Items needed in the IMAGE CDF files in order to calculate the geographic latitude and longitude of each pixel.
Parameters needed in the CDF files
1) Spacecraft Orbit parameters in CGI coordinates.
2) Spacecraft Attitude parameters in CGI coordinates- This is what we interpret to be the spin axis coordinate
(direction of true spin axis at Instrument snapshot time)
3) Angle of Instrument Look-Direction from Nadir – This is what we interpret to be the spin phase angle. On Polar,
this is the despun angle.
4) Instrument Field of View – unique to each instrument.
5) Instrument Offset angle – unique to each instrument. This is the angular difference in pitch and yaw (or x and y
in image coordinates) that exists between the calculated central pixel look direction and the onboard measured
direction. This will be known only after we launch and may change if there are thermal issues or mechanical shifts
as a function of time. On Polar, this has been constant to the degree that we can measure. This may also only be
needed for one direction (perpendicular to the spin plane) because the offset in the spin plane can be adjusted with
definitions to the spin phase angle. On Polar, the UVI has a mean instrument offset for each system (primary and
secondary) and different offsets for each filter position.
5) Distortion of Telemetered Image – unique to each instrument. At this time, we are assuming zero distortion. This
is used to determine the angle that any pixel will view relative to the central pixel. On Polar, the distortion is zero to
the degree that we can measure.
Brief summary of how the latitude and longitude are determined for each pixel. This is done in two parts. First,
calculate the look direction for each pixel. Then calculate the latitude and longitude of each pixel that views the
Earth.
Look Direction
Calculate the position vector of the s/c. Defined as the direction from the center of the Earth to the s/c.
Calculate the look direction vector in the s/c frame (LSC). Defined as a unit vector coplanar with the s/c attitude
vector and the position vector.
Calculate the look direction at the time of the image. This is defined as the look direction in the s/c frame rotated
about the spin axis by the spin phase angle.
Transform from the s/c frame to the instrument frame of reference. This uses the offset angle for each instrument
and the average angular filed-of-view, in x and y, of each pixel.
Transform from the instrument frame of reference to the image plane frame of reference by using the distortion of
telemetered image. Not done with Polar/UVI
Lat and Lon location for UVI
See the routine FGEODE for details. Geodetic coordinates are defined from the normal to the geode surface.
Geographic (geocentric) coordinates assume the earth is a sphere and are defined from the center of the sphere.
The look direction for the specified pixel (LPIX) is calculated from the look direction of the center of the UVI field
of view (L0) by two successive rotations in row and column directions. Each pixel is assumed to have a fixed
angular width. The angular distance from the center of the pixel to the center of the FOV is calculated and then L0
is rotated into LPIX.
The secondary and primary detectors have different orientations and require different rotations between L0 and
LPIX.
Geocentric lat/lon values are calculated for the intersection of the look direction for the specified pixel (LPIX) and
the surface of the earth. The geocentric values are then transformed into geodetic values.
The intersection of LPIX and the earth is calculated first in GCI coordinates and then converted to geographic
coordinates. The conversion is by means of IC_GCI_TO_GEO. This routine and its supporting routines, was taken
from the CDHF (Command and Data Handling Facility) and is part of the ICSS_TRANSF_ORB call. These
routines take into account the nutation of the earth’s rotation and the oblateness of the earth’s surface.
The viewed emissions are assumed to originate EMIS_HGT km above the surface of the earth. See GET_SCALAR
for details.