GPS and Barometric pressure sensor recommendation please

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Hi all:
I am in the process of designing a project for school. This project
requires a GPS sensor and a barometric pressure sensor to determine
position in space. I have not much knowledge about sensors yet but I am
trying. I have spent several hours googling a few terms and so far I
came up with the Garmin 18 GPS sensor, and I am still not sure which
barometric pressure sensor I will use. These sensors have to communicate
to a basic stamp 2pe microprocessor. My question to you guys is: do you
know of any sensors (GPS and/or barometric) that I should research for
this project? Or may be you could tell me from where I could scavange
these sensors. Any help will be greatly appreciated.

Sonoman

Re: GPS and Barometric pressure sensor recommendation please
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To make a cargo bomb, you really need only one of these variables. Why
do you want to know both?

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Re: GPS and Barometric pressure sensor recommendation please
...
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It really is the end of an age of innocence. From now on do we only
help those who use their Real Name and email address??!! - RM


Re: GPS and Barometric pressure sensor recommendation please
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I was a little harsh, but a bit more information about the project would be nice.
Optional, but so is assistance.

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Re: GPS and Barometric pressure sensor recommendation please
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You ARE aware that GPS will give you altitude for free, right?

Re: GPS and Barometric pressure sensor recommendation please

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It's less accurate than position, and that's bad enough in many situations.

Paul Burke


Re: GPS and Barometric pressure sensor recommendation please
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I use a GPS unit that has a quoted accuracy of 4 metres for 50% of the
time.  This corresponds to the true position being within a ~50 square
metre area centred on the measurement result.  If you consider that
the surface area of the earth being some 510*10^12 square metres then
localising your position to 1 part in 10^13 is not bad.

S-V

Re: GPS and Barometric pressure sensor recommendation please
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It is if you need to know whether you'll meet that 20m high cliff at the
bottom or the top, or where the corner of the house you are going to
build is NOW, after all, it will be there all the time, not just 50% of
the time, and 4m away could be on next door's land.

Paul Burke



Re: GPS and Barometric pressure sensor recommendation please

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Also altitude is quoted on all GPS systems that I have ever seen, to
have a much greater margin of error than surface coordinates.

Re: GPS and Barometric pressure sensor recommendation please
snipped-for-privacy@nowhere.net wrote in message
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Altitude error will always be worse on a GPS system, at least until a
method is devised so that the receiver can see the satellites that are
below it. ;-)

1 part in 10^13 is pretty darn accurate.  IIRC when I was at college
the international definition of the second (the most accurately
defined standard unit) was accurate to 1 part in 10^12.  If you need
greater accuracy then you have to perform long term filtering of the
position data from a fixed installation, then you can get to the sub 1
metre fix.

S-V

Re: GPS and Barometric pressure sensor recommendation please
  > 1 part in 10^13 is pretty darn accurate.  IIRC when I was at college
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That's like saying that fifty quid I owe you is peanuts compared with
the size of the world economy, so I'll keep it.

Paul Burke


Re: GPS and Barometric pressure sensor recommendation please
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IIRC,  at sea level a 1 millibar change in atmospheric pressure
corresponds to about 8 meters elevation change.  An uncalibrated
pressure sensor  may have daily elevation drifts of about 80 meters
with variations in barometric pressure.

I've been using the UBLOX GPS MS-1E for about a year now.  When I have
5 or more satellites in view,  the altitude numbers are usually stable
to 5 meters or less.   However, it is important to realize that GPS
elevation numbers are actually computed as height above a theoretical
geoid, and the accuracy of the elevation depends a lot on the accuracy
of the geoid model---which is generally supposed to be good to within
60m worldwide, I think.


For a barometric sensor,  you can pick from a number of 5V units
from Motorola that have 0 to 4.5V output for 0 to ~15PSI.  See the
Digi-Key catalog.

Of course, everything gets easier and more accurate if you can get
reference data for a known location.

Mark Borgerson


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No, I was not aware. As far as I know it will give you longitude and
latitude which is like the X and Y axis. That sounds very interesting
and I will look into it. Thanks for the tip.

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And, not to forget, an unbelievably accurate clock, too.  Physicists
have been known to use a GPS receiver in experiments weighing
thousands of tonnes (i.e. which are essentially impossible to move
even if you were to try rather hard) just because they wanted to know
the exact time.

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Most "consumer-grade" GPS receivers do indeed work like that.  They
don't trust themselves enough (or can't afford the increased amount of
processing) to extract altitude, too.  Instead, they use a reference
ellipsoid or terrain altitude information as a constraint to extract
the (lon/lat) position.

--
Hans-Bernhard Broeker ( snipped-for-privacy@physik.rwth-aachen.de)
Even if all the snow were burnt, ashes would remain.

Re: GPS and Barometric pressure sensor recommendation please
  A bit confused,  the natural 3D solution of GPS is in ECEF
 (earth centered earth fixed) coordinates.  So latitude and longitude
  are dependent on various models,  as well as altitude.

  The usual 3D solution requires four satellites to solve four unknowns,
  position and time.  If there are only 3 satellites available,  then
  one can assume an altitude to solve for three unknowns.  This is
  normally a fall back feature.


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Re: GPS and Barometric pressure sensor recommendation please
On 2 Aug 2004 15:54:29 GMT, Hans-Bernhard Broeker

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I haven't seen a consumer-grade GPS which doesn't provide altitude.
Even the $99 USD Delorme unit calculates altitude.

--
Al Balmer
Balmer Consulting
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Re: GPS and Barometric pressure sensor recommendation please
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I dunno about "most". I have a cheap, bottom-of-the-line Garmin eTrex,
and it solves for altitude. Some of the really old first-generation
units might not, I guess (IIRC some of them only had three rx channels
anyway).

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  Keep in mind most civilian GPS recievers will not resolve an altitude above
60,000 feet.  Depending on what you're doing, this may or may not be a purchase
consideration.

--
Linux Registered User # 302622                         <http://counter.li.org

Re: GPS and Barometric pressure sensor recommendation please
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This is incorrect, the actual regulation prohibits exporting receivers
capable of reporting position at greater than 60,000 feet *AND* 1000
knots.  Some GPS manufacturers interpreted the AND as a logical OR in
the past.  These days, most GPS chipsets will continue reporting
position above 60,000 feet if speed is less than 1000 knots, and vice-versa.

Marc


Re: GPS and Barometric pressure sensor recommendation please
Personally I haven't used any, but I heard quite positive things about
the Lassen SQ from Trimble (http://www.trimble.com/lassensq.html ).
Regards and good success!
johannes

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