RPG geolocation:
The pixel azimuth and elevation was retrieved from the L15G derived from the RPG NRT
matching, under the assumption of the orbit and satellite axis parameters used in the level 1.5
processing and contained in the L15 NANRG files. Only the longitude and latitude returned by the
fitting was used, no other parameters returned by the fit were considered.
Under these assumption the retrieved optical model varies both from column to column and from
file to file.
The variation in the column azimuth position was analysed. This was done by taking the retrieved
azimuth for each pixel and column away from that for a reference file (chosen to be the first file of
that type, total or shortwave, for that day). Values for the relevant pixel and column were
subtracted in each and the difference thus allowed the daily variation column position to be
analysed. Variation in the parameter is caused by column to column noise in the start of line
position as well as E-W drift of the whole image. If the axis and orbit parameters are correct then
this subtraction performed should yield the same difference for all pixel of a given column as the
variation in the azimuth angle between pixels is a constant function of the optics. An error in the
parameters used to derive the optical model from the longitude and latitude can introduce slow
apparent variation in the relation between pixel azimuth, but should not result in ‘noise’
RAL (white) and RMIB correlation for total (red) and shortwave (green).
Parameters returned by the RGP minimisation. White – L15G value, Red – intilisation value,
Green – optimised value (TOT), Blue – optimised value (SW). Parmeters are radsat – radius of the
satellite from Earth centre, col_drad – change in radius per column in the image
Pixel azi – azi pix 128
Optimised (black) and NANRG (blue) geolocation parameters
This next section analyses the pixel azimuth and elevation retrieved from the RGP reprocessing
lon-lat and NANRG spin and satellite position parameters. First a probability distribution of pixel
position are derived from the data. These may be derived in three ways: (a) from the retrieved
azimuth and elevation, with a nominal column azimuth subtracted. The nominal column azimuth
assumes a constant column spacing and a constant starting point of the scan. (b) from the
azimuth differences from pixel 128 for each column retrieved and the retrieved elevation and (c)
from the azimuth difference and elevation difference from pixel 128 for that column.
The purpose of this analysis is to determine what quality of geolocation could be achieved given
the parameters currently available in the level 1.5 determination and an updated optical model
based on the RGP reprocessing.
Analysis (a) actually presents a worse case that using the level 1.5 parameters with an updated
optical model, because the level 1.5 geolocation does have extra information available on the
Spread of pixel position probability distribution determined for the RGP reprocessing matching
geolocation. Y axis shows distance from most probable position, deviation may be either azimuth
or zenith or both, it is not a combined distance. Colours indicate percentage of cases represented
by the part of the position prob. distribution which is greater than the y scale value from the most
probably position.
Purple: > 1% of pixel; Blue: > 5%, Cyan: > 10%, Green: > 25%, Orange: > 50%, Red: > 60%.
Thus the figure indicates that for most GERB pixels 60% of cases (red) differ from the most
probably pixel position by more than 0.1 of a GERB pixel (or 40% of cases are within 0.1 of a
GERB pixel of the most probably position). Similar the Green contour which lies below 0.33 of a
pixel for most pixels indicates that the zenith and azimuth retrieved by the RGP reprocessing
geolocation 75% of cases vary by less than 0.3 of a GERB pixel from a fixed point. These results
have only a nominal column azimuth subtracted from the azimuth retrieved from the lon lat
returned by the RGP reprocessing geolocation using the NANRG orbit and attitude parameters.
The nominal azimuth of the column is 10.05 – FINDGEN(282)*0.07039. Note: first two and last
two columns excluded.
As above but for position probability distribution derived for pixel azimuth relative to that of pixel
128. This plot shows that if information on the staring azimuth of the column is available in addition
to the NANRG parameters and an update to the optical model for pixel 10-230 only 10% of
occurrences of the RGP matching deviate by more than 0.2 of a GERB pixel in azimuth and/or
elevation from the nominal value.
As above but both relative azimuth and zenith (relative to pixel 128) are used to derive the pixel
position prob. distributions.
Comparison of the preceding two figures would seem to indicate that there is not a great deal to be
gained by using relative zenith as well as azimuth. However knowing a starting point azimuth of
each column will considerably help the geolocation accuracy that can be obtained using the current
NANRG parameters along with an updated model derived from the
June 2004
One day of data (June 1st 2004) nominal subtraction only
One day of data relative azimuth
Comparison between the reprocessing geo and the optimal geo, colours show distance difference
in pixels, where the difference is calculated from the azimuth and elevation of the retrieved banana.
Dark blue 5%
Light blue 10%
Cyan 20%
Green 30%
Yellow 40%
Reed 50%
White 100%