An Iowa National Guard exercise in late April for the first time saw the use of a common digital mode among military, Amateur Radio, and Military Auxiliary Radio System (MARS) participants on the 60-meter interoperability channels. Military standard communications mode MilStd 188-110 was pressed into service to pass digital messages during Exercise Stable Mercury. Because Amateur Radio operators on 60 meters are not symbol-rate limited, all parties were able to use a common digital mode at a higher data rate to pass traffic. For RTTY or digital operation, radio amateurs must transmit on the center frequency of 60-meter channels with a bandwidth no wider than that of a USB signal.

The April 23 – 24 communications exercise involved the deployment of Guard units across numerous incident command posts to operate cooperatively with federal, state, local, and auxiliary units. The scenario for the drill was based on an actual severe weather event that occurred 20 years ago, and the April exercise used radar feeds and storm spotter reports taken from the June 29, 1998, Iowa Derecho to inform this training event. A derecho is an extended straight-line windstorm associated with a fast-moving cluster of severe thunderstorms.

Exercise planner and retired Colonel Rob Hedgepeth, KE0GSN, stated that a major training objective for Exercise Stable Mercury was to train in sending voice and digital messages among the various exercise participants via HF radio. The rationale was that introducing a common digital protocol would increase message throughput over what could be achieved using only voice modes.

MARS volunteers Mitch Winkle, AB4MW, and Steve Hajducek, N2CKH, prepared an Amateur Radio version of the software package that MARS members use to interoperate with military units employing the MIL STD 188-110 serial PSK modem embedded in the AN/PRC-150(C) HF transceiver and its associated “chat” software. Iowa District 1 Emergency Coordinator Paul Cowley, W0YR, led the effort for his state’s ham radio community to load and configure the M110 program in time for the exercise.

Northern Command Interoperability Communications Planner Mark Jensen, WA6MVT, noted that the five 60-meter spot frequencies are the only designated channels where federal, military, MARS, and Amateur Radio operators are permitted to operate together.

Amateur Radio rules impose a symbol rate limit of 300 baud below 29.7 MHz, restricting the types of digital modes that may be used. No such limitation applies on the 60-meter interoperability channels, however, allowing the Amateur Radio community to use the higher-rate Serial PSK mode that MARS and the military use. The M110 program employs a sound card mode, similar to other ham radio community software, and allowed 1,200-baud symbol data rate for this exercise.

The FCC proposed revising the Amateur Service Part 97 rules in response to the ARRL’s so-called “Symbol Rate” Petition for Rule Making (RM-11708), filed in late 2013. ARRL had asked the FCC to change the Part 97 rules to delete the symbol rate limits in §97.307(f), replacing them with a maximum bandwidth for data emissions of 2.8 kHz on amateur frequencies below 29.7 MHz.

Participating organizations in Exercise Stable Mercury included the Iowa National Guard’s Joint Planning Group, Joint Operations Center, 671st Troop Command, Iowa Joint Incident Site Communications Capability, 71st Civil Support Team, Iowa Air Guard’s 132nd Wing Mobile Emergency Operations Center, and the Iowa Department of Homeland Security and Emergency Management Emergency Operations Center, among other entities.

The first Exercise Stable Mercury was held in 2014. — Thanks to Army MARS Program Manager Paul English, WD8DBY