Could a handheld GPS interfere with the INS system?
 

Someone on the SLF forum asked about GPS use during an airline flight...and I was wondering:

Could a GPS system like a Garmin 496 (or similar) interfere with any of the equipment in the cockpit? I would have thought that since they are usually used in aircraft anyway there wouldn't be a problem.

I take it airliners still use GPS updated INS rather than just straight GPS?

I hope the answer is 'no' ! I use my Garmin on most flights and it is very rare to find explicit advice in the inflight mag or elsewhere on whether or not it is prohibited. They all go on about mobile phones and now also anything with earphones, but rarely mention GPS.
On a recent BA flight the announcement was to 'check in the Highlife Mag for more details' ...but there were none.
So until I hear otherwise I will still be the pillock who taps on the cockpit door and whispers, "psst, turn left at Belgium, mate".

As a rule, devices that transmit or receive data on the electromagnetic spectrum may cause interference. Portable radio, TV, Bluetooth, Wi-Fi, and GPS are all potential sources of interference. The other day I was flying and the DME readings were all over the place. I remembered that my cell phone was left on. I turned it off and the DME eventually settled down. Please, unless you're flying a glider or light sport aircraft, keep your GPS off. If you really want to know your speed, ask before or after the flight. The crew will gladly entertain such questions.

That goes against the practice of the vast majority of GA in this country. GPS is used (even in the handheld type) a lot in Group A aircraft alongside all the other VOR/ILS/DME/ADF stuff in the cockpit. Mobile phones interfering with the DME is a completely different issue though.

I asked the question not because I want to use GPS during a commercial flight but because if it has the potential to interfere with airliner systems it has the potential to interfere with the avionics in my plane as well...which is very worrying.

Im probably wrong but I was under the impression the a GPS unit only received the signal from the satelite and did not transmit anything! And as such the only risk of interference would be from the electromagnetic field generated by electronics. i.e. very small

Small point of order here but GPS nor any ground based nav aids actually update INS. The INS, IRS ADIRU etc. only interface with the respective FMS and do not receive any updates directly from any ouside sources. If your really brave you can do an INS update by flying over a navaid and do a manual update, but that's not the same as you had implied. For all of you who that anyway, forgive me for being redundant.:ok:

Thanks for clearing that up- sorry I don't know a huge amount about INS systems...although I had heard the phrase 'GPS updated' before and assumed that that was what it meant. I just tacked on the question about INS because I was curious.

While your GPS is not an active transmitter it is classified as a passive transmitter. I read some guidance from one manufacturer that said they should not be allowed. Basically anything with an antenna is a risk. Of course how much of a risk is questionable but if it hasn't been specifically approved then you are adding risk by using it.

Quote Spooky: "Small point of order here but GPS nor any ground based nav aids actually update INS. The INS, IRS ADIRU etc. only interface with the respective FMS and do not receive any updates directly from any ouside sources."

Depends on the system. On the three large aircraft types I have flown which had GPS updating, the IRSs all used very different methods of updating position. My current type uses the FMC to produce a blended position which is weighted depending on the quality of the information fro a Multi Mode Receiver which has an inbuilt GPS along with a VOR, DME and ILS. The FMC tells you on the progress page which position it is using, it's nearly always 'GPS Primary'. The type before that used an IRS position which was updated by GPS and the type before that had an IRS with a GPS embedded in it, so the IRS position itself was a blended position.

From my experience, I would doubt a GPS set would disrupt the aircraft electronics at any stage of the flight. But of course you should pay regard to the current airline policy. Mine tells the passengers, 'No electronic items are to be used until the seat belt signs are turned off after take off - and they must be turned off again when the seat belt signs are back on for landing".

But also, I haven't found a GPS set which works in a cabin. Not that I've tried very often!

