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GPS units are very poor with altitude - on the other hand the GPS system 
has become the prime source for navigation information in military 
reconnaissance aircraft. Many Low Earth Orbit satellites use GPS as a 
prime source of navigation. GPS also works well in the open ocean. I 
just finished a project where we showed that by using multiple GPS units 
on an aircraft and processing the output of each unit to produce a fix - 
we can do better than the INS system on board. Three years ago I worked 
with Air Force Research Labs in NM on a free flight LEO satellite that 
derived all it's nav and timing from GPS. We were able to locate radio 
frequency emitters on the ground to amazing accuracy from this 
navigation data supplied by GPS.

The reasons for this seeming dichotomy is on the ground in a tree canopy 
or non-flat terrain you do not have connectivity with a sufficient 
number of satellites where as in the air you do. You also have some 
issues with multipath where you are connected to a reflected path rather 
than the direct path - especially with the birds that are low on the 
horizon. Barometric altitude is actually inferior to GPS altitude if you 
have sufficient spacecraft coverage.

While you might get better results by taking your lat/long to a map 
program - you suffer from an error known as "layover." Layover is the 
result of fixing a location on a two dimension grid that is above or 
below the grid. The receiver measures the range from multiple spacecraft 
to itself. When the altitude is in error this forces a layover error on 
the lat/long measurement. Depending on the geometry - the layover error 
can actually be larger than the altitude error. The layover shows up as 
a bias error rather than a random error - that is you true location my 
not be within the error ellipse size calculated by your device. Layover 
cannot be compensated for after the fact because to do so would require 
the altitude error - which is unknown. This is an unavoidable fact of 
life of the use of GPS for instantaneous location measurements on the 
ground. That problem goes away in many of the survey units where you can 
set them to take a long term average and leave them in the same location 
to do that over several hours. Doing that will give you amazing accuracy 
- 10 meters - in both lat/long and altitude. When DGPS is used or you 
have a military unit that uses the good code - you can get down to sub 
meter. That's the good news - the bad news they have to stay in the same 
location for a couple hours.

The bottom line is a GPS receiver is no better than the number of 
spacecraft it has connectivity to. The more the better. When you are on 
the surface of the earth it's difficult to get sufficient connectivity 
to enjoy the accuracy the GPS system supports unless you are willing to 
stand still and endurance riders generally aren't willing to stand still 
;-) . GPS is fine for hacking around - however, I get very nervous when 
I hear a ride has been measured by GPS (unless it is done correctly with 
a survey unit). I just get ready to ride about 5 to 10 percent further 
than advertised.

Truman

April wrote:
> Hi, Marlene,
>
> I don't bother with altitude readings from a GPS unit that doesn't 
> have a barometric altimeter built in. When I upload my data to 
> MotionBased.com, I use their known elevation data rather than the unit 
> elevation data and get more reliable data after-the-fact.

--

“He who fights with monsters might take care lest he thereby become a 
monster. And if you gaze for long into an abyss, the abyss gazes also 
into you.” Friedrich Nietzsche, Beyond Good and Evil


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