Viewshed from a geostationary satellite

Svetlonio's Avatar


23 Jul, 2020 05:23 PM

I have a satellite dish installed on top of a camper van, and when traveling on forest service roads it's sometimes time hard to find spots where my satellite is in an unobstructed portion of the sky. I am trying to figure out the best way to use CalTopo to help me with this; I have tried using the "viewshed analysis" layer putting the eye altitude at geostationary orbit, but that does not seem to work (I guess it's not optimized to analyze half the globe, at least not on the free version). Right now I am doing some trigonometry to put the eye closer to the ground, but that means that the analysis gets increasingly inaccurate the further away I get from my reference point.

I am wondering if there is a better method to do this, and whether the desktop / mobile applications would work better for this purpose. Thanks!

  1. Support Staff 1 Posted by Meghan on 24 Jul, 2020 02:54 PM

    Meghan's Avatar

    Have you tried using DEM shading to analyze canopy coverage? You can go to Add New Layer (left side bar)> DEM shading> Add New Rule. Select Canopy and set it to 0% to 10%. Add this rule and then save the layer. It could potentially help you visualize and identify areas that have an unobstructed view of the sky!


  2. Meghan closed this discussion on 24 Jul, 2020 02:54 PM.

  3. Demetrio Girardi re-opened this discussion on 24 Jul, 2020 03:53 PM

  4. 2 Posted by Demetrio Girard... on 24 Jul, 2020 03:53 PM

    Demetrio Girardi's Avatar

    I did not know about that feature, and will try it out (thanks!), however,
    I'm mostly trying to get past mountains and hillsides - it's relatively
    straightforward to read an unenhanced topo map for canopy, but harder to
    judge whether any of the peaks to the south are tall enough and at the
    right angle to block the satellite, if that makes sense.

  5. Support Staff 3 Posted by Julie on 24 Jul, 2020 05:34 PM

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    Hi Demetrio,

    I think you are stuck doing some geometry, but I can give you tools that might help. I assume you know the angle above the horizon where the satellite is located, or are able to calculate that? Also assuming that you could find it on a 360* axis?

    I'm just theorizing here, but first you could draw a bearing line from the specific site you are considering to the direction of the satellite. Make the line plenty long enough depending on how high in the sky the satellite is (ie closer to the horizon will need a longer line.) Once the line is drawn, click on profile or terrain stats for that line. Find the highest point on the line and calculate the angle from your position to the highest point. If that angle is less than the given angle to the satellite, then you have the view.

    I think you could combine Viewshed Analysis with the above strategy. Set the eye altitude to the height off the ground of your satellite dish. If your bearing line intersects with any of the red highlighted areas, those are the points that you need to test the angle on. Basically, the closest place that the bearing line intersects a viewable location based on the analysis, you'll need to find the elevation difference between your starting point and that intersection, then calculate the angle and compare that to the satellite angle.

    However, CalTopo can't calculate those angles for you.


  6. Julie closed this discussion on 24 Jul, 2020 05:34 PM.

  7. caver456 re-opened this discussion on 24 Jul, 2020 05:38 PM

  8. 4 Posted by caver456 on 24 Jul, 2020 05:38 PM

    caver456's Avatar

    Hi Demetrio - I'm just another user, but tried some things based on your
    interesting question. This only works for an effective point source of
    light at infinite distance, i.e. the sun, so you'd have to figure out the
    general region you want to be in then figure out the angles of the
    satellite to that region, but: Add New Layer --> Custom Relief seems to
    work well. When you set the azimuth and zenith then hit 'add lighting', it
    fills out the text box with something like '315 by 45'. Just on a wild
    guess, hoping to increase the multiplier / 'brightness' of the light, or
    really to increase the contrast of the relief, I tried appending ' by n'
    e.g. '315 by 45 by 5' or whatever multiplier you want - I tried 5 and 10
    and they seem to be doing the trick - the shadows are almost all black,
    which might be more useful... but only if you know the region ahead of time
    and can calculate its angles. Julie and Meghan, does that sound right or
    is it doing something different?

  9. Support Staff 5 Posted by Julie on 24 Jul, 2020 05:42 PM

    Julie's Avatar

    Tom, thanks! I like your line of thought there. Curious to hear if it works for you, Demetrio.

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