Steven J. Franke, K9AN

Joseph H. Taylor, K1JT

3314 E. Anthony Dr., Urbana, IL 61802; k9an@arrl.net

272 Hartley Ave, Princeton, NJ 08540; k1jt@arrl.net

The MSK144 Protocol for
Meteor-Scatter Communication
Here’s a full description of the modulation, message structure,channel coding, and special operational features of the new
meteor-scatter mode implemented in WSJT-X.

Meteor-scatter communication was firstdescribed1 in the pages of QST in 1953 as ameans for communicating on dead 15 and20 m bands. Hams soon realized that evenmore 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 “bluewhizzer” 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 dayof 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 ofhigh-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 asecond. Modified cassette tape recorders
saved the received audio and played it backat low speed, for decoding by ear. By thelate 1990s, personal computers were used tosend 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.2 Soon afterward K1JT introduced
computer program WSJT 3 with the FSK441
protocol, the first amateur digital mode
designed specifically to communicate withthe shortest and most frequent meteor pings.
FSK441 uses 4-tone frequency-shift-keying

and noncoherent demodulation. Its charactertransmission rate is 147 cps, and it provides
reliable copy for signals a few dB above thenoise in a 2500 Hz bandwidth. Since 2001
hundreds of thousands of MS contacts havebeen 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). Whendesigning this protocol we gave high priority
to considerations of transmission speed,
sensitivity, and decoding efficiency. The
final design choices for MSK144 are wellmatched to the nature of MS signals on theamateur 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 apractical speed limit for the typical 2500 Hz
bandwidth of amateur SSB transceivers. The
generated MSK144 waveform ensures that
decoders can use coherent demodulationand 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 introductionin the summer of 2016, MSK144 has rapidly
become the world-wide mode of choice foramateur MS contacts.
In this paper we present technical detailsof the MSK144 protocol and describe

its motivation and underlying design
philosophy. We begin by describing themodulation, frame structure, and errorcontrol coding, paying particular attention
to the spectrum and envelope shape ofgenerated waveforms. We include on-theair spectral measurements and the resultsof simulations that establish the decoding
sensitivity and false-decode rate. We thenpresent some details of the MSK144 decoder
as implemented in the popular open-source
computer program WSJT-X,4 followed bysome 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 ofrequired information in North American
VHF contests; a special short-message
format useful at 144 MHz and higher
frequencies; and a tool for measuring andcompensating frequency-dependent phaseshifts in the receiver passband.
Modulation and Coding
MSK stands for minimum shift keying,a form of continuous-phase frequency-shift
keying (FSK) with shift equal to half thebaud rate. MSK144 uses message frames of
144 bits and modulation at tone frequencies
1000 and 2000 Hz to transmit channel
symbols at keying rate of 2000 baud. The
resulting audio waveform can be viewed as aform of offset quadrature phase-shift keying(OQPSK) with individual pulses shaped likethe first half-period of a sine wave. Using
this OQPSK viewpoint, the continuous-time
QEX July/August 20171

