A New GPS Question

[hevansrv7a]

My wife and I flew NWA to Puerto Rico today. I had a looksee at the A320 panel and a chat with the pilots. They use 3 cross-checked inertial guidance systems and the usual land based stuff, but no GPS. They told me that above 20,000 feet that GPS is not accurate. They were proud of only a half-mile position error at the end of a medium length flight. I said my hand held would beat that and that's when they said no, not at altitude. OK, so at cruising altitude I pulled out my 496 that I had in my carry on because it has the street level mapping for PR. It said the position error was 76 feet, speed 475 kts and so on.. OK, that's pretty bad compared to down low, but it's not half a mile, is it? Thanks to the new era in commercial flight, I could not share this with them during the flight and could not cross check Lat-Long at a given moment.

So, are there any real experts out there who can shed light on this question? OK, I know RV's don't fly above 20k, but the question is at least interesting.

[Yukon]

Our inertial navs on the older 737's are updated constantly by DME/DME fixes.
Without the dme fixes, the inertial data becomes very inaccurate over time.

All newer installations use GPS as well as DME. I have never heard of the altitude issue.

[panhandler1956]

That 20,000' thing is BS, we routinely use GPS as primary nav in our business jet FMS platforms and it is much more accurate than our inertials. The older corporate jets used inertials as primary, now they are all GPS primary and IRS (inertial reference system) as backup. With the new RNAV arrival and departures we have to have relatively accurate nav info in the flight levels (18,000 and above).
The IRS is also used for EFIS platforms although some of the smaller jets us AHRS as it is cheaper. The AHRS is similar to what all these fancy experimental EFIS systems use.

I have friends at work that could talk all day long on the subject of RNP (required navigational performance) as it relates to GPS, IRUs, etc. We operate 400+ corparate jets of all flavors.

[Kevin Horton]

I have no idea where they get the stuff about GPS not being accurate above 20,000 ft. Complete BS. I've participated in many flight test programs that used GPS at altitudes up to 51,000 ft, and there has never been any concerns about its accuracy.

They don't have GPS because the A320 was designed before GPS was available for IFR. The navigation system uses triple inertial reference systems updated using DME-DME. It is more than accurate enough for enroute, terminal and non-precision approaches (except for a very small number special approaches that require higher navigation performance).

Airbus later added an optional GPS, but many airlines that had older aircraft didn't buy the optional GPS on the newer ones, as they wanted to have all the aircraft in the same configuration (lowers pilot training costs, etc). The airlines have been very, very slow to adopt GPS. There are many older airlines still flying around using VOR navigation, as the airline bean counters aren't prepared to spend the money to upgrade the avionics to add GPS.

[Ironflight]

We are now using GPS on both the Space Shuttle and International Space Station as another navigation source (we use several, and compare them), and I am perfectly comfortable using it when needed - and our accuracy requirements are pretty high. It did take several years of tweaking and playing around to get good solutions when you're traveling 18,000 mph, but once you figure that out - GPS rocks!

Paul

[cytoxin]

im going on a big jet ride, can i get it through security or should i leave it at home. i dont want a DRE at the gate (see prostate thread for further info on the DRE)

[Wes Hays]

I took my 396 with us when we flew commercial to Alaska. Pulled it out above 10K ft. and it picked up the sats out of the window. It was really a kick following us along and seeing where we were. I don't think there will be a problem at all. We took it through Canadian and American customs with out any questions at all.

Have a great time with it!

Oh by the way, no DRE's either.

[Brambo]

I fly the A320/A319 for United and I can tell you that the GPS works just fine above 20,000'. All of our 319's have GPS and most of our 320's also. There are still a few A320's that use the old DME/DME update.

Bill Rambo
RV-7A

[craigvince]

Originally Posted by cytoxin

im going on a big jet ride, can i get it through security or should i leave it at home. i dont want a DRE at the gate (see prostate thread for further info on the DRE)

I have ferried many a small plane by traveling one-way on a big plane. I've never had any problems with any of my flightbag contents. (I leave the Leatherman tool and portable flare gun behind now, though).

[hevansrv7a]

You guys are the best. Thanks for clearing that up!