Shared Part 90 Spectrum Means Monitoring Before Transmitting

Walkie TalkieOn May 16, 2017, the Federal Communications Commission issued a Notice of Violation (NOV) to Utility Mapping Services Inc. (UMSI) for its operation of a Trimble R8-Model 2 Global Navigation Satellite System (GNSS) Receiver modem on 461.075 MHz, a Part 90 UHF shared frequency, in a manner that caused interference to co-channel users.

By way of background, units such as this Trimble unit are utilized by surveyors.  The UHF radio portion of the unit is utilized for data transfer.  Because the data modem transmits over Part 90 frequencies, a license for that operation must be obtained by the user.

For those not familiar, the license application is submitted to one of the FCC’s designated Frequency Advisory Committees (FAC).  The FAC (also called a Frequency Coordinator) selects an appropriate frequency within the band where the unit operates, and for which the entity has eligibility.  That application is then sent to the FCC for grant.

As a general rule, Part 90 frequencies in the UHF Band (450-470 MHz) are only assigned on a shared basis, pursuant to 47 C.F.R. Section 90.173(a).  Therefore, licensees are expected to cooperate in use of their assigned spectrum, in order to avoid causing harmful interference, pursuant to Section 90.173(b).  Traditionally, this has meant that licensees must “listen” for other users before they transmit (either manually or through electronic means), and limit communications to the minimal practical transmission time, pursuant to Section 90.403(c).  Licensees failing to do so may be fined by the FCC.

It should also be noted that stations on shared frequencies may not continuously transmit.

Thus, it is clear the units such as the Trimble R8-Model 2, when using shared UHF spectrum, must monitor the frequency for co-channel emissions before transmitting.  It doesn’t appear that the units themselves have the ability to do so automatically (Motorola calls it “polite mode” for Motorola equipment), so users must manually monitor before transmitting.  However, given the number of complaints which I’ve heard about in the field, it is clear that many users do not do so.

In the early 2000s, our office, in conjunction with our client, the Personal Communications Industry Association (PCIA, now WIA) attempted to address the issue of low power data systems on Part 90 non-public safety UHF frequencies, by proposing the creation of certain pools of frequencies on what had previously been known at the UHF “offset” frequencies.  With the support of the Land Mobile Communications Council (LMCC), the FCC adopted issues an Order creating these pools.  The pools are as follows:

Group A1 – Coordinated frequencies, voice and non-voice on a co-primary basis. Base, mobile and operational fixed stations authorized of various power levels.  More limited power within 50 miles of the Top 100 major urban areas.  Max 75 foot antenna height.

Group A2 – Similar to A1, without the Top 100 limitation.

Group B – Coordinated frequencies.  However, non-voice operations are primary, and voice operations are secondary.  Six watt power limits for base station, two watts for mobiles. Max antenna height of 20 feet.

Group C – Non-coordinated frequencies for itinerant use (operation of a radio station at unspecified locations for varying periods of time).  Voice and non-voice co-primary.  Six watt and two watt limitations as per Group B.

Group D – Central Alarm frequencies.

As you can see, Group B and C frequencies are the perfect home for these survey operations.  Group B, in particular, is appealing because of the primary non-voice designation.  While monitoring before transmit is necessary to avoid harmful interference to primary users, there is no need with regard to secondary users.  For this purpose, a secondary user may not cause interference, and must accept interference, from a primary user.  Thus, it is highly recommended that survey operations utilize Group B frequencies to the extent possible.

In a “Tech Tips” article for an industry magazine in 2008, Robert Reese reported the difficulty in finding a clear UHF channel to use in California for his Pacific Crest Corporation radios.  He also discussed alternatives.  Either way, it is important that users adhere to the FCC’s Rules when using such devices, in order to avoid the time consuming and potentially expensive NOV process.

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