Unicom and Multicom
I’ve known for years that at non-towered airports you make calls on the Unicom frequency and often there is someone at the airport who will give wind and quiet-flight advisories. I never gave much thought to how the frequencies were assigned or when a station was a Unicom station or Multicom station. However, a recent Aviation StackExchange question piqued my interest.
The AIM covers this in 4−1−11. Designated UNICOM/MULTICOM Frequencies but it is a bit sparse and confusing. It looks like the AIM got its information from the FAA section of the CFRs Title 47: Telecommunication, PART 87—AVIATION SERVICES Subpart F—Aircraft Stations.
As near as I can tell, there is one official Unicom frequency per airport and it is assigned to the airport operator or other entity on an exclusive use basis. This is the frequency that is listed in the A/FD. Other operators (usually FBOs) may also have Unicom frequencies, but they are not listed in the A/FD. Apps like ForeFlight usually have these frequencies.
Brief keyed RF signals (keying the transmitter by momentarily depressing the microphone “push-to-talk” button) may be transmitted from aircraft for the control of automated unicoms on the unicom frequencies listed in paragraph (y)(3) of this section, or for the control of airport lights on the following Unicom frequencies are 122.700, 122.725, 122.800, 122.950, 122.975, 123.000, 123.050 and 123.075 MHz.
Multicom on the other hand is the default frequency for pilot communications for airports without a control tower and without a Unicom frequency. It is 122.850 or 122.900 MHz. The default Multicom frequency for airports with a control tower or FSS on the field is 122.950 MHz.
The A/FD often lists 122.950 as the Unicom frequency. So in practice these terms appear to be used interchangeably.
The FAA document, AC 90-42F – Traffic Advisory Practices at Airports without Operating Control Towers, while a bit outdated does confirm that the default frequency for airports without a control tower or Unicom station that the CTAF is 122.9.
You can find out who has a license for a base station on the FAA website. They have several different search sites that you can play around with.
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The ASRI Ground Station Administration Service provides for the coordination, assignment, and licensing of VHF Aeronautical Enroute frequencies in the 128.825 to 132.000 MHz and 136.500 to 136.975 MHz spectrum to eligible aviation business customers IAW Part 87 of the FCC rules. It appears from looking at FBO frequencies in ForeFlight that many larger FBOs are using their services.
Just for fun I tried looking up base stations at my home airport. The airport had a registered base station for 122.95 but they let it expire. According to the A/FD 122.95 is the Unicom frequency. A flight school on the field had a base station on 122.85 but they closed and the license expired. The fuel service still has licenses for 122.85 and 122.95 but they only monitor the ASRI frequency now.