EMISAR

EMISAR is a dual frequency (L- and C-band) fully polarimetric (4 complex channels per frequency) Synthetic Aperture Radar (SAR) system developed for remote sensing applications.

The system also has a single pass cross-track interferometric mode for topographic / elevation mapping applications and a repeat pass interferometric mode for high resolution elevation mapping and change detection applications.

The system is used for DRCS applications but is also available for scientific partners and, to a limited extend, also for customers. The latter may contact Professor Erik Lintz Christensen  for further information on costs and availability.  

Background

Since 1989 Electromagnetics Institute (EMI) has flown a C-band, vertically polarized, Synthetic Aperture Radar (SAR) with a 2 by 2m resolution and an 80 km range. A full swath, full resolution real-time processor was completed in 1992. The C-band system has since been upgraded to full polarimetric capability (i.e. amplitudes and phases of HH, VH, HV, and VV polarization). An additional L-band system with full polarimetric capability and the same high resolution and image quality was completed and tested early 1995. C-band single pass cross-track interferometry capability was added in 1996. Repeat pass interferometry at both L-band and C-band is facilitated by the system providing very accurate flight track control.

The EMISAR system is presently operated on a Gulfstream G3 aircraft of the Royal Danish Air Force. The G3 is a twin engine jet, with a 6000 km range. The SAR is nominally operated at an altitude of 41,000 ft. The 3-axes stabilized antenna system (supporting L- or C-band) and the radar INU are installed in a pod mounted below the fuselage, which facilitates rapid system installation and dismount. Additional flush mounted antennas have been installed to provide the C-band single pass interferometric capability. 

The development of the C-band SAR was sponsored by the Thomas B. Thriges Foundation while the enhancement of the system to dual frequency (C- and L-band) polarimetric capability was supported by the Danish Technical Research Council and the European Union, represented by the Institute for Remote Sensing Applications of the Joint Research Centre (JRC), Ispra. The project has also benefited extensively by the aircraft and other support from the Danish Defense and in particular the Royal Danish Air Force.

The major application of the system is data acquisition for the research of the Danish Center for Remote Sensing (DCRS) which has been established at Electromagnetics Institute on funding from the Danish National Research Foundation. During 1994 and 1995 the SAR system was used to acquire polarimetric data for EMAC (European Multi-sensor Airborne Campaigns) arranged by ESA.

The EMISAR sensor

EMISAR can collect SAR data in different modes dependent on the application. For all modes the data are absolutely radiometrically calibrated:

Polarization modes

  • Quad, where the transmit polarization is alternately horizontal (H) and vertical (V), and data from all four transmit and receive polarization combinations are stored on tape.
  • Dual, where the transmit polarization is alternated from pulse to pulse, and only the two co-polarized channels are stored on tape.
  • Single, where only one polarization is transmitted and received.
  • Single transmit, dual receive channel mode, primarily intended for cross-track interferometry (XTI) applications where the receivers are connected to two different antennas.

Resolution modes

In range, optional filtering and subsampling (decimation) can be carried out before the data are stored in the range line buffers, i.e. resolution can be traded for swath width. The range line buffers hold 8192 samples. In azimuth, pre-filtering to 1.5 m sample spacing is normally applied to limit the data rate but a finer sampling is possible, e.g. 0.75 m sample spacing if either: 1) only one frequency is recorded; 2) only 2 polarizations are recorded at each frequency; or 3) the swath is reduced. Normally, data are collected on tape with 1.5 m x 1.5 m sample spacing, and the unprocessed slant range swath width is 12 km. In wide swath modes the sample spacing is 3.0 m x 1.5 m or 6.0 m x 1.5 m (range and azimuth), and the corresponding swath widths are 24 km and 48 km.

The sensor is normally operated with the antenna pointing at zero Doppler but data recording at a fixed stabilized squint angle is also supported. This feature is useful when the aircraft drift angle is outside the maximum possible antenna yaw correction range.

Dual frequency capability

The full range of different modes are supported for both L-band and C-band. Simultaneous operation at both frequencies is also fully supported except for the fact that simultaneous L- and C-band polarimetry necessitates the swath to be reduced or the resolution to be degraded. The reason is that the present pod can only accommodate one stabilized antenna. Dual frequency data is presently acquired either by changing the antenna between two mappings, or by mounting the L-band antenna in the pod and using one of the flush mounted antennas for C-band polarimetry. However, the present azimuth pre-filters do not support the data from the flush mounted antennas with range varying Doppler centroid.

Interferometric capability

EMISAR supports single-pass interferometry (XTI) as well as multi-pass interferometry (RTI). Two flush-mounted C-band antennas provide a 1.14 m long baseline oriented 34 degrees with respect to the horizontal plane. The antennas are fully polarimetric, so either HH or VV polarized XTI data can be collected. Since the antennas are connected via a switch matrix the system can be operated in multiple XTI modes: 1) single baseline (transmit on one antenna, receive on both), 2) double baseline (transmit and receive on one antenna then transmit and receive on the other), and 3) dual baseline (transmit on one antenna and receive on both, then transmit on the other and receive on both, thus collecting both double and single baseline data simultaneously).

RTI is supported via a flight control system capable of keeping the actual flight track within a few meters of the desired track. The SAR control computer gets navigation information from the aircraft P-code GPS receiver, computes the track displacement and displacement rate, and emulates an instrument landing system (ILS) receiver which is connected to the aircraft flight director computer.