[Alberg30] GPS Question

[Robert Kirk]

Bob... GPS errors are the combination of a bunch of things including 
atmospheric refraction which you mentioned and multipath which is flaky 
around buildings & other obstacles. Differential signals broadcast by the 
Coast Guard do reduce the refraction effect but only down to the tens of 
meters level, not inches. Remember when the Government intentionally 
jiggled the clock accuracy to a couple of hundred yards to confuse the 
Russians and civilians? That was called "Selective Availability". 
Differential was very useful the because it eliminated that intentional 
error. But the government dropped SA a couple of years ago, but you still 
have geometry errors.

By the nature of the geometry of the satellites, GPS is excellent in 
horizontal position (where you are on the map) but only so-so in altitude 
accuracy; a couple of hundred feet error in altitude is common. Like taking 
visual bearings at sea on two landmarks almost in a straight line, the 
intersections are sloppy. Also,since two side-by-side GPS's are doing their 
computations from a different overdetermined set of satellite observations 
they can also come up with slightly different positions, given the "noise" 
of a few yards in the calculation.

You should get better than 30-40 feet position accuracy and a couple of 
hundred feet vertical accuracy with the multichannel GPS's from the last 
few years.

The GPS companies are now selling units with a little built-in aneroid 
altimeter to help the altitude problem, which doesn't seem too useful to 
me; If you want an altimeter buy a separate one. Not useful for boaters in 
any case.

If you have one of the new "WAAS Capable"  GPS units, which use additional 
correction data from a different satellite system, your accuracy should 
improve to around 10 feet an the horizontal and maybe that good vertically. 
(WAAS is something like a satellite based Differential Correction. It was 
supposed to replace Ground Controlled Approaches and allow airplanes find 
the runway and land automatically while the pilot is having a beer, when 
the system gets a few more satellites up in the next couple of years. I'm 
not sure the pilots believe this, though.)

Right now, surveyors with very expensive GPS systems which use more of the 
GPS satellite signal and post processing, are able to get a position to 
within an inch or two without even needing WAAS. I assume you are not using 
a surveyor's GPS, though :-)

Bob Kirk
Isobar #181

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