representation of an MSK144 signal can be written as=s ( t ) I ( t ) cos ωc t − Q ( t ) sin ωc t
where the waveforms I ( t ) and Q ( t ) are called the in-phase andquadrature components of the signal, respectively. In WSJT-X
the MSK144 waveform is generated with carrier frequency=
f c ω=
1500 Hz, and the resulting audio signal is transmitted
c / 2pas upper sideband by a standard single sideband (SSB) transmitter.
Example waveforms for I ( t ) and Q ( t ) in Figure 1 show how
the in-phase and quadrature signals are created from half-sine pulses
with 1 ms duration. Note that the Q ( t ) waveform is shifted by half asymbol relative to I ( t ) ; therefore half-symbols appear at the start andend of the Q ( t ) waveform in this plot. Pulses with positive polarity
represent bits with value 1, negative pulses represent 0. With theconvention that bit indices begin at zero, even-numbered bits are senton the Q channel, odd-numbered bits on the I channel. The waveformshown in Figure 1 represents the bit sequence 01100110101101010.
Waveform Spectrum and Envelope
Figure 2 shows the average audio spectrum of an MSK144 signal as
generated by WSJT-X. The carrier frequency f c = 1500 Hz has beenchosen to center the main spectral lobe in the available bandwidth of atypical SSB transceiver. The generated audio waveform has constant
amplitude; however, band limiting by the transmitter’s audio and RF
filters will remove all spectral components above 3 kHz, so only themain lobe will be transmitted and the waveform will no longer haveconstant envelope. Table 1 shows some examples of the amount ofenvelope variation caused by different amounts of filtering. Figure
3 shows the measured spectrum of a received MSK144 signal at
the receiver’s audio output, as well as the spectrum of receiverbackground noise with no signal present.
Linear amplification is necessary to reproduce faithfully theenvelope variations introduced when the MSK waveform is filtered.
Such an amplifier will maintain the sidelobe-free spectrum producedby the transmitter filter. On the other hand, if the signal is hard-limited
and then amplified by a nonlinear amplifier, spectral sidelobes will
reappear, but at lower levels than were present before filtering.5 In
general, we expect that the MSK144 waveform will tolerate nonlinear
amplification without generating excessive amounts of splatter;
however anyone contemplating use of a nonlinear amplifier should
conduct tests to verify that the amplifier does not cause excessive
spectral broadening.
Frame Structure and Channel CodeAs in all other WSJT-X modes with FEC, MSK144 user
messages are compressed into exactly 72 bits. Transmissions
consist of a sequence of identical frames that carry these bits alongwith synchronizing and error-correcting information. Each frame
includes a 72-bit user message, an 8-bit cyclic redundancy check(CRC) computed from the message, and 48 bits of error-correcting
redundancy. The resulting 72 + 8 + 48 = 128-bit codeword iscombined with two 8-bit sync words to form a 144-bit message
frame. The frames are constructed as {S8, D48, S8, D80}, where S8
represents an 8-bit sync word and D48, D80 represent the first 48 andlast 80 bits of the 128-bit codeword. The 8-bit CRC is used by the
decoder to detect and eliminate most false decodes. At 2000 baud, theframe duration is 144/2000 = 0.072 s.
The 80-bit combination of message and CRC is mapped to a
128-bit codeword using a binary (128, 80) Low Density Parity Check(LDPC) code designed specifically for MSK144. We chose an LDPC
code because these codes provide state-of-the-art performance andcan be designed with virtually any desired number of data bits andparity bits. Moreover, they can be decoded with low computational
cost compared to other code types used in WSJT-X — specifically,
the long constraint-length convolutional codes used in JT4, JT9, and
WSPR and the Reed-Solomon code used in JT65. As a consequence

2 QEX July/August 2017

Figure 1 — Short segment of the in-phase, I (t ) (solid curve) and
quadrature, Q (t ) (dashed curve) components of an MSK144 waveform.
The in-phase waveform contains 8 contiguous half-sine pulses.
The quadrature waveform is offset by half of a pulse. The offset andhalf-sine shaping ensures that the envelope of the MSK144 signal isconstant, before band limiting by filters in the transmitter.

Figure 2 — Average spectrum of MSK144 audio signals generated
by WSJT-X using 1.5 kHz audio carrier frequency. Transmitter audio
and RF filters will limit the bandwidth of the transmitted signal to,typically, 300-2700 Hz, which includes most of the main lobe of thisspectrum but no sidelobes.

Figure 3 — Solid curve: received spectrum of the message “K9AN
AA2UK R‑05” transmitted by AA2UK and received by K9AN. Dottedcurve: spectrum of background noise as shaped by the frequency
response of K9AN’s receiver.

