GPS 2D vs 3D Fix

Hallo, I have arducopter 2.0.49 on my Quad + Copter.

 

Today I made aflight and only get a 2D fix.

But in the log I see I have 9 Sat´s.

 

What is the differenz between 2D and 3D fix and what can couse this?

 

thx

druckgott

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Replies

  • There is a BIG difference between being able to track 9 satellites and being able to use all 9 for navigation. Once the receiver is able to get a lock on a satellite, it is "receiving it." But if there is RF noise, there may be a bit error in the received message from the satellite. A full message takes 30 seconds, and a single bit error can cause that frame to be thrown out. Until the receiver is able to get a full message with no errors (and get repeat messages without errors every few minutes) it can not use that satellite in the navigation solution.

     

    I would make sure that your GPS is as far away as possible from any RF transmitters (including your RC transmitter and your XBee/Telemetry system, any WiFi routers, 1.5 or 3 GHz laptop computers) and wait for 3D lock.

  • Works a whole lot better with a battery or a battery mod. otherwise it has to re-download the entire almanac every time....

    Franco

  • i've had between 6 and 9 sats and no 3D fix a number of times with that gps. it always gets a 3d fix in the end but sometimes takes a while

  • Moderator

    Try using this before you go flying, should be part of everyone's preflight this sort of thing.

    http://www.navcomtech.com/Support/Tools/satellitepredictor/main.cfm

    I think it should be part of GCS giving a go, no go depending on whats in the sky. Especially important in fixed wing craft that are banking for a while !

  • Developer

     

    A 2D fix gives only longitude and latitude. It needs a minimum of 3 satellites.

     

    A 3D fix gives full longitude latitude + altitude position. It needs a minimum of 4 satellites.

     

    It is quite strange that you got only a 2D fix with 9 satellites. I've never seen this. Perhaps a temporary GPS system problem in your area or a very bad sat constellation distribution.

     

    If you see this again, take a small portable GPS with you next time, and see if it does get a 2D or 3D fix.

     

     

  • relates to the geometry of being able to pin point a uav position. example: if all the sats were in the same location you might not get a fix at all.

    if there is not a good spread of sat positions in relation to the earths globe then the accuracy of the uav position or "fix" is reduced. 3D is the best and you should be able to get 3D outside. Is the gps view of the sky obstucted by wires or copter frame etc?

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