GPS and Barometric pressure sensor recommendation please

To make a cargo bomb, you really need only one of these variables. Why do you want to know both?

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

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

Bryan Hackney wrote: ...

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

Personally I haven't used any, but I heard quite positive things about the Lassen SQ from Trimble

Regards and good success! johannes

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.

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

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.

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.

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

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:

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

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

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

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.

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 writes:

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

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Al Balmer
Balmer Consulting
removebalmerconsultingthis@att.net

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).

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

Circuit Cellar had an article about an altimeter datalogger in their February

2001 issue. It won 3rd prize in their Design2K contest.

Good luck with your project.

Regards David.