I have known Harold Feld, Public Knowledge's Senior Vice President, for a long time and agree with him on many issues. He has even been a past client. But the above Twitter exchange indicates a not unusual viewpoint in the spectrum policy area and clearly was an undercurrent in the recent consultant's report that rationalized decimating the FCC's spectrum enforcement resources.

Notwithstanding vague promises made to NAB it is pretty clear that broadcast pirate enforcement will be a low priority for the new "modernized" field enforcement at FCC. Pirate broadcasting is often intermittent and at odd hours, requiring both staff overtime and travel, two things that are very scarce at today's underfunded FCC. New equipment designed to decrease necessary staff time would also be helpful, but equipment money is also very scarce.

So let me address the safety implications of pirate broadcasting, rather than view it solely as just an economic inconvenience to incumbent broadcasters. Pirate broadcasters distract audiences and also sometimes even carry paid ads!

But all properly licensed FM stations are subject to technical rules also. While some of them may be about "preserving monopoly of radio licensees", many also deal with preventing interference to other non broadcast radio systems. (The ongoing saga of interference from certain licensed FM stations to 700 MHz LTE base stations due to a regulatory loophole (along with FCC distraction on other issues) is a concrete example that each radio service does not live in a totally independent silo from other services!

It is doubtful that all pirate FM stations buy transmitters that meet all Part 73 technical standards with respect to out-of-band emissions and harmonics. Thus there is a real risk of interference to other radio services without some reasonable check and balances.

But why the reference to FAA and aviation safety in the title? Returning readers may recall that FAA concerns about interference to aircraft Instrument Landing System(ILS) receivers has been a recurring topic here. The ILS localizer signal is in the 108.10 - 111.95 MHz band just above the FM broadcast band. Two FM signals having the correct mathematical relationship to the ILS frequency in the area can cause overload in the ILS receiver through the mechanism of "intermodulation" and cause interference in the receiver on the ILS frequency. This is the same basic technical problem in the long running Nextel/public safety interference issue. (It is also the issue behind a lesser known controversy involving alleged interference from T-Mobile base stations to SiriusXM receivers in New York and your blogger discusses the technical issues in depth in p. 72-91 of this public filing at FCC.)

FM stations that are properly licensed use frequencies, powers, and locations that have been reviewed by FAA during the application process to make sure they do not pose a new inference risk to ILS receivers in aircraft. Pirate radios are very unlikely to check with FAA. (FAA's Airspace Analysis Model computer program that does the analysis is publicly available so they might check, but I doubt it.) While it is generally assumed that intermodulation interference can only occur when there are 2 strong signals present with the correct mathematical relationship with the victim signal, that is not literally correct. As shown in ITU-R Recommendation SM.1009-1 the interference power generated in the receiver by the inevitable nonlinearities is the weighted sum of the power of the 2 FM frequencies that have the correct relationship with the ILS frequency. (This is called Type B interference in the jargon of ITU-R.)

If one of these FM broadcast band frequencies comes from full power FM station and is very large at the location of the ILS receiver in the air, only a negligible amount of power at the other frequency is needed to cause interference if they have a certain mathematical relationship. While the math is clear, it is counterintuitive that a low power signal could cause such interference. Many years ago while I was working at FCC on this issue and the main contact with FAA on it I asked them to demonstrate this phenomenon which they kindly did for me and several other FCC officials at their Technical Center in Atlantic City.

Thus a modest power pirate FM could have the right location, power, and frequencies to cause Type B interference to the safety critical ILS signal.

Like FCC, FAA is not a perfect agency. Almost every other aviation regulator in the world limits the likelihood of this interference by both addressing the issue of FM station power, frequency, and location near airports and by mandating receiver immunity standards prescribed in ICAO Annex 10 to the Convention on International Civil Aviation, Volume I – Radio Navigation Aids, Section 3.1.4 Interference immunity performance for ILS localizer receiving systems (2006). However, as we have discussed earlier, FAA has refused to implement their standards for US aircraft in domestic service and has blocked FCC implementation of them. Thus the FAA's Airspace Analysis Model that predicts interference assumes that receivers more vulnerable than the ICAO standards are in use. But as pirate radio stations proliferate without effective enforcement there is a real risk that one of them might just land at a power, frequency, location triple that poses a threat to aviation safety but is not addressed since pirates don't have to deal with spectrum regulations!

If this is the new reality of "field modernization" then I urge FCC to have a pragmatic discussion with FAA,who were consulted on aspects of the consultants study but probably not this one, that FM pirates are a low priority and they might want to increate the immunity of the US air fleet (foreign aircraft already comply) to this type of interference by implementing the 2006 ICAO immunity standard.

Tags: FM/ILS, enforcement