FCC Relaxes Limits for Some Safety-Related Radars
Applications include vehicle braking systems and airport use.
The FCC has wrapped up a couple of proceedings relating to three types of specialized radar devices in the 76-77 GHz band: one for vehicle radars, which function as part of automatic braking systems in cars; one for detecting “foreign object debris,” or FOD, on airport runways; and one for tracking aircraft and service vehicles around airport runways and ramp areas. All of these applications are approved for unlicensed use.
The 76-77 GHz frequencies are among the very highest that the FCC has authorized for any purpose. They are particularly well suited to short-range radar applications. Outgoing signals tend to form tight beams, even from antennas just a few inches across; and the signals tend not to travel far, as almost any kind of matter will block them.
The current vehicle radar rules are complicated, as we explained last year, with power limits that depend both on (a) whether the vehicle is stopped or moving, and (b) whether the radar is aimed forward or in some other direction. Adopted back in 1995, those rules tried to limit the amount of radio-frequency energy to which pedestrians would be exposed – due, for example, to idling cars at stoplights. The following year, the FCC modified its RF exposure rules, relaxing the exposure limits in some bands (including frequencies above 1500 MHz) and thereby permitting increased power.
It took the FCC until now to get back to the vehicle radars, but manufacturers may decide the wait was worth it.
Under the revised rules, vehicle radars can now operate at up to 100 watts average power, aimed in any direction, moving or stopped. The only drawback is a maximum peak power of about 316 watts. True, this seems like a lot; but radar signals tend toward high peak power, compared to average power. Complying with the peak limit may require pushing the average down to well below the maximum.
Airport FOD radars, proposed just six months ago, have also been made subject to these technical rules. Where the FCC had originally proposed authorizing these at 77-81 GHz, they are now limited to a narrower band just below, at 76-77 GHz.
Airport radars for monitoring activity on the ground have been the subject of an FCC waiver, but never had their own Notice of Proposed Rulemaking. That’s okay, though, because the same notice that proposed the vehicle radar rules also contemplated allowing any radar application whatsoever in the 76-77 GHz band. While not going that far, the new rules do approve radars for airport ground control at 76-77 GHz, once again under the same technical rules.
FCC actions that rely on the radio-frequency exposure limits tend to be controversial because many people believe the limits allow more exposure than may be healthy. Until now, concerned persons could limit their own RF exposure by cutting back on cell phone use and objecting to wireless meter readers in their basements (although in fact the meter devices are not a significant source of exposure). Avoiding exposure from cars and at airports is going to be more difficult. But there are many – us included – who are concerned more about car accidents and aircraft mishaps than about incidental exposure to radio waves. Speaking for ourselves, we think the FCC got it right.
(Curiously -- and uncharacteristically -- the FCC's order does not specify when the revised rules will become effective. That's probably just an inadvertent oversight that will be corrected in due course. Check back here at CommLawBlog.com for updates.)
What about the related rulemaking, ET Docket 10-23, involving Level-Probing Radars (LPRs) at 5,925-7,250 MHz? That makes LPRs co-channel with TV STLs at 6,875-7,125 MHz. The Commission proposed allowing the EIRP of Part 15 LPRs to be increased from -21.3 dBm to +7 dBm--a mind boggling 28 dB.
But not to worry: Part 15 users are sternly admonished only to use their increased-power LPR with the transmitting antenna pointed downwards, and no mobile operation is allowed. Oh, and the devices have to be "professionally installed," but there is no definition of what constitutes professional installation in the proposed rules.
Engineers for the Integrity of Broadcast Auxiliary Services Spectrum (EIBASS) was the only entity to file comments addressing the potential for interference to 7 GHz TV BAS operations.
Dane E. Ericksen, P.E., CSRTE, 8-VSB, CBNT
Hammett & Edison, Inc., Consulting Engineers
EIBASS Co-chair
Dane -- As I noted in comments in the LPR proceeding, the mind-boggling 28 dB you mention is a measure of peak power. The increase in average power, which is usually a better measure of interference potential, is 8 dB -- in the downward-pointing antenna boresight. Predicted average emissions at 6 GHz off to the sides, that might potentially cause interference, are 13 dB below the general unlicensed emissions limits. I honestly don't believe LPRs are a realistic threat to 7 GHz BAS operations.
But only if the LPR antenna is, in fact, aimed downwards. It's a Part 15, unlicensed device, so I worry about inadvertent non-downward pointing operation by a user who has no idea that the frequency is co-channel to TV STLs. I agree that a downward-pointing LPR has low interference potential.