GPS and Barometric pressure sensor recommendation please

"Vadim Borshchev" <vadim.borshchev@127.0.0.1> wrote in message
news:opsb339nqgy1ubid@news...
> On Mon, 02 Aug 2004 01:27:08 -0400, Sonoman <billgates@microsoft.com>
> wrote:
>
> > 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.
>
> Vaisala radiosonde might be what you are looking for.  It contains
> temperature (two of them),  humidity and barometric sensors.  RS80 variety
> also comes with 8 channel GPS transiever (it transmits Doppler shifts, so
> you'd need to figure out how to obtain the position from them).  They are
> quite cheap on ebay, around 10 quid.
>
> The links to check:
>
> http://www.google.co.uk/search?q=vaisala+rs80+radiosonde
> http://www.zfx.de/sonde/
>
>
>    Vadim

Please note that the sonde requires a special receiver/processor
to decode the downliked position and weather data. The GPS receiver
in the sounding system is able to resolve movements, but not
absolute position, which is not needed in its primary purpose.

The budget permitting, I think that the solution is a credit-card
GPS module. They are available from most primary GPS manufacturers.

There is one more point to take into account, if the altitude is to
be measured with a pressure sensor: the reference pressure must
be set to match the current weather situation (called QNH or QFE).

Tauno Voipio
tauno voipio (at) iki fi

Rick Merrill wrote:
> Bryan Hackney wrote:
> ...
> 
>>
>> To make a cargo bomb, you really need only one of these variables. Why
>> do you want to know both?
>>
> 
> 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
> 

I was a little harsh, but a bit more information about the project would be nice.
Optional, but so is assistance.

-- 
------------------------------------------------------------
Creepy, Soulless Gigolo for President ? NOT !
---------------------------------------------
THK is one weird, weird something.

In article <2n6kekFso8bfU2@uni-berlin.de>, paul@scazon.com says...
> Lewin A.R.W. Edwards wrote:
> 
> >>I am in the process of designing a project for school. This project 
> >>requires a GPS sensor and a barometric pressure sensor to determine 
> > 
> > 
> > You ARE aware that GPS will give you altitude for free, right?
> 
> It's less accurate than position, and that's bad enough in many situations.
> 
> Paul Burke
> 
> 
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

Tauno Voipio wrote:

> "Vadim Borshchev" <vadim.borshchev@127.0.0.1> wrote in message
> news:opsb339nqgy1ubid@news...
> 
>>On Mon, 02 Aug 2004 01:27:08 -0400, Sonoman <billgates@microsoft.com>
>>wrote:
>>
>>
>>>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.
>>
>>Vaisala radiosonde might be what you are looking for.  It contains
>>temperature (two of them),  humidity and barometric sensors.  RS80 variety
>>also comes with 8 channel GPS transiever (it transmits Doppler shifts, so
>>you'd need to figure out how to obtain the position from them).  They are
>>quite cheap on ebay, around 10 quid.
>>
>>The links to check:
>>
>>http://www.google.co.uk/search?q=vaisala+rs80+radiosonde
>>http://www.zfx.de/sonde/
>>
>>
>>   Vadim
> 
> 
> Please note that the sonde requires a special receiver/processor
> to decode the downliked position and weather data. The GPS receiver
> in the sounding system is able to resolve movements, but not
> absolute position, which is not needed in its primary purpose.
> 
> The budget permitting, I think that the solution is a credit-card
> GPS module. They are available from most primary GPS manufacturers.
> 
> There is one more point to take into account, if the altitude is to
> be measured with a pressure sensor: the reference pressure must
> be set to match the current weather situation (called QNH or QFE).
> 
> Tauno Voipio
> tauno voipio (at) iki fi
> 
> 

So what you are telling me about the pressure is that I have to 
calibrate the barometric pressure sensor every time before I use it to 
take into account the weather changes? Interesting.

I did not think that was necessary for my application. I want to know 
the altitude of each point in relation to each other and not against a 
reference (i.e. sea level), unless the weather changes in between 
readings I think I'll be ok without calibration, don't you think? I just 
don't want to complicate my life if I don't have to. Please advise.

> So what you are telling me about the pressure is that I have to
> calibrate the barometric pressure sensor every time before I use it to
> take into account the weather changes? Interesting.
>
Yes,  this is standard procedure for mountaineers, who often use barometric
presure an hight indication. 19th century technology, no batteries needed,
smaller and lighter than GPS.

Wim

  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.


Hans-Bernhard Broeker <broeker@physik.rwth-aachen.de> writes:
>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.

On 2 Aug 2004 15:54:29 GMT, Hans-Bernhard Broeker
<broeker@physik.rwth-aachen.de> wrote:

>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.

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
removebalmerconsultingthis@att.net

> > 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. 
> 
> Most "consumer-grade" GPS receivers do indeed work like that.  They

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).

Sacre Vert wrote:
> 
> 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.
> 

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

On Mon, 02 Aug 2004 01:27:08 -0400, Sonoman <billgates@microsoft.com> wrote:

>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

http://home.germany.net/100-173822/d_logg2_engl.htm
Circuit Cellar had an article about an altimeter datalogger in their February 2001 issue.
It won 3rd prize in their Design2K contest.
http://www.circuitcellar.com/design2k/winners/third.htm


Good luck with your project.

Regards
David.