WAAS coverage issue

Dave Siciliano

Dave Siciliano

Final Approach
Joined
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Display name:
Dave Siciliano
As posted on AvSig.

Best,

Dave
===============================================

Quote:
FAA Predicts Erosion of GPS WAAS Service Due to Intelsat GEO Failure


Federal Aviation Administration (FAA) officials say that loss of control over an Intelsat geostationary (GEO) carrying a GPS Wide Area Augmentation System (WAAS) transponder could subject users in the National Air Space (NAS) to temporary outages for the rest of this year, beginning within the next two to four weeks as the GEO drifts out of a useable orbit.


Intelsat S.A. announced the anomaly in Galaxy 15 (G-15) on April 8. Although the communications services provided by G-15, located at 133 degrees west longitude (WL), have not been affected, according to Intelsat, the satellite apparently is not responding to commands by controllers. The anomalous condition began on April 3, according to the FAA.


The Luxembourg-based Intelsat is moving an older spacecraft (G-12) that serves as a backup for G-15 from its location at 123 degrees WL. However, G-12 does not have an L-band transponder, which is needed for WAAS transmissions.


​

Full story: http://tinyurl.com/y38ag27

The satellite going down is the westernmost WAAS transmitter. 16 airports in Alaska will lose WAAS coverage entirely. Due to the loss of redundancy, the rest of us will experience intermittent WAAS failures when one of the two remaining satellites goes off-air for system maintenance. A replacement isn't expected to be operational until December.
 
And shutting down ground based navigation systems is a good idea exactly why?
 
mantakos said:
Because something broke? Doesn't that happen regardless of whose name is on the side? Because I seem to remember things breaking...
-harry
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And what is Intelsat's response going to be versus an owner-operator's?
 
Ghery said:
And shutting down ground based navigation systems is a good idea exactly why?
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You couldn't do an approach with even close to the accuracy as WAAS with LORAN anyway, or with a VOR, if that's what you are referring to.
 
mantakos said:
I would presume that Intelsat's response is going to be whatever the contract requires it to be.
-harry
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Exactly. Intelsat could care less that pilots in Alaska or elsewhere won't have WAAS, they only care what's in their contract.
 
Trapper John said:
Exactly. Intelsat could care less that pilots in Alaska or elsewhere won't have WAAS, they only care what's in their contract.
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And the people whose job it is to care about pilots who fly into those 16 airports in Northwest Alaska (an area that is largely unpopulated) are responsible for ensuring that the contract contains provisions that properly represent what they consider to be an appropriate level of concern.

Clearly, the contract those people drew up provides no redundancy over those 16 airports, and that pretty clearly indicates a willingness to allow a single failure to eliminate that coverage.

In other words, the "concern" was written into the contract, and this is how much concern there was. If the FAA had a level of concern greater than this, then they would have written that into the contract, because these are not unforeseeable events.
-harry
 
Trapper John said:
Exactly. Intelsat could care less that pilots in Alaska or elsewhere won't have WAAS, they only care what's in their contract.
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It's not like they chose to have the satellite fail. You know, they do have operational lifetime's. But, Lockhed-Martin are the people in charge of getting the lease on a satellite for the WAAS program, so it doesn't surprise me. I don't understand why the FAA doesn't put their own satellite up there for WAAS. They could do that, and put XM style weather on it, for the general public to use. Wait, that would make too much sense. I just find it funny we use systems for navigation that were launched on non-US vehicle's. Heaven forbid we use them to launch people, but to use them day to day, its fine.
 
mantakos said:
And the people whose job it is to care about pilots who fly into those 16 airports in Northwest Alaska (an area that is largely unpopulated) are responsible for ensuring that the contract contains provisions that properly represent what they consider to be an appropriate level of concern.

Clearly, the contract those people drew up provides no redundancy over those 16 airports, and that pretty clearly indicates a willingness to allow a single failure to eliminate that coverage.

