Page 30 January February 2018 TCA

Figure 3: Screenshot of the PSK
Reporter website using decoded
FT8 transmissions to show
summertime multi-hop sporadic-E
paths on 50 MHz around 1500 UTC
on July 28, 2017.
Usage patterns of FT8, JT9, JT65,
and WSPR makes it advantageous
for the decoders to focus not just
on a single frequency, but on a
frequency range covering several
kilohertz. Our decoders are
organized to scan a range of
frequencies up to 5 kHz, if the
receiving hardware supports it,
finding all signals in the specified
mode and decoding and displaying
the results. For FT8, JT65, and
WSPR, the present decoders go
one step further, taking advantage
of the fact that when a signal with
strong FEC has been decoded, we
can calculate its transmitted
waveform exactly. An amplitude‑
scaled version of that waveform
modulation, and synchronization MSK144 has rapidly become the can be subtracted from the
schemes are the same as for the slow dominant mode for Amateur meteor‑ received data and the decoder executed
JT9 modes. JT9 fast modes have proven scatter contacts. on the remainder, thereby possibly
useful for such propagation types as decoding weaker, previously hidden
ionospheric scatter and weak double‑hop Decoders and Sensitivities signals. This approach has proved very
sporadic E on the 6‑meter band. Together with additional details effective. In these modes, WSJT-X
published in the WSJT-X User Guide and frequently decodes weak signals lying
MSK144 – FEC is implemented by within 1 or 2 Hz of much stronger ones.
augmenting the 72 message bits with an the open source code, Tables 1 and 2
eight‑bit CRC calculated from the and the preceding paragraphs define the The various WSJT-X modes have better
message bits. The resulting 80‑bit various protocols supported in WSJT-X. sensitivity than traditional modes such as
augmented message is mapped to a For these protocols to be useful, each CW for three main reasons: They use
128‑bit codeword using a (128, 80) LDPC one needs an efficient decoder. The efficient modulation schemes tailored to
code designed by K9AN specifically for algorithms we have implemented use the targeted types of propagation; they
this purpose. Two eight‑bit synchronizing soft-decision decoding, and to the best use detection bandwidths matched to the
sequences are added to make a message of our knowledge, they are the most protocol keying rate, and they benefit
frame 144 bits long. Modulation is Offset sensitive practical algorithm for each from coding gain provided by each
Quadrature Phase‑Shift Keying (OQPSK) code. For JT4, JT9, and WSPR, we use specific error‑correcting code. Table 1
at 2,000 baud, so the frame duration is the Fano algorithm, as implemented by shows detection bandwidths for the slow
72 milliseconds. Compared to FSK441, KA9Q (see Note 3), and for JT65 we use modes range from about 1.5 Hz – 6 Hz.
the mode widely used for digital meteor‑ the Franke‑Taylor algorithm, recently Noise power is proportional to bandwidth,
12
11
scatter since its introduction in 2001, published in QEX. There is a full so each of the slow modes has an
MSK144 has the advantages of strong description of the QRA64 decoder, advantage of 10 dB or more when
error correction, an effective character written by IV3NWV (see Note 8), and we compared to the typical 50 Hz “ear‑and‑
transmission rate about 1.7 times faster, expect to publish details of the MSK144 brain” bandwidth of the very best weak‑
and significantly better sensitivity. and FT8 decoders soon. signal human CW operators.
28