February 11, 2000
Dear AIRSAR Customer,
Enclosed please find the AIRSAR along track interferometry (ATI) experimental dataset. Although AIRSAR has had the capability to collect ATI data since the late 1980’s, we have not had an operational ATI data processor that allows us to meet customer demand for these data. In the past several years a group at JPL have been working on an ATI processor that we hope will allow us to make ATI data more accessible and available. This experimental dataset represents the first step in that process.
The full dataset for a full 60km scene would be require over 40 GBytes of disk space. Therefore, several looks have been taken for these data and the files deemed most useful for distribution have been selected.
As with any experimental product there are caveats that come along with these data. These data are not fully calibrated. In particular:
The geo-location of these data assume that the data all reside at sea-level, which will not be true for land within the scene. Moreover, the motion-compensation algorithms rely on a fixed elevation reference. Where the scatterer elevation varies from that reference, a differential phase will be introduced into the inteferogram. This should have no effect on ocean scenes, but will introduce phase variations following the topography of the land in a scene. These phase variations should not be interpreted as arising from motion of scatterers over the land.
There is an unknown global phase offset for each interferogram. This propagates into the velocity maps as an unknown velocity offset. The relative velocities are accurate, but the absolute velocity is unknown.
There are clear indications of motion-compensation/phase-center alignment errors in these data. We are continuing to work to improve the data quality.
The velocities are not "un-wrapped" – any interferometric phase-difference greater than 360 degrees introduced by scatterer motion will map to a modulo-360 degree phase difference. The velocities reported are also modulo the ambiguous velocity for each baseline and frequency.
We remind users of ATI data that only the component of the scatterer’s velocity in the look direction of the radar is measured. The incidence angle map may be used to interpret this velocity component in terms of horizontal (or vertical) velocity components.
A list of the data files and their description is attached to this letter, along with a diagram showing the location of the AIRSAR ATI antennas on the aircraft.
Since we are in the early stages of providing these types of data, we would appreciate any feedback that you can give to us on the utility of these data sets for your research.
We look forward to providing more of these data in the near future.
Sincerely,