Oct 4, 2004
Product ID Site - Flight Line Length (km) Bandwidth (C,L,P) Mhz
ts1809 costarica221-1 60 40-40-40
 
URL to backscatter image: http://blacks.jpl.nasa.gov/PRECISION/ts1809.jpg
 
The standard AIRSAR data files are included on the CD-ROM/DVD along with browse GIF image files.  Please note that due to large file sizes, CD-ROM’s may be too small to hold all the data, and the data might be spread across multiple discs.  For more information about what these files are and the format, please visit the http://airsar.jpl.nasa.gov web page.

We applied a P-band filter to reduce the impact of interference in the P-band imagery. However, there may be a small amount of p-band interference still present in the data. In addition, in some cases, the P-band filter is not effective. Because the P-band filter may adversely impact the calibration of the data, we provide both the filtered and unfiltered files.

This data was acquired in ping-pong mode, which means that the XTI antennas were alternately used to transmit and receive this effectively doubles the baseline (potentially increasing the height accuracy), but decreases the SNR. Either the normal or the doubled baseline can be used in processing the data. Usually, we use the long baseline, but if there are problems, we use the normal baseline. The long baseline was used in processing this data.

The AIRSAR DEM product is in meters and represents the elevation of the terrain above a spherical approximation to the WGS-84 ellipsoid. The radius of this approximating sphere is calculated using a so-called "peg point".

Common errors in the imagery:

It is not unusual for the processed DEM to suffer from multi-path effects that we currently cannot correct. These artifacts manifest themselves as vertical along track banding (of a few meters) in the topography.

In areas of low correlation (sometimes associated with uncompensated motion, but usually in areas that are shadowed or subject to layover), we sometimes have difficulty unwapping the phase, which results in holes in the imagery.

Uncompensated motion can effect the calibration of the imagery (looks like cross track streaks). Usually this occurs if the flight was very turbulent, or if the motion data was not properly tracking the motion of the DC-8.

If we have sufficient tie points, we may be able to detect errors in the geolocation of the data relative to the map data. In that case, we will update the location of the imagery, but we will note in this letter if the correction was necessary. It will be noted below under Tie-pointing information.

Tie-pointing information
3 Ground control points (GCPs) for the ts1809 product were obtained from SRTM DEM 90m Postings scale maps, which have 5m contour intervals. We made our best effort to find GCP's that span the cross-track dimension of the imagery. The offset between the datum of the maps and the WGS84 ellipsoid for this product was 0.00000 meters. This offset was applied when calculating the absolute height of the AIRSAR DEM. Therefore, the height values of the AIRSAR DEM will differ from the height values of the topographic maps by this amount.

During tie-pointing, we applied the offset between the datum of the maps and the WGS84 datum that AIRSAR uses, and compared these heights with the heights read off the AIRSAR DEM.

We then calculated the height offset/tilt that produces a DEM in best agreement with the map values. At this stage, the geo-location of the GCP's is not considered.

The c-band height offset applied was -42.5395 m, and the phase correction corresponding to the tilt correction was 13.0412 cycles.

The mean and standard deviation of the height difference between the map values and the image values is -1.40426 m and stdev =0.973398 m at c-band.



You may want to apply additional GCPs as part of your analysis.
Additional Comments
phase unwrapping errors have resulted in some gaps in the imagery.
If you have any questions, please contact Bruce Chapman at:
Tel: 818-354-3603
Fax: 818-393-5285
E-mail: bruce.chapman@jpl.nasa.gov