Level probing radars at 6, 26, and 80 GHz would have adequate power for a wide variety of industrial applications.

The FCC has proposed new rules for “level probing radars” (LPRs) in three bands: 5.925-7.250 GHz, 24.05-29 GHz, and 75-85 GHz. LPRs are downward-aiming radars used to determine levels of materials at industrial installations. Some are mounted inside those enormous tanks that dot the industrial landscape, to tell the operators how much liquid is inside. Other LPRs are used outdoors – at quarries, for example, to measure piles of gravel, or at nuclear power plants, to monitor the water level in the ponds used to store highly radioactive fuel rods. There are thousands of potential applications. The new rules would apply equally to in-tank and outdoor radars.

The FCC is easing its way into this area very gradually. More than two years ago, it proposed rules to allow in-tank radars in the 77-81 GHz band, and granted a waiver pending the rulemaking. Without having reached a decision on the original questions, the present Further Notice of Proposed Rulemaking expands the proceeding to add outdoor LPRs and more frequency ranges. Up in the nosebleed part of the spectrum, the FCC had earlier proposed radars for airport use at 78-81 GHz, to detect debris on the runways, and a relaxation of the vehicle radar rules at 76-77 GHz to allow non-vehicle applications and higher power.

An LPR typically transmits a train of very short pulses, with relatively long separations in between. For historical reasons, the FCC’s technical rules are more hospitable to continuous transmissions, such as those used to carry voice and data signals. The same rules, when applied to a pulsed transmission, effectively require operation at greatly reduced power. That lower power is sometimes adequate for measurement of highly reflective surfaces, but otherwise has largely prevented the successful operation of LPRs.

The newly proposed rules, being specifically geared to LPRs, should allow the downward-aiming transmitter to provide adequate power for a wide variety of applications. To protect other spectrum users from interference, the FCC has proposed much more stringent limits on radio-frequency emissions from the sides of the device and upward. Those stray emissions can be due either to properties of the antenna or to reflections from the material being measured. In the 24.05-29 and 75-85 GHz bands, they are limited to the same very low levels that are permitted for an iPad or a digital toy: 70 billionths of a watt. In the 5.925-7.250 GHz band they must be lower still, at about 3 billionths of a watt.

Comments and reply comments will be due 30 and 60 days, respectively, after publication in the Federal Register. We will let you know when that happens.