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'[ot]: GPS differential corrections for home brewer'
2002\11\19@111212 by rad0

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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\19@113252 by Dr Martin Hill

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To do this properly you need to do it on a per satellite basis.  You can't
just take the error from one position and use it as an offset for another
receiver as you can't be sure they are both using exactly the same
satellites to get their positions.

Martin

{Original Message removed}

2002\11\19@114116 by rad0

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OK,

I'm not yet totally discouraged...

What's the format?  Could it still be done with
a TX RX short range setup?

You could send a correction for whatever bird
is coming in.

I don't remember if you have access to the individual
satellites with a cheap gps receiver?  Anyone?

Thanks

----- Original Message -----
From: "Dr Martin Hill" <spam_OUTmhillTakeThisOuTspamRACE-TECHNOLOGY.COM>
To: <.....PICLISTKILLspamspam@spam@MITVMA.MIT.EDU>
Sent: Tuesday, November 19, 2002 10:35 AM
Subject: Re: [ot]: GPS differential corrections for home brewers??


> To do this properly you need to do it on a per satellite basis.  You can't
> just take the error from one position and use it as an offset for another
> receiver as you can't be sure they are both using exactly the same
> satellites to get their positions.
>
> Martin
>
> {Original Message removed}

2002\11\19@114554 by Alan B. Pearce

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>To do this properly you need to do it on a per satellite
>basis.  You can't just take the error from one position
>and use it as an offset for another receiver as you can't
>be sure they are both using exactly the same satellites
>to get their positions.

I can see where you're coming from with this, but I understood the trick was
you used the data stream from a receiver that is at an accurately known
point, and compared your data stream with that data stream. Hence any data
from satellites that the reference receiver can see, and my receiver cannot
see, and vice versa, get excluded from the equation.

I do not know how many satellites each receiver has to have in common, but
the I understood the technique is good for relatively small distances anyway
(10's of miles). I may have this all wrong though, so please correct me.

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2002\11\19@125426 by John Walshe

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As far as I can make out it is a question of what accuracy you want. For mm
accuracy I'm sure Dr Hill's comment holds, but if you just want cm/m
accuracy then a static/base unit in the locale of the area of interest would
be sufficient (it is likely to be using the same satellites anyway). I was
using a Leika DGPS system just this weekend and the correction signal was
beamed from a site in Wales (Holyhead ->I was mapping in the centre of
Ireland ~250miles away!). The repeatability of the resultant map was quite
good - enough to show that the mappers did take the same path when doing the
work. Of course when the differential signal was lost the plot jumped west
by about 5m.
I'm working on a system similar to your idea but because of the terrain I
will have a centrally located unit, static and just logging position (this
should wander with the dither). The second (mobile)unit for the mapping I
will carry with me and at the end of the day I'll subtract one data set from
the other to get (hopefully) a more accurate map (and hopefully zeroed on
the static location to boot). I only need 1~2m accuracy so I'm hoping that
will do.
I'd be interest to hear your - or anybody else's comments on my proposal.

John

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2002\11\19@130846 by Jim

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Do the units any of you guys use make use of the
"WAAS" (Wide Area Augmentation System) data
transmissions?

Some of the newer hand-held GPS receivers do, and
some made a year or so back can be re-flashed
to yield this function (a friend updated his awhile
back for this functionality) ...

WAAS can yield better accuracy (but probably not to the
degree that DGPS can).

WAAS data, btw, is transmitted in the same BW (or
spectrum) as standard GPS signals - but from a
geosynchronous bird (actually a couple of them) ...

RF Jim


{Original Message removed}

2002\11\19@133959 by Josh Koffman

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I haven't built my own, but there are those that have.
http://home1.gte.net/clseng/DGPS_index.htm is a pretty good site. Also
try googling for homebrew dgps.

Josh
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rad0 wrote:
> I am wondering if it is possible to make a home brew gps
> differential transmitter?

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2002\11\19@134559 by rad0

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I've looked over these, but I wasn't actually talking about
'actual' DGPS.

I was talking about a home brew DGPS-like system.

Thanks


{Original Message removed}

2002\11\20@163022 by Barry Gershenfeld

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>I'm not yet totally discouraged...
>
>What's the format?  Could it still be done with
>a TX RX short range setup?

RTCM-104 is the magic word to find specs on Differentla GPS

Barry

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