John Clark

John Clark

Well Known Member
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
 
lostpilot28 said:
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.
Click to expand...

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_Area_Augmentation_System
 
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.
 
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
 
glenn654 said:
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?
Click to expand...

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.
 
Jamie said:
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.
Click to expand...

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.
 
Tomasz said:
......

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.
Click to expand...

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.
 
surely...

...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
 
the gps units are already receiving a satellite signal. to get a correction signal through a similar satellite (although maybe on a different set of frequencies?) doesn't take any new hardware(other than the waas satellite :D

i understand it is a real time correction factor, I don't understand how your internet connection would provide the same correction factor; that correction factor needs to be updated regularly and locally
 
For what it's worth, GPS without WAAS is accurate "to within 25 feet." That means better than 50% probability that accuracy is BETTER than 25 feet.

In the first Gulf War they delayed the start of the war, partly to get more satellites launched. I had a GPS unit that was very nearly a prototype. We were driving around the desert, stopped for a bit and I took a check point (the unit could store 10 - amazing!). Anyway, I didn't realize it but my sunglasses fell off the seat into the sand.

Hours later we tracked back to the spot by GPS, me calling out instructions to my driver. I told him to slow, then stop. I opened my door and my glasses were 6" to my right on the ground.

Morale of the story? In an emergency, you can fly non-WAAS precision glide path to a safe landing. Just stay a teensy bit high on GP, land a bit long, and be happy.
 
gps reliance on 'them'

....seems we have put all our eggs in the GPS basket, don't it?

one wonders if a meteor shower, some solar flares, and we'll be wishing we had our Loran and VOR's back again.

we all think 'someone' out there has created and is maintaining a wonderfully complex technology so we can find our way to the corner store in our SUV's.

beware the potholes.
 
flyboy1963 said:
....seems we have put all our eggs in the GPS basket, don't it?

one wonders if a meteor shower, some solar flares, and we'll be wishing we had our Loran and VOR's back again.

we all think 'someone' out there has created and is maintaining a wonderfully complex technology so we can find our way to the corner store in our SUV's.

beware the potholes.
Click to expand...

Hey, most people fly VFR most of the time. My charts work just fine, GPS is super nice but its not all there is.
 
flyboy1963 said:
....seems we have put all our eggs in the GPS basket, don't it?

one wonders if a meteor shower, some solar flares, and we'll be wishing we had our Loran and VOR's back again.

we all think 'someone' out there has created and is maintaining a wonderfully complex technology so we can find our way to the corner store in our SUV's.

beware the potholes.
Click to expand...

It's a pretty big basket when you consider 24 or so satellites orbiting the earth. As explained previously, WAAS just fine tunes the accuracy. Where WAAS really counts is in the vertical distance, which is a requirement for GPS approaches.

I too, have had moving map GPS since the first releases to the public back in the early ninties. By around 1996, the moving maps would show detailed runways, highways, the interstate, etc. And without WAAS, the GPS was close enough to show my road vehicle entering an interstate on ramp, the first hour I used it. Boy, was I impressed! Since then, my mind hasn't changed. GPS has only gotten better, and provides real time information for pilots, that Loran or radio navigation (VORs) can't even come close too.

L.Adamson --- RV6A/ Garmin 696/ XM weather
 
Danny7 said:
Hey, most people fly VFR most of the time. My charts work just fine, GPS is super nice but its not all there is.
Click to expand...

I fly VFR too. But it's great to know the weather patterns in real time, for distances of several hundred miles away, or more. I also like the fact that my GPS in conjunction with my fuel metering system, is always providing up to date info on my fuel usage & reserves. With XM weather, I get continual updates on altitude settings, as well as ASOS, AWOS, ATIS at airports far in advance of what the comm will do. It also shows current TFR's as well as the exact boundaries. This last, year, they were popping up all over the place.

And for boundaries shuch as restricted military airspace, and airspaces such as the Grand Canyon, you can't beat GPS, as it's all precision within mere feet. In areas such as these, GPS allows me to scan for other aircraft, and enjoy the view, rather than my head stuck in the cockpit triangulating VOR's while scanning the chart.....because I might infringe on airspaces if I don't. :)

L.Adamson --- RV6A/ Garmin 696/ XM Satellite weather
 
You must log in or register to reply here.