ATM Modernization, Business & GA, Commercial, Military

Avionics System Design: Potential Perils of PEDS

By Walter Shawlee 2 | January 1, 2000

The issue of PED (personal electronic device) interference with onboard avionics systems becomes more complex and contradictory as one digs deeper into prior documents, conflicting reports, and tests both conducted and not conducted.

The primary operational directive on this issue is FAR 91.21. It prohibits any portable electronic device operation except voice recorders, hearing aids, heart pacemakers, shavers and (most importantly) "any other portable electronic device that the operator of the aircraft has determined will not cause interference with the navigation or communication system of the aircraft on which it is to be used." Determination of this "non-interference" is left to either the air carrier or, in non-carrier aircraft, the pilot in command.

This regulatory solution is far from optimal. It results in totally different standards on the same aircraft type between different operators and further convinces naturally skeptical passengers that the issue is simply not serious.

Pilots and flight crews do not have radio frequency interference (RFI) test labs at their disposal to assess and identify these issues. Carriers do not have the resources either. Lack of clearer regulation has led to extremes of acceptable operation and left aircrews with poor and ineffective defenses against problem devices.

Cell phone use in particular, either on the ground or in the air is arbitrary in its permission or denial between aircraft operators. I see no benefit or purpose in prohibiting cell phone use on the ground, as it is both legal and represents no potential flight hazard. Conversely, in-flight use is unacceptable. It is both an illegal use of the radio itself and an uncertified and unapproved transmitter in the aircraft.

This seems clear to me. I am troubled that it is difficult to understand and widely misrepresented by those favoring cell phone use. Once the engines whine, the passenger radios need to go off—and this has nothing to do with airlines seeking extra income from onboard phones.

In August of 1996, RTCA issued DO-233, an executive summary of the PED issue, and used DO-160C test procedures for its conclusions. The summary states that the chance of interference from cell phones is slight, but it hedges its bets, stating that even that slight chance is potentially hazardous during take-off and landing. RTCA’s recommendation is for a more strict FAR 91.21 that prohibits PED use during any critical flight phase.

It is worth noting that not all involved in SC-177, the special committee that arrived at these conclusions, agreed with the "slight" conclusion. Some felt that different testing methodology would show significant potential interference under certain conditions. Whatever they felt, they have been completely overtaken by technology, and the problems today are much different and more significant than anything they tested.

Since 1996, two very important factors have entered the picture: CPU speeds of portable computers, laptops, palmtops and other devices have skyrocketed, and digital technology has crept into many more devices, from hand-held games to CD players. The energy of portable systems has also increased, with new advanced battery systems and extensive use of switching regulator technology.

The result has been a dramatic shift in emission signatures, with very pronounced levels at 133 MHz due to increasing central processing unit (CPU) clocks, and various multiples and submultiples of this, and 100/125MHz bus speeds, plus all possible mixing products. The push for lighter laptops, palmtops, and more digital toys in particular has meant less metal, more plastic and more resulting emissions from the finished product, especially under near field conditions.

The rush to lower prices has also compromised attention to emission issues. Positioned inches from a laptop screen, we lit up a Fluke 1AC-A volt-alert because fields were generated by the screen backlighting supply. This becomes interesting, considering the probe is normally used to find live 115 volts AC lines and has to literally touch a 115-volt AC line to light up. So much for low emissions!

The use of 100 MHz local bus and 133 MHz clock multiples is especially significant; it produces clear cut, unambiguous in-band interference with both nav and com systems. Furthermore, with primary clocks of 366 MHz and up, the harmonic content is now more significant at frequencies likely to cause interference with very low level receivers such as GPS, as well as direct in-band interference to glideslope receivers. We had no difficulty measuring these signals from laptops with nothing more advanced than a short piece of wire.

For some useful facts and to determine if interference actually is a possible threat, we constructed a simple double shielded chamber from two aluminum transit cases. We insulated the inner one from the outer, to make a Faraday shield, and tested this to be sure we had at least 80dB of rejection of ambient RF (we did), and that external signals within range our HP8554B spectrum analyzer were eliminated. This gave us an inner shielded cavity of about 3 cubic feet (0.085 m3), free from outside influences. We then ran an 18-inch (45.7-m) long Z-fold monopole into the shielded case (one 90� fold in each axis, X, Y and Z) to pick up radiated emissions from our test objects. The signal from the monopole was sent back through a double shielded RG223/U coax cable to both a spectrum analyzer and AM Comm in turn (both representing 50 ohm loads) to see if anything meaningful could be observed.

The chamber allowed us to reject virtually all ambient RF and gave us a clean sample of the unit under test radiated emission to work with. For the first test sample, I threw in a new 366 MHz laptop, and booted it up. The spectrum analyzer display showed tremendous narrowband activity around 100-133 MHz, 200-366 MHz and many sharply defined mixer products and harmonics, to well over 1 GHz. Levels were routinely -90 dBm and sometimes up to -80 dBm.

Most importantly, (as I explained in previous columns) the signatures were totally software dependent and changed significantly in terms of frequency and level as the unit performed different operations. When the coax was connected to the input of the AM comm, channel interference was spectacular, but very well defined, totally blocking some specific frequencies and producing no interference on others. This agreed exactly with the nature of the emission signature on the spectrum analyzer.

The channels affected became totally unusable, but closely located channels were left unaffected. This is an important consideration because of a recurring theme: Reproducing the problems reported by flight crews was quite difficult when attempting to prove or disprove PED-induced interference. Exactly why this is so should now be a bit clearer.

Arguably, aircraft wiring does not consist of 18-inch bare wires strung throughout the cabin over each passenger, neatly attached to the radio system. Coupling is also less tight °than we created in our test chamber. However, to this point, the main counter argument has been that no significant in-band emissions existed from PEDs, and thus they could not be a threat.

In addition, their emissions were felt to be too low to be problematic. This simply is no longer so. Newer high-speed devices create a much more troubling situation, and we no longer can be so sanguine about possible interaction, since such interference can now be reproduced by anyone inclined to do so.

The coupling of this in-band signal to either radio feedlines, antennas, or related hardware may still be minor, and individual systems in specific locations may be unable to create problems in a given aircraft. However, the effects of many emitters, all in-band, and with distributed locations is a more serious situation. It cannot be disregarded so easily. There is no DO-160C or Mil-Std-461 model for this interference mode, thus no universal, accepted methodology for this measurement exists.

I believe the problem is being attacked in completely the wrong way. Aircraft operators should not certify the suitability of passengers’ PEDs, any more than they should select suitable neck ties for passengers. But they should be allowed to reject devices (and maybe a few ties), based on their experience.

If PED manufacturers intend for their device’s use in aircraft, they should be required to show safe operation. Their devices should produced with clearly reduced emissions in any band of potential interference. My view of inarguably safe levels for aircraft use would be less than -115dBm at one foot from the device, between 200 KHz and 1.4 GHz worst case, any axis, and a safe compass distance of 1 foot (0.305 m).

I believe many makers would be keen to introduce compliant machines, as it offers a new way to differentiate their product for better margins. If there are no takers, it just indicates that the risk is more serious than we think, and should not be born by the operators in any case.

Walter Shawlee may be reached by e-mail at walter2@sphere.bc.ca

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