Table 1
Representative examples of envelope variation obtained by filtering a simulated MSK144 frame. Envelope
variation increases significantly as transmitter bandwidth narrows.
No filtering
0-3 kHz
0.3-2.7 kHz
0.5-2.5 kHz

Average power
1.0
0.996
0.988
0.959

Peak envelope power
1.0
1.27
1.32
1.50

we can use many decoding attempts on each chunk of data, with eachattempt concentrating on a different subset or weighting of the data.
This procedure leads to a significantly higher probability for decoding
weak or noisy signals.
The selected (128, 80) LDPC code is defined by a 48 × 128
parity-check matrix in which all elements are 0 or 1. The matrix has
exactly three ones in each column and eight in each row. (Thus, only
3 × 128 = 8 × 48 = 384 of the 6144 entries are equal to 1; it’s this low
density of nonzero elements in the matrix that gives the LDPC codeits name.) Each of the 48 rows of the parity check matrix defines aparity check equation. The eight 1s in each row determine which of
the 128 codeword bits are included in that parity check. Each parity
check is carried out by summing, modulo 2, the indicated codeword
bits. All 48 of these sums will be zero for a valid, error-free codeword.
Starting with the parity-check matrix we derive a 48 × 80
generator matrix that will determine 48 parity bits for any 80-bit
message word. The 1s in a particular row of the generator matrix
determine which of the 80 message bits to sum, modulo 2, to produce
a parity bit. Additional details of the way we have implemented theseprocesses, including concise definitions of the two matrices, can befound in the open source code6 for WSJT-X.
Code Performance
Figure 4 summarizes performance of the MSK144 protocol
when used with the soft-decision decoder currently implemented in
WSJT-X. As shown by the curve labeled Pc, single received frameswith SNR > 0 dB are nearly always decoded correctly. (Here andelsewhere in this paper, signal-to-noise ratios are measured in a
2500 Hz standard reference bandwidth.) At SNR = ‑1.5 dB about 40%
of received frames are correctly decoded, and about 0.1% of received
frames will yield an incorrect codeword. Most of the incorrect
codewords will be rejected because the falsely decoded message will
not pass the CRC test. For SNRs near the 50% decoding thresholdabout 1 in 200 such incorrect codewords — about 5 per one-million
decoding attempts — will accidentally have the correct CRC and will
be displayed as undetected false decodes. The measured probabilityof undetected false decodes is plotted as a function of SNR as Pi,
scaled up by a factor of 105 to make it visible on the plot. The false
decode rate falls to negligible levels when SNR > 0 dB.
Figure 5 shows the fraction of noisy received codewords thatwill be decoded as a function of the number of hard errors in thecodeword. Redundancy provided by the parity bits allows decodingof most synchronized frame-length bit sequences with fewer than 10
hard errors, and a small fraction of those with as many as 15 errors.
We note that without the error-correcting code, even one hard error
in the 72-bit packed message would reduce its uncompressed content
to garbage.
A number of significant design choices have been built into
the code/decoder combination. One such parameter is the numberof iterations the decoder is allowed to try before it gives up. Moreiterations take more time, but (up to a point) will produce more
decodes. However, the probability of false decodes increases
dramatically if the maximum allowed number of iterations becomes
too large. Another choice involves the detailed design of the code

Peak-to-average power ratio
1.0
1.28
1.33
1.57

Max-to-Min power ratio
1.00
1.91
2.29
3.31

Figure 4 — Probability of a correct decode Pc and incorrect
decode Pi as a function of SNR for standard MSK144 messages.
These curves were generated by numerical simulation, based on
107 simulated noisy received frames. The results represent ideal
performance assuming perfect frequency and time synchronization.
Coherent averaging of N messages will slide both curves to the leftby the amount 10 log10 N.

Figure 5 — Fraction of correct decodes as a function of the numberof hard errors in a received codeword.

