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I sent in an article explaining much of this to EN for when they had 
some extra space. Hopefully they will print it.  The GPS was developed 
by the USAF to provide all weather day night navigation and time 
transfer for the US military. The development started in the about 1972 
if I remember right. The Navy had their own satellite navigation system 
since about the early 60's.  This was a low cluster of low orbiting 
satellites, know as TRANSIT. The trouble with this was it was in a low 
earth orbit so it was not always visible and had a revisit time of about 
90 minutes. This fine for ships but little else. Because of the cost of 
the system, the President directed that the GPS also be designed to 
allow civilian use.

About the same time the USSR started development on their system which 
goes under the name GLONAS - which stands for Global Orbiting Navigation 
Satellite System . The name of the GPS systems is NAVSTAR GPS which 
stands for, Navigation System with Timing and Ranging (NAVSTAR) Global 
Positioning System (GPS).  GPS and GLONAS are very similar both in 
design and capabilities and both are still on orbit and are active 
programs.

The GPS system has two channels one for civilian use (C code) and one 
for military use (P or Y code). The military channel is protected and 
cannot be used excepted by an authorized user, it is more accurate and 
it is higher in power (when despread) so the visibility is better. The 
military user uses both channels to remover any atmospheric delays that 
will result in a location and time transfer error. The civilian user 
cannot do this. Contrary to popular belief altitude is not a big issue 
with the GPS system, after all is used to guide cruse missiles, aircraft 
and smart bombs. The issues with altitude is primarily related with 
civilian ground base use. Without visibility to four or more space craft 
(three dimensional mode) you cannot derive altitude information. Without 
visibility to three space craft (two dimensional mode) your GPS receiver 
will not work.

The GPS satellite constellation is 24 satellites in circular 
semi-synchronous (approximately 12 hour) orbits. This is known as Medium 
Earth Orbit, or MEO. With this constellation, a receiver in mid 
latitudes can see at least four spacecraft above 15 degrees elevation 
and 6 above 10 degrees.  If there is blockage in one or more directions, 
such as being in a canyon, or in the mountains, etc., the ability to see 
sufficient spacecraft will be impacted. This is particularly a problem 
if the blockage is from southwest to northwest or southeast to 
northeast. Because of the orbit there are not satellites to the north or 
south of the user. Also with low visibility angles diffraction of the 
atmosphere becomes an issue. This results in biased estimates of the 
range to the space craft ( the path of the electromagnetic wave is bent 
at these low angles and takes longer to get from the space craft to the 
ground than is predicted by the actual distance between the receiver and 
spacecraft). The military receiver using both channels can remove this 
effect, the civilian receiver cannot remove this inaccuracy.

This blockage of visibility to space craft is the key issue. This is why 
an external antenna on top of the helmet works better than the internal 
antenna in the unit on the side of your horse, the horse, human and 
trees are also blockage. In aircraft, ships and military ground based 
users, the GPS system is integrated with the already existence internal 
navigational systems as an augmentation. The GPS can not be available 
for a number of minutes and this will not impact the accuracy since INS 
systems have very good (better than GPS actually) short term accuracy 
stability.  In general any trail measured for an endurance ride will 
turn out to be about 10% longer than it actually is. In hilly terrain in 
the woods it might be as much as 20% long.

As far as Talladega, it was a pre-measured and pre marked existing horse 
trail and the total mileage was accurate. It just a darn tough ride.

Truman

Laurie Durgin wrote:

> I remember the discussion a while ago about distance inacuracies if 
> you have a lot of hills, (mountains, canyons for example)Someone said 
> it marked the Grand Canyon alot shorter than it was? Had to do with 
> Linear distance.satilittes.Orginally created for ships?
> Makes me wonder afte doing the Talladeaga, Al. ride. They GPS ed it 
> for the distance, but there were people there saying it didn't feel 
> right. It felt longer to me too. And that is a hilly, winding ride. 
> Makes you think. ..




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