In other words, the "concern" was written into the contract, and this is how much concern there was. If the FAA had a level of concern greater than this, then they would have written that into the contract, because these are not unforeseeable events.
-harry
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That means no WAAS. It doesn't mean the end of the world for those airports - it may mean they are LNAV only for a while.
 
mantakos said:
If the FAA had a level of concern greater than this, then they would have written that into the contract, because these are not unforeseeable events.
-harry
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That assumes FAA (or whoever) has enough bargaining power with the satellite vendor to be able to get the vendor to agree to such a condition. That's not always the case when there are only one or two vendors available to do business with.
 
Trapper John said:
That assumes FAA (or whoever) has enough bargaining power with the satellite vendor to be able to get the vendor to agree to such a condition. That's not always the case when there are only one or two vendors available to do business with.
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Bargaining power comes from a willingness to write a bigger check.

What is it that an "owner-operator" could do in such a situation that the government cannot pay a contractor to do?
-harry
 
mantakos said:
Bargaining power comes from a willingness to write a bigger check.
Click to expand...

It's not always that simple.

What is it that an "owner-operator" could do in such a situation that the government cannot pay a contractor to do?
-harry
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Oh, probably something like reserving a transponder as a spare when one craps out...
 
Trapper John said:
Oh, probably something like reserving a transponder as a spare when one craps out...
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The government can't pay a contractor to put things on a satellite?

(BTW, in this case, the transponder is fine, the problem is that the satellite cannot be controlled and won't maintain its orbit).
-harry
 
mantakos said:
Because something broke? Doesn't that happen regardless of whose name is on the side? Because I seem to remember things breaking...
-harry
Click to expand...

One VOR breaks.. or one ILS breaks it takes out a few approaches..

One sattelite breaks and 1/3'rd of the land mass of the US is uncovered (Alaska), and the rest of the nation is intermittently impacted.
 
mantakos said:
The government can't pay a contractor to put things on a satellite?

(BTW, in this case, the transponder is fine, the problem is that the satellite cannot be controlled and won't maintain its orbit).
-harry
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I think he was obliquely referring to the older generation sat that predated WAAS...
 
Doggtyred said:
One sattelite breaks...
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Yeah, and if that satellite was "owned/operated" by a government agency, then ... what? It wouldn't have broken?
... and 1/3'rd of the land mass of the US is uncovered (Alaska)...
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16 airports in Northwest Alaska, none of which have LPV approaches yet any way.
...and the rest of the nation is intermittently impacted.
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For 5 minutes, 3-5 times a year, until December.
-harry
 
mantakos said:
The government can't pay a contractor to put things on a satellite?

(BTW, in this case, the transponder is fine, the problem is that the satellite cannot be controlled and won't maintain its orbit).
-harry
Click to expand...

Why is that such a big issue?

At geosync altitude, the orbit won't decay (a sat at 500 km will stay up for 20 years without intervention...at 22k miles, it'll take a while!) and the position of the WAAS satellite is not important to the calculation, it simply broadcasts data out. It's not like a high-density com-sat that you need to point a dish at either, the signal is low bandwidth and you don't need a directional receiver.
 
JeffDG said:
Why is that such a big issue?

At geosync altitude, the orbit won't decay (a sat at 500 km will stay up for 20 years without intervention...at 22k miles, it'll take a while!) and the position of the WAAS satellite is not important to the calculation, it simply broadcasts data out. It's not like a high-density com-sat that you need to point a dish at either, the signal is low bandwidth and you don't need a directional receiver.
Click to expand...

One of the risks is that an uncontrollable satellite will drift out of it's box and put neighboring satellites at risk.

On WAAS specifically, the satellite does have a gain antenna directing the signal to earth - meaning that if the satellite drifts and rotates so the beam is not aimed at the proper place, the signal-noise ratio will become unacceptable.

A loose satellite is a bad deal...
 
Doggtyred said:
One VOR breaks.. or one ILS breaks it takes out a few approaches..