QEX July/August 20173

itself. Increasing the density of the paritycheck matrix yields a code that requires
more iterations to decode or, equivalently,
produces fewer decodes for a given numberof iterations. On the other hand, a more dense
code that produces half as many decodes
may give one fifth as many false decodes.
We created a number of codes with different
densities and explored the performanceof each one for different iteration limits.
Ultimately, we chose parameters that yield
good Pc and Pi performance with the smallest
number of decoder iterations. We believe
the chosen code is close to optimum for the
purpose at hand.
Optional Short-Message Format
On a given meteor-scatter path theduration of a ping is proportional to theinverse square of operating frequency. Pings
at 144 MHz are therefore about 1/8 as longas those at 50 MHz. Most of a frame must
be received in order to decode its message,
so pings shorter than about 70 ms, common
at 144 MHz and higher bands, are too short
to convey a decodable standard MSK144
frame. To utilize even shorter pings, theprotocol includes optional short messages
that can be used to send signal reports andother necessary QSO information after callsigns have been exchanged. Short frames
are 20 ms long and consist of 40 bits: an
8-bit sync word, 4 bits to convey message
information, 12 bits representing a hash of
the string consisting of the DX call sign
followed by the home call sign, and 16 parity
bits for FEC. There are just 9 supported
messages, namely R‑03, R+00, R+03, R+06,
R+10, R+13, R+16, RRR, and 73. The
12-bit hash serves two purposes: it gives thereceiving operator high confidence that a
decoded frame was indeed intended for him,
and it is used to reject most false decodes.
We designed a binary (32,16) LDPC codefor the MSK144 short-message frames. The
16 × 32 parity-check matrix has exactly three
1s in each column and 5, 6, or 7 in each row.
Performance measurements for this code(including verification of the hash test) areplotted in Figure 6. The decoding thresholdis almost the same as for the long code, but
the peak probability of false decodes is about
30 times larger — a consequence of the shortcode’s smaller block length.
Implementation in WSJT-X
MSK144 is one of seven distinct
operating modes in program WSJT-X.
An overview of the design purposes andoperating characteristics of each mode has
been published in the QST article of Note 4,
and many more details can be found in the
WSJT-X User Guide.7 We limit the discussion
here to some particular features of MSK144
as presently implemented in WSJT-X.

4 QEX July/August 2017

Decoder Details
With pings longer than about 100 ms
we can coherently sum multiple frames
to improve the signal-to-noise ratio. The
current MSK144 decoder can average up to 7
frames, about half a second of data. Coherent
averaging of N frames improves sensitivity
by 10 log N dB, so sensitivity improves
rapidly for longer pings. Coherent averages
of N = 2, 3, 4, 5, and 7 frames with constantsignal level yield gains of 3, 4.8, 6, 7, and
8.5 dB, respectively. These amount to very
worthwhile improvements, even when thereal-world signal levels are not constant over
the averaging interval.
Coherent demodulation requires properalignment of channel-symbol boundaries,
precise knowledge of a received signal’s
carrier frequency, and an estimate of carrier
phase. Averaging over multiple frames
increases the necessary frequency precisionin proportion to total signal duration. To
ensure that carrier phase does not varysignificantly over the averaging interval,
7-frame averaging requires that frequency
be known to better than 1 Hz. In effect,
the decoding algorithm must search all
possible frequency offsets in some tolerance
window using a 1 Hz step size, at the sametime searching over time offsets to establish
symbol synchronization. For the longest
frame averages, the decoder’s execution
time is dominated by this synchronization
requirement.
WSJT-X implements a version of thesum-product algorithm 8 for soft-decision
decoding of the noisy received symbols.

