[aprssig] RE: [OZAPRS] GBAS

Gerry Creager gerry.creager at tamu.edu
Sun Feb 10 08:53:33 CST 2008

GBAS is a rather generic term describing what the US FAA is doing for 
Local Area Augmentation Systems.  Darryl's descriptions are correct but 
a little incomplete, in that, as well as determining precise ephemeris 
for the constellation, you can also determine tropospheric delay and 
ionospheric parameters, removing several other sources of enhanced error 

With GBAS, aircraft (at least in the US; haven't looked at the rules 
elsewhere) can effect high-precision instrument landings (CAT-III 
coupled approach for you pilots), or other enhanced precision approaches.

IN a later post, Bob mentions calculating and transmitting a local 
offset.  While this is of passing interest, simple offset calculation 
didn't address the issues associated with PR and PRR (and doppler) 
correction directly at the receiver that could be affected by multipath 


Darryl Smith wrote:
> 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.
> Darryl
> -----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
> http://www.tech-software.net
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Gerry Creager -- gerry.creager at tamu.edu
Texas Mesonet -- AATLT, Texas A&M University	
Cell: 979.229.5301 Office: 979.458.4020 FAX: 979.862.3983
Office: 1700 Research Parkway Ste 160, TAMU, College Station, TX 77843

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