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Jet Propulsion Laboratory California Institute of TechnologyAIRSAR Airborne Synthetic Aperture Radar
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The AIRSAR Project

The Airborne Synthetic Aperture Radar (AIRSAR) is an all-weather imaging tool able to penetrate through clouds and collect data at night.  The longer wavelengths can also penetrate into the forest canopy and in extremely dry areas, through  thin sand cover and dry snow pack. AIRSAR was designed and built by the Jet Propulsion Laboratory (JPL) which also manages the AIRSAR project.JPL, a division of the California Institute of Technology, is a lead research and development center for the National Aeronautics and Space Administration (NASA). AIRSAR serves as a NASA radar technology testbed for demonstrating new radar technology and acquiring data for the development of radar processing  techniques and applications. As part of NASAís Earth Science Enterprise, AIRSAR first flew in 1988 and continues to conduct at least one flight campaign each year, either in the United States or on an international mission.AIRSAR

A Flying Laboratory  AIRSAR instrument (panels behind wing) mounted aboard a modified NASA DC-8 aircraft.  During data collection, the plane flies at 8 kilometers over the average terrain height at a velocity of 215 meters per second.

 


The AIRSAR Instrument

AIRSAR is a side-looking radar instrument and can collect fully polarimetric data (POLSAR) at three radar wavelengths: C-band (0.057 m), L-band (0.25 m), and P-band (0.68 m).AIRSAR can also collect two types of interferometric data: cross-track interferometric dataAIRSAR (TOPSAR), which are sensitive to topography and along-track interferometric (ATI) data, which can be used to measure ocean surface currents.

AIRSAR Instrument-Antennas  The AIRSAR radar antennas on the back part of the DC-8 aircraft.  These antennas are used to collect data in all three modes.  L-band ATI uses an additional antenna (not shown) located in front of the wings.

 

 

AIRSAR Operation Modes

AIRSAR provides  three operational modes:

POLSAR:      P-, L-, C-band full polarization

TOPSAR      XTI1:C-band VV, C-band DEM, P- and L-band full polarization

††††              XTI2:C- and L-band VV and DEMs, P-band full polarization

 ATI†††††††††   C-band and L-band;VV polarization only


POLSAR Data

In POLSAR mode, fully polarimetric data are acquired at all three frequencies.Fully polarimetric means that radar waves  are alternatively transmitted in horizontal (H) and vertical (V) polarization, while every pulse is received in both H and V polarizations. AIRSAR POLSAR data are sensitive to the geometry (including vegetation) and dielectric properties (water content) of the terrain.

 

Land Classification  Polarimetric data over coastal Thailand in an area of mangrove forests along the coast, agriculture and fish farming (square ponds).


TOPSAR Data

 In TOPSAR mode, AIRSAR collects interferometric data using C- and L-band vertically-displaced antenna pairs to produce digital elevation models (DEM's). The radars which are not being used for interferometry (P-band for XTI2 or P-band and L-band for XTI1) collect quad-pol data co-registered with the C-band DEM.  Interferometric data can be collected in "ping-pong" mode, where each antenna is used alternately for transmit and the effective baseline is doubled, and in "common-transmitter" mode where only one antenna is used for transmit.

AIRSAR

Topographic Mapping  Topographic radar data over the Ray Mine in central Arizona.  Colors show elevation information from the mine pits in the center of the image up through the surrounding mountains.  These data are being used for mine site and watershed characterization associated with environmental assessment of the area.

 

ATI Data (Experimental)

Data collected in the along-track interferometry (ATI) mode can be used to measure ocean current velocities.Two pairs of antennas, one C-band and one L-band, separated along the body of the plane, are used to collectAIRSAR ATI data.

Measuring Current Movement  Along-track interferometry radar data collected in the area of the Golden Gate Bridge, San Francisco, California.  Colors in the water indicate differences in the direction and amount of current movement as water flows into and out of the bay.

   

 

AIRSAR Technology Research

AIRSAR enables researchers to develop and test new radar technologies for spaceborne operations. Polarimetric SAR instrumentation demonstrated by AIRSAR was flown as the shuttle imaging radar-C (SIR-C) mission in 1994.  More recently, the February 2000  Shuttle Radar Topography Mission mapped the world with a technique similar to, and demonstrated by, AIRSARís TOPSAR mode. An 80MHz bandwidth option - achieving 1.5m resolution - has been incorporated into AIRSAR in support of a future spaceborne radar mission. 

Currently under development are modes which combine polarimetry with cross-track interferometry and along-track interferometry.

 

AIRSAR Data Applications

AIRSARís polarimetric and interferometric capabilities provide data for a wide range of applications including biomass estimation, soil moisture measurements, vegetation classification, land-use classification, slope estimates for natural hazard studies, along-track interferometry to measure coastal currents, wetland and  flooded forest classification, natural hazard monitoring, geologic mapping, and glacier studies.

AIRSAR Data Collection

AIRSAR data are collected for NASA-funded investigators as well as for other organizations. Once the area and dates for the flight season are scheduled and approved by NASA Headquarters, JPL plans data collection at selected investigatorís sites and works with the investigators to optimize their collection.  Data are processed at JPL and  delivered  to the investigators for analysis.  All processed data are archived at JPL and catalogued on-line (http://airsar.jpl.nasa.gov.)

 

Table: AIRSAR Data Specifications

Parameters

20 MHz

P-L-C POLSAR and TOPSAR

40 MHz

P-L-C POLSAR and TOPSAR*

80 MHz

L-band POLSAR only

Data Swath Width (range)

15 km

10 km

5 km

Slant Range Resolution

6.7 m

3.3 m

1.6 m

 

DEM Posting

10 x 10 m

5 x 5 m

NA

Pixel spacing(x,y) 10x10 m 5x5 m 3x3 m

RMS Height Accuracy (z)

     C-band :                  1-5 meters           1-5 meters              NA

     L-band                    2-10 meters           2-10 meters           NA

Image Calibration (magnitude)

    Absolute:                  3 dB                       3dB                    3dB

Relative:   0.2 dB cross polarization, 1.5 dB between frequencies

* FCC prohibits collection of P-band 40MHz within the United States