Will the US turn off/degrade GPS System??

[fatboy slim]

Was just wondering as we climbed out on our RNAV SID driven by our twin GPS recievers how well civilian aviation kit is set up if the Yanks decide to degrade the GPS during this upcoming spat. Will the 'puters know there is less accuracy if that is the case??

[Lucifer]

They used to upgrade the system to full during conflicts to allow the best for weapons systems, not degrade it! Besides it is no longer degraded at all.

[expedite_climb]

Lucifer - AFAIK there are two different systems. The mil one never has any degradation, yet the civvie one does.

Reason - the enemy might use the civvie one to hit back.

[feet dry]

Indeed Fatboy...

What you refer to is called selective availablity which was 'switched off' a few years back under Bill Clinton. This meant in theory your £90 Garmin would be as accurate as the whizzy bits of kit you have up front.

It would be perfectly possible for the good ol' boys to dial back into the system a little error so that johnny foreigner is not entirely certain which bit of desert he is in.

[Iron City]

The policy is that SA is not used and the publication of the policy in all the places where SA is now referenced (like SARPS, MOPS, MASPS, etc) is just a matter of time, I understand.

If anything is done to the plain GPS signals you will find it in a NOTAM or at the U.S. Coast Guard Navigation Center web site or any of a bunch of other places.

Would be odd to use SA and at the same time allow WAAS, which has just finished it's I don't know how many 9's availability/goodness exercise to continue to operate and improve navigation solultions, and it is not run by the military folk.

[Rwy in Sight]

Hello fellow Ppruners

A PPL holder told me that during the 1999 NATO attack in Yugoslavia his GPS unit believed itself to be in Spain rather that Greece where it was at the time.

Anyway I did check with a friend and he said his GPS was functionning OK this morning.

And BTW about this £90 Garmin any ideas where I can purchased. I looked at the Pilot shop in London but the cheapest unit was over £110 !!!

Rwy in Sight

[Memetic]

I'd only worry if you were flying in the gulf region (!) with non Mil kit. It's possible to regionally degrade the signal.

See:

http://clinton4.nara.gov/WH/EOP/OSTP/html/0053_2.html

Additionally, we have demonstrated the capability to selectively deny GPS signals on a regional basis when our national security is threatened.

As for a £90 garmin, buy it in / from the us and update the firmware to the UK / Europe verosion. :0

[Lucifer]

Aha, you learn something new every day.

[OzExpat]

I think you need to be careful about the concept of the "£90 Garmin" being "as accurate as the whizzy bits of kit you have up front". The only other comment I'd offer is that, even though SA has been turned off, all the TSO129 GPS equipment in use is still likely to have "SA = On" hard coded into the calculations.

[©hris]

Apparently a GPS blackout has been in effect for some time now over the Middle East.

An aquaintance of mine recently flew over Iran and some neighbours (as a pax) and said his handheld GPS worked the whole time until hit middle eastern territory.......food for thought.

[Dan Winterland]

SA can be regional, varied in it's inaccuracy and is done from the transmitter (satellite) end. Furthermore, GPS jammers are commercially available and probably in use in Iraq due to the Coalition use of GPS guided munitions. GPS signals are easy to jam due to the very low transmitter power and the long time needed to transmit the time signal which the receiver uses to calculate it's position.

The military signals use a different frequency to the one your Garmin receives and can also use encryption to negate the effects of jamming.

I flew 40 miles North of the Iraq border yesterday and our FMCs were still happy to use GPS updating over DME/DME and raw IRS data - showing an ANP of 0.08NM.

A handheld GPS in the back of an airliner is going to fairly unreliable for lots of reasons.

[Jump Complete]

Dan Winterland points out that a hand-held GPS in the back of an airliner is going to be inaccurate.. fairly obivous, I guess, but it is it allowed?
The reason I ask is there is a letter in this months Pilot magazine in which a reader says he turned on his Garmin III unit next to a window and was told to turn it off by cabin crew as it would interfere with the navigation equipment. He says "Am I missing something, or do radios in airliners work differently from those in light aircraft?" In other words I guess, how is it acceptable in a G/A aircraft and not in an airliner? Any comments?

