Failure: Bad GPS Pos

Posted by Edgar Scott on January 24, 2014 at 5:30pm

This is getting where I can no longer even test in my back yard at all, 30 minutes and hdop is still 2.7  I cant get below 2.0, usually its about 2.7 at my house.  This is a new issue that seems to have started with 3.1

Randy mentioned a battery may have died, but Im not sure where it is, is it on the APM, or the GPS?

Can I thru a PID just change the HDOP to be 3.0 or would that be bad?

I am not sure why this has never been an issue unless it was lowered when 3.1 came out.

Thanks, Ed

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Replies

  • Ron LaBorde July 7, 2014 at 6:39pm

    My phone indoors gets an hdop of 1.5 while my quad outdoors gets 2.5! 

    I might have to check the battery and see if it's bad or if there is a broken trace or solder joint. It used to work great but it has been crashed...

  • F. C. Bearsch June 20, 2014 at 6:07pm

    You all mention a battery for the GPS, it is inside the housing? 

    Today I had the alert and the a bad po error I assumed that I tried to arm to quickly while in loiter mode, however when I reset I received the same error pre arm GPS hdop... 

    So is the their a battery under the housing for the GPS that can go out?

    • Dean Miller > F. C. Bearsch June 21, 2014 at 7:56am

      There is a battery attached to the underside of the GPS board, inside the housing.  This battery should last many years and will not affect your position error anyway.  A dead battery will only affect satellite acquisition times (a lot). The battery is only there as a timekeeper, like a watch without a screen,  so when the GPS is turned on it knows what day and time it is so it knows which satellites to look for.  It sounds like you simply did not have enough satellites to get a good fix, remember trees, buildings and mountains all affect signal quality and sometimes it just takes more time to get a good fix.  You should be using a ground station (telemetry required) to see what the GPS HDOP values are doing and how many satellites you have.  If you already took the GPS out of the housing be very careful with the screws, they strip the plastic easily.

    • F. C. Bearsch > Dean Miller June 21, 2014 at 3:44pm

      Mine is less than 30 days old... I was testing arm in loiter and I think once the copter has a pre arm message it locks out a rearm until a reset...

      I think I got that whipped I have been letting it get a 3d fix and pass pre-aem with a back up battery then switch so I am certain of the charge...

      Then I switch and I think I did not let it get a GPS 3d fix and tried to arm in loiter

  • Dean Miller January 24, 2014 at 7:38pm

    Hey Edgar, I live in an area surrounded by mountains so I have a hard time getting enough satellites to give a good HDOP, I usually fly with a HDOP of 1.8 to 2.4, I changed the GPS_HDOP_GOOD value in the full parameter list to 2.5 (so it will pass pre-arm check) with no apparent ill effects but its not really recommended by the experts.  I also generally have better HDOP values once I'm up in the air. I'm not sure how far you can push the limits before you run into GPS accuracy problems, 3.0 is getting up there, but give it whirl.

  • Developer
    Randy January 24, 2014 at 6:07pm

    The minimum "good" GPS hdop is configurable using the GPS_HDOP_GOOD parameter (as mentioned on the pre-arm check wiki page).  No guarantees that the loiter or auto control will be any good though!

  • Admin
    Thomas J Coyle III January 24, 2014 at 5:49pm

    @Edgar,

    There is a backup rechargeable battery on the uBlox LEA-6 GPS that maintains volatile memory that holds the satellite data.

    This data allows for a warm start from power on rather than starting with no data in the memory which is a cold start.

    If this battery dies then the GPS has to start from scratch (cold start) every time it is powered up which can make for a prolonged time before a 3D lock is achieved.

    Regards,

    TCIII ArduRover2 Developer 

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