The Federal Communications Commission has opened an inquiry seeking public comment on a petition filed by LightSquared (News - Alert) that essentially argues GPS receivers aren't entitled to protection from interference.

LightSquared recently has argued that GPS receivers use spectrum allocated to LightSquared, in a possible bid to establish what might be called an “interference equivalence” argument. The possible upside would be that LightSquared interference that affects GPS receivers could then be positioned as equivalent to GPS signals bleeding over into LightSquared spectrum.

LightSquared says GPS receivers, such as cellphones, car navigation units and aviation equipment, improperly use its licensed frequencies because they were built with inadequate filters and therefore infringe on LightSquared’s authorized spectrum.

As with many other essentially political arguments, LightSquared then might be able to neutralize or diminish GPS interference claims.

The FCC (News - Alert) said comments must be submitted within 30 days, for a deadline of Feb. 27, 2012. After that, the FCC will accept responses to those comments for 15 days, or until March 13, 2012.

Engineers probably will scratch their heads, even as lawyers for LightSquared try to make the argument that there is some sort of legal equivalence between the bleeding of signals from LightSquared into the GPS bands, and similar sideband energy from GPS receivers extending into the LightSquared bands.

Signal strength is an important issue here, as one might argue it is scientifically provable that LightSquared signal bleeds over into the GPS band, while GPS signals likewise spill over into the LightSquared spectrum.

What is more germane is whether the errant signals cause harm to the other licensee.

In a broad sense, the issue is that all radio signals have an intended “center frequency” as well as a number of weaker “side lobe” signals that spread out both above and below the center frequency.

It likely is incontestable that some measurable signal bleeds over into the other licensee’s spectrum.

The more specific issue is that “transmitters” and “receivers” operate at vastly different power levels. So the practical implications are what matters. Does one licensee’s spectrum use actually lead to measurable interference with another licensee’s applications?

It might in face simply be a fact that some amount of measurable energy is detectable outside a licensed band. Some would argue that scientific fact is not the problem the FCC is trying to prevent.

Radio signals decay in logarithmic fashion (think about your Wi-Fi router signals at home), which is to say, rapidly with distance. A satellite, TV tower or mobile cell site, which has to send signals a long distance, therefore has to push out signals at high power levels, even though most of the receiving devices only need a vastly-weaker received signal to operate.

So one issue is not just the behavior of radio waves, but the differential power levels. That is why the interference caused by LightSquared simulated cell sites obliterates intended GPS signals when the GPS units are in close proximity to a cell site.

You might also infer the reverse, namely that power levels make a practical difference when a GPS handset is close to a mobile cell site. The GPS unit, when transmitting, does have a signal footprint that will spill over into the LIghtSquared band.

The difference is that the GPS transmitted power levels are so low, relative to the strong cell tower signals that actual interference does not occur.

It isn’t clear whether the new FCC inquiry means there is serious rethinking at the Commission, or is simply a way to ensure that the process is on stronger ground when the inevitable lawsuits are filed.

It would, in any case, appear likely that among the submissions in the comment period will be data suggesting that incidental side lobe energy caused by GPS receivers is not equivalent or material in its impact, compared to the side lobe energy levels emitted by a LIghtSquared cell site.

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