[amsat-bb] satellite los footprints

[Joseph Armbruster]

I can not decide how to implement ground footprints with my google earth satellite tracker.  I figured, since I can't make up my mind, I should get a second (and third, and fourth) opinion.  For this thread, I would like to discuss how satellite ground-footprints should be implemented.  A quick brainstorm led me to three possible implementations (I am leaning towards 3).  For each of these, I assume that a geographic line-of-sight footprint is desired with no RF characteristics taken into consideration:

option 1 : assume a spherical earth model and project a polygon downwards towards the footprint

- note: this is obviously the easiest approach but will result in the most error

option 2 : assume an ellipsoidal earth model and project an irregularly shaped polygon downwards towards the footprint

- note: this is arguably more difficult than option 1 and would result in less error

option 3 : use a digital elevation model and an ellipsoidal model to cull-out regions that are not visible due to geographic features and project an irregularly shaped polygon downwards towards the footprint

- note: In this case, our footprint polygon would have holes cut out for the regions that are culled out by mountain ranges, canyons / etc...  Obviously, this would be the most difficult to implement but would likely be the best visual representation.  The problem is, I would never dream of distributing DEMs for the entire Earth with my tool, even DTED0 would be absurd in my opinion.  I could make the elevation queries accessible using a web-service, but then the user would be tied to the internet.  The other option would be to allow the users to download their elevation data into a cache, then the tool would just load / use it.  This way the user would only have to obtain the elevation data for their region of interest.  Maybe that would be the best approach?  I am open to suggestions!

If you have any experience visualizing footprints, please let me know.  I would be interested in hearing your lessons-learned.  These are what the line-of-sight indicators look like right now:  Google Earth Satellite Tracker - Line of Sight Update

I am open to comments and suggestions,
Joseph Armbruster