New GPS buyers are frequently concerned about the accuracy (or lack of it) of
the altitude readout on their newly purchased GPS. Many suspect their equipment may even be defective when they
see the altitude readout at a fixed point vary by many hundreds of feet. This is NORMAL.
Generally, Altitude error is specified to be 1.5 x Horizontal error specification.
This means that the user of standard consumer GPS receivers should consider +/23meters (75ft) with a DOP of 1
for 95% confidence. Altitude error is always considerably worse than the horizontal (position error). Much
of this is a matter of geometry. If we (simplistically) consider just four satellites, the "optimum"
configuration for best overall accuracy is having the four SVs at 40 to 55 degrees above the horizon and one (for
instance) in each general direction N, E, W, and S. (Note: You will get a very BAD DOP if the SVs are
at the exact same elevation. Luckily, this is a rare occurrence.) The similar "best"
arrangement for vertical position is with one SV overhead and the others at the horizon and 120 degrees in azimuth
apart. Obviously, this arrangement is very poor from a signal standpoint. As a result, of this geometry the calculated
solution for altitude is not as accurate as it is for horizontal position. Almost any calibrated altimeter
will be more stable at reading altitude than a GPS.
GPS altitude measures the users' distance from the center of the SVs orbits. These
measurements are referenced to geodetic altitude or ellipsoidal altitude in some GPS equipment. Garmin and most
equipment manufacturers utilize a mathematical model in the GPS software which roughly approximates the geodetic
model of the earth and reference altitude to this model. As with any model, there will be errors as the earth is
not a simple mathematical shape to represent. What this means is that if you are walking on the seashore,
and see your altitude as 15 meters, you should not be concerned. First, the geodetic model of
the earth can have much more than this amount of error at any specific point and Second, you have the GPS
error itself to add in. As a result of this combined error, I am not surprised to be at the seashore
and see 40 meter errors in some spots.
DGPS operation (where available) will dramatically improve the performance of
even low cost GPS receivers. Horizontal accuracy of +/ 5 meters and altitude accuracy of +/ 10 meters (relative
to the WGS84 geode) with suitable DGPS receivers and low cost GPS receivers such as the Garmin GPS12XL can be
expected.
In any case, it is extremely unwise to overly depend on the altitude readout of
a GPS. Those who use GPS altitude to aid in landing their small plane should have their insurance policies paid
up at all times.
