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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