FCC moves to allow 77-81 GHz radars inside storage tanks.

Driving through an industrial area, looking at the huge storage tanks that dot the landscape, don’t you wonder how the users of the tanks know how much stuff is inside? Neither did we, until recently. If we thought about it at all, we imagined some kind of giant float, like the thing in the back of the toilet, except much bigger. But that couldn’t be very reliable, considering it’s not all that reliable in the toilet, either.

Many of the big tanks have a better system. A radar device sits up on the ceiling, looking down. It transmits short pulses, measures the time they take to come back, and computes the distance to the surface inside. Using radar avoids problems of corrosion, contamination, and the endless repairs needed for moving parts in the vicinity of liquids.

Other equipment inside the tank – valves, agitators, filling pipes, and so on – also reflects radar signals, as do the tank walls themselves, and that can lead to errors. One solution is a radar beam narrow enough to miss the other structures and strike only the liquid directly below. But then those pesky laws of physics intervene. The most feasible way to produce a narrow beam, using a reasonably sized antenna, requires transmitting at high frequencies.

The manufacturers of tank radars favor the 77-81 GHz band. This is well up in the nosebleed portion of the spectrum, among the very highest frequencies the FCC has ever authorized. From an engineering standpoint, it is an excellent band for the purpose. Not only do signals at these frequencies form tight beams, but they are absorbed by metal and concrete tank walls, and so do not escape the tank to threaten interference to receivers outside.

From a legal standpoint, though, the band has a problem. Everyone agrees that tank radars should be authorized on an unlicensed basis, like Wi-Fi and cordless phones. But 77-81 GHz falls within a list of “restricted frequencies” that are closed to unlicensed transmitters. Most frequencies on the list accommodate highly sensitive services such as GPS, search-and-rescue, and satellite downlinks. But when the FCC drew up the list in the 1980s, it deemed everything above 38.6 GHz to be restricted. There was little activity up there in those days, and the FCC reasoned it could always de-list frequencies later, if the need were to arise. And in fact the Commission has since done so for several individual bands, for certain purposes. But otherwise the blanket prohibition on unlicensed operation above 38.6 GHz remains in force.

As a result, despite a near-unanimous view that unlicensed 77-81 GHz tank radars will not cause interference, the FCC is legally barred from allowing them under the present rules. The FCC has accordingly launched a proceeding to change its rules to permit these radars to operate. It also issued a temporary waiver allowing use of these radars, under certain conditions, while the rulemaking is pending.

The only opposition so far comes from a group of radio astronomers, who have primary rights at 77-81 GHz. Fearing interference to their telescopes, they asked for a mandatory separation of two kilometers between tank radar installations and the astronomers’ sites. The FCC indicated its intention to deny the request, on the ground that any slight leakage from inside the tanks would be too weak to cause interference.

The rule change is obviously the right thing to do. But it points up a broader problem: the need to abolish the blanket prohibition on unlicensed operation above 38.6 GHz. Rather than continue to exempt various bands piecemeal, the FCC should decide once and for all which bands really do need protection and drop the rest from the restricted list. Why does this matter? The initial request on 77-81 GHz tank radars reached the FCC in November, 2006. (Two more followed in April and July, 2007.) Three years is a long time to wait for proposed rules and a temporary waiver, especially for a technology widely considered to be non-interfering.

More is at stake here than just tank radars, important as those are. As the lower part of the spectrum fills up, and as prices drop for equipment that uses the upper reaches, demands on the over-38.6 GHz segment will rise. Amending the list of restricted frequencies now will help to avoid unnecessarily long delays in the future.