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'[EE]: GPS guided rover'
2001\04\27@122654 by Joan Ilari

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I have built a PIC-controlled rover. Now I need to give it
some kind of moving capability. Does it make any sense to
try to make it to find its location through a GPS ? I have
looked in the Net for GPS accuracy and I have seen very
different values, depending on the technology used. For
systems using a stationary beacon besides the satellites,
precisions of cms. can be achieved. Has anybody tested this ?
Are low level commercial systems (that is, not too expensive
ones) capable of this ?

If GPS cannot be used, has somebody any idea to know the
position of my rover (besides IR beacons and triangulation) ?

Thanks !
--------------------------------------------------------------
    Joan Ilari                 spam_OUTjoan.ilariTakeThisOuTspamterra.es
    Barcelona                  Voice:  +34 93 431 96 39
    Spain

"I've seen things you people wouldn't believe. Attack ships on
fire off the shoulder of Orion. I watched C-beams glitter in
the dark near the Tannhauser gate. All those moments will be
lost in time, like tears in rain. Time to die..."
                                              -Blade Runner-
---------------------------------------------------------------

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2001\04\27@130042 by David VanHorn

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At 06:18 PM 4/27/01 +0200, Joan Ilari wrote:
>I have built a PIC-controlled rover. Now I need to give it
>some kind of moving capability. Does it make any sense to
>try to make it to find its location through a GPS ? I have
>looked in the Net for GPS accuracy and I have seen very
>different values, depending on the technology used. For
>systems using a stationary beacon besides the satellites,
>precisions of cms. can be achieved. Has anybody tested this ?
>Are low level commercial systems (that is, not too expensive
>ones) capable of this ?

You're talking DGPS, which is expensive.
Civilian GPS will get you to within maybe 10', and of course it only works
outdoors.

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2001\04\27@130454 by Don Hyde

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GPS units are normally good for a few tens of meters.  The centimeter
accuracies used in continental drift experiments etc. involve averaging over
long timespans.

> {Original Message removed}

2001\04\27@234014 by Russell McMahon

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> I have built a PIC-controlled rover. Now I need to give it
> some kind of moving capability. Does it make any sense to
> try to make it to find its location through a GPS ? I have
> looked in the Net for GPS accuracy and I have seen very
> different values, depending on the technology used. For
> systems using a stationary beacon besides the satellites,
> precisions of cms. can be achieved. Has anybody tested this ?
> Are low level commercial systems (that is, not too expensive
> ones) capable of this ?
>
> If GPS cannot be used, has somebody any idea to know the
> position of my rover (besides IR beacons and triangulation) ?


In practice GPS can give you much better accuracy than the specs seem to
suggest.
Also, the removal last year of the purposefully added "selective
availability" error makes the real accuracy much better than before.

The RELATIVE short to medium term accuracy is well under 1 metre.
That is, if you know your exact starting point then you will be able to
establish your subsequent positions within a metre or perhaps less.

Differential GPS is not especially hard to do. DGPS uses a fixed GPS base
station which know its exact location to provide an error signal that allows
you to correct the error in received GPS information. In addition to the
rover's GPS receiver you need a radio (or similar) link from the base
station GPS to the rover. Some receivers are provided with input modes which
allow a standard DGPS correction "sentence" to be input.
You could almost certainly do your own DGPS. I'm not sure how useful it is
with the removal of selective availability. It will correct for path errors
but I'm not sure how consistent these are between different locations which
are small distances apart.

Using a bare GPS unit I have carried out some measurements of a vehicle
moving around local streets, turning 360 degree loops at intersections,
returning to the same street from a different direction etc. The RELATIVE
accuracy is well under 1 metre. That is, on a relatively narrow residential
road you can clearly tell which side of the road a vehicle is on, and even
where on the road the vehicle is. This is an off the shelf modern domestic
GPS unit - it's the unit used by Rand Macnally (spelling?) with their
automotive map program. Cost is about $US70 AFAIR. It has some degree of
position stabilisation in software but you could probably add this yourself
with software for a unit which didn't have it.

What I am not (yet) aware of is how/if  the relative accuracy changes with
time and whether the position accuracy degrades as distance from the
starting point increases. As I understand the system, neither should change,
but they may.
While you could not complain if the results were ever worse than you hoped
for I feel that the achievable accuracy is liable to be adequate for gross
rover positioning in many cases with added input from local sources such as
wheel movement and steering integration.



