danish high-resolution polarimetric C-band synthetic aperture radar (SAR).

The Danish polarimetric SAR for remote sensing applications

Presents the Danish polarimetric SAR system, EMISAR, and the approach taken in the system design to achieve a reliable high performance system. The design and implementation of the antenna system as well as the analog and digital hardware are discussed. The SAR utilises a dual polarised microstrip antenna with probe fed patches and the antenna exhibits a modified cosec-squared pattern with high polarisation discrimination. The two transmitted polarisations are time multiplexed at the transmitter and at the dual channel (V- and H-polar) receiver front end while azimuth pre-filtering is implemented by independent circuits for the four parallel channels (VV, VH, HV, and HH). The system has been designed to permit both C- and L-band data to be recorded on the same high density digital tape. The paper presents the performance of the system including measured resolution, peak- and integrated sidelobes, polarimetric channel imbalance and cross-talk.<<ETX>>

EMISAR single pass topographic SAR interferometer modes

The Danish Center for Remote Sensing (DCRS) has augmented its dual-frequency polarimetric synthetic aperture radar system (EMISAR) with single pass across-track interferometric (XTI) modes. This paper describes the system configuration, specifications and the operating modes. Analysis of data acquired in 1995 indicates that height resolutions better than one meter should be attainable at least in areas with benign topography where overlay shadows and phase unwrapping errors are minimal. Calibration procedures applied to correct for multipath and limited channel isolation are discussed.

A high resolution polarimetric L-band SAR-design and first results

An L-band polarimetric SAR system has been developed as part of the dual frequency (L- and C-band), polarimetric, airborne EMISAR system. The SAR features a unique combination of fine resolution (2/spl times/2 m) and wide swath (9.3 km). The transmitter power is 6 kW. From a flight altitude of 41,000 ft the range of the radar is 64 km with a noise equivalent sigma naught of -20 dB. The antenna is a stacked microstrip patch array with the feed structure on the back side of the antenna panel to reduce unwanted radiation. The cross polar level is below -35 dB. The polarization switch is a relatively conventional PIN diode switch matrix able to sustain the 6 kW peak power from the transmitter still exhibiting low loss (0.3 dB) and high isolation (more than 50 dB). Thus system cross talk (between polarizations) is dominated by antenna cross talk and is some -35 dB. Polarimetric imagery has been acquired over the EMISAR calibration scene: An agricultural site in Denmark featuring a range of different fields and forested areas as well as several trihedrals and dihedrals. Based on the imagery data, sensor performance is assessed. The L- & C-band polarimetric EMISAR instrument is one of the key sensors in the JRC EARSEC initiative aiming at supplying state-of-the-art remote sensing data to European scientists.

EMISAR: C- and L-band polarimetric and interferometric SAR

EMISAR is a C- and L-band fully polarimetric (i.e. 4 complex channel per frequency) synthetic aperture radar designed for remote sensing with high demands for resolution (2 m), polarization discrimination, and absolute radiometric and polarimetric calibration. The present installation has one 3-axes stabilized antenna (C- or L-band) and two flush mounted C-band antennas providing the system with cross track and repeat track interferometric capabilities.

Microstrip antenna arrays for airborne high-performance, polarimetric SAR systems

Many dual-linear polarization antenna arrays, e.g. for polarimetric synthetic aperture radar (SAR) systems, are required to have very low cross-polarization levels. In order to improve the cross-polarization performance of the entire array over that of the basic dual-polarization element from which the array is constructed, a well-known element mirroring technique has often been employed. This technique improves the cross-polarization performance of the array, but it may also cause generation of grating lobes. A new principle is described for obtaining very low cross-polarization levels in dual-linear polarization antenna arrays and simultaneously avoiding the generation of grating lobes. This paper reviews the new principle and presents its implementation in a wideband, dual-linear polarization microstrip antenna array used in the Danish airborne L-band polarimetric SAR system “EMISAR”. The antenna array, comprising 8 × 2 probe-fed stacked patch elements, exhibits a 10 % bandwidth, a peak directivity of 18 dBi and a cross-polarization level of 40 dB below the co-polar. Suggestions for further enhancements to the novel principle are given.

Science data collection with polarimetric SAR

Discusses examples on the use of polarimetric SAR in a number of Earth science studies. The studies are presently being conducted by the Danish Center for Remote Sensing. A few studies of the European Space Agencys EMAC programme are also discussed. The Earth science objectives are presented, and the potential of polarimetric SAR is discussed and illustrated with data collected by the Danish airborne EMISAR system during a number of experiments in 1994 and 1995. The presentation will include samples of data acquired for the different studies.