Surely the following post by radeng in the Pax & SLF Forum answers the question: (Including you, Doors to Manual ;))

radeng
Over 250 posts so far. Perhaps I should click here and order a Personal Title

Join Date: May 2001
Posts: 414


If the GPS receiver is a direct conversion type, then the biggest possible cause of interference is to the on board GPS. Not desirable. But if it's a superhet, then we have a potential problem. If it meets FCC (US) requirements on oscillator radiation, it could well have an Intermediate Frequency such that the local oscillator blocks a DME channel. If it meets EU standards, possibly not - but only possibly, depending upon which stanadrds the manufacturer has decided to declare it compliant to.
As far as cell phones are concerned, any LO radiation if the receiver uses high side injection could kill a DME channel. A complication arises in that the immunity of cabling on a nice new aircraft is quite likely to be different to that on a 10 or 20 year old aircraft where corrosion, movement etc have happened.
The problem is that as far as aircraft systems are concerned, you KNOW what frequencies are involved. Bringing in unknowns (because you DON'T know the frequency plan of every possible bit of equipment) is another matter. Add to that the fact that there isn't just the fundamental radiation to worry about, but the presence of intermodulation products when authorised transmissions are made from the aircraft, there is a VERY good reason for banning the use of these things.
If you wonder about my qualifications, 'radeng' is a reduction of 'radio engineer'.

Contacttower
"I would have thought that since they are usually used in aircraft anyway there wouldn't be a problem."
To be blunt, but I hope not unkind, what you think is not relevant. Aviation legislation prohibits the use of any electronic/electrical equipment on aircraft for which that aircraft is not approved. The approval may be a general one, as in mobile phones, or may be more specific.

On a technical note, there are many aircraft systems that are susceptible to interference from electromagnetic interference "EMC", FADECs are a particular case. In the modern world, most electronic devices are digital such as laptops, GPS, mobiles, calculators, etc., and clearly an aircraft and its systems cannot be tested for interference from all specific devices.

You can imagine the case where "Doors to Manuel" is sat in the back of a nice modern twin donk and decides to switch on his GPS. Immediate effect, both FADECs cough and splutter and the wicks go out. As far as I can see, the only good thing that will come out of the scenario is that he will know precisely where the crash is.

Bottom line, any equipment to be used on an aircraft must have been tested and proven not to interfere with aircraft and its systems. That is an expensive business but cheaper than finding out the hard way.

Off at a tangent here, but I once had a girlfriend who had a cochlear implant (effectively bionic hearing). A magnet and electrodes are implanted within the head - the purpose of the electrodes are to stimulate the cochlear thus allowing the deaf to hear (supposing the auditory nerve is OK), and the magnet is there to hold a transmitter firmly in situ outside the skull.

The advice given by the cochlear implant centres is that this device should be turned off for TO and landing however when asked, neither Boeing or Airbus provided a conclusive answer. To me this seemed to be a problem as here was a SLF who needed to be transmitting in order to hear the safety briefing.

So

Not quite the same, but we did about 2.5 hours of testing with her in a Cherokee - try as we might, it was impossible to get her to interefere (well, interefere with the avionics - she was a natural pilot when interefering with the controls, and incidentally the deaf who have hearing corrected with a cochlear implant are NOT precluded from a class 2 medical).

The next step was on an A320. I was also SLF on this occasion, and pre departure talked through my tests with the P1. The result was that he was happy to contine our test on this flight. We put her "down the back", as far away from the e-bay as possible. All was fine.

Its just a thought should anyone encounter a cochlear implant PAX in the future.

And one other point - I have no idea what frequency range these contraptions work within, but the various cochlear implant centres are always willing to field intelligent questions.

The thing that worries me is that if there is a real possibility of mobile phones interfering with critical A/C systems, then I do not believe that the present procedures, which simply involve an announcement that all mobiles should be switched off whilst engines running to be sufficiently robust. I regularly travel as SLF on Airbuses, and it is a regular occurrence for PAX to remember to switch off phones during flight, or for phones to go off. This probably represents the tip of the iceberg. I have to confess that on one occassion I have also found after the flight that I had inadvertently forgotten to turn my mobile off. So even where there is no intention to deliberately keep one's mobile on, there will be a significant failure rate in compliance.
From my own experience on smaller A/C, I usually keep my mobile on as the last line of defence backup for Radio failure. (Being over cautious on all trips other than local instructing details I also have a hand held battery operated transceiver and GPS - Call me paranoid!). I only turn the mobile off if I'm doing a "critical" instrument approach in IMC conditions. The only interference that I have experienced is the pip pip in the headset, presumably the Mobile interacting to find the relevant cell transmitter. I accept however that an A/C like the PA31 has much less critical electronic dependancy than an Airbus which is completely fly by wire.
Perhaps cabin crew should check all mobile phones when they check the boarding passes? The difficulty with this would undoubtedly be delays to boarding, and arguments with owners of "multifunctional" devices which have a flight safe mode.
One further thought, it has been announced that certain carriers will be running services which are enabled to handle mobile phones in flight. How is this consistent with the previously stated interferences to DME/FADEC? Will there be better electronic isolation of the A/C systems to facilitate this?

