WAAS Satellite failure

[John Clark]

From AOPA:

WAAS satellite failure cuts signal to one.
One of two satellites providing Wide Area Augmentation System (WAAS) coverage is experiencing technical issues and will soon discontinue its broadcast. The FAA does not anticipate any immediate impact to service, but the remaining satellite will be the only one broadcasting the WAAS signal in space.

Because GEO signals will be single string, there may be service interruptions if the GEO uplink stations switch from primary to backup. These switchovers are rare events, but if one occurs it may take up to 5 minutes to fully restore LPV service.

The full story is on the AOPA Aviation eBrief.

John Clark ATP, CFI
FAA FAAST Team Member
EAA Flight Advisor
RV8 N18U "Sunshine"
KSBA

[Danny7]

http://www.aopa.org/flightplanning/a...T.mc_id=ebrief


looks like waas gps is vulnerable to complete loss if the remaining sat fails. there is supposed to be a backup by december.

[lostpilot28]

Originally Posted by Rivethead

I thought WAAS was a land based component of GPS.

I thought it was a mix of land-based antennas communicating with a bunch of WAAS satelites. I'm surprised that there were only 2.

[Jamie]

Originally Posted by lostpilot28

I thought it was a mix of land-based antennas communicating with a bunch of WAAS satelites. I'm surprised that there were only 2.

The ground-based stations are used to keep WAAS accurate. Basically the stations are at a known, surveyed location and they collect information about the accuracy of the GPS signals being received at the station. This information is collected and sent to the WAAS satellites. Your GPS receiver receives the WAAS signal which is sent separately from the other GPS signals. The WAAS satellites are commercial satellites that serve other purposes as well and the government leases time on them (unless things have changed).

http://en.wikipedia.org/wiki/Wide_Ar...ntation_System

[Jamie]

By the way, this is why I don't like WAAS GPS being the sole source for aircraft navigation. Way too vulnerable of a system. GPS in itself has built-in redundancy but WAAS redundancy is very limited.

[glenn654]

So to clarify this for myself, if the WAAS GPS signal fails the standard GPS will not continue to provide guidance for the WAAS GPS? They're separate systems with specific receivers?


Glenn Wilkinson

[Jamie]

Originally Posted by glenn654

So to clarify this for myself, if the WAAS GPS signal fails the standard GPS will not continue to provide guidance for the WAAS GPS? They're separate systems with specific receivers?

If those two satellites (the commercial ones sending down the WAAS signal) go you will not be able to shoot an LPV approach because your GPS receiver will no longer have a WAAS signal.

I'm not an expert (although I have written some GPS software for business customers) but that's the gist. Don't think about it as WAAS GPS -- think of WAAS as a completely separate service that sends out correction data for GPS.

[Tomasz]

Originally Posted by Jamie

I'm not an expert (although I have written some GPS software for business customers) but that's the gist. Don't think about it as WAAS GPS -- think of WAAS as a completely separate service that sends out correction data for GPS.

That's exactly what it is. GPS errors are mostly due to current atmospheric conditions and as such are quite stable. Stable enough that special GPS receiver can get it's position and compare it to it's surveyed known location. Then it calculates the difference and says 'hey, the GPS lies to us about 5ft to the east'.

Then the system gets that info and sends it via the satellite back to your GPS Waas enabled receiver. The receiver calculates GPS position and then modifies if by the amount sent by Waas.

Because the GPS error drifts slowly it gives the system enough time to calculate error and loop it back without loosing too much accuracy.

There are special systems using radio to transmit the GPS error. They are much, much faster (but with very limited range) than the satellite loop and can provide position with accuracy down to fractions of an inch.

[Danny7]

Originally Posted by Tomasz

......

Because the GPS error drifts slowly it gives the system enough time to calculate error and loop it back without loosing too much accuracy.

There are special systems using radio to transmit the GPS error. They are much, much faster (but with very limited range) than the satellite loop and can provide position with accuracy down to fractions of an inch.

just so. I had a surveyer come out to my property, we were close to a ridge. when he could get the local correction signal, he'd guarantee the marker pin within an inch. without it was 3-4 inches. good enough for 80 acres.

[Andy_RR]

...the WAAS data could be made available to critical locations (i.e. GPS-based instrument approaches etc.) via ground-based communications networks (a.k.a. t'internet) and re-broadcast at said locations for local consumption - who needs to rely on satellites with only dual redundancy?

A