Why is SpacecraftAzimuth 180 degrees out for two nearly identical LROC R images?
I have been trying to understand the output of campt. One of the stumbling blocks has been SpacecraftAzimuth. The documentation is ambiguous so I have been trying to understand it by trial and error. I thought I had it figured out until I found two nearly identical images with totally different SpacecraftAzimuth values:
When I run campt on LROC image M1223450772R, I get a SpacecraftAzimuth of 179.871 . When I run it on M1218738444R for the same lat lon, I get 1.564 .
M1223450772R and M1218738444R are nearly identical in all other respects except for sun angle and 2 months separation in time. Does SpacecraftAzimuth have something to do with sun angle?
#1 Updated by Aaron Curtis about 2 months ago
My new theory is that SpacecraftAzimuth is constrained to the plane of the image and described by three points: 1) At the same line as the requested GroundPoint, at the highest sample value (right edge of image). 2) The requested GroundPoint. 3) Some projection of the spacecraft position onto the image which is different than the GroundPoint... maybe the lat / lon projection?
If this theory is right then the reason it goes from 180 to 1 is because point 3) has moved from slightly left of point 1) to slightly right of it.
Would be great if somebody can confirm that I've guessed correctly about these points.
#2 Updated by Lynn Weller about 2 months ago
I can't give you the affirmative answer you are looking for, but maybe the attached slide will help until someone with a better understanding can get to this post. The slide is from an ISIS beginner's workshop. I have access to it in house, but it seems it ought to be online somewhere (under our Wiki section), but I'm not seeing this particular workshop there. I'll see if someone can add it since it has a ton of useful information.
#4 Updated by Aaron Curtis about 2 months ago
Thanks Tammy, that image confirms my newfound understanding and explains the numbers. The glossary entries for all of these azimuth things really should be re-written for clarity. To begin with, get rid of the "... angle from a point of origin to the direction of the .." sentences. They are very confusing and lead one to waste time wondering how you can have an angle from a point to a direction (you can't). Also, I presume the Sun Azimuth glossary entry should be renamed to SubSolarAzimuth.
I would recommend the definition of SpacecraftAzimuth be written:
The angle between points A, B, and C in the plane of the image, measured clockwise if looking from the spacecraft direction, where:
* A is the point on the image co-linear with the spacecraft and the center of the planet. (Actually, is it a point on the image or the ground surface? I'm not quite sure.)
* B is the requested image or ground point of interest
* C is the point at the right hand edge of the image (maximum sample) halfway down the image (number of lines / 2)
The other Azimuth glossary definitions could be similarly improved.