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Re: [Paparazzi-devel] Re: Differential GPS through Paparazzi

From: Tom Kent
Subject: Re: [Paparazzi-devel] Re: Differential GPS through Paparazzi
Date: Wed, 18 Mar 2009 09:43:15 -0500
User-agent: Thunderbird (Windows/20081209)

The way GPS works the errors in the signals from each satellite usually
change in a continuous (and relatively slow) manner.  So I wouldn't
worry too much about a second or two of could have an impact,
but it should be negligible.

The bigger problem is that this whole system doesn't work very well with
standard GPS devices.

Each satellite that the GPS receives from has its own error, which is
basically (but not actually) independent from all the other satellites'
error.  Thus you can only say "the base station is off by ___ meters in
X, Y, and Z, so we must be off by the same amount" if you can see the
EXACT SAME SATELLITES...if even one different satellite are used by the
ground station and aircraft (which if the aircraft is up in the air
higher than the ground station is almost guaranteed) then this method
will not work.

The solution is to get a GPS receiver that will give you data on a
satellite by satellite basis and use that in the ground station and the
aircraft.  Use the ground station to determine each satellite's error
and send that to the aircraft.  The aircraft should then correct for
that error in the input data it gets from each satellite BEFORE it
calculates its position.  This method will account for most, but not all
of the error in the GPS system.

Tom Kent

David Buzz Carlson wrote:
> DGPS is _normally_ done with another receiver at a known GPS location. 
> This receiver then computes the difference between what it received at a
> certain time and what the actual location is.  It then sends that data
> to the airborne unit.  Because of this, there is a slight lag in when
> the data is received and when it can actually be used by the airborne
> platform.  I'm not sure if that lag would prevent you from using it how
> you propose, but it would definitely be an interesting project.
> Buzz
> James Maday wrote:
>> Hello Marcus-
>> Having purchased to sets of the TWOG bundle, my first idea would be to
>> use the second bundle's GPS attached to either our landing net or the
>> GCS for differentiation with the aircraft's GPS.  If we attach the
>> second GPS to the landing net, we could get really accurate landings I
>> think.
>> If necessary we would purchase a different GPS receiver, but hopefully
>> we don't need to.  I guess the question you asked me, Marcus, is one I
>> should pose to readers as I'm unable to definitely answer it myself:
>> what sort of secondary GPS receiver is needed to run a DGPS system
>> with Paparazzi?  How would the two GPS coordinates be manipulated in
>> the software to more accurately obtain altitude?
>> Thank you for reading and responding,
>> -James Maday
>> Saint Louis, MO, USA
>> >Hello James,
>> >
>> >what would be the source for the differential signal?
>> >Do you suppose to get it throught the uplink from
>> >the GCS, some kind of local receiver on the AP or
>> >some kind of network-device on the AP?
>> >
>> >Marcus
>> On Tue, 17 Mar 2009 00:54:09 -0500, James Maday <address@hidden
>> <mailto:address@hidden>> wrote:
>> >> Hello Paparazzi fans-
>> >>
>> >> I'm working with a team developing a UAV which will very soon
>> employ the
>> >> Paparazzi Autopilot (AP) for navigation.  We have only just begun
>> >> programming our airframe.xml, but before we get far I'm hoping to
>> answer
>> >> this feasibility question: is differential GPS (DGPS) usable by the
>> >> Paprazzi
>> >> system for altitude measurement?
>> >>
>> >> I've searched the mailing list and didn't find any previous talk of
>> using
>> >> DGPS with Paparazzi, so I figured I'd poll the audience to see if
>> anyone
>> >> has
>> >> tried and can offer advice.  Thank you very much!
>> >>
>> >> To answer my question here are some basic project details.  The UAV
>> must
>> >> maintain 10-15 feet (roughly 3 - 4.5m) above the ground following
>> >> predetermined waypoints for 10 hours.  We have two sets of the TWOG
>> with
>> >> LEA_5H GPS device from PPZUAV.  We are using a laptop w/ Intrepid
>> Ubuntu
>> >> Linux for the GCS.  An example of a UAV that uses DGPS in its AP is
>> the
>> >> ScanEagle by Insitu/Boeing.  (
>> >> )
>> >>
>> >> -James Maday
>> >> St. Louis, MO
>> >>
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