This iterative decoding algorithm accepts
a numerical reliability measure for each of
the received symbols. Each iteration updates
the symbol reliability based on the degree towhich the parity check sums are satisfied.
After every iteration, hard decisions are madeabout the value of each symbol based on the
updated symbol reliabilities; if the result isa valid codeword, the algorithm terminates.
If no codeword is found before completinga predetermined maximum number of
iterations, the algorithm “times out” andreports a decoding failure.
The MSK144 decoder in WSJT-X Version
1.7 operates in near real-time by looking
at small overlapping chunks of data andcompleting all decoding attempts before thenext chunk arrives. Each chunk contains 7
message frames, equivalent to about 0.5 sof data. For each chunk the decoder firsttries to synchronize and decode the best
single message frames. If this fails, it tries
combinations of 2-, 3-, 4-, and 5-frame
coherent averages. Finally, it tries a coherent
average of all 7 frames. Older and slower
computers may not be able to keep up withthe demands of the MSK144 decoder whenusing the “Deep” decoding setting on the
WSJT-X user interface. Selecting “Normal”
decoding will eliminate the longest and mosttime-consuming 7-frame average to savetime. The “Fast” decoding setting eliminates
both 5- and 7-frame averages. Omitting
the longest averages reduces sensitivity
somewhat, although the penalties are modest.
Phase Equalization
Most superheterodyne SSB transceivers
use narrow filters optimized for good shape

Figure 6 — Probability of a correct decode Pc and incorrect decode Pi as a function of
SNR for the MSK144 short-message format. These curves were generated via numericalsimulation, based on 107 simulated noisy received frames.

factor without regard to phase linearity.
Group delay variation across the passband
smears out MSK144 pulses, causing
intersymbol interference. WSJT-X includes aphase equalization facility that can be used tocorrect any group delay variation contributedby the receiver. When a received frame
has been successfully decoded, this tool
generates an undistorted waveform whose
Fourier transform can serve as a frequencydependent phase reference to compare withthe phase of the received frame’s Fourier
coefficients. Phase differences between
the reference waveform and the received
one will include distortions contributedby the originating station’s transmit filter,
the propagation channel, and filters in thereceiver. If the received frame originatesfrom a station known to transmit signals
having little phase distortion (say, a station
known to use a properly adjusted softwaredefined-transceiver) and if the received
signal is relatively free from multipath
distortion so that the channel phase is close
to linear, the measured phase differences will
be dominated by the local receiver’s phase
response.
The phase-response tool in WSJT-X fitsa low-order polynomial to the measured
phases. The saved polynomial coefficients
can then be used to correct the phases in allreceived signals, effectively flattening thereceiver’s group-delay response. Careful useof this capability can improve the decoder
sensitivity by significant amounts. As an
example, Figure 7 shows the phase responseof the TS-2000 receiver at K9AN, measured
using a signal transmitted by the softwaredefined-transceiver at KØTPP. Phase errors
as large as p/2 radians (90 degrees) are foundnear the passband edges, relative to midband values. The smooth curve in Figure 7
is a 4th-degree polynomial fit to the measured
data. The improved decoder sensitivity canbe judged from the eye diagrams of Figure 8,
which contain overlaid plots of 72 receivedsymbol amplitudes constituting one frame ofthe signal from KØTPP. The upper part of
the figure shows the diagram before phasecorrection; the lower part after applying thefitted phase equalization curve. Notice thatthe eyes are more “open” after equalization.
Wider eye opening means fewer bit errors
at low SNR, and a higher likelihood of
successfully decoding a noisy received
frame.
If the operator is careful to ensure thatthe applied phase correction accurately
represents the receiver’s response, then thephase equalization derived from one strong
reference station will improve decoding of
signals from most other stations. It should
be noted that phase equalization is not likely
to improve decoding performance for those
who use SDRs with linear-phase receive
filters.
Standard Operating Procedures
In North America the highest MSK144
activity levels are found on 6 meters. QSOs
are carried out with alternating transmit/
receive (T/R) sequences 15 s long; a hundred
watts and a modest antenna up 20 feet

Figure 7 — Filled circles: measured phase differences between thecomplex spectra of a locally generated reference waveform and theaverage of several strong frames received from KØTPP. Smooth
curve: 4th-order polynomial fit to the measured data.

