Using Linear interpolation to locate a new sat position

Discussion in 'Free To Air (FTA) Discussion' started by Pixl, May 7, 2013.

  1. Pixl

    Pixl Topic Starter Senior Member
    Pub Member / Supporter

    Feb 27, 2010
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    Traverse City, Michigan
    I use a variation of Linear interpolation between two known points to locate a new sat position. This works if you already have some sats set in your positioner box, if you are starting from scratch it won't help much. Lets say I have 72w and 83w set in my box and I want to find 77w. I move to 72w and note that the positioner box count is 159. Now I move to 83w, the count is now 206. The count difference between those sats is 47. The orbital difference is 11 degrees. Divide 11 into 47 and I have 4.27 counts per degree in this part of my sky.
    Now there are 6 degrees between 83w and 77w. 6 times 4.27 gives 25.6 counts from 83w to 77w. My dish is still pointed at 83w with a count of 206. I back up 26 counts to 180. At this position I am pointed at or very near 77w.
    Do a blind scan then peak up the position and set it in the positioner.

    Since the counts per degree are not linear across the sky, there is a slight error using this method. But you will be close enough to find signal every time. You should re-calculate the counts per degree between the two nearest known sats for each search.

    Here is a link to more information on Linear interpolation.
  2. northgeorgia

    northgeorgia SatelliteGuys Family

    Nov 14, 2011
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    North GA
    Wow, this is interesting! I wondered if a moderator could make this a sticky and place it into the FAQs? That'll be useful for the day when, hopefully, I can get a motorized C-band set up :)
  3. KE4EST

    KE4EST FTA or Bust
    Staff Member HERE TO HELP YOU!

    Aug 9, 2004
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    Far West -- Western North Carolina
    I have used this method many times.
  4. Vondertrenk

    Vondertrenk Active SatelliteGuys Member

    Aug 30, 2007
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    I used that method also, but thank you for sharing
  5. Lone Gunman

    Lone Gunman SatelliteGuys Family

    Mar 19, 2010
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    Yup, I do this also and it is fairly accurate in sat to sat movement but as was noted, at the far ends of travel those numbers per degree will change. On my dishes I have the actuator mounted on the West side so when I go back East the closer I get to the East limit that count is much less, but, it can be somewhat predicted if you step from sat to sat across the arc. Example: On this 10ft Winegard I get 9 counts between 43w and 45w but from 137w to 139w I get 23 counts so there can be a big difference depending on where you are in the arc.

    When I installed my Vbox X dish controllers I made a list of all the sats that I was going to use then once I found them I put the number location on that sheet so I'd know how to get back to each one. I referenced the East limit for my "zero" point, ie, counter zeroed on the East limit switch, which gave me a "known" starting point. That way if your count gets corrupted you can just manually run to the East limit, zero the box and then move to each satellite location number and reprogram it in just a few minutes.

    FYI the old GI920 had a feature that would do this via it's software and when you went looking for a new sat then it almost knew the exact location it would be. And not only that, but once it went to that satellite it would "auto peak" that signals skew and position.
    #5 Lone Gunman, May 9, 2013
    Last edited: May 9, 2013
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