       Russell McMahon

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2001\04\28@025659 by Damon Hopkins

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you could also do what the cars do and add in your own variables like
velocity into the equation to get faster updates since the GPS signals
are only once per second.

Damon Hopkins

Joan Ilari wrote:
{Quote hidden}

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2001\04\28@094354 by Matt Bennett

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Russell McMahon wrote:
>

> The RELATIVE short to medium term accuracy is well under 1 metre.
> That is, if you know your exact starting point then you will be able to
> establish your subsequent positions within a metre or perhaps less.

This is highly dependent upon your reciever- many won't give the proper
level of precision to get 1 m accuracy.  (

{Quote hidden}

To do differential GPS properly, you need to have DGPS enabled
recievers.  You need to compare the pesudo ranges for each satellite,
and then compute a solution, which isn't a trivial computation- I've
never done it, but from looking at the equations, I believe that it is
beyond the ability of a PIC.  Differential GPS without SA is most useful
for the correction of ionospheric errors other apparent minor variations
in the GPS system.  As long as you have a short separation between the
two recievers, you're in good shape.  (The shorted the better, 10 Km or
less preferred)  The ionospheric errors start to matter when you're
getting in the 1m range of position accuracy.

You can just use one stationary ordinary GPS reciever and use it's
reported position as a reference point for a second moving reciever, but
that will only work accurately if both recievers are the same sort and
are looking at *exactly* the same satellites (pseudo DGPS).  If they
have different satellites in view, their position solutions will vary
slightly, with different GDOP (geometric dilution of precision- which is
where the accuracy numbers on the GPS come from).  The different
position solutions will kill any sort of enhanced accuracy you are
trying to get.  Also, if the two reciever's switch satellites at
different times, you will very likely get large discrete jumps in your
reported position.

Many recievers will let you control the minimum horizon angle at which
it will consider a satellite for using in it's solution, using this, you
can be pretty sure you're looking at the same set, but by doing this,
you limit your choice of satellites, and possibly increase your GDOP.

Matt

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2001\04\28@105359 by Russell McMahon

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From: "Matt Bennett" <mjbspamKILLspamHAZMAT.COM>
To: <.....PICLISTKILLspamspam.....MITVMA.MIT.EDU>
Sent: Sunday, 29 April 2001 01:40
Subject: Re: [EE]: GPS guided rover


{Quote hidden}

OK - buy the Talon receivers then :-).
Seriously, they are cost competitive with many apparently similar models.

Even relative beginners will find GPS fairly weasy to handle. Apply power (6
to16v or so in this case) and read NMEA output strings in serial ASCII at
4800 bps.
Strings are easy to analyse - even for a PIC probably - even with limited
storage available as largely all you need here are coordinates.




Russell McMahon

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2001\04\28@213119 by James R. Cunningham

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Garmin sells an add-on dgps unit about the size of a deck of playing cards for
about $400-500 US which is compatible with all their regular units and will
reduce the horizontal scatter from about 3-4 meters down to about 1-2 meters
when within reach of a dgps transmitter (virtually the entire US, but I don't
know about other countries).  It's true that gps units only work outdoors
(except when near large windows), but I can lay mine on the seat of my plane,
and it does very nicely.

Jim

David VanHorn wrote:

{Quote hidden}

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2001\04\28@214834 by Alexandre Domingos F. Souza

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>know about other countries).  It's true that gps units only work outdoors
>(except when near large windows), but I can lay mine on the seat of my plane,
>and it does very nicely.

       What is used in the "shell" of your plane??? Fiberglass??? :o)

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2001\04\28@221823 by James R. Cunningham

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Aluminum.  It's a Piper PA28-150.  It also works well in the same position in a
friend's PA28-260, which is also aluminum.  It gives up when placed on the floor
though.

Alexandre Domingos F. Souza wrote:

> >know about other countries).  It's true that gps units only work outdoors
> >(except when near large windows), but I can lay mine on the seat of my plane,
> >and it does very nicely.
>
>         What is used in the "shell" of your plane??? Fiberglass??? :o)
>
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