[Scan-DC] 72 MHz Time Signals

[Andrew Clegg]

Perhaps a bit esoteric, but the 72 MHz band contains multiple digital signals that are part of campus-based time distribution systems.


The general architecture appears to be a base station that's connected to GPS or NTP, which then transmits a 72 MHz time signal to other clocks on the campus, to keep them all synchronized. These systems are generally at hospitals, schools, and other locations where each indoor clock may not have easy connectivity to GPS or Internet.


The ones I'm seeing appear to be based on the Primex system (https://support.primexinc.com/hc/en-us/articles/360022470074-Overview-of-the-72MHz-ClassicSync-Solution), which is easy to distinguish because this system only transmits from 39 minutes past the hour until 6 minutes past the hour, and is silent otherwise.


These systems operate under the Part 90 Industrial/Business pool under a primary fixed service allocation in 72-73 MHz.


The 72 MHz signals I can hear from my QTH in Ballston, and the licensees for each frequency based on the FCC's License View database, are below. I can't tell for sure which location I'm receiving on each frequency, except for 72.760 MHz, which is assigned to only one licensee, and is also the strongest signal from my QTH.


  *   72.100 MHz: Shrine of the Most Blessed Sacrament, Fairfax County Schools, Kaiser, Navy Federal CU, Metropolitan Washington Airports Authority, Reid Temple Christian Academy, Washington Advent Healthcare
  *   72.140 MHz: Red Cross, Fairfax County Schools, Association for Supervision and Curriculum Development, Charter School Birney Campus, Southern Maryland Hospital
  *   72.200 MHz: Fairfax County Schools, Holton Arms School, Kaiser, Georgetown Visitation Prep School, Red Cross
  *   72.280 MHz: Fairfax County Schools, Navy Federal CU, Red Cross, Immanuel Church, The French International School
  *   72.340 MHz: Fairfax County Schools, Red Cross
  *   72.400 MHz: Red Cross, Fairfax County Schools, Navy Federal CU, Haynes Charter School
  *   72.640 MHz: Fairfax County Schools, Kaiser Tysons, Kaiser Largo
  *   72.760 MHz: Sibley Memorial Hospital
  *   72.840 MHz: Inova Alexandria Hospital
  *   72.880 MHz: Johns Hopkins (?)

I also hear a signal on 72.060 MHz, but it is not consistent with the Primex signal format on the other frequencies, and also does not conform to a silent period from :06 - :39. There is no licensee for this frequency out to 100 km distance listed in License View. I'm not sure what this signal is.


73 & Merry Christmas to all,
W4JE