I suspect that the really simple answer is that nobody wants to find out for sure because it MIGHT end the airline monopoly on telephone call charges. I think that, if anyone had ever wanted to run a properly constructed scientific trial on any PED (Portable Electronic Device), the first problem they'd encounter outside of the airline itself, is the myriad frequencies and potential interactions between the various types of PED.

Now, as one who designs instrument procedures, I know that the tolerances can (on occasion :eek: ) be right on the limit. Think about that for your next take-off and landing - then ask yourself... do I really want to jeopardise any of those OCCASIONALLY tight tolerances?

On a personal note, I don't think that anyone really knows the answer to this so it's undoubtedly safer to err on the side of caution. I'm not yet convinced, one way or the other on the issue of PEDs, but I sure don't want to find out the HARD way.

I have to call you on that one. There is no IRS/IRU/ADIRU that updates from any external source...in any civil aircraft, be it Honeywell, Litton, etc. The FMC (FMS) will up date from GPS/DME/LOC/VOR.

Can you expand on your aircraft type and specific model? I suspect it is either a 777 or 747 as both of these use the MMR. The question as I understood it was, can the IRS/IRU/ADIRS be updated by the GPS. To my knowledge the answer is No. The blended positions from the FMC (FMS) are not a result of any IRU, GPS updating, but rather a blened position as a result of all the information gathered. On the 777 you can see these individual values on the POS REF page along with the MAP Display.

A Garmin GPS has been approved for use in aircraft...it has been tested by the FAA...and it says so in the instructions. OK what I think isn't really relevant but if a GPS can mess with stuff in an airliner it can mess with stuff in my all-singing, all dancing airways certified spamcan (not that I own one) which is worrying.

Spooky,

Not sure that all civil aircraft do the GPS / inertial integration in the FMS. Check out the Nothrop Grumman LTN-101E, both in terms of what it does and what aircraft it's on.

Well after looking up as much as possible regarding the LTN-101E I can only see reference to "sole means of navigation with GPS accuracy". Typical double speak so just what is happening here that is unique to this design has escaped me? I suspect that the GPS is an integral part of the IRU technology and unlike the Honeywell product line which typically uses a data port, the Litton system has it integrated into the IRS???

Thanks for the reference!

interesting discussion


JezzBaldwin:---hearing aids/ and pace makers are excepted form the PED ban under the FARs

Here is one airline's web site remarks concerning GPS.


You can use these while the plane is at the gate with the doors open, after the plane has taken off and reached its cruising altitude and before it begins to descend for landing, and after the plane has landed and is taxiing to the gate.

Personal digital assistants
Personal computer games
GPS (global positioning satellite) systems

Your flight attendant will announce when it is safe to use these devices.

When people note that some aircraft (even light aircraft) already have GPS, remember that these are not bought at Radio Shack and pasted onto the panel. They are integrated into the aircraft systems, and so proper precautions are taken at the design stage, so they do not affect each other.

Now I don't know, but MAYBE Garmin has engineered their handheld aviation units so that they don't affect plane systems. That's the only way I can imagine you could safely use it in a Cessna. In this case, the equipment would have met the relevant certification standards. This is unlike waking into a store and picking a unit off the shelf and using it in a plane.

Of course...I should have stated in my question that I am talking about GPS systems as sold in pilot shops from reputable aviation accessories manufacturers like Garmin or Bendix...I wasn't talking about 'any old GPS' if you see what I mean.

