On the 76-81 GHz Screen: More Radar, Less Amateur?

FCC proposes increased radar uses of the band, to the possible detriment of radio astronomers and amateur radio.

Just ten years ago, the upper reaches of the millimeter wave bands - the "nosebleed" spectrum - were mostly empty. There is a lot of potential capacity up there, but technical constraints made it difficult to design equipment capable of operating at those frequencies. Since then, though, a corps of clever engineers have overcome the problems. Applications have proliferated. In addition to point-to-point communications at 71-76, 81-86, and 92-95 GHz, we have several uses of radar: vehicle radars at 76-77 GHz, airport radars at 76-77 GHz (for detecting service vehicles and "foreign object debris," or FOD, on the runways) and at 78-81 GHz (FOD only), and industrial "level probing" radars at 77-81 GHz.

All of these radar applications are presently unlicensed under Part 15 of the FCC's rules, except for the airport FOD radars at 78-81 GHz, which require individual licenses under Part 90.

The poor propagation in this frequency range limits radar applications to a few tens of meters, but the short, millimeter-scale wavelengths provide excellent precision - a welcome property if, for example, your car has a radar-based automatic braking system. The limited range also allows the same frequency to be reused just a short distance away. (See this link for an introductory discussion of radar and associated regulatory issues.)

The FCC is now thinking about liberalizing its rules. The proposals include:

• adding vehicle radar at 77-81 GHz, which is becoming the global standard;
• adding 76-77 GHz to FOD radars, for a total frequency range of 76-81 GHz;
• allowing fixed radars at 76-81 GHz - for example, to monitor tunnels or bridges for stopped vehicles, provide collision warning for ship-to-shore cranes, and offer train detection for automatic control functions; and
• allowing aircraft-mounted 76-77 GHz radars to operate while on the ground to prevent collisions between wingtips. (Who knew this is a problem?)

Of great interest to us regulatory lawyers (but maybe nobody else) is a proposal to consolidate all of the 76-81 GHz radar applications, except level probing radars, under Citizens Band rules. This "license by rule" regime deems all users to be holding licenses, even though nobody gets an actual piece of paper from the FCC. In principle this would give radar users somewhat greater protection against interference than unlicensed operation under Part 15. In practice, though, since we don't foresee many radar users actually filing interference complaints, the change will not make much real difference.

One major victim of these proposals will be the radio astronomers, who have not only an allocation for the 76-81 GHz band but also the most sensitive receivers on the planet. Their biggest threat is the vehicle radars, which go anywhere that vehicles do. The FCC proposes to give the astronomers co-primary status across the entire band (they are now secondary at 77.5-78 GHz), but this will have no practical effect on motorists driving near the radio astronomers' observation sites, which will be sitting ducks for interference.

Another allocation at 76-81 GHz belongs to the Amateur Radio Service, whose licensees are allowed to transmit at power levels far higher than the radars discussed here. This could pose a threat to the radars, particularly the vehicle radars, whose malfunction due to interference from amateur operations would raise grave safety concerns. The FCC earlier suspended amateur use of the 76-77 GHz segment to protect the first generation of vehicle radars. Now that it seeks to extend the vehicle radars over the entire 76-81 GHz band, it also proposes to delete the entire amateur allocation, and perhaps replace it with other spectrum nearby. We expect this idea may generate some controversy.

The NPRM does not specify comment and reply periods, which in any event would not begin to run until publication in the Federal Register. Watch this space.