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In places like Australia that don’t have SBAS they have implemented GBAS at a couple of the major airports to allow suitably equipped aircraft to fly 3D RNP approaches…. In Europe we have EGNOS SBAS and so our TSO146 navigators can do such approaches (LPV) right out of the box….whereas the GBAS corrections are transmitted locally via VHF…. I’m sure there is a good reason that the corrections are transmitted this way instead of via a pseudo-SBAS frequency…which would presumably allow a TSO146 receiver to be used seamlessly…Does anyone here know more about this?

EGPD / OMDW / YPJT, United Kingdom

The GPS antenna is not sensitive downwards. So these corrections, to use efficiently should be transmitted from above the aircraft. So you wouldn’t be able to use that antenna very succesfully anyway.

It might be standarised for other usages as well. For example farmers, road construction often use GBAS system for their autopilot systems.

JP-Avionics
EHMZ

Does anyone here know more about this?

The cost of leasing a transponder on a geostationary satellite which covers Australia?

Administrator
Shoreham EGKA, United Kingdom

Presumably too much….they were in discussions with the Indians to point a transponder at Australia, but nothing came of it…. The problem is small population / big country….same size as the US with 5% of the population….also the user-pays philosophy wouldn’t have helped….

EGPD / OMDW / YPJT, United Kingdom

GBAS is much simpler for local applications than SBAS. Basically, A GBAS ground station just compares GPS and known real position and broadcasts the difference, giving a correction that is very accurate (a lot more than SBAS) but only in a small area., say 20-30 miles.

SBAS would not only need a transmitter on a geostationary satellite, but a network of stations to calculate the corrections for the continent over a wide area, and the redundant comms infrastructure required. I believe that the ground segment of SBAS is more expensive than the satellite hardware.

So if you have a continent which is mostly empty and want to cover the few large airports, GBAS will be a lot cheaper.

As to the original question – the SBAS corrections are quite complex, and involve broadcast of specific corrections for atmospheric conditions, satellite positions, etc – they are not a simple “add 0.00012 degrees to your latitude” correction. Also, for GBAS you will need to assure that each station in a given area has a unique frequency, and transmitting from a ground station you have very different attenuation than from space and it varies a lot less over a given area – so Using the same frequencies than a space segment is out.

Biggin Hill

Yes, so 3D RNP is probably out for light aviation in Australia…which is a shame considering Australia was one of the first (the first?) countries outside N America to embrace GPS approaches…and because the US and Europe have SBAS it is unlikely an affordable solution will ever be developed to allow use of GBAS corrections…

EGPD / OMDW / YPJT, United Kingdom

When the new GPS satellites are in place with L5, it is my understanding that a dual frequency L1/L5 without SBAS will provide equivalent performance to the L1/SBAS. So the next generation of GPS navigators will not require the aircraft to be inside an SBAS service volume to fly LPV types of approaches. I expect this will be the case in the 2020 to 2025 timeframe.

KUZA

That’s really amazing.

I wonder if current “W” boxes will be capable of supporting this mode, with just software changes?

Administrator
Shoreham EGKA, United Kingdom

Peter wrote:

I wonder if current “W” boxes will be capable of supporting this mode, with just software changes?

I would be very surprised if any of the boxes currently on the market could be software upgraded, given that L5 is on a completely different frequency (~1GHz instead of ~1.6GHz). And that is actually the point of the exercise.

LSZK, Switzerland

AnthonyQ wrote:

Yes, so 3D RNP is probably out for light aviation in Australia…which is a shame considering Australia was one of the first (the first?) countries outside N America to embrace GPS approaches…and because the US and Europe have SBAS it is unlikely an affordable solution will ever be developed to allow use of GBAS corrections…

The first certified GPS precision approach (operational) was done in Norway in 2007. It is called SCAT-1 (GLS) and is now in operation on most/all? of Avinor’s smaller airports. I think GBAS has superseded SCAT-1.

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