new grass user with lat/lon location problem

[ms_vorce at ccmail.pnl.gov]

GRASS Users,

I'm a new GRASS (and GIS) user working out how to geo-register data taken from 
an airborne SAR.  I have the lat/lon of the SAR entenna, the depression angle 
(it's a side-looking SAR), and the range in meters from the antenna to the 
center of the n x m patch of ground data is being collected on.  I need to find 
the lat/lon of the center of that patch to 1 meter accuracy, if possible.  It's 
easy to determine what the horizontal distance is given the depression angle and 
range.  What I'm not sure about is how do I use that distance (I also know the 
compass direction) to find the patch center's lat/lon?

Some things I've thought of trying are learning what the Earth's circumference 
is at that latitude (46.392218) and dividing by deg, min, sec, sec fraction to 
find the distance-to-degrees conversion.  But I'm unsure what kind of error this 
would introduce for the area I'm interested in (South Eastern Washington state).

Any suggestions would be greatly appreciated,
Marie Vorce