WAAS question

[captkirkyota]

Ok, in my readings I have not seen any definitive answers to these, or at least I don't remember seeing any, so.....

Does WAAS use RAIM?

 

Does WAAS only use the Geo Sats? I think no, it uses all, but......

 

What happens if one of the Geo Sats goes down?

( I remember reading that one will cover the Whole U.S. so only one is actually needed, but also that there are supposedly 2 more additional Geo sats up there as back ups anyway. But where exactly I read that?........)

 

 

These next questions I can give vague answers too, but would like anyone who can tell the answers better than I can to please provide input.

 

The diff. between vnav/lnav approach vs WAAS?

And why is WAAS better?

Again I can give a super basic vague answer, but would like a more refined answer from someone.

[Gomer Pylot]

WAAS is Wide Area Augmentation System, a method of providing increased accuracy to the GPS receiver, mostly in the vertical plane. It was invented to provide sufficient altitude accuracy to allow precision GPS approaches. RAIM is something else, a method of monitoring GPS data to predict whether sufficient accuracy will be available in the future, and is independent of WAAS, indeed it existed well before WAAS was implemented. WAAS makes it more or less redundant. It uses both geostationary satellites and ground stations to correct for atmospheric anomalies, which affect the speed of light, and thus introduce inaccuracy. The GPS satellites have to know the time so precisely that the effects of relativity, caused by both their velocity and the lack of gravity at the satellites has to be calculated. The speed of light (and radio waves are just light waves at a different wavelength) is a constant only in a perfect vacuum, and is slowed by material objects, such as air, water, and anything else. In order to get sufficient accuracy, ground stations send corrections to the geostationary satellites, which in turn send those corrections to the GPS satellites and to the receivers.

 

Precision, or VNAV approaches, with glideslope information, are only possible when WAAS is available.

[captkirkyota]

Thanks G.P.,

I know that stuff already however.

I did however just a few mins. ago through some online stuff find out that WAAS does use FDE-fault detection and exclusion which they call synonymous w/RAIM, as you said redundancy.

At this point a WAAS approach is not considered a precision app., they intend them to be one day, they are calling them at this point a "semi precision" app.

The diff. between LNAV/VNAV and WAAS app. is that the first because it is not WAAS, ie does not receive the GEO sat signals, it has more strict MDA/DA's whereas WAAS has Localizer performance standards with vert guidance, thus lower DA's. So basically one has localizer performance, the other doesn't.

That's what I've been able to find.

Jury still out on what happens if a GEO sat fails.

Yes WAAS receives from all sats as well as GEO sats all at the same time.

[Eggbeater]

Sounds like you have most of it researched. I believe the answer to your original question is that RAIM is a continuous check of your navigation signal's reliability BY your GPS unit. The difference between RAIM and WAAS/LAAS is that the WAAS/LAAS systems are separate ground units that receive the satellite signal and compare it to a precisely surveyed point, after which the error is used to correct all GPS receivers in the area.

 

There are three types of GPS/RNAV approaches: LNAV only, LNAV/VNAV(Lateral Nav/Vertical Nav), and LPV(Localizer performance and vertical guidance). I never understood the difference between LNAV/VNAV and LPV approaches until one of our company check airmen explained it to our IFR ground class.

 

LNAV as its name suggests, provides only lateral navigation to your GPS unit similar to a VOR approach and uses step-down altitudes in the same way. LNAV/VNAV approaches are also considered non-precision approaches and provide a vertical "glide-slope" that can be used to navigated a suggested vertical glide path. The reason it isn't considered precision like an ILS approach is that the signal given to your aircraft's unit does not get more sensitive as you get closer to the runway.

 

The LPV approach fixes all that by providing both a vertical and lateral path that taper like an ILS' funnel to precisely navigate you to the end of the runway, therefore it is considered a precision approach. If the approach plate states all three approaches are available, then the equipment is in place for anyone to use all approaches - the only variable is your aircraft's equipment. Many GPS units coming out are equipped for LPV approaches, and the only thing preventing most 135 operations from using it are the company's OPSPECS approval and training for pilots. I'm pretty sure that many airlines and fixed wing corporate companies are already using LPV, but I don't think any helicopter companies have started using it yet.

 

The beauty of the LPV approach is that unlike the ILS, which requires extensive ground equipment that must be maintained and monitored, it can be placed in many airports that normally would not have a precision approach. I would be interested to see how extensive the new NEXTGEN airspace system utilizes them. The old(current) system uses ILS approaches backed up by non precision approaches like VOR and GPS, while I've heard the next system should primarily use LPV approaches with ILS' as back ups.

 

However, it seems the GPS system relies too heavily on one link (the satellites) and outages could put the entire sytem at risk. I hope this reply wasn't too long; I just always wondered about WAAS, and didn't find enough aviation related information to it out there.

[lelebebbel]

The diff. between LNAV/VNAV and WAAS app. is that the first because it is not WAAS, ie does not receive the GEO sat signals, it has more strict MDA/DA's whereas WAAS has Localizer performance standards with vert guidance, thus lower DA's. So basically one has localizer performance, the other doesn't.

That's what I've been able to find.

Jury still out on what happens if a GEO sat fails.

Yes WAAS receives from all sats as well as GEO sats all at the same time.

 

An LNAV/VNAV approach is not possible without WAAS. You will only get lateral guidance -> LNAV minimums apply.

 

An LPV approach is similiar to an LNAV+VNAV approach, except that the glide path is defined and surveyed more precisely, so the obstacle clearance area around the glide path can be smaller and the approach minimums can be lower. (there is no localizer involved in an LPV approach, its just a name)

 

With a WAAS unit, the receiver calculates if the signal quality is good enough for LPV (if applicable on the approach to be flown).

If the receiver loses the WAAS signal, it will tell you that you are now on an LNAV approach, or -worst case- that signal integrity is lost completely (RAIM flag, "INTEGR" on garmin GPS units) and you have to go missed.

[captkirkyota]

Egg and lel,

too long, NO! WAY! these are exactly what I was looking for, better more in regular person English rather than the technical lawyer speak I've been reading, you've basically told me how to tell what I already know in a much better way than I was currently able to tell it back, thank you, thank you, THANK YOU! these answers are perfect!

[kodoz]

Not that you care now that you're Mr CFII, but check the AIM 1-1-19, paragraph 3..."When GNSS is not using integrity information from WAAS or LAAS, the GPS navigation receiver using using RAIM provides GPS signal integrity monitoring."