The International Cessna^® 170 Association

Well, I finally succumbed and purchased the WAAS upgrade for my 430. Shooting a GPS approach into Halfmoon Bay and getting a RAIM warning just prior to the final approach fix convinced me to spend the money for the upgrade. I would not have gotten the RAIM warning (at least in this particular instance) with the WAAS upgrade. Now I am just waiting for the pain to subside.

The pain will diminish quite a bit when you shoot your first LPV approach to a 250' minimum!

John those were my exact thoughts. If we only had some WAAS approaches around here. Oh that's right, the FAA is just getting around to approving them for helicopters. Go figure.

Robert could you tell us why you got a RAIM warning and why it wouldn't have happened with WAAS?

Bruce Fenstermacher wrote:....Robert could you tell us why you got a RAIM warning and why it wouldn't have happened with WAAS?

One reason, as I understand it, is that the WAAS satellite itself effectively becomes part of the GPS constellation, and can substitute in some way for a GPS satellite that is out of service. The accuracy of the fix is part of the RAIM calculation, and WAAS certainly improves that.

John is correct. Without the WAAS upgrade I am actually required to conduct a RAIM check prior to departing IFR.

Hmmmm well actually we GPS approach users are all required to look at RAIM before filling the flight plan to that we can adequately plan the flight should RAIM not be available and the last time and the most important time to check is just before the actual approach. All Garmin IFR GPSs do this RAIM check, that is one of the things that make it an IFR certified GPS.

You can check RAIM prediction now at several flight planning sites like FltPlan.com http://www.fltplan.com/

I'm not so sure the WAAS satellite actually assists in the navigation IE RAIM or as a replacement for another regular satellite but adds a correction for a known point to the equation.

So I'm still not clear why a WAAS receiver would allow an approach or have RAIM when a non WAAS unit wouldn't.

Disclaimer:
I'm required to go through annual 135 GPS training. Besides testing it consist of about 4 hours of a computer droning on about GPS and how it works from the beginning of GPS time. To say it is hard to stop your eyes from glassing over and your head hitting the table would be an understatement. I did eventually pass the tests with at least an 80% so the details I'm discussing could well be the 20% I got wrong this year.

According to Garmin documentation (http://www8.garmin.com/manuals/1294_WFD ... ctions.pdf), RAIM prediction is only required for oceanic/remote flight segments if you have a GNS430 or 530 with software version 3.XX or higher, and an approved GPS antenna. (See page 3 of the referenced document.) That's regardless of whether you're operating under part 91 or 135. Oceanic/remote means you're more than 200NM from the nearest airport. And of course, individual operators can require it regardless.

I can't remember where I read that the WAAS satellite itself gives your receiver an additional LOP beyond those it gets from the GPS satellites, so I can't prove it. But it doesn't seem far-fetched to me, if the WAAS satellite incorporates an accurate clock and its ephemeris is known. (One clue is that the GNS430W displays signal strength bars for the WAAS satellites right alongside the bars for the GPS satellites on the Satellite Status Page.) Anyway, I do believe that's a major part of why WAAS enhances the integrity of your GPS solution to the point that RAIM prediction is not required.

John,

I could be wrong but think that you have RAIM regardless if you have WAAS or not. You also have a fault system for WAAS.

RAIM is a system that automatically compares two solutions looking for a difference and requires 5 satilites. 6 satilites are required to figure which satilite is faulty. WAAS is a system were corrections are included and received by satilite but I don't believe the WAAS satilites are actually part of the triangulation used for navigaton.

From Garmin WAAS RAIM/FDE PREDICTION PROGRAM INSTRUCTIONS http://www8.garmin.com/manuals/1294_WFD ... ctions.pdf

• 1.1FAULT DETECTION AND EXCLUSION (FDE)
  The GARMIN G1000 with GIA 63W, the GNS480/CNX80, and the GNS 400W/500W Series products incorporate Fault Detection and Exclusion (FDE) features, satisfying the requirements of TSO-C145a/C146a and “GPS Oceanic/Remote Navigation” per FAA AC 20-138A Appendix 1.
  FDE consists of two distinct parts: fault detection and fault exclusion. Fault detection (RAIM) detects the presence of an unacceptably large pseudorange error (and presumably, position error) for a given mode of flight or a satellite failure which can affect navigation. Fault detection is synonymous with RAIM (Receiver Autonomous Integrity Monitoring). Upon the detection of a fault, fault exclusion follows and excludes the source of the unacceptably large pseudorange error, thereby allowing navigation to return to normal performance without an interruption in service. FDE functionality is provided for oceanic, en route, terminal, and non-precision approach phases of flight. The FDE functionality adheres to the missed alert probability, false alert probability, and failed exclusion probability specified by TSO-C145a/C146a.

(The red emphasis above is mine.)

So it would seem WAAS receivers still have a RAIM prediction but it is now called FD which is part of FDE.

And so I would believe that if you didn't have RAIM for an approach with a non WAAS receiver you wouldn't have FDE with a WAAS receiver and won't be able to fly the approach.

You're right, Bruce. I think what I was talking about, and what I thought you mentioned before, was the requirement to do a RAIM prediction as a pre-flight action. That's no longer required if you have v3.xx or higher software and an approved antenna installation, for enroute or terminal segments.

John

John I did say that and the preflight planning RAIM prediction is still required by our part 135 operations specifications so I assumed it's required for all.

I'm so confused. Uncle!!!!

John

jrenwick wrote:I'm so confused. Uncle!!!!

John

Are your eyes glassing over and eye lids getting heavy. Don't let your head hit the table, it hurts.

Bruce Fenstermacher wrote:Are your eyes glassing over and eye lids getting heavy. Don't let your head hit the table, it hurts.

Thanks, Bruce. I always respect the voice of experience!

John

Bruce,

The AOPA produced an article on RAIM that I blieve answers most of the RAIM questions.

http://www.aopa.org/advocacy/articles/2 ... brief.html

Robert Eilers wrote:Bruce,

The AOPA produced an article on RAIM that I blieve answers most of the RAIM questions.

http://www.aopa.org/advocacy/articles/2 ... brief.html

Yes...but does it answer the other questions in life? Can a member of a certain Pilots Assn use it?
Example from the AOPA site: "Often RAIM predictions are fairy short..."