A New GPS Question

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.

That's not correct

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.

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.

Just a little higher....

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

so can i take it on the big plane

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 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.

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

Portable GPS on comm flights

cytoxin said:

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)

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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).

Wow and Thanks!

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

You PULLED it OUT?!?!

hevansrv7a said:

... 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...

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You PULLED OUT electronic equipment and turned it on during flight?!? Are you MAD?!? I am AMAZED that the airplane was even able to land at all! You could have caused a MID-AIR COLLISION by confusing the airliner's delicate navigation electronics with the spurious signals emitted by your so-called GPS unit! You could have been responsible for a hundred people being at the bottom of the ocean after the airliner would have nosed over to follow the interfering navigation signals. PLEASE DON'T EVER DO THIS AGAIN!!!



You got nerve though... I am afraid of the flight attendants seeing me checking my cellphone for the time! I hate flying commercial!

--JCB

I have used my E-trek GPS for years

I always carry it when going commercial air. After 10K I hold it to the widow (I always get the window) and watch the alt/ground speed. I want to make sure the guy up front is doing a good job.

But last year while flying US air the flight attendant told me that I couldn't use a GPS, because it interfered with the planes navigation. I didn't argue (didn't want a sky marshal setting next to me for the rest of the flight) with the attendant.

Has anyone else been told not to use GPS on commercial flights?

Kent

kentb said:

Has anyone else been told not to use GPS on commercial flights?

Kent

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I use to use my GPS III on the plane but in the last couple years, they always say that portable GPS systems cannot be used. Also trying to explain to the security people what it was turns out to be more of a hassle than it was worth.

American Airlines web site lists portable GPS as a no-no during all phases of flight.

This may have been true early on

In the early days of GPS this might have been true.

GPS is essentially a ranging technology. The satellite broadcasts a signal that contains the time the signal left the satellite. The receiver receives the signal, compares it to its internal clock, and calculates it's distance from the satellite. (That's a simplistic interpretaion. There are a lot of compensating calculations in between).

Thus with a single GPS satellite a receiver can define itself somewhere on a known sphere (with the satellite at the center). With two satellites the GPS receiver is somewhere on the intersection of 2 spheres (i.e. a circle) With three satellites a GPS receiver locates itself on the intersection of three spheres (two possible points). Typically one of those points is somewhere in space, so theoretically a GPS receiver can pin a 3d fix with 3 sats. However, that little $5 clock inside the GPS receiver isn't exactly as accurate as the atomic clock inside the satellite, so a 4th satellite is used to compensate for the cheap clock in the GPS receiver.

In a 3D fix, altitide is almost always the least accurate dimension. When you think about it the reasons are straightforward. The earth blocks out 180 degrees of the vertical axis so the geometry of the satellites on that plane is always poorer than the 2 horizantal axis (Lat and Lon).

In the early days of GPS, when it was rare to have 4 visible satellites, the GPS receivers would use a mathmatical representation of the Earths surface as one of the 3 spheres necessary to calculate the fix. In those days it was not uncommon to have the only available satellites bunched up together low in the sky with extremely poor geometry so that single channel sequencing GPS receivers had a dickens of a time trying to resolve Lat and Lon, let alone altitude. (Think of trying to identify the intersection of three spheres where the centers are very close together) In that environment it would not be inconceivable that GPS was of limited usefulness, particularly with respect to calculating altitude.

However, with todays full constellation and 12 channel parallel receivers I wouldn't think GPS would have any problem at altitude.

(This latter part is speculation on my part. I'm not a mathematician or engineer. I was merely a close bystander to the early development of commercial GPS systems)

John Allen

I'll chime in with the UAL guy and say that the GPS in the Airbus 318 and 319's I fly work very well. Our estimated position error is usually in the neighborhood of .09 miles on a 3-1/2 hour flight.

I recently purchased a Garmin 496 and brought it along on a DEN-LAX flight hoping to plot a really cool Google earth track. Unfotunately, the GPS absolutely would NOT pick up a signal sitting in the cockpit of the Airbus with the windows closed.

When we arrived in LA, I took the GPS to a window seat in the cabin and it almost immediately picked up a bunch of satellites. I then opened one of our sliding windows in the cockpit and didn't have any problem getting a satellite. We've got a bunch of tiny heating element wires and supposedly a bunch of protective coatings in the windows up front to protect us from all sorts of UV and all that stuff. Apparently it works on GPS signals, too.

