Altitude measurement with GPS

A handheld 3 satellite GPS receiver probably uses a solid state barometric pressure sensor (silicone chip with a diaphram with a strain gage) to indicate ~height.

The larger GPS receivers track (8+) satellites within the GPS/survey equipment and do triangulation for elevation (with more error) than X/Y coordinates. Their elevation readings take time as well to 'settle' into an accurate value. Cost is proportional to the number of functions, convience, accuracy and speed.

In theory you can set a GPS for a known altitude reference point, like a plane altimeter is set to the local airport barometric pressure at the start of every flight. but I do not know if this is availabe in the lower cost HH units?

No. there is no any more pressure sensor in the handhold gps. They can seek digital storing in the date base compare with signal accepted from sarellite, after calculation inside and get the parameter of position, include high level.

I believe that you need 4 satellites to measure altitude, three for position.

if you don't mind altitude having a tolerance of 50 feet, you are ok with 4 or more satellites and a Bass Pro Shop GPS device. If you are measuring elevation you need Lieca Survey equipment.

Surveying grade GPS systems use Differential GPS or Real Time Kinematic systems can accurately measure elevations to 1 cm level accuracy. The accuracy improves as the number of satellites acquired goes up. These systems typically have a base station over a fixed point and a rover to acquire the data points. I have used Topcons GPS system for over three years and have not had any problems with variation in data. The Topcon system uses differntial corrections to compensate for ionospheric changes during the survey. Handheld units would most likely have variation due to ionospheric disturbances caused by solar radiation fluctuations and solar flare events. All of the hand helds I have seen use triangulation to calculate the position and elevation. Four satellites is the minimum for height calculations, but the error will be large.

No, you are not right,

GPS satellites are in space, therefor no pressure is measured, as there is no pressure there. The GPS "knows" it's position in reference to the earth surface and other GPS satellites. your GPS receiver receives the GPS signals, mathematically calculates it's position on the earth and let you know your position and altitude by retrieving this information from it's own database. Understanding that, you now know that an altitude difference due to changing weather is impossible. As a matter of fact, this is one of the best virtues the Global Positioning System has, is being totally independent of any change in climatic conditions. I guess you were not in the exact same position when you took those measurements. Hope it helps.

Wangito.

Some handheld GPS models *do* have barometric altimeters (e.g., Garmin eTrex Summit).

Correct, for some odd reason, I thought that he refers to high end GPS equipment. Most probably, not so...

Wangito.

This is one of my fields of expertise.

I doubt it is pressure related. Hand held GPS are not going to be very accurate because they are not designed to be.

More likely it is something called Dilution Of Precision (DOP). There is a vertical and horizontal component (VDOP and HDOP). Both fall under something called GDOP, which is the configuration of the constellation of stateliness at any one tine.

There are enough satellites to guarantee a minimum of 6 visible at any one time. However, the position of those satellites changes with respect to each other and so does the GDOP. The worst case is when the satellites are at a single point. The pest case is when they are at right angles to each other.

Horizontal position is always more accurate than vertical position because you can't see satellites below you. That is why you see larger variations of vertical component.

Aircraft can also use baro assisted altitude from their plane to improve vertical accuracy, but I don't know of any handheld units that contain barometric pressure sensors. Read your manual and see.

You need a minimum of four satellites to measure 3-D and five to get time (PVT).

By the way, the last poster claimed that GPS is immune to weather is not quite true. While ionospheric effects make the bulk of the error, tropospheric effects do play a part and are part of the error adjustment calculations performed on airborne GPS systems. Tropospheric effects are compensated rather crudely using temperature and humidity as the main components if memory serves.

You are absolutely right, Anonymous Hero, about weather effects on GPS, However from the way the question was posted I thought that going into such details, will be going too far.

Wangito.

Am not an expert so please explain...

You wrote: However, the position of those satellites changes with respect to each other and so does the GDOP.

If they are stationaries how come they change position?

Wangito.

I think only an equatorial satellite at the right distance can aappear stationary compared to an earth observer, they call it a geosynchronous orbit.

http://en.wikipedia.org/wiki/Geosynchronous_orbit

GPS is in these orbits

http://www.kowoma.de/en/gps/orbits.htm

Things may have changed a bit in the past 18 months regarding handheld GPS and altitude (barometric- or GPS-based). This MB Wiki article give an excellent, brief description, IMHO: http://wiki.motionbased.com/mb/GPS_Barometric_Altimeter

Everyone of you muppets think you know the answer but none of you do have the definite answer in which case I suggest that you quit spreading false info. Godan muppets, the whole lot of you!!