The GPS systems I am referring to are ones designed and certified for use in aircraft.

Our three onboard INS units all receive GPS updating, and it's not a small point. We're limited in class II nav to certain time constraints without that GPS updating.

GPS signals are extremely weak - less than the ambient noise level. It is indeed a wondrous feat that your GPS chip can pick up the signal at all, let alone guide you round the skies. My guess therefore is the GPS electronics would be very low noise generators, otherwise the GPS wouldn't be able to work. I understand there are different frequencies involved and that the GPS has to generate frequencies to beat, but noise is noise and you imagine a GPS must be the quietest thing that isn't a mouse.

This INS thing is a bit of a red herring. ContactTower seemed to be saying his GPS might upset things. Perhaps another GPS might be the most prone. That GPS might show some misreading which could corrupt the FMS's opinion of where it was. That sequence of events is unlikely. GPS is digital so you can't 'bend' the reading, and wherever navigation information is blended there is going to be some process which rejects 'odd' inputs. The loss of a valid GPS input is something that would be designed for.

The title of this thread asks about interference to INS, I thought INS was a bit-o-magic in a box somewhere that keeps track of G forces, how could GPS interfere? Waiting to be educated...:uhoh:

Has there ever been a proven case of interference with aircraft systems from a passenger carried device? References please.

Last i read on this, there was a reported case of laptop interference in some 757 and Boeing even sent up the plane afterwards, empty of course, with the laptop to see if they could reproduce it, and couldn't.

I have been developing electronic products since the 1970s, hardware and software. Much of the stuff written above about susceptibility to EMI is nonsense; if it wasn't there would be transport planes crashing everywhere because there will always be a few people leaving their phones on.

Having a no-gadget policy is fine as a policy but claiming it has a proven technical basis is bogus.

When we develop aircraft systems, we have to pass both EMC emissions and susceptibility tests. The FAA are rightly over-cautious about allowing anything on-board that has not been tested for emissions. This is applied to test flights and limits the test equipment we can use, which can be a pain.

I doubt whether any electronic gadget not specifically marketed for aircraft has been tested, and therefore, no matter how low the emissions really are, they will not technically be certified for flight.

Of course, someone back in row 50 in economy is so far from the EE bay that their equipment is less likely to be a problem than someone in first class (or the cockpit).

GPS updating of IRSs
 

Spooky,

My current type, the A320 uses the blended position referencing to the most accurate position, usually the GPS. I used to fly the 747-400 which did something very similar, but just different enough to differentiate the types. I've also previously flown a 747 Classic which used a Marconi FMS900 fed by receivers various and a Litton IRS (Type unknown). The Litton was position updated by the FMS, usually the GPS position. Also, I've flown a military large type which had a military FMS800 (can't remember the manufacturer) fed by what was known as an EGI. Embedded GPS IRS which did update the IRS position from the GPS constantly and was the most accurate system of all.

Since they have been mentioned in this thread, I make this post about them, rather than the Garmin type GPS receivers that started the topic.

In Brunei, the mobile phone system is crude, to say the least. The handsets are always making search transmissions and at full power. When they can't get a response they ramp up the power and try again. At high power outputs the transmission power is sufficient to break through on my tape player. Yes, you read that correctly, the tape player. It produces a deafening burst of sound so loud that it makes an unsuspecting passenger jump.

Now, in an aircraft, if you leave a mobile phone switched on, it will still search for a pole to link up with. Even over the ocean. Transmission power that is sufficient to be picked up and amplified by the pick-up in a tape player will definitely cause interference to aircraft electronic systems. Unless and until they fit mobile phone transponders into aircraft to allow the phones to connect on low power, let there be no confusion. No-one can safely leave a mobile phone switched on in an aircraft.

Your key words here are "update the IRS position". It does not up date the IRS. I'm amazed at how many people hold this misconception. The GPS signal is used to upgrade the FMC position, not modify the IRS position. As you have stated, the FMC (FMS) takes all this information, weighs it for logic, and then applies it to the FMS position. Typically you can look and see what nav reference is doing the heavy lifting to come up with your position.