I suggested hanging the Garmin's external antenna out the window for the ride home, but the captain didn't think it was such a great idea!

Rod Woodard
Loveland, Colorado (KFNL)

RWoodard said:

I recently purchased a Garmin 496 and brought it along on a DEN-LAX flight hoping to plot a really cool Google earth track. Unfotunately, the GPS absolutely would NOT pick up a signal sitting in the cockpit of the Airbus with the windows closed.

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had the exact same experience with another Garmin, a 296, in the cockpit of an MD80 while riding with a Captain friend of mine. Absolutely no signal. Zilch.

On the other hand, there are great Bluetooth receivers these days, and another Captain friend of mine who flies the 319/320/321 usually keeps it in his shirt's pocket and gets a 10-satellite fix. Of course you need a PDA or a PC to store the track, as these receivers have no memory and keep no log of the NMEA stream. I've had good experiences with a RoyalTek RBT-2001, based of the SiRF III chipset.

Ciao, Luca
(clearly not an airline pilot!)

Hmmm.

My phone is a PPC-6700 which is a combination PocketPC/Telephone. It's setting next to me all the time anyway (with the transmitter turned off, of course). It's got Bluetooth so all I'd need is a snazzy Bluetooth-enabled GPS. 'Course I'd never do this because the FAA prohibits the use of Personal Electronic Devices while inflight.

On the topic of Bluetooth... Some pilot I know (we'll call him Bob) was messing around one flight and electronically searched the aircraft for any operating Bluetooth devices. Bob found someone's Bluetooth phone or PDA or whatever in the back. Bob couldn't connect to the device, but could send a file. Bob typed up a little message in MS Word and beamed it to the passenger. According to Bob's PDA, it was received.

Unfortunately, Bob was so excited about finding a Bluetooth device, his message wasn't very creative. Next time, Bob says he's going to claim to be the hot blonde in 12D--should liven up the trip for somebody! Bob will be standing at the cockpit door peering through the peephole watching for action in the back!

Oh that mischievous Bob...



Rod Woodard
Loveland, Colorado (KFNL)

Rod,

you just made me chuckle!

Had a similar experience a while back, using a Bluetooth device in flight. Infact, it was that same BT GPS unit I was mentioning in the previous message, I was simply dumping the flight track on my Palm PDA, so that I could later track the flight on Google Earth, pretty much like what you wanted to do from the cockpit with your own Garmin.

You have to realize that Alitalia has now given a Thinkpad laptop to all the flight crews, both Captains and Pilots, so that all the forests in the world can be saved by wasting a little less paper... Basically all of the manuals, be it aircraft, company or whatever else, are loaded on this laptop in searchable PDF format, and crews connect those Thinkpads to docking stations available at the briefing center in order to have them updated.

The laptop also has Bluetooth, and clearly the crew onboard that A321 that just departed from Rome towards Brussels that evening (crew was folks I did not know well) must have switched on their laptop, as I saw the Purser frantically going around in the cabin asking all the passengers whether they had any mobile phone switched on... I am sure that their Bluetooth scan window popped up with my GPS receiver and they tried to connect to it, but since it was already bound to my PDA (which is hidden from BT scans) they thought it was a mobile phone...

So, big brother was watching me from the cockpit... now I am working on a wired GPS so they cannot scan me!

Ciao, Luca

So aggravating!

kentb said:

...But last year while flying US air the flight attendant told me that I couldn't use a GPS, because it interfered with the planes navigation...Kent

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Stop and think about how lame the argument is that your cellphone / laptop / GPS receiver / broadcast receiver / etc is going to interfere with the planes navigation equipment...

