[aprssig] RE: [OZAPRS] GBAS

Darryl Smith Darryl at radio-active.net.au
Sun Feb 10 04:59:30 CST 2008

dGPS works by sending PR (Pseudo-Range) and PRR (Pseudo-Range Rate)
corrections for each satellite, based on the what signals would be needed to
give a correct 3D satellite position. Therefore the PR and PRR Corrections
are single dimensional, and assumed to be identical in each direction. This
is a fair assumption.

There are a number of sources for the errors, some of which are cause a
velocity offset (PR) between the GPS and the Satellite Vehicle, and other
which cause an acceleration error (PRR) between the GPS and the SV

Once you have four base stations you can do some really really smart things.
It took me a while to work this out. There are two operational modes...

	* Receive the same satellite data on four receivers and correlate
the signals determining PR and PRR's
	* Receive data from each satellite on each receiver and correlate
the results.

You can use the four GPS units on the ground together to act as a reverse
GPS unit. That is they receive a transmission from a GPS satellite, and
correlate the data to determine the EXACT orbital parameters, as opposed to
the approximate ones that are available downloaded from the GPS satellite
live, or the ones that are available for post processing. 

Once you have the exact position of every GPS satellite you can remove one
of the sources of error on GPS, that is the lack of precisely known position
of the satellite. These precisely known orbital parameters can then be used
to generate better position reports.

You can also generate sector based dGPS data that is more accurate within
the local area. But I think that this is secondary. I think the orbital
parameters are the primary use for GBAS.


-----Original Message-----
From: ozaprs-bounces at aprs.net.au [mailto:ozaprs-bounces at aprs.net.au] On
Behalf Of Andrew Rich
Sent: Sunday, 10 February 2008 9:04 PM
To: Aprssig
Cc: ozaprs
Subject: [OZAPRS] GBAS

Some may be interested to know that in Australia, we are about to implement

So what is GBAS ?

Funny name.

GBAS is a set of 4 GPS receivers on the ground, which are in highly accurate
surveyed positions.

The GPS error is uplinked to aircraft in the area.

I suppose it is DGPS, but with 4 ground receivers instead of 1.

The idea being as you fly into an area that supprts GBAS, your position
accuracy suddenly shoots up.

Andrew Rich VK4TEC
vk4tec at people.net.au

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