is sufficient for making lots of meteorscatter contacts. By informal convention
the standard 6 m “calling frequency” is50.280 MHz. As described in our earlier
mentioned QST paper of Note 4, most QSOs
start with someone calling CQ and proceedroughly as follows:
1. CQ K1JT FN20
2.
K1JT K9AN EN50
3. K9AN K1JT -01
4.
K1JT K9AN R+03
5. K9AN K1JT RRR
6.
K1JT K9AN 73
Each station continues sending a given
message until receiving the next message
from the QSO partner. The signal reports
conveyed in messages number 3 and 4
are measured signal-to-noise ratios in dB.
By longstanding tradition, especially forweak-signal work on VHF and higher
bands, a minimal contact is considered
complete and suitable for logging after
both call signs, signal reports or someother previously unknown information, andacknowledgments have been exchanged.
The final “73” message is a courtesy; inthe above example it lets K1JT know that
his final acknowledgment was received andthe contact is complete. Many users like toexchange 13-character free-text “chit-chat”
at this point.
As an example of on-the-air usage ofMSK144, the WSJT-X screen shot in Figure
9 shows a sequence of messages received at
K1JT after he called CQ on 50.280 MHz.
In a transmission starting at 11:30:00 UTC,

Figure 8 — Eye diagrams for the received in-phase (left) and
quadrature (right) symbols for a signal from KØTPP received by
K9AN. Upper curves are with no phase equalization; lower curves areafter equalization using the polynomial fit shown in Figure 7.

QEX July/August 2017

5

W5ADD answered the CQ. Figure 10 isa snapshot of the WSJT-X horizontally
scrolling spectrogram showing this ping(lower panel) and another one several
sequences later that carried the message“K1JT W5ADD R‑02” (upper panel).
These two pings were of moderate strength,
SNR = 5 and 4 dB, respectively, and theone in the top panel is about 70 ms wide;
nevertheless these and even much weaker
pings (see the “dB” column in Figure 9) aredecoded without error. From Figure 9 you
can see that when the QSO with W5ADD
was complete N9BX and KA9CFD called as“tail-enders,” and two more QSOs followedin quick succession.
Split Operation using “CQ nnn”
The sporadic nature of meteor pings
makes it possible for many stations toshare a frequency with little interference.
Decoding several different stations in thesame receiving sequence lets you see “whoelse is on,” in addition to the station you may
be working. However, when the frequency
gets too busy it’s a good idea to spread out.
WSJT-X provides a mechanism for doing thiswhile preserving the important advantage of
having a known “meeting place” for initiating
contacts. CQ messages may include three
digits between the CQ and call sign. For
example, K9AN might send “CQ 290 K9AN
EN50” on 50.280 MHz to indicate that hewill listen for replies on 50.290 MHz, andcontinue the QSO there. Note that offset CQs
are not the same as typical “split” operation
on HF bands. When operating split, each
operator transmits on one frequency andlistens on another. MSK144 QSOs using“CQ nnn” are used to move a contact off
the calling frequency to a single offset
frequency where both stations will transmit
and receive. WSJT-X has facilities that allow
it to recognize “CQ nnn” calls and reset thetransceiver’s dial frequency automatically, as
required for both stations.
Contest Mode
North American VHF contests use
Maidenhead grid locators as multipliers andrequired exchange information. MSK144
offers an optional contest mode in whichgrids are exchanged and acknowledged
instead of signal reports. The standard
message sequence then becomes
1. CQ K1JT FN20
2.
K1JT K9AN EN50
3. K9AN K1JT R FN20
4.
K1JT K9AN RRR
5. CQ K1JT FN20
The acknowledgment “R” in message
number 3 is conveyed by using the fact that
propagation modes suitable for MSK144 are
generally effective only out to distances oforder 1300 miles. To convey the message

6 QEX July/August 2017

Figure 9 — Messages received at K1JT in a sequence of three quick MSK144 QSOs on 6
meters.

Figure 10 — Small portions of the WSJT-X real-time spectrogram showing pings
received from W5ADD in sequences starting at UTC 11:30:00 (lower) and 11:32:30(upper). The corresponding decoded messages are those in the first two lines of thescreen shot in Figure 9.

