FCC seeks voluntary cooperation in protecting airport weather radars from interference.

Having occasionally criticized the FCC for what might be perceived as over-enforcement – here, for instance, or here, or here – we welcome the chance to balance the record by crediting the Commission when it takes a kinder, gentler approach.

Not everyone gets Internet service through the phone company or cable company. Some subscribe to a wireless Internet service provider, or WISP, that installs a radio link to the user’s home or business. Many WISPs favor three particular frequency bands that do not require an FCC license, yet allow relatively high power operation that can reliably cover distances of several miles.

Seeking to add capacity, the FCC began a proceeding in 2003 to enlarge one of these bands, at 5 GHz. The frequencies it sought to add were occupied, as is most usable spectrum – in this case by federal government radars.

With the help of federal spectrum experts, the FCC wrote rules that allow WISPs and others to use the band, so long as their radios are capable of detecting and avoiding the radar signals. That took some time. Another delay ensued while the FCC, the government users, and the manufacturers worked together on test procedures to evaluate whether candidate devices in fact met the detect-and-avoid requirements. But eventually everything was in place.  The FCC began issuing equipment certifications, and WISPs began offering service in the band.

But a problem arose.

Some of the radars began registering interference. And some of those, called Terminal Doppler Weather Radar (TDWR) systems, are used to monitor weather conditions near airports. This was worrisome, as good weather information is an important component of aviation safety.

The FCC and the FAA investigated. Sure enough, the interference turned out to be caused by WISP transmitters, particularly those operating high off the ground, in direct line of sight with the TDWR units.

Some of the WISPs were using non-certified transmitters. That is a plain violation of the rules, and the FCC has taken enforcement action. But other affected WISP transmitters are certified. Having met the technical requirements for coexistence with TDWR, they cause interference nonetheless. That is another violation of the rules, which prohibit an unlicensed transmitter, such as the WISPs’ units, from interfering with any other kind of radio equipment. The offending transmitter must cease operation until the problem is fixed. Failure to do so can draw fines from the FCC.

For once, though, rather than gear up for all-out enforcement, the FCC showed restraint. In a letter to WISPs and their manufacturers, the FCC provided a list of the TDWRs that use the band, including their latitude and longitude, frequency, elevation, and tower height. It worked with an association of WISPs to disseminate that information. It encouraged the association to set up a database of WISP locations to help track down interference. And it asked the WISPs voluntarily to avoid TDWR frequencies when operating within 35 km or line-of-sight with a TDWR site. In short, it treated the WISPs as professional equals, and sought their help in a common cause – a welcome trend.

In the meantime, the FCC has suspended new certifications for outdoor transmitters in this band, and is working with the FAA and the manufacturers on new technical standards to better protect the TDWR systems.

There is, we must note, a less pleasant alternative available to the Commission, an alternative which the letter hints at, but just barely. Two people signed the FCC letter. One is Julius Knapp, Chief of the FCC’s Office of Engineering and Technology, co-architect of the 5 GHz rules, and a likely proponent of the voluntary approach. But the other is P. Michele Ellison, Chief of the Enforcement Bureau. Her presence on the letter provides a sobering reminder: If the interference does not stop through cooperative efforts, the WISPs might expect something in the mail on blue FCC letterhead.

Thanks to David Case at Cisco for bringing this item to our attention.