Electronic devices on aircraft

Go and read the WSJ piece first. I’m going to pull out a couple of items in there just or explanation.

The Federal Aviation Administration allows pilots to use iPads and other electronic devices to replace charts and manuals in the cockpit, powered up during takeoff and landing. But the FAA says it can’t test all the different gadgets passengers may bring on board. The agency worries a multitude of devices could pose more danger than a single iPad for pilots.

So they can use iPad’s and “other electronic devices” in the cockpit. That’s interesting because they must be shutting off the RF chips on those devices when they board the aircraft. Every cell phone, touchpad and the like has the ability to disable to disable the Broadcom or Qualcom RF chip inside the device and you can still read documents, play music, etc. that’s stored on the device.

Cell phones run around 1.8GHz to 2.0GHz. WiFi is in the 2.4GHz range, right in there with microwave ovens. Not to mention most radars are in that range too from around 20cm down to 10cm.

But here’s the thing, with any RF transmitter you have harmonics, parts other frequencies where there is resonance but is suppressed by filtering and tuning. They’re usually down anywhere from 30db to 70db, and they are divisible, so 2.4GHz is 2,400Mhz, with harmonics at 1,200MHz, 600MHz, 300Mhz, 150Mhz, and so on. If we do the same for cell phones, 1,800MHz, 900Mhz, 450Mhz, 225Mhz, 112.5Mhz.

That last one in each series is what could potentially interfere with aircraft communications that run around 127MHz AM, those being 150MHz and 112.5MHz

You also have to understand the difference between amplitude modulation and frequency modulation. Aircraft use mostly Amplitude Modulation which makes it MORE susceptible to interference.

Picture a sine wave.

In amplitude modulation (AM)you vary the height of the peaks and valleys, the time period stays consistent, i.e. there is a complete varied wave every x seconds.

In frequency modulation (FM) you vary the time period while the peaks and troughs remain the same. There is a complete wave on a variable time basis. The animated graphic below demonstrates:

So it is possible that mobile devices could interfere with communication and navigation gear.

Below is a basic list of all the radio frequencies applicable to ILS:

Instrument Landing System (ILS)

Localizer: 108.1-111.95 MHz
Glide Slope: 329.15-335.0 MHz

VHF Omnidirectional Range (VOR)

108.0-117.95 MHz

Distance Measuring Equipment (DME)

Air: 1025-1150 MHz
Ground: 962-1213 MHz

Tactical Air Navigation (TACAN)

Air: 1023-1152 MHz
Ground: 960-1215 MHz

Airport Surveillance Radar (ASR)

Primary Surveillance Radar: 2700-2900 MHz
Secondary Surveillance Radar, Transponders, ADS-B and Univeral Access Transcievers
Transponder (air): 1090 MHz
SSR (ground): 1030 MHz
UAT (air & ground): 978 MHz (USA only)

Air Route Surveillance Radar (ARSR) 1215-1350 MHz

Do you notice any frequencies in that list that might be 2nd, 3rd or 4th order harmonics of the frequencies used by mobile devices? Uh huh.

And the admonishment not to use cell phones is interesting. At speeds up to about 200MPH cell service can hand off fairly well. But once you start moving faster, it becomes more of a trick. So your phone would have to use more power because the towers are more distant.

So you’re probably better off turning the radios off on your mobile devices when you’re flying.

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