One sattelite breaks and 1/3'rd of the land mass of the US is uncovered (Alaska), and the rest of the nation is intermittently impacted.
Click to expand...
Dammit, it's NOT uncovered. It's IMPAIRED, meaning GPS still works but accuracy suffers. LNAV-only minima will still be available as far as I can tell.
 
mantakos said:
Because something broke? Doesn't that happen regardless of whose name is on the side? Because I seem to remember things breaking...
-harry
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Well Harry to be honest not a single one of my satellites have broken:D
 
JeffDG said:
...and the position of the WAAS satellite is not important to the calculation, it simply broadcasts data out.
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Position is important, actually. A WAAS-enabled GPS can use the signal from the WAAS satellite itself as another reference in solving the position-in-space problem. That improves coverage reliability as you need one less GPS satellite in view to calculate your position & altitude.
 
Joe B said:
Position is important, actually. A WAAS-enabled GPS can use the signal from the WAAS satellite itself as another reference in solving the position-in-space problem. That improves coverage reliability as you need one less GPS satellite in view to calculate your position & altitude.
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I wonder if the system allows for WAAS data to be used from a SV that's offline for position? IOW can a WAAS SV flag itself in a way that allows the correction datastream to be considered valid?
 
gismo said:
I wonder if the system allows for WAAS data to be used from a SV that's offline for position? IOW can a WAAS SV flag itself in a way that allows the correction datastream to be considered valid?
Click to expand...

Position of the WAAS satellite might be invalid, but the correction data stream would still be valid, if you were able to receive it.
 
JOOC, what is done with old worn out geostationary satellites? Is there a process for parking them someplace so the existing location can be reused, or are we gonna run out of GEO locations after some time?
 
Makes sense. Kinda scary to see that only 1/3 actually make it to the graveyard orbit though.
 
If the WAAS payload is still active and the SV is drifting out of position both the navigation and corrections signals could still be valid. The navigation signals for all GPS SVs (including WAAS) contain detailed information on where the SV is and where it is going (this is called ephemeris). If the SV is drifting, the drift will be reflected in the ephemeris and your receiver will simply use that in its position calculation.

"I wonder if the system allows for WAAS data to be used from a SV that's offline for position? IOW can a WAAS SV flag itself in a way that allows the correction"

Yes, the position of the SV is not important to the integrity of the signal.

From the FAAs point of view WAAS provides two services: 1) Wide area position corrections to GPS; 2) An indication of the integrity of each signal (GPS or WAAS). The FAA probably places more value on the integrity function than the position correction.
 
vontresc said:
JOOC, what is done with old worn out geostationary satellites? Is there a process for parking them someplace so the existing location can be reused, or are we gonna run out of GEO locations after some time?
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too bad we coudn't have some target practice and shoot them down. :D

**obviously under the assumption no persons or property is hurt on the ground**
 
PA44-LUV said:
too bad we coudn't have some target practice and shoot them down. :D

**obviously under the assumption no persons or property is hurt on the ground**
Click to expand...

The problem with that idea is that just because you break a satellite into extra pieces doesn't mean it is coming down. Now you just have more satellites in a less predictable orbit. Now, if we could de-orbit them, either by slowing them below obital velocity so they come down and burn up, or by speeding them up beyond escape velocity (much greater delta V) and aiming them at the Sun, we would get them out of the way of the next generation of geostationary slot holders.
 
COFlyBoy said:
If the WAAS payload is still active and the SV is drifting out of position both the navigation and corrections signals could still be valid. The navigation signals for all GPS SVs (including WAAS) contain detailed information on where the SV is and where it is going (this is called ephemeris). If the SV is drifting, the drift will be reflected in the ephemeris and your receiver will simply use that in its position calculation.

"I wonder if the system allows for WAAS data to be used from a SV that's offline for position? IOW can a WAAS SV flag itself in a way that allows the correction"

Yes, the position of the SV is not important to the integrity of the signal.

From the FAAs point of view WAAS provides two services: 1) Wide area position corrections to GPS; 2) An indication of the integrity of each signal (GPS or WAAS). The FAA probably places more value on the integrity function than the position correction.
Click to expand...
I was wondering that too. I think the problem is that satellite isn't pointing the correct direction to reliably send a signal to Earth (wsuffa's contribution to this thread).
 
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