Re: RC: GPS

["Truman Prevatt, PhD" <truman.prevatt@netsrq.com>]

The GPS - also known as NAVSTAR - was a satellite program launched by the US Air Force to provide world wide time transfer of  UTC (which is the agreed upon world wide time as maintained by atomic clocks located a several locations world wide and maintained in France ) to within a few nanoseconds. This was it's primary role. It turns out that world
wide accurate time transfer is very difficult to achieve thanks to a little problem call relativity. It was designed for military use but a separate commercial capability was designed into the system which is what we use.

In order to accomplish that it had to know the location of the location the time was being transferred - to within the time transfer error desired. Multiple satellites that transmit multiple signals are used in order to accomplish this. The timing from the signals is estimated and the locations of the space craft is used to calculate both the
location and the time at the location. This is all the GPS system supports. Accurate location and accurate time.

All things such as distance traveled, speed, etc. are calculated in the particular GPS unit from the location and time data derived from the spacecraft and are calculated by software in the units.  First the signals are very weak so as not to interfer with anyother signals in it's frequency range. For those who care the commercial signal power is
-150 dBm - which is well below the electromagnetic noise floor. The signal is encoded so that the receiver can find it and derive information from it.  The signal reception can be impacted by blockage, like foliage in a forest, inside a house or car, in a canyon. You are required to collect the two signals from four spacecraft to get decent
performance. The second drawback with the handheld units is at the frequency of the signals, a 1/4 wave antenna is about 4 inches long. The antenna in these units is very inefficient.

If you can insure that you can collect two channels for four spacecraft the whole time you are measuring a trail then you might have a chance of doing a fairly decent job - but I probably still wouldn't trust it.

NAVSTAR does a very good job supporting what it was designed to do, calculate your location and give you accurate time. It can help you navigate to a specific location. It can help you follow a route you program in. It can do a lot of things, but how good a job it does on anything else depends on a lot of factors. I is much more capable of providing
accurate information to an airplane in flight than a hiker on the ground. I would very skeptical of any distance measured on the ground in varying terrian by any of the commercial hand held units.

Truman

"guest@endurance.net" wrote:

> Beth Swier www.horsieracer@aol.com
> HELP!  I bought a Magellan 310, and it does not seem to be working particularly well.  All I really want a GPS for are the odometer and speedometer.  I checked the accuracy with my truck and it was not correct.  Are GPS's accurate for measuring distance on horseback?  If so, is the 310 adequate or should I purchase something else?  Please advise.
>                                            Thank You,
>                                                Beth

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