If you were flying in a non-GPS equipped aircraft, there would be a limit on how many hours you could operate without some sort of FMS update, typically a VOR, before you could continue. In the GPS equipped aircraft this update is faily constant except at very high latitudes, thus on the B777 the time of operation is limited to 24 hours and not likely to effect any stage lengths in todays operations.

We have three INS units, two of which are managed through a FMC, and crosslinked. Each INS has it's own dedicated GPS GNS input directly to the INS and the FMC has it's own GPS input as well.

The third INS, a Litton 92, has a dedicated GPS input which we also don't see. the Litton is only good for (I believe) 6 hours Class II without the GNS input, but it's good indefinitely with it. It doesn't work through a FMC or FMS; it's autonomous and has dedicated GPS input to the INS itself.

Perhaps, but it is not updating the LTN92. It may be updating the position that is derived and displayed but it does not update the IRS.

What's latitude got to do with it?
 

Why's that then?

GPS signals are spread spectrum
 

GPS signals are "extremely weak" because they are modulated in direct-sequence spread spectrum (DSSS), that is why they can be under the ambient noise level. They are coded Code Division Multiple Access (CDMA). GPS was a military development and the system is designed to a high jamming margin against intentionally jamming. It is highly unlikely or impossible that one GPS receiver can unintentionally be jammed or blocked by spurious emissions of another GPS receiver on board.

I once used my GARMIN handheld GPS on board a A320 in mid flight to get some waypoints, but I had asked the captain before and he approved it.

Spooky, you're talking semantics here. My (basic) Litton 92 notes tell me that the IRS position can be updated by an external source. In the fit I was referring to, this souce was the FMS900 which excluded the radio nav aids if the GPS source was accurate. Threfore as far as I'm concerned, the IRS is being updated by the GPS. And as far the EGI is concerned, it's a combined position. An IRS position enhaced by GPS or a GPS position stabilised by the IRS? I don't know. But it did receive the encrypted Y channel GPS signals, so it was in a different league to the fit on airliners.

Actually, it is. The Litton is taking the GNS inputs, using them for updating, and providing a continuously updated and corrected position using those inputs. It is not processing through a FMS or FMC. The INS is not inputting as a sensor into a multi-sensor unit which then combines the signals. The GPS inputs to the Litton to update it, and the Litton provides the position.

Dan, it was the Rockwell Collins FMS800.

The Embedded GPS / Inertial Navigation System did indeed include a Y-code capable GPS as well as a Laser INS. But that was only a small part of the sytem; unfortunately the system which you and I were familiar with was only a partial version of the full version used on the Pacer Craig KC 135.

The 'heart' of the system was the CDU. This allowed the navigation method to be selected from either INS1 (the LINS) or INS2 (the old Carousel IV). Either could also be blended with GPS, or, in extremis, GPS-only was possible. However, using a steering signal from GPS-only often causes aberrant heading demands as the system updated, so should not be coupled to the autopilot in NAV mode.

If the whole thing went tits-up, it was, of course, still possible to bypass the CDU and use the old C-IV to drive the autopilot - always assuming that the food-powered interface device had backed up the waypoints correctly. Some of them simply couldn't be bothered.......

And, just for old time's sake, I give you:

GPS has a spreading gain of 20,000 times. A 'good ' receiver, even if direct conversion, won't radiate more than 2 nanowatts IF it meets reasonable standards. The GPS signal is a long way down on that. The problem comes with anything that doesn't meet reasonable standards for radiation, and there's a lot of CE marked stuff where the CE stands for Chinese Export. A good professional GPS should never be a problem - it's the cheapo cheapo that might be. Regrettably, we have a whole load of European rules on these matters that aren't enforced anywhere except Germany.
There have been some documented cases in the US where the use of portable VHF FM broadcast receivers on airliners has interfered with the VOR because of the relatively high levels of local oscillator radiation allowed. There was a very good article some years back in the IEEE Spectrum magazine on that.

Thanks Dan and Guppy. I'm going take your word for this GPS/IRS updating until I learn something new, which hopefully at this old age I can still do. Very interesting and Thanks Again!