Endurance.Net Ridecamp Archives

The survey GPS units cost in the upwards  to a 1000 dollars. They have 
much more efficient antennas and better (lower noise) electronics than 
the hand held uints. They are fairly accurate, however, the real time 
read outs are not all that accurate. The accuracy is obtained by 
downloading the position data (along with the error statistics 
concerning the fix based on the satellite signal strengths) in to a 
mapping program. This program now calculates your track. This tract can 
be used to estimate distance. This distance if fairly accurate - 
depending on which map database you use. The more expensive units have 
more memory and a larger sampling rate ( samples fixes are taken more 
often).

By using the highest resolution map database ( supplied by NIMA ) you 
can get very good track distance estimates. The realtime distance 
estimates even the good ones give are on the order of  up to about 10% 
off - normally 10% short. Using the fix data and a map is much more 
accurate.

With some of the hand helds and a good mapping program you can do pretty 
good if you are careful to stop when you do not have space craft 
coverage and wait till you do. If you throw into too much data with 
marginal spacecraft coverage - you will be hosed.

The NAVSTAR GPS system was designed to provide accurate time transfer 
and accurate location information. That is what it does - all the rest 
is calcualted from this information. It is how it is calculated that 
determines the accuracy.

Truman

Catfish Daniels wrote:

> Hi Tina,
> Good point.  I was so shocked at the discrepancies that I took my 
> mountain bike and rode the same trails - or I should say, until I got 
> stuck on a big hill and the sand was too deep.  So I joined a friend 
> of mine on his four wheeler and came up with the very same result.  
> Let me point out, however, that my "double-checking" was really 
> unnecessary, because as soon as you loose signal, or even receive a 
> weak signal, which has only happened to me twice before (in places you 
> don't really want to ride anyways), it beeps and your watch says "weak 
> signal".
>
> Anyway, hope this answers your question. :-)
>
> Catfish



=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=

Ridecamp is a service of Endurance Net, http://www.endurance.net.
Information, Policy, Disclaimer: http://www.endurance.net/Ridecamp
Subscribe/Unsubscribe http://www.endurance.net/ridecamp/logon.asp

Ride Long and Ride Safe!!

=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=