GPSRFI (2024)

This is a question that generates a lot of comment and confusionon the newsgroup, but few people have the necessary experience togive an answer based on reasoned engineering experience. Here aremy answers and comments.

1) GPS Receivers are known to radiate RF energy from theoscillatorsand computer logic circuits inside the GPS. Won't this possiblyinterferewith the navigation equipment on airplanes?

Answer:
Every electronic device with a computer or oscillator insideradiates*some* radio frequency energy. The key word is *some*. Acellulartelephone, ham radio transmitter, business band radiotransmitter,CB radio or similar transmitting device is DESIGNED to radiatesignificantamounts of RF energy. They *DO* have the potential to interferewithvarious Avionics equipment and SHOULD NEVER be used on board anaircraftduring flight. Many airlines prohibit their use at all timeswhileon board a commercial airliner.

With other electronic devices, such as laptop computers,electronicgames, AM and FM Radio receivers, and the like, theanswersare not so black and white. There are documented cases of AM/FM radioscausing interference with Avionic systems and as a result, AM/FMradio receivers are generally prohibited. Most electronic gamesarecheaply built and do not have the same FCC mandated requirementsfor shielding as do other electronic systems such as computers and GPSreceivers.. Even though these game devices are low power,theyhave been accused of interference with Avionics during the terminalnavigationphase and so are prohibited except during cruise (typically above10,000feet). Laptop computers are now in widespread use by passengersinairplanes. While these devices are relatively high overall powerconsumption (in the 10 watt range) they are generally welldesigned,with adequate shielding. As a result, laptop computers areallowed to be used during the cruise portions of virtually allpassengerairline flights.

Well then, What about GPS Receivers?

GPS receivers are very low power devices. The typical TOTALenergy used is less than a one watt rate. If all of this powerwereconverted to RF, then there WOULD be a problem.However,the fact is that a) only a very tiny part of this energy is capable ofbeing converted to power that *might* generate RFI and b) the designersof GPS receivers are required to consider RFI during the design so thatthe finished product passes FCC Part 15 Class B (residential) EMI(ElectroMagnetic Interference) tests. (Note: Contrary to somepeople'sintuition, Class B (residential) EMI testing is considerably morestringent than Class A (Industrial/Commercial) EMI tests. This isbecause in residential situations, a neighbor may be just 10 feetaway whereas in industrial environments, neighbors are usually furtheraway.) c) By design, (or happy accident), the "spurs"generated by a GPS generally fall outside the communicationsfrequenciesused by Aircraft and so have not been a problem even though a few"spurs"exist. (If no one is listening on a frequency where a potentialRFIsignal exists, then there is no interference.)

What is the TRACK RECORD of GPS receivers in so far as EMI isconcerned?Garmin and Magellan tell us that there has never been a REPORT of anyproblemcaused to any avionics system as a result of the use of a GPS on anaircraft.No pilot or engineer on the GPS newsgroup has ever reported a GPSreceivercausing interference to an avionics system on an aircraft.About 80% of the airlines in our survey permit the use of handheld GPSreceivers by passengers. I believe it is safe to say, "IfEMIfrom a handheld GPS were a problem *somebody* would have noticed it bynow". My conclusion therefore is that the use of a low power GPSreceiver on an aircraft is substantially less likely than a laptopcomputerto generate significant amounts of EMI and since laptop computers arejudged"safe" then GPS receivers are "safer".

2) But SOME airlines do not permit the use of GPSreceivers.Why is that if they are "safe"?

Answer:
Some safety officials are more cautious than others. Someare less technically competent than others. If there is a"potentialhazard", no matter how insignificant, it is always easiertosay "no" if you have no basis for a decision. In the case athand,to say, "yes, it is safe to use a GPS on board an aircraft"requires a substantial amount of technical knowledge and expertise in acomplex field. To say "no" you cannot use a GPS on board anaircraftrequires no such skills and is "safe" in many contexts..(Note:One aviation industry EMI expert who reviewed this materialsuggeststhat my reasoning above is overly harsh. He states that *some* safetyexpertsare fearful that some FUTURE Avionics equipment may incur harmfulinterferencefrom a GPS and once the GPS "barn door" is open, it will bedifficultto close it.)

3) If a GPS is safe, why can't I use it on an airplaneanyway,even if the pilot says NO?

Answer:
This would be a) unwise, b) illegal and c) dangerous.Never presume that you have more authority than the Captain of aship!He is responsible for the lives of his passengers and likely hasknowledge and experience about his aircraft and/or equipment and/orthisparticular flight that no one else has.. The use of a GPS by apassengeris NOT worth a confrontation and a possible visitation from the policeor FBI when you land..

4) I have used my GPS during takeoff and landing and nothinghappened.Why can't I keep doing it?

Answer:
Takeoff and landings are the most dangerous parts of a flight.Even though the likelihood of an EMI problem from a GPS receiver islow,there is no reason to take a chance on interference with a criticalsystemduring a critical moment during takeoff and landings. The lifeyousave could be mine (and yours).

5) You are WRONG! I put my GPS a foot or two from myscanner and I can hear "spurs" at several frequencies.Therefore,the GPS CAN cause EMI problems.

Answer:
Yes, you can hear a "spur" at some discrete points with awideband scanner. However, these "spurs" are very low powerand the typical scanner receiver is not well shielded. The energythus radiated can only be DETECTED a few feet away and a) such signalswill not normally cause problems with communications receivers sincethesignal energy dissipates as the SQUARE of the distance between theradiatorand the receiver. Probably the main reason why these "potential"RFI signals have not been a problem is that these "spurs" have not beenfound to fall on frequencies used for aircraft navigation andcommunications.Such "spurs" have the "potential" to cause problems, but due tothesmall signal levels and the frequency of the GPS receiver spurs,they have SO FAR not been a source of interference to Avionicssystems.(Again: If they HAD been a problem, someone would havenoticed.)

The BOTTOM LINE (According to Joe)

The interference potential of handheld GPS receivers isminimal.However, no passenger should EVER operate a GPS receiver on boardan aircraft except in strict accord with the directives of the flightcrew.

Now as to FAA rules and regulations:

The text of FAA Part 91.21 reads as follows (emphasis added):

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http://www.airweb.faa.gov/REGULATORY_AND_GUIDANCE_LIBRARY/RGFAR.NSF/0/5D4AEFD672582D15852566CF006135DC?OpenDocument

Joe Mehaffey