Figure 11 — Messages received at K1JT in a QSO with K9AN using short-format messages.

fragment “R FN20”, WSJT-X encodes thelocator as that of the diametrically opposite
spot on Earth. The receiving program
recognizes such a locator as special —it would imply an implausible distanceapproaching half the circumference of the
Earth — so the software does the reverse
transformation to the original grid and inserts
the implied “R ” in the message displayed tothe user.
Short Messages
As described above, MSK144 supports
short-form messages that can be used after
QSO partners have exchanged both callsigns. Short messages consist of 4 bits
encoding R+report, RRR, or 73, together
with a 12-bit hash code based on the orderedpair of “to” and “from” call signs. Whenshort messages have been activated on the
WSJT-X user interface, messages 4 to 6 in thestandard sequence are modified as follows:
1. CQ K1JT FN20
2.
K1JT K9AN EN50
3. K9AN K1JT -01
4.
<K1JT K9AN> R+03
5. <K9AN K1JT> RRR
6.
<K1JT K9AN> 73
Individual call signs are replaced by a
12-bit hash code in the transmitted frame,
and the substitution is indicated onscreen by
enclosing the call signs in < > angle brackets.
When the receiving program decodes thealready known (and therefore expected)
hash code, it displays the known call signs
in angle brackets. Figure 11 is a screen shot
showing messages received by K1JT in a
QSO with K9AN using short messages. The
short-message feature is intended for use on
144 MHz and higher bands, where very shortpings make them especially beneficial.
A third party monitoring short message
transmissions will not necessarily know
the call signs of the stations involved in
the ongoing QSO. WSJT-X implements an“SWL” mode that lets a listener monitor
short messages exchanged by two otherstations. When SWL mode is turned on, thedecoder remembers recently copied call signs
and compares received hashes with a list ofhashes calculated from all ordered pairs ofcall signs currently in the list. If the received
hash is found in the list, the decoder prints
the decode with the “guessed” call signsin < > angle brackets. Accepting received
messages with any hash found on a list
increases the probability of printing a false
decode in proportion to square of the numberof remembered call signs. To decrease thenumber of spurious decodes printed in SWL
mode, the software only prints decodes after
the associated hash has been received at least
twice in a receive sequence.

Conclusion
MSK144 is a highly efficient protocol
for conducting minimal QSOs via meteor
scatter. Indeed, we believe the protocol’s
effective character transmission rate,
occupied bandwidth, and sensitivity are close
to optimum for the stated purpose, whileremaining consistent with the capabilities ofstandard amateur SSB transceivers.
We thank Bill Somerville, G4WJS, for
helpful comments on an earlier version ofthis manuscript.
Steve Franke, K9AN, holds an Amateur Extra
class license. He was first licensed in 1971 andhas previously held call signs WN9IIQ andWB9IIQ. An early and abiding fascination
with radio science led to his current positionas Professor of Electrical and Computer
Engineering at the University of Illinois in
Urbana-Champaign. Steve is a member of
ARRL and a Fellow of the IEEE.
Joe Taylor was first licensed as KN2ITP
in 1954, and has since held call signs K2ITP,WA1LXQ, W1HFV, VK2BJX and K1JT. He
was Professor of Astronomy at the University of
Massachusetts from 1969 to 1981 and since thenProfessor of Physics at Princeton University,
serving there also as Dean of the Faculty forsix years. He was awarded the Nobel Prizein Physics in 1993 for discovery of the firstorbiting pulsar, leading to observations that
established the existence of gravitational waves.
After retirement he has been busy developing
and enhancing digital protocols for weak-signal
communication by Amateur Radio, including
JT65 and WSPR. He chases DX from 160 meters
through the microwave bands.
Notes
O. G. Villard, W6QYT, and A. M. Peterson,
W6POH, “Meteor Scatter — A Newly
Discovered Means for Extended-Range
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