Do GPS units emit signals or just recieve?

I am not as technologically well versed as many of you so perhaps this seems too easy
but I thought that GPS units simply read signals from the satilites and displayed the results. I didn't think that they actually emitted any signals of their own. especially since it would be trying to communicate with a satilite all the way in outer space. (my other logic was that if the military used it and it emitted a signal then any soldier using one would give away his position)
Here is where I got confused. GPS units are on the list of devices you are not allowed to use during flight on commercial aircrafts to avoid disrupting the plane's navigation system. If a GPS doesn't emit anything that how would it disrupt the plane.

I am very curious what is really coming from my GPS unit (if anything)


PS does turning the WAAS on or off make a difference?

They only recieve signals. More to follow;

Certain electronic devices are prohibited in use on IFR airplanes (which is what any airliner is) because of the fields they create, it has nothing to do with transmitted signals, but any time you have an electonic device, it has an electronic field. There's all sorts of wires running in aircraft, not to mention one that controls the horizontal situation indicator, and it runs out to the wingtip usually where there is a remote compass and a "flux gate" thingie which measures the variation of the earths magnetic field, and automatically subtracts it and displays the correct magnetic heading (a compass doesn't work because when you turn, it will actually whip around in the opposite direction at first because of the magnetic field). So this, along with thousands of other critical things, are passing through the fuselage of the airplane, near where you are using the GPS, they want to eliminate ANY change that an electonic device will cause any erroneous indication. Airplanes also have plenty of electronic devices of their own that are creating fields, but they actually calibrate certain instruments to take that error out, so again, when you introduce a new electronic device, it can throw instruments off.

Even if you were operating a boombox, cd player, GPS, cell phone, and a plethora of other things, the chances it would do something are very rare, but even if there's a .003% chance, the FAA wan'ts to eliminate it. While extremely rare, air-transport accidents always make the news, and always have an impact on travel, even though they are way safer than cars or even general aviation (light planes).

They actually operate on time, the time is sent from the satallites, and the time is different based on the distance of each satallite. The GPS then solves a complex equation to find a "solution" to the unknowns in the equations. In other words, it needs to find a solution to each of the 3 different times, which equalls a definite point on the globe.

Newer GPS that use Differential GPS and a few others (WAAS and LAAS) also recieve signals from either a ground based transmitter or another (stationary) satallite. Some of the error for GPS comes from the Ionisphere, and the Ionisphere doesn't make up an "even" layer in the atmosphere, so by transmitting this "error" to the GPS units, they can get even more accurate.

Finally, some GPS models do not use "position fixes" to determine your instantaneous velocity, they actually use doppler effect from the signals recieved. By knowing which signal is being "compressed" from doppler effect, you can determine the instaneous velocity in whatever direction it is happening. Position fixes are most definitely used, but as a matter of display some of the better ones will use the doppler effect.

There's 24 satallites, 3 spares. They transmit on 1.3 and 1.5 gigahertz if I remember correctly, there's two frequencies transmitted, one is for the military and one is for civilians. There is no error anymore, and your "baseline" guarentee for accuracy is 100 feet at least 95 percent of the time, it's much better with some sort of "differential" GPS, as most are these days. The military GPS units can compare BOTH signals to find out the ionispheric error instanteously, but this didn't prove to be the "holy grail" that they thought it would be. There's a lot more information that is transmitted than just time, but the time is the critical information that it uses. If a GPS has been off for many months, it has to download an "almanac" that contains predicted satallite locations and health among other things, so that's why it sometimes takes a long time to "initialize".

if it only recieves then why is it on the list of "Dangerous" devices to use on a plane. I would like very much to see my speed and even location as I fly over the country side

if it only recieves then why is it on the list of "Dangerous" devices to use on a plane. I would like very much to see my speed and even location as I fly over the country side

See my first post, it was in the "more to follow" part :D

at least untill it obsesses about some other little thing. like
"what would it be like to forget how to move my leg?"

at least untill it obsesses about some other little thing. like
"what would it be like to forget how to move my leg?"

I flew IFR for around 4 hours today, on the east coast, in the "soup" (clouds)...

Well, not really, it was in a simulator here in Arizona :D, but it's pretty realistic. Practicing a two-crew operation type thing.

It's funny how you completely forget about your "legs" and other various body parts as your mind effectively "shuts out" those kinds of concerns...

Because your GPS is a high frequency device that leaks radio emissions. So it broadcasts but not intentionally.

Unless it's a Garmen Rino, which does broadcast its location to buddy units.

I used a gps on a commercial flight once, watching it show 500+ mph was sort of weird.

Monte

I used a gps on a commercial flight once, watching it show 500+ mph was sort of weird.

Monte

and that's a good example of the doppler-effect instaneous velocity function :D

Wow, some of you guys are WAY too smart!

They only recieve signals. More to follow;

Certain electronic devices are prohibited in use on IFR airplanes (which is what any airliner is) because of the fields they create, it has nothing to do with transmitted signals, but any time you have an electonic device, it has an electronic field. There's all sorts of wires running in aircraft, not to mention one that controls the horizontal situation indicator, and it runs out to the wingtip usually where there is a remote compass and a "flux gate" thingie which measures the variation of the earths magnetic field, and automatically subtracts it and displays the correct magnetic heading (a compass doesn't work because when you turn, it will actually whip around in the opposite direction at first because of the magnetic field). So this, along with thousands of other critical things, are passing through the fuselage of the airplane, near where you are using the GPS, they want to eliminate ANY change that an electonic device will cause any erroneous indication. Airplanes also have plenty of electronic devices of their own that are creating fields, but they actually calibrate certain instruments to take that error out, so again, when you introduce a new electronic device, it can throw instruments off.

Even if you were operating a boombox, cd player, GPS, cell phone, and a plethora of other things, the chances it would do something are very rare, but even if there's a .003% chance, the FAA wan'ts to eliminate it. While extremely rare, air-transport accidents always make the news, and always have an impact on travel, even though they are way safer than cars or even general aviation (light planes).

Imagine the added risk if every 2nd person was running such a device. With the plethora of new electronic devices this is a real possibility. The electrical noise on the plane would be fierce. Also, the complexity of certifying devices as OK for flight would be enormous. Who would run the certification program. How would airlines check all the devices to ensure they were certified? The safest thing to do is to dissallow their use during the most critical times, take-off and landing.

Here GPS is used on small planes as a part of the navigational systems.

Jm who first responded seemed to describe the danger as coming from the ever so slightly altered magnetic field that is generating from being a electronic device. which would mean a GPS device and a Gameboy would be equally dangerous since each runs on batteries and create a very small magnetic field.

pacman however says something about leaking radio waves. do all other electronic devices do that too ie a Gameboy? what is high frequency about a GPS device?
( I am remembering a scene in "Mission Impossible" where a disk drive was emitting a frequency, is that what we are talking about.)



Because your GPS is a high frequency device that leaks radio emissions. So it broadcasts but not intentionally.

Unless it's a Garmen Rino, which does broadcast its location to buddy units.

Radio receivers convert the high frequency input to a lower frequency by mixing it with their own generated frequency (and using the difference in frequency). So a GPS makes hi-freq radio waves, but designers try to keep the signal in the case with shielding. (Even your computer is designed to minimize output radiation - notice the FCC label)

Gameboy - a computer - similar.

Here GPS is used on small planes as a part of the navigational systems.

And the unit is properly sheilded to avoid leaking signal. Your pocket GPS unit is not. As JM has done an admirable job of explaining many of the technical details, I will not bother to here.

As far as GPS vs. Gameboy, the Gameboy does not emit RF (radio frequency) energy. That is why you can use it.