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Definitions of AIRSAR Modes and Bandwidth Selections

The Airborne Synthetic Aperture Radar (AIRSAR) has three main modes of operation. These are POLSAR, cross-track interferometry (XTI), also called topographic SAR or TOPSAR, and along-track interferometry (ATI). In the POLSAR mode, polarimetric data are collected at three frequencies. The data products resulting from POLSAR data are in the slant range projection.

In the cross-track interferometry mode (XTI), data are collected to generate digital elevation models (DEMs) at C-band and L-band. Data products resulting from XTI data are in the ground range projection; polarimetric data collected in an XTI mode are co-registered to the DEM and are also in the ground range projection. The along track interferometry (ATI) mode is considered experimental and the processor for ATI data is still in development.

Definition of Radar Modes:
Polsar--> P-, L- and C-band polarimetry (HH, HV, VH and VV polarization combinations).
ATI* --> L- and C-band double-baseline along track interferometry in VV polarization only.
XTI1 --> C-band single-baseline cross track interferometry in VV polarization only. L- and P-band polarimetry.
XTI1P--> C-band double-baseline cross track interferometry in VV polarization only. L- and P-band polarimetry.
XTI2* --> L- and C-band single-baseline cross track interferometry in VV polarization only. P-band polarimetry.
XTI2P*--> L- and C-band double-baseline cross track interferometry in VV polarization. P-band polarimetry.

* : ATI and L-band XTI are operating as experimental modes. Announcements will be made when these modes are deemed operational.

XTI Mode Definitions:

For terrain mapping of areas with relatively small height variations, the double-baseline XTI mode (XTI1P or XTI2P) is suggested. This mode will improve the height accuracy almost by a factor of two in flat areas with a large backscatter. The tradeoff for better height accuracy is an approximate 15% reduction in the cross track swath.

For terrain mapping of areas with large height variations (mountainous or high relief areas), the single-baseline XTI mode (XTI1 or XTI2) is suggested. This mode will improve SNR reducing height estimation errors due to phase unwrapping errors. In addition we are collecting polarimetric interferometry (POLTOP) at C-band. This results in DEM data at four polarizations and can be used to study interferometric phenomenology.

Bandwidth Selection:

AIRSAR operates at either 20 MHz or 40 MHz bandwidth. In addition, we have an L-band polarimetric antenna that collects high resolution data at 80 Mhz. Currently when we collect L-band 80 MHz data, we are not able to collect C-band data. Due to FCC regulations, P-band data can only be collected at 20 MHz within the continental United States.

20MHz bandwidth: for POLSAR data, the azimuth pixel spacing is 9.26 meters and range pixel spacing is 6.6 meters (18 looks taken during processing). For TOPSAR data the azimuth and range pixel spacing are either 5 or 10 meters.

40MHz bandwidth: azimuth pixel spacing is 9.26 meters and range pixel spacing is 3.3 meters (18 looks taken during processing). For TOPSAR data the azimuth and range pixel spacing are either 5 or 10 meters.

80MHz bandwidth: azimuth pixel spacing is 4.63 meters and range pixel spacing is 1.67 meters (9 looks taken during processing).

Table of Modes/Bandwidth versus Cross Track Swath (Based on a nominal aircraft radar altitude of 8km)

Mode Bandwidth (MHz) Swath (km)
POLSAR / XTI1 / XTI2* 20 15
  40 10
ATI* / XTI1P / XTI2P* 20 15
  40 8.5
HI RES SAR (L-band) 80 5
POLTOP (C-band) 40 5

* ATI and L-band XTI are operating as experimental modes. Announcements will be made when these modes are deemed operational.
 
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