Consider an airliner on approach to ORD. Consider how many sources of RF energy are impinging upon the aircraft at that delicate electronic moment. It is about 15 miles line of sight from Sears Tower which has most if not all of Chicago's TV transmitters - that is literally megawatts of effective radiated power. Consider all of the broadcast FM stations that are even closer - there's 50 kilowatts each and they are even closer (some of them) to the airplane and they are on frequencies near to the ILS receivers. Consider the thousands of cellphones in operation at any moment within say, 1 mile of the airplane. Consider all of the cellphone towers supporting those cellphones - some of which are probably located at ORD itself. Consider all of the two way radios in use at the airport. Consider all of the various two-way radios in operation at any moment around the airport - police, fire, commercial and amateur. And consider all of the millions of microwave ovens on the ground leaking RF energy. And all of the wireless networks and the spurious signals being radiated by thousands and thousands of PCs that are directly underneath the airplane while it is on apporach. And, consider all of the other aircraft radios on adjacent frequencies all in direct line of sight of the aircraft. With all that RF floating through the air, it is amazing that we are not all RF roasted to well done perfection.

Now consider what the powers that be consider to be a threat to the electronic safety of the airplane - tiny spurious signals that *might* be leaking from your receiver. Keep in mind that your GPS receiver was manufactured in accordance with FCC regulations that limit the emission of spurious signals down to a very low level. The chances that your GPS receiver could generate a signal that could somehow override the satellite signals or the localizer or the glideslope are practically nil.

So why do they tell us not to use our electronic devices during flight? Control. Must control passengers. Passengers bad, Airline staff good.

--JCB

OldAndBold said:

So why do they tell us not to use our electronic devices during flight? Control. Must control passengers. Passengers bad, Airline staff good.
--JCB

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I flew into MSP a few weeks ago from Boston on the morning of a blizzard and we got as far as about 100 feet from the jetway. And we sat. Finally a guy in a seat a few rows up pulls out his laptop and starts working.

The stewardess, errr, flight attendant, goes nuts and tells him to shut it off "because we're not at the gate yet."

I shudder just to think of how close we came to dying that day.

My biggest pet peeve w/ flight attendant electronic instructions? "OFF" is not a position.

I'm VERY glad to see that the FCC is still nixing the idea of cells on commercial aircraft.

OldAndBold said:

Consider an airliner on approach to ORD. Consider how many sources of RF energy are impinging upon the aircraft at that delicate electronic moment. It is about 15 miles line of sight from Sears Tower which has most if not all of Chicago's TV transmitters - that is literally megawatts of effective radiated power.

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Also consider that all there is an aluminum skin between all these external radiation sources and the avionics and wiring. The aluminum fuselage acts like a Faraday cage, providing a significant amount of protection to the avionics. But your personal electronic device (PED) is inside the fuselage, and much, much closer to the avionics and wiring.

If the avionics and wiring, etc are properly designed and installed, and there are no flaws in the shielding etc, there is very little risk of a problem from a PED that have passed the normal FCC tests. But, consider that a certain percentage of aircraft will have bad coax, bad wiring, etc. And a certain percentage of PEDs may have been improperly assembled, or they may have been dropped, which could shift the internal shielding. If the bad PED is in a location that is close to the flaw in the aircraft's shielding, then it is possible to have a problem.

Several years ago I spoke with the captain of a Challenger business jet who told me an interesting story. They were in cruise, when suddenly the VOR indications in the cockpit started doing very strange things. He sent the copilot back in the cabin to see if anyone was using a PED. He found that the CEO's son was playing with a Game Boy. The Game Boy was turned OFF, and the VOR indications returned to normal. The Game Boy was turned back ON, and the VOR problems returned. Game Boy OFF for the rest of the flight.

The problem is real, but it takes specific circumstances to allow it to happen. In cruise, the consequences are low enough that it makes sense to allow the use of PEDs. At low altitude, the potential consequences are a fatal accident, so it makes sense to prohibit use of PEDs.

Kevin Horton said:

Also consider that all there is an aluminum skin between all these external radiation sources and the avionics and wiring. The aluminum fuselage acts like a Faraday cage, providing a significant amount of protection to the avionics
. . .
The problem is real, but it takes specific circumstances to allow it to happen. In cruise, the consequences are low enough that it makes sense to allow the use of PEDs. At low altitude, the potential consequences are a fatal accident, so it makes sense to prohibit use of PEDs.

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Actually, I wasn't suggesting the operation of PEDs during approach. Just pointing out that the airplane is exposed to far more EMI from outside than anything that is inside. Your comment about the effects of the fuselage is valid though. Nonetheless the antennas are all outside and exposed to all that EMI. And after all, the thing we are talking about operating during flight (a GPS receiver) is specifically intended to be operating onboard an airplane(!)

