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'GPS differential corrections for home brewers??'
2002\11\20@011247 by Brooke Clarke

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Hi:

There are a number of different flavors of DGPS:

(1) "poor man's DGPS" - using a low cost GPS receiver at a known location,
finding it's position error and subtracting that error from a roving GPS.  The
problem is that the error will be different if the two receivers are not using
the exact same set of satellites.

(2) Code DGPS - the error in the range and range rate to each sat from the base
station is sent to the rover where it is subtracted.  My old Motorola VP had
this built in and these are now available heavily discounted.  This can be done
in two ways (a) in real time using a wireless link or (b) you can record the
base and rover data and post process.  I did the latter back when SA was turned
on the the results showed a huge improvement over the raw rover data.  NMEA is
the standards organization that specifies the different "sentences" used for
DGPS.  The U.S. is almost covered with low frequency transmitters broadcasting
DGPS corrections (even in the center of the U.S., not just along the coast).
Most of the sporting type GPS receiver manufacturers make receivers for this
signal and there are some home brew receiver plans on the internet.
There's someone in the S.F. bay area that's broadcasting corrections on the
internet, but I don't remember which type.

(2A) Now that SA is turned off a single receiver can be used to correct it's
self.  There is a patent on this, but the idea is to go to a known location,
stay there until you know the errors on each sat, then you can go for some time
using the corrections you found at the known point.  This works because the
errors move much more slowly now than when SA was on.

(3) Carrier phase DGPS - this is the method used by surveyors and depends on
knowing the phase of the signal from each sat and resolving the integer
ambiguities on each one.  Once this is done you can do mm type work either in
real time or post processed.

My GPS info is at:<http://www.pacificsites.com/~brooke/electron.shtml#Sputnik>

Have Fun,

Brooke
http://www.prc68.com

Date:    Tue, 19 Nov 2002 10:11:40 -0600
From:    rad0 <spam_OUTrad0TakeThisOuTspamATTBI.COM>
Subject: [ot]: GPS differential corrections for home brewers??

Hello,

I am wondering if it is possible to make a home brew gps
differential transmitter?

I know on my gps board, a small ashcroft, the corrections are
input on a single pin, but I do not know how or in what format?

So, what's the format, how is it done?

And, I was thinking of using the inexpensive TX RX modules for
transmitting the correction from a stationary second gps(from a
known position), after receiving and calculating the error correction?

Is this a reasonable approach?


thanks

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2002\11\20@023402 by rad0

picon face
----- Original Message -----
From: "Brooke Clarke" <.....brookeKILLspamspam@spam@pacific.net>
To: <rad0spamKILLspamATTBI.COM>; "pic microcontroller discussion list"
<.....PICLISTKILLspamspam.....MITVMA.MIT.EDU>
Sent: Wednesday, November 20, 2002 12:11 AM
Subject: GPS differential corrections for home brewers??


> Hi:
>
> There are a number of different flavors of DGPS:
>
> (1) "poor man's DGPS" - using a low cost GPS receiver at a known location,
> finding it's position error and subtracting that error from a roving GPS.
The
> problem is that the error will be different if the two receivers are not
using
> the exact same set of satellites.
>
what kind of accuracy can you get from the above??




> (2) Code DGPS - the error in the range and range rate to each sat from the
base
> station is sent to the rover where it is subtracted.  My old Motorola VP
had
> this built in and these are now available heavily discounted.  This can be
done
> in two ways (a) in real time using a wireless link or (b) you can record
the
> base and rover data and post process.  I did the latter back when SA was
turned
> on the the results showed a huge improvement over the raw rover data.
NMEA is
> the standards organization that specifies the different "sentences" used
for
> DGPS.  The U.S. is almost covered with low frequency transmitters
broadcasting
> DGPS corrections (even in the center of the U.S., not just along the
coast).
> Most of the sporting type GPS receiver manufacturers make receivers for
this
> signal and there are some home brew receiver plans on the internet.
> There's someone in the S.F. bay area that's broadcasting corrections on
the
> internet, but I don't remember which type.
>
I actually have one of this type of receivers, how acurate is it? (above)



Thank you.

--
http://www.piclist.com hint: The PICList is archived three different
ways.  See http://www.piclist.com/#archives for details.


2002\11\20@024040 by rad0

picon face
----- Original Message -----
From: "Brooke Clarke" <EraseMEbrookespam_OUTspamTakeThisOuTPACIFIC.NET>
To: <PICLISTspamspam_OUTMITVMA.MIT.EDU>
Sent: Wednesday, November 20, 2002 12:11 AM
Subject: GPS differential corrections for home brewers??


Brooke,

I checked out your site, there is too much there for me right now.

But, lets say you have a receiver at a known location, calulating the error
for each sat in view.

Then you up load this to a rover, how accurate would the rover be? and
for approx how long?


Thanks

--
http://www.piclist.com hint: The PICList is archived three different
ways.  See http://www.piclist.com/#archives for details.


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