FCC Lets Radar Speak Volumes - In Tanks and Maybe Outdoors, Too

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.

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Comments (4) Read through and enter the discussion with the form at the end
Dane Ericksen - March 29, 2012 10:48 AM

The FNPRM proposes allowing Part 15 Level Probing Radars (LPRs) at 5.925-7.250 GHz to increase their allowable EIRP from -21.3 dBm (Paragraph 13) to +7 dBm (Paragraph 24); that's a 28.3 dB increase-- almost three orders of magnitude. The Commission concludes that licensed incumbents in these bands won't receive interference because the Commission will tell users to

1. Only install "downward" pointing antennas (Paragraph 26)
2. Be "professionally" installed (Paragraph 34)
3. Only used at "fixed locations" (Paragraph 34)
4. Prohibit hand-held use (Paragraph 34)
5. Not "marketed to residential customers" (Paragraph 34).

Right.

So how will the FCC enforce these restrictions? If it's a Part 15 device, there is no control of the device once it has been sold. And since when would the Commission have jurisdiction over how a Part 15 device is marketed? Unlike the marketing of cell phone jammers, which are illegal in all cases, the sale of these high-power LPRs would be legal, so I don't think that the FCC would be able to restrict "marketing," especially by third parties. Remember how one manufacturer of Part 74, Subpart H, Low Power Auxiliary wireless microphones claimed with wide eyes that it had no idea that its products were being marketed to ineligible nuclear power plant (NPP) operators? And what about another manufacturer of high-power, licensed-use-only wireless micrphones, shamelessly marketing its products to generally ineligible churches?

Now if the high-power versions of LPRs have a) a motion detector to prohibit hand-held use and b) an angle detector to ensure the device would only radiate when an integral transmitting antenna really was aimed downwards, then I think the IX threat to licensees in the 6.5 and 7 GHz TV BAS bands would be small. And there would be no need for either "professional installation" (whatever that means), or unenforceable marketing restrictions.

Mitchell Lazarus - March 29, 2012 11:02 AM

In response to Mr. Ericksen -- The proposed rules also limit sideways and upwards emissions to very low levels. These are the only emissions that realistically can cause interference to other users. The FCC can confirm compliance with these limits as part of the certification process. Unlike wireless microphones, these devices don't have any obvious non-compliant applications.

Dane Ericksen - March 29, 2012 11:24 AM

The limits on sideways (non-main beam) power are only meaningful if the LPR has built-in features that keep the device from inadvertently being operated hand-held (i.e., in motion) or with the antenna not aimed downwards. It's a Part 15 device, meaning that it can be marketed by third parties and sold to users who may have no appreciation of why the user manual says only to be installed at a fixed location and with the antenna pointing downwards (and an angle limit needs to be specified; "downwards" is a pretty vague term).

Don Wright - April 30, 2012 3:20 PM

It seems that the broader issue with outdoor use will be "ripple effect", not the traditional systemic effect, but that of the kind caused by wind blowing across the surface of an outdoor tank or pond. On a flat liquid surface the downward beam would be reflected back vertically into space theoretically. On a windblown surface one would expect a lateral reflection as each ripple passes through the beam and it is deflected at an angle proportional the angle of the face of the ripple. Granted the timing of the ripple and the radar pulse would have to be coincident, but there would be a momentary lateral transmission at this point in time. If the PRF is very low this is probably a non-issue but for a high PRF it could be a concern.

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