Re: GPS to calculate mileage

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

Actually if one simply processes the GPS signals directly - not relying
on the current comerical processing available - horizian geometries are
the worst for altitude.  Geometerties where the space craft's subvehicle
point are clost to the given location are optimal for altitude.  The
critical problem with GPS and horizon geometries is the frequency of the
signal and the effects coming through the atmosphere.  Military users
have compensation for this while comerical users don't.  

The commerical use of GPS will support the current accuracies as
specified by the US Government.  There is a limit - even with
differential GPS.  

The WGS84 standard - soon to be replaced by WGS96 - is the standard
which GPS data is calculated and reported. It is the reference in which
GPS ephemeris, and the geodetic constants are reported. These constants
are those that best describe the shape of the earth in the 1985 survey. 
These ephemeris and constants are required for the calculation of
location from the timing mesurements calculated by the receiver. Local
references are antiquated.  This is a big problem in the commerical
world.  The USGS is still back in the 40's on some of their surveys. 
Even some U.S. military systems are using WGS72, but that is changeing
by direction.  Within a short period of time all U.S. Government users
will by at WGS96 and the GPS ephemeris and the geodetic constants will
also reflect the WGS96 model and the problems facing the commerical
users will be even more componded.

My team recently performed a test on airborne nav systems where a GPS
(military accuracy) was compaired to a GPS augmented inertial nav system
(to remove long term drift)  as comparied to a day time star tracker
system.  The GPS nav system performed the worst and the star tracker
system the best.  GPS is good and is a giant step forward in
distributing time world wide and is a big advance in providing location
to commerical and military users, but I would not bet the farm on the
distance it measures - especially in the commerical mode.

Truman

Duncan Fletcher wrote:
> 
> Yes and no. I don't believe the biggest problem with altitude is WGS84 datum
> to local reference datum but the same thing that affects all measurements -
> selective availability (the fudge factor the military throws in the time
> data to deliberately make it inaccurate). The second major problem is
> satellite geometry and resulting signal strength (the best geometry for
> altitude involves 3 of the satellites being on the horizon which is not the
> case for horizontal - satellites on the horizon have poor signal strength
> assuming they are not block entirely). In any case, within the geographical
> confines of a endurance race, the difference between geoid and reference
> altitude will be very vary within a very small number. Most units also have
> at least horizontal conversions from one datum to another built in - I have
> never questioned whether the vertical datums are built in (450 feet in error
> vertically to me is a lot more critical than 300 feet horizontally, so I
> don't rely on the altitude which I would use only as means of finding
> horizontal location from topos anyway).
> 
>
-- 
Truman Prevatt
Mystic “The Horse from Hell” Storm
Rocket a.k.a. Mr. Misty
Jordy a.k.a. Bridger (when he is good)
Danson Flame - hey dad I'm well now and ready to go!

Brooksville, FL