NEW AVIRIS ORTHOCORRECTION PROCESSING AND PRODUCTS
Last Modified 070214, J. W. Boardman
Questions/Comments, contact Sarah.R.Lundeen@jpl.nasa.gov

A new production orthocorrection system has been implemented for AVIRIS data.
Here we briefly describe the new system, its products and detail how they
differ from previous AVIRIS geocorrection files.

The new system uses the same IGM, GLT lookup-table methodology for data
rendering, but employs a full three-dimensional ray tracing and a 30m spatial
resolution digital elevation model for complete orthorectification.  Each pixel
in the imagery is individually ray traced using the best-estimate of sensor
location and attitude until it intersects the digital elevation model.  The
spatial fidelity of the data should be much improved, especially in areas of
rugged and variable across-track terrain.  The previous geocorrection code only
applied a single ground elevation for each scan line of data and those
elevations were determined from interpolation of the very coarse GTOPO30
digital elevation model.  The new ortho code does a full 3-d ray tracing for
each pixel and uses the USGS NED 1 arc second (roughly 30m) digital elevation
data.

While we expect the data to be improved over previous versions of AVIRIS
geocorrection, we do not expect the production data to be accurate or precise
at the subpixel level.  Time and sensor specific angular and spatial offsets
and biases affect the solution.  We have used data from multiple flight lines
in April 2006 to solve for average values for these sensor-to-body parameters.
While the solution showed these parameters to be mostly stable, so the mean is
a good substitute, there was some variation from line to line.  As such, fine
tuning of the ortho model on a line by line basis is still suggested if one
requires subpixel accuracy and precision.  But such line-specific processing,
using ground control points from outside data sources, is well beyond the scope
of the AVIRIS production facility.

The new IGM file (*_ort_igm) is changed in two ways from the previous IGM
files.  First, the new file has three bands, instead of two.  The third band is
the elevation of the ray tracing intersection, our estimate for the ground
elevation at each pixel center.  This third band can be passed through the
ortho GLT (*_ort_glt) to build a map-correct digital elevation model for
draping and/or modeling of the AVIRIS spectral data.  The second change to the
IGM files is the projection system used for the x/y map data in the first two
bands.  Previous versions of the orthocorrection code reported pixel center map
locations in UTM WGS-84 coordinates in the first two bands.  The new code uses
WGS-84 longitude and latitude instead of UTM x-y values.  Band 1 of the
*_ort_igm is WGS-84 longitude and band 2 is WGS-84 latitude (both in decimal
degrees).

In addition to the orthocorrection upgrade there is a wholly new data set
(*_obs and *_obs_ort) that contains pertinent parameters relating to the
geometry of the observation and illumination conditions.  The two versions of
this file are before and after GLT-application.  The file *_obs is in the raw
spatial format and matches the uncorrected AVIRIS data.  The *_obs_ort file has
been rendered using the *_ort_glt lookup table and matches the orthocorrected
imagery.  This observation parameter file has ten bands.  The ten bands, in
order, are: path length (sensor-to-ground in meters); to-sensor-azimuth (0 to
360 degrees cw from N); to-sensor-zenith (0 to 90 degrees from zenith); to-sun-
azimuth; to-sun-zenith; phase (degrees between to-sensor and to-sun vectors in
principal plane); slope (local surface slope as derived from DEM in degrees);
aspect (local surface aspect 0 to 360 degrees cw from N); cosine i (apparent
local illumination factor based on DEM slope and aspect and to-sun vector, -1
to 1) and UTC time (decimal hours for mid-line pixels).  We encourage anyone
using these auxiliary parameters, especially for science-critical applications,
to verify them against other models and sources as this is a new implementation
and an experimental data set.

One additional change in the data delivery is the creation of two separate
ephemeris files.  As before, these are binary files with 6 values stored in
double precision float (Intel byte order) for each AVIRIS scan line.  The six
values store the attitude (roll, pitch and heading) and position (x,y,z) for
the sensor at the mid-point in each scan line.  The file *_eph has the position
data in a WGS-84/NAD83 UTM x,y,z coordinate system.  The file *_lonlat_eph
stores position in WGS-84 longitude, latitude and elevation.  Both ephemeris
files have the raw GPS ellipsoidal elevation converted to orthometric
elevations via use of the NGA EGM96 global geoid model.