[©hris]

Well if a handheld is 'aviation' approved then it should work just fine on an airliner? Right?

If a GPS just receives signals, how could it interfere with the Aircraft systems? Just the same as my little cd player doesnt affect anything!
True that tests were done proving that there is sometimes interference, but on the in the case of hundreds of laptops and radios and cd players being used in specific locations near the flight deck all at the same time. Or so I am told anyways.

-Chris

[bluskis]

Iraq are indeed jamming GPS, so best not to rely on it if you are out and about in that country.

[Memetic]

If a GPS just receives signals, how could it interfere with the Aircraft systems? Just the same as my little cd player doesnt affect anything!

Without getting into the how immune avionics should be from interference I think I might be able to throw light on why a GPS reciever is seen as a potential problem. (Anyone with more up to date tech knowledge correct me from here on in!)

Although the GPS "just receives signals" it is the way in which it does it that is seen to be the problem. Most (all?) GPS units use a superhet (http://home.attbi.com/~radiowarren/hetbasic.html) circuit to make the reciever more sensitive. This circuit generates a signal that is added to the incomming GPS signal, basically to boost it to a useful level. It is this signal that is seen as having the potential to interfere with systems.

The same applies to most modern radio recievers (Superhet or PLL) hence why we are not allowed to use them on board.

The link has a pretty good explanation of the resons and workings of the circuit.

[englishal]

Yes, I have seen a fault with a GPS antenna cause a GPS receiver to re-radiate knocking out all GPS's within a mile radius. We couldn't believe either that the receiver could cause this, so it took us ages to find the fault.

Just come back from the Caspian, 200nm north of Terhan, 500 from Baghdad and GPS was working fine there.

Cheers
EA

[arcniz]

I believe the mathematics of GPS signalling at the source and the subsequent position computation at the receiver are sufficiently arcane and complex that very specific odd-shaped segments can be diddled out of usefulness if the folks in charge so desire.

[Keef]

No, the mixer signal in a superhet is NOT to boost the signal. It's to shift it to another frequency. There is almost always at least one stage of RF amplification in a superhet before the mixer stage (can't speak about GPS receivers), so the leakage of the mixer signal would be minuscule.

I think the real reason for banning portable GPS in an aircraft is that it's simpler to ban everything. Avoids arguments.

[OzExpat]

I don't mean to interrupt this highly technical discussion but I'm hoping nobody will object to me digressing back to the original question. It seems that I've been one of many people who asked this question officially. The answer has now been given by the United States, in terms that I was expecting though I still had to ask anyway!

Here is the answer :-

U.S. Policy Statement Regarding GPS Availability, March 21, 2003
* The United States Government recognizes that GPS plays a key role around the world as part of the global information infrastructure and takes seriously the responsibility to provide the best possible service to civil and commercial users worldwide. This is as true in times of conflict as it is in times of peace.
* The U.S. Government also maintains the capability to prevent hostile use of GPS and its augmentations while retaining a military advantage in a theater of operations without unduly disrupting or degrading civilian uses outside the theater of operations.
* We believe we can ensure that GPS continues to be available as an invaluable global utility at all times, while at the same time, protecting U.S. and coalition security requirements.

I would like to publicly acknowledge and thank CASA for obtaining this statement for me.

[Few Cloudy]

If you jam GPS during a war, maybe you shouldn't be surprised if the odd smart bomb isn't so smart and lands in your market place. I wonder if this was considered?

[hptaccv]

...but if I remember correctly, the gps signal is put together out of a p code and a c/a code, where we, as civilian users can only utilise the c/a code. Military users (i.e. smart bombs etc.) lock onto the p-code giving them a higher level of accuracy. Selective Availability was used to further degrade the c/a code, but was switched off (as mentioned before..).

please correct me if I'm wrong!

[fatboy slim]

Thank-you expat and CASA

[Few Cloudy]

hptaccv,

Not sure what part it was that the Iraqis were jamming - but they were using Russian jammers and the USA didn't like it and bombed them out of existence, according to reports last week.