I hate flying on airliners. Hate it hate it hate it. I will drive 800 miles to avoid using an airline. It will be interesting to see how far I will be willing to fly the RV to avoid the airlines - I suspect the decision will have much more to do with avoiding flight over the Rockies or whether the flight is out of the country.

--JCB

OldAndBold said:

I hate flying on airliners. Hate it hate it hate it. I will drive 800 miles to avoid using an airline. It will be interesting to see how far I will be willing to fly the RV to avoid the airlines - I suspect the decision will have much more to do with avoiding flight over the Rockies or whether the flight is out of the country.

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I've somehow turned into one of those road warriors who travels every week. Luckily for me, it's an easy flight, lots of direct service, DFW-SNA. However, flying anything short of a turboprop (ok, maybe a Malibu or P210) just isn't going to work - too much weather, terrain and plain distance between here and there.

And so I have developed this love/hate relationship with American Airlines. I love that they get me across two time zones and a lot of mountains in less than three hours. I love that I'm whatever Super Grand PooBah status to where I get to select exit row seats ahead of time and sit with extra legroom and a power port for the laptop (somehow, the airplane is the only time I get to watch movies. I sure can't work when we encounter moderate turbulence, but watching movies is easy ).

I hate that even 737s can't deal with mesoscale complex thunderstorms like we got last week and I get delayed. I hate having to go to DFW and suffer the indignities of the TSA. But most of all, I hate that I'm out of town when my airplane sits in the hangar, doing nothing. As my wife mentioned to my father-in-law one day "No, the little airplane isn't the problem. It's the big airplane that's the problem."

Anyway, I've never tried to use a GPS as self loading cargo - er, pax. What I frequently do is take along WAC charts to try and navigate VFR in the flight levels. It's actually a pretty educational experience. Who else does this?

the_other_dougreeves said:

. . .
Anyway, I've never tried to use a GPS as self loading cargo - er, pax. What I frequently do is take along WAC charts to try and navigate VFR in the flight levels. It's actually a pretty educational experience. Who else does this?

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When I was a student pilot I would take the VFR sectional with me and try to follow the flight from Yuma back to LA. Got lots of dopey looks and remarks from fellow passengers. I flew a lot in and out of the SF Bay area for a while and I got to a point where I could generally know exactly where I was anytime after the airliner cleared the Sierra Nevadas or got north of about Morro Bay - I could figure which town was which, where the airports were and figure out which approach the pilot was taking. They'd put flaps out as they went over Livermore on an approach to Oakland, or on the way into SJC I'd know whether they would be landing from the north or the south. Or into SFO I'd recognize all the little airports on the way up the shoreline.

What really bugged me though was flying over Tracy and being able to spot my house - and knowing that as I flew over it that because of time spent at the airport and traffic and the commute home it would be another three to four hours before I would be walking in the front door!

Geez I miss flying in CA...

Kevin Horton said:

Several years ago I spoke with the captain of a Challenger business jet who told me an interesting story. They were in cruise, when suddenly the VOR indications in the cockpit started doing very strange things. He sent the copilot back in the cabin to see if anyone was using a PED. He found that the CEO's son was playing with a Game Boy. The Game Boy was turned OFF, and the VOR indications returned to normal. The Game Boy was turned back ON, and the VOR problems returned. Game Boy OFF for the rest of the flight.

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similar experience that a friend of mine at Alitalia had a few years ago, when he was F/O on the MD11. He now is an MD80 Captain, and has also been building his RV-8 for some 5 years now, hopefully this summer is the right one!

Humming over the ocean in the NAT tracks at night, the fuel computer all of a sudden starts to move fuel from one wing to the other for no apparent reason (the MD11 has a fuel computer that *tries* to keep the best CG and balance for the plane at any given time). The fuel keeps on moving, and then the plane really starts to be unbalanced... flight attendand told to go check for PEDs, kid playing a Gameboy found, Gameboy switched off, fuel computer stops pumping fuel, realizes that the plane is now *really* unabalanced, and starts pumping the fuel back in the right tank... kid told to switch it on again, fuel computer gets confused again, so Gameboy off again.

So they have the kid moved a few rows to another empty seat, Gameboy goes on and nothing happens to the fuel computer...

After this story, I keep very careful control of my actions on PEDs in flight... this stuff happens for real, unfortunately!

Ciao, Luca