1 October 2003
processing. The data at this level would require image calibration and additional proc-
essing. See Appendix F for a list of airborne system sensors.
3-7 Airborne Geometries. There are several ways
in which airborne
can be controlled. Transects should always be flown parallel to the principle plane to the
sun, such that the BRDF (bi-directional reflectance distribution function) is symmetrical
on either side of the nadir direction. The pilot should attempt to keep the plane level and
fly straight line transects. But since there are always some attitude disturbances, GPS and
IMU (inertial measuring unit) data can be used in post-processing the image data to take
out this motion. The only way of guaranteeing nadir look imagery is to have the sensor
mounted on a gyro-stabilized platform. Without this, some angular distortion of the im-
agery will result even if it is post-processed with the plane's attitude data and an eleva-
tion model (i.e., sides of buildings and trees will be seen and the areas hidden by these
targets will not be imaged). Shadow on one side of the buildings or trees cannot be elimi-
nated and the dynamic range of the imagery may not be great enough to pull anything out
of the shadow region. The only way to minimize this effect is to acquire the data at or
near solar noon.
3-8 Planning Airborne Acquisitions.
a. Planning airborne acquisitions requires both business and technical skills. For ex-
ample, to contract with an airborne image acquisition company, a sole source claim must
be made that this is the only company that has these special services. If not registered as a
prospective independent contractor for a Federal governmental agency, the company may
need to file a Central Contractor Registration (CCR) Application, phone (888-227-2423)
and request a DUNS number from Dun & Bradstreet, phone (800-333-0505). After this, it
is necessary for the contractee to advertise for services in the Federal Business Opportu-
contractor is by riding an existing Corps contract; the St. Louis District has several in
place. A third way is by paying another governmental agency, which has a contract in
place. If the contractee is going to act as the lead for a group acquisition among several
other agencies, it may be necessary to execute some Cooperative Research and Develop-
ment Agreements (CRDAs) between the contractee and the other agencies. As a word of
caution, carefully spell out in the legal document what happens if the contractor, for any
reason, defaults on any of the image data collection areas. A data license should be
spelled out in the contract between the parties.
b. Technically, maps must be provided to the contractor of the image acquisition area.
They must be in the projection and datum required, for example Geographic and WGS84
(World Geodetic System is an earth fixed global reference frame developed in 1984). The
collection flight lines should be drawn on the maps, with starting and ending coordinates
for each straight-line segment. If an area is to be imaged then the overlap between flight
lines must be specified, usually 20%. If the collection technique is that of overlapping
frames then both the sidelap and endlap must be specified, between 20 and 30%. It is a
good idea to generate these maps as vector coverages because they are easily changed
when in that format and can be inserted into formal reports with any caption desired later.