GPS Question..Please help
 

During my ATPL viva i was asked this question

Whats the difference between the aircraft GPS reciever and the Gps in the cellphone
Why cant we use Cellphone for gps in flight ?
i was confused :\

i answered about the integrity of the GPS with the FMGS they were not looking for that but were looking for a technical answer ..

please shed some light and help me find a tactical answer

Airbourn gps aims to acquire 4 satelites to triangulate lat, long positions and also altitude. Ground based gps such as cell phones obviously dont require altitude hence will acquire 3 satelites.

found it .................Assisted GPS is the answer

Thanks chuzwuza for taking time to answer my question.:ok::):)

Airbourne GPS must have a RAIM function too.

Dunno, does it have to? At least there is no way to check RAIM status on the 737NG, there is simply no RAIM indication. It might still use it internally, or not, who knows, after GPS is not necessary to fly GPS approaches in that type ;)

GPS is certainly required for RNAP AR in the NG. And, you very well may not pass muster for "regular" RNAV IAPs without GPS.

I thought the Indian DGCA had started to grow up.. And refrain from asking silly questions?

Anyway, most probably the answer he was getting at is that a cellular telephone device *can* use assisted GPS where it triangulates it's position from the cell towers. This is used to speed up the acquisition of positional information.

Most cell phone handsets will try to acquire as many GPS satellites as possible, so the answer about it not computing a 3D position is incorrect.

To be honest the question is stupid, as there is no right answer unless he specifies a make, model and version number of a particular handset.

Even then there can be significant differences, as India found out to its detriment with the Blackberry, you can buy a version with security services standard end to end encryption. The GOI were far from happy about that!

@aterpster, i wasn't talking about RNP AR, just GPS/GNSS approaches. And yes, we do not need any GPS for those, nor for any RNAV approaches with more than RNP 0.10. You are correct however, for any RNP AR with RNP 0.1 or lower we do need GPS, if the RNP is higher than that and if DME/DME coverage is sufficient we don't need it again. Quite academic though as i haven't seen any complete GPS outage yet and since the NG does not have any RAIM requirement nor indication we do not need to care about that either.

RNP AR is pretty uncommon still in most parts of europe. Within our route network we have only one airport that has an RNP AR (soon two, we fly both currently, the second one as trial), and that is innsbruck. However, i have to admit that our route network is pretty small with only around 170 destinations.

@PT6A, assisted GPS does not only speed up position updating by triangulating via cell towers, most assisted GPS installations can do a far more first position update by checking for known WLAN installations, which helps a lot especially in cities. And they do download the current almanach data via the cellular data network which helps the GPS receiver to get a faster fix on satellites.

Denti,

Afirm on what you said..Also depending on the specific device, will depend if the device needs the network for the GPS to work.

Some devices outsource their processing to the network, while others retain the ability to function as a standalone GPS when not connected or outside of the network coverage.

Denti:

You've lost me. I thought everyone was playing from the same RNP AR book. RNP AR in the book varies from RNP 0.3 to as low as 0.1.

In addition, in the U.S. for RNAV IAPs DME/DME is specifically prohibited on the chart. DME/DME is permitted on some SIDs and STARs, where the DME geometry has passed flight inspection.

Because regulations prohibit the use of cellphones in flight :-)

Sorry could not resist...
It's as good an answer as any to a vague "trick" question.

BTW: One other advantage cell phone GPS has is that the cellpone already has a fairly precise time of day - this also makes satellite aquisition faster.

About $199,995, I think. ;)

Not quite sure what part of the world your rules are based but in my world:

RNP AR starts at RNP .3and requires operator specific certification to go below that to a minima of .11 for the B737CL and .10 for the A320. Dual GPS and Dual FMC mandatory for RNP AR

RNAV (GNSS): Ends at RNP .3 and can be Single GPS and Single FMC minimum with alternate ground navigation capability available for missed approach. With a Dual GPS/FMC ground aid not required.

RNAV: DME/DME availability required to be demonstrated as able to supply an acceptable geometric update for FMC

A stand alone aircraft GPS unit does need RAIM however with a GPS as a "sensor input" to an FMC equipped aircraft the FMC will calculate an ANP (Boeing) or EPE (Airbus) which will indicate the confidence in the calculation of a position.

Wow, now I have a headache this early on this one!
but here goes,

RAIM is built into the GPS system on the aircraft, and is part of what is balancing all the the different sat connections and detecting faults the pseudorange measurements.

When you look at RAIM predictions,they are configured with 3 modes, Fault Detection, FD(5 sats req'd), Fault detection and exclusion, FDE (6 sats req'd), and if Selective Avail, SA is turned on.

For 3D navigation, you need a min of 4 sats, but will have no RAIM fault detection.

The RAIM function that you will see in the box is represented with the HIL number. Depending on your box, and RNP level set, the box will alarm when the HIL number reaches a certain threshold, the HAL, telling you that you horizontal integrity is getting near the limit Horizontal Alarm Limit of the RNP level.
The HIL, HAL, and HPL numbers are some voodo combination of the secret sauce inside each box, and given the potential combinations of algorithms and factors, I have no idea how alarm limits are decided.

As an example, during flight validation testing, with the Smiths box, RNP level 0.3, I had the box alarm at HIL of .51 in a 737, while in a 320,with a Honeywell box on RNP 0.1 it alarmed at .67....

Hope that helps! RAIM prediction tools

That's such a bad question to ask, in my opinion. It's WAY too open-ended. I could write a 10,000 word essay on that. I was taught as an instructor that good questions were clear, concise and specific. That question isn't. Because it's so open-ended you've now received a ton of different answers below! I can understand why you're confused about the question.

Triangulate is not correct. GPS uses trilateration - Trilateration - Wikipedia, the free encyclopedia

Or, more usually, multilateration.

Trilateration is a fallback when only 3 satellites are visible and, as a previous poster pointed out, won't give you an altitude.

Difference between GPS etc
 

Does airborne GPS not also use WAAS (Wide Area Augmentation System) to achieve greater accuracy for ILS procedures ?

This would not be a feature of cellphone GPS use.

Interestingly, Garmin uses this (WAAS) extensively on their products including the small Etrex Vista unit that has served me well in gliding for many years.

Not all aviation GPS units are WAAS capable. Airlines don't typically have WAAS, while it's commonplace in the GA / flight training world.

Actually, cell phones do use WAAS if they receive the signal (depending on cell phone model of course), however it is not a big marketing thing as there is no real additional use for groundbased ops as there is in airborne application. The only thing is an increased accuracy.

GPS on phones and iPads and things.
 

The poblem is, as I see it, is: Despite the accuracy questions or whether it has RAIMS capability or Vertical Nav (Elevation) or not - the main probs are whetther or not you are receiving a phone network coverage signal = for, if there is no signal due to altitutde, coverage or whatever, then you won`t get your GPS working, or am I wrong?