The Heathkit Most Accurate Clock (HMAC) is a "radio based" time clock reference that syncs up to the WWV transmitter in Fort Collins, Colorado any time there is a band opening on the 5, 10 or 15 MHz frequencies. My receiver is located 1227.71 miles (over land) at a heading of 80.18 degrees from the WWV transmitters, and it takes the signal ~6.586 msec to reach me (which is corrected for by the HMAC receiver). On average, the clock syncs up [locks into the 100Hz sub-carriers which carry the 1 baud data bit stream and the 1kHz minute sync pulse] about five times per day.

When I was originally preparing for my Technician Amateur Radio License test in March 1991 and studying about the propagation models, I wondered how you could ever determine when the propagation was "useable." You would have to have a constant transmitting source and be listening all the time to see if the propagation was "up". It then dawned on me that we already have that setup with the WWV broadcasts and that the HMAC could be modified to monitor and track these changes in propagation. This was the motivation for my creating the HMAC project.

To achieve this goal of monitoring the bands, in 1991 I set out and designed and built a Z80-based microprocessor system, created the digital interface to the HMAC clock, wrote the assembly language code to run the system (more than 3800 lines of Z80 assembly language code) and created the "HMAC Propagation Monitoring System." The only limitation of this system is that the radio can only listen to one frequency at a time and not all three frequencies simultaneously.
You can view sync-up history from each year beginning in 1991 in either graphical format or the raw data text file that is generated by the monitoring program by selecting the year to the right.