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Question about GPS_HDOP

Posted by Carlos on July 29, 2015 at 3:42pm

I sometimes get high GPS_HDOP values (i.e. above 2) so naturally my copter (3DR X8+) fails the pre-arm check. However, once the copter goes up a few meters,  I imagine the signal will get better in most instances. So I was wondering if it’s possible to take the HDOP off the pre-arm check and making it a failsafe, so once the copter reaches its flying altitude it checks the hdop and if it’s too low it does an RTL. So in effect making the hdop kinda like a geofence. Or maybe program the copter to have one HDOP value when it’s on the ground (e.g. 350) and another when it reaches it’s first waypoint up in the air (e.g. 200).

Thanks

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Replies

  • Drew Curtis July 31, 2015 at 2:15am

    If you want to use RTL then you need a good position to begin with. If you have a HDOP above 2 you could have position errors of 20m+ at your home location. When you began to fly the HDOP will get better, but that original position would still be wrong.

    HDOP scales up the position errors, and multipath errors can become much worse with high a HDOP. It's calculated the geometry of the satellites location in the sky. If you upgraded your hardware to use GPS+GLONASS it would reduce the HDOP; twice as many satellites. If you have twice as many satellites even in the same location in the sky it will reduce it by HDOP/sqrt(2).

    • Carlos > Drew Curtis August 1, 2015 at 9:39pm

      Thanks, I hadn't considered the scaling of position errors. In my (somewhat limited) experience adding GLONASS doesn't help all that much. Having more satellites doesn't help much if you are in a narrow valley or if you are surrounding by lots of trees.

      • Drew Curtis > Carlos August 3, 2015 at 12:08pm

        Having more satellites would almost always help you in terms of HDOP. You'd have twice as many satellites and HDOP represents the spread of the satellite geometry in the sky. If you have a limited view if the sky you'd still see more satellites using both GPS and GLONASS, and that's the time when you need more the most. If you get into using more constellations (GPS+GLONASS+Beidou+Galileo) there are diminishing returns where it doesn't help you much after the first ~10 satellites you're using. The second constellation does help a lot. 

      • Moderator
        Gary Mortimer > Carlos August 2, 2015 at 12:53am

        Waiting longer to take off does give the GPS 15 minutes to catch up with the almanac.

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