The MSK 144 Protocol for

Meteor-scatter communication was first described in the pages of QST in 1953 as a means for communicating on dead 15 and 20 m bands. Hams soon realized that even more impressive results could be obtained at 6 and 2 m, where background noise levels are much lower and useful low-elevation gain is obtainable with relatively modest antennas. Early meteor-scatter (MS) contacts used CW and relied on relatively rare “blue whizzer” meteor trails that last several seconds or longer. Today we can use a fast digital mode with built-in error correction to make contacts on VHF bands any day of the year, out to 1300 miles or so, using meteor-induced “pings” shorter than 0.1 s — with no dependence on weather, solar activity, position of the moon, or fickle band openings. European hams pioneered the use of high-speed CW (HSCW) in the 1960s and 1970s, using Morse code at speeds 10 to 40 characters per second (cps; 10 cps=120 WPM) to convey short messages using pings as short as several tenths of a second. Modified cassette tape recorders saved the received audio and played it back at low speed, for decoding by ear. By the late 1990s, personal computers were used to send and receive HSCW at speeds as high as 150 cps. Shelby Ennis, W8WN, described the state of the art in HSCW circa 2000 in QST. Soon afterward K1JT introduced computer program WSJT 3 with the FSK441 protocol, the first amateur digital mode designed specifically to communicate with the shortest and most frequent meteor pings. FSK441 uses 4-tone frequency-shift-keying and noncoherent demodulation. Its character transmission rate is 147 cps, and it provides reliable copy for signals a few dB above the noise in a 2500 Hz bandwidth. Since 2001 hundreds of thousands of MS contacts have been made with FSK441 on the VHF bands, and some even as high as 432 MHz. Today’s computers are considerably more powerful than those of 2001. This rapid technology advance has enabled us to develop MSK144, a practical protocol for meteor scatter that uses bandwidthefficient modulation and cutting-edge tools for forward error correction (FEC). When designing this protocol we gave high priority to considerations of transmission speed, sensitivity, and decoding efficiency. The final design choices for MSK144 are well matched to the nature of MS signals on the amateur VHF bands and the characteristics of today’s amateur transceivers. The effective transmission rate of 250 cps makes good use of very short pings and can be shown to be a practical speed limit for the typical 2500 Hz bandwidth of amateur SSB transceivers. The generated MSK144 waveform ensures that decoders can use coherent demodulation and even coherent averaging over multiple message frames. Using these techniques, we find that some MSK144 signals can be decoded with signal-to-noise ratios as low as −8 dB in the standard 2500 Hz reference bandwidth. Following its public introduction in the summer of 2016, MSK144 has rapidly become the world-wide mode of choice for amateur MS contacts. In this paper we present technical details of the MSK144 protocol and describe its motivation and underlying design philosophy. We begin by describing the modulation, frame structure, and errorcontrol coding, paying particular attention to the spectrum and envelope shape of generated waveforms. We include on-theair spectral measurements and the results of simulations that establish the decoding sensitivity and false-decode rate. We then present some details of the MSK144 decoder as implemented in the popular open-source computer program WSJT-X, followed by some special operational features of the program in MSK144 mode. These features include semi-automated tools for convenient use of a standard MS calling frequency; a contest mode to facilitate exchange of required information in North American VHF contests; a special short-message format useful at 144 MHz and higher frequencies; and a tool for measuring and compensating frequency-dependent phase shifts in the receiver passband.