On Sat, 16 Jul 2005 10:30:26 -0700, Peter Bennett wrote:

On Sat, 16 Jul 2005 08:43:33 -0400, Anchor
wrote:



There seems to be some confusion between WAAS and DGPS.

Yes, I can see that from your message...

WAAS is on on some GPS satellites, not all. If a GPS can see a WAAS
equipped satellite, it has WAAS information for that satellite. Clearly
if a satellite, WAAS or non WAAS, is in shadow the GPS cannot use that
satellite. A WAAS GPS will need 3 or more WAAS satellites to produce a
WAAS fix. Three or more satellites but less than 3 WAAS satellites in
view generates an ordinary fix. This has nothing to do with whether or
not you are in harbor.

WAAS correction data is transmitted by two (Inmarsat?) geosynchronous
satellites, not by any of the normal GPS satellites.

WAAS collects correction data from a network of reference stations in
the US, and uploads that data (after some processing) to the
satellites for rebroadcast. The correction data is only useful at
location inside, or near, the reference station network - if you
attempt to use the data in distant locations, you may increase
positioning errors, rather than reduce them.

Some GPS manufactures refer to a WAAS fix as a differential fix. This is
a poor choice of nomenclature because a WAAS fix is a WAAS fix, not a
differential DGPS fix.

WAAS is a form of differential correction, so I believe it is valid to
consider a WAAS-corrected fix a differential corrected fix.

Hello Peter,

Absolutely but here in is the cause of the confusion:
1. WAAS - satellite based differential
2. DGPS - terrestial based differential

WAAS is not DGS and DGPS is not WAAS, but they are both differential.

Surveyors survey sub divisions to to 2 cm accuracy by establishing a local
DGPS system which consists of
o ground station
* dual frequency L1/L2 GPS receiver
* RTCM differential correction calculating computer
* RTCM differential correction transmitter w/antenna
(400-500 MHz UHF)
o survey station (handheld)
* dual frequency L1/L2 DGPS receiver
* RTCM differential correction UHF receiver

The ground station is set up where the position is known with precision,
like right on top of a survey benchmark. Turn it on, enter the lat, lon,
elevation and the system is operational.

Options include the RF power and directional or uni-directional antennas.

My brother in law makes his living doing DGPS surveys.

Greg Blair