A New GPS Question

[OldAndBold]

Originally Posted by hevansrv7a

... 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...

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

[kentb]

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

[Brantel]

Originally Posted by kentb

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

Kent

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.

[Mel]

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

[fliier]

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

[RWoodard]

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)

[lucaberta]

Originally Posted by RWoodard

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.

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!)

[RWoodard]

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)

[lucaberta]

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

[OldAndBold]

Originally Posted by kentb

...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

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