[amsat-bb] Re: satellite los footprints

Greg D ko6th.greg at gmail.com
Mon Mar 25 21:49:26 PDT 2013

Further to what Gus writes, I think Method #3 will suffer from two 
assumptions, giving an impression of precision when less should be 
expected. First, you are highlighting the shadows that mountains and 
other terrain should give, but which are only applicable to visible 
light. Radio waves bend and knife-edge diffract over and around these 
things, so you're eliminating areas from being in serviceable view when 
they could be interesting to try, if not perfectly useful.

Second, local obstructions such as buildings, trees, and other stuff 
that aren't represented on Google Earth can be a big factor in the 
success of any satellite pass, especially if / when we ever get back 
some microwave capability in orbit. I have a small video camera mounted 
on my Az/El rotor boom because I have this huge oak tree immediately 
behind my house, and it was critical to know where it was - one large 
limb in particular - compared to AO-40, in order to make a contact in 
that direction. That tree is big, but I suspect not sufficient to show 
up on Google Earth.

My recommendation is that you go with simple and easy now, and update it 
later when you need something with more precision, for example, if / 
when we get a high orbit target to aim precisely at. Maybe we'll have a 
Google Backyard View by then.

Greg KO6TH

Gus wrote:
> I would suggest you go with #1 or #2. The added complexity of method 
> #3 probably won't pay any significant dividends in practical terms. 
> You could always implement #3 for version II. :-)
> Will you be considering squint? Frankly, I'm not sure any current 
> satellites are using antennas where squint would play a part.
> Regards...
> On 03/25/2013 11:15 AM, Joseph Armbruster wrote:
>> 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
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