Persistent sleuth Michael Marcus obtains, posts document shedding light on recurring interference to airport weather radars.

In our recent blog post about an AT&T wireless Internet service causing interference to an airport weather radar in Puerto Rico, we asked whether the FCC had charged AT&T with the wrong offense. Because the transmitter operated outside its FCC-certified frequency range (among other problems), the FCC determined it did not qualify for unlicensed operation, and so fined AT&T for not having a license – even though AT&T could not have obtained a license for that service.

Our friend Michael Marcus, a spectrum-savvy engineer (and former FCC official), asked a different question: how did the transmitter get to be operating on a non-certified frequency? Where most of us would be content to mull this over in our idle hours (if it occurred to us at all), Marcus is made of different stuff. He not only took the question to the highest reaches of the FCC, but managed to get some answers.

Modern radio transmitters, like most other modern devices, are controlled by software. The FCC recognizes a category of transmitters called “software defined radios,” or SDRs, which can be legally updated or modified by software changes, including those downloaded over the air. But most transmitters do not qualify as SDRs. Once certified by the FCC, their properties have to be locked in. The software is supposed to be secure against changes, particularly those that would take the transmitter out of compliance and lead to, say, interference to airport radar.

AT&T’s Puerto Rico transmitter was certified for operation over 5735-5840 MHz, but it was being operated at a frequency outside that range. Moreover, the transmitter lacked the required capability to listen for weather radar signals, and if it found them, to avoid the frequencies on which they occur – a feature called “dynamic frequency selection,” or DFS.

The transmitter was manufactured by Motorola, which knows how to comply with FCC technical rules. But the transmitter was non-compliant when FCC inspectors found it in operation – on a non-certified frequency and lacking DFS – in AT&T’s Puerto Rico system. Moreover, the FCC has identified other non-compliant transmitters operating in the same band. In every case we know of, the transmitter was made by Motorola, and all came from the same “Canopy” product line. 

What went wrong?

One clue comes from a “consent decree” between Motorola and the FCC released in April 2010. Motorola paid $9,000 to settle charges that it violated FCC rules relating to DFS requirements in the frequency bands relevant here. The document contains no specifics. But it does mention that the FCC’s Enforcement Bureau asked Motorola for certain information related to the suspected offense on April 20, 2009. And that Motorola responded on May 20, 2009.

That was all the opening Marcus needed. In August 2011, he filed a request under the Freedom of Information Act (FOIA) seeking a copy of Motorola’s May 20, 2009, response letter.

Motorola had requested and received confidential treatment of that letter, under a FOIA provision that protects trade secrets and confidential commercial information. The FCC notified Motorola of Marcus’s document request (as the FCC’s FOIA protocol requires). Motorola provided the FCC with a version of the letter from which it redacted the parts it said would harm its competitive position, and also sent a copy of the redacted version directly to Marcus. The Enforcement Bureau issued an order (not publicly released) agreeing with Motorola that the FCC need not disclose the redacted material.

Marcus appealed that decision to the full Commission, asking for access to specific redacted information. He argued, in part, that release of the information is in the public interest because it may shed light on the root causes of interference from the Canopy transmitters into airport radars, and thus promote air safety.

The FCC’s decision gave Marcus a partial victory. One question in the FCC inquiry asked about a particular transmitter used in Puerto Rico – although not the same one that got AT&T in trouble. The one mentioned in the inquiry lacked an FCC ID number, which is required evidence of required FCC certification. The FCC asked if the transmitter was certified, and if so, under what FCC ID number, and why the number was not marked on the device. The FCC also asked when and where various Canopy transmitters were marketed in prior years, and under what FCC ID numbers. The answers to these questions would not harm Motorola’s competitive position, the FCC ruled, so Marcus was entitled to obtain them.

Marcus has now posted Motorola’s response on his blog. Motorola’s explanation: it thinks it sold the unit outside the United States (for which no certification or FCC ID is required), after which the unit was “somehow imported” back into the U.S. If true, that would put the responsibility squarely on whoever imported the unit and sold it domestically, and also on the people who installed and operated it despite the missing FCC ID.

The mystery continues, as does occasional interference to airport radars. Users of the Canopy transmitters have no easy way to check the operating frequency or the presence of DFS capability. But those within the U.S. can – and should – look for an FCC ID number on the unit. If the number is missing, the user is on notice that the device is not only unlawful, but also a potential threat to air safety. For the sake of all air travelers, please turn it off.