Taylor & Walker / Whispering Around the World

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WSPRing Around the World
Joe Taylor, K1JT and Bruce Walker, W1BW
Amateur Radio would be much less interesting if our communication channels were always
predictable and reliable. In fact, we often don’t know where in the world our signals may becopied. If vagaries of the ionosphere and MF and HF propagation fascinate you, you’ll surely
enjoy using WSPR and its associated web site, WSPRnet.org.
WSPR (pronounced ―whisper‖) is an acronym for ―Weak Signal Propagation Reporter.‖With a computer program of this name and a standard SSB transceiver, you can participate in aworld-wide network of low-power stations exchanging beacon-like transmissions to probe
potential propagation paths. Most participating stations transmit as well as receive, although
short-wave listener (SWL) activity is also common. In principle, and with the propagation gods
willing, everyone can copy and be copied by everyone else who is currently active with WSPR
on the same band.
When a global picture of all these connections becomes available, things get especiallyinteresting — and that’s the purpose of WSPRnet. Most stations using WSPR are configured toautomatically upload their reception reports to a central database at WSPRnet.org, in real time.
By pointing your browser to WSPRnet you can get nearly instantaneous reports of where and at
what signal strength you’re being received, and view the results plotted on a world map.
In today’s ham jargon, WSPR is another soundcard mode. Its setup requirements are similar
to those of, say, PSK31. WSPR transmits and receives, but it does not support normal types ofon-the-air conversation. Instead, it sends and receives specially coded, beacon-like transmissions
aimed at establishing whether particular propagation paths are open. Transmissions convey acallsign, station location, and power level using a compressed data format with strong forward
error correction (FEC) and narrow-band, four-tone frequency-shift-keying (FSK). The FEC
greatly improves chances of copy and reduces errors to an extremely low rate. The signal
bandwidth is only 6 Hz, which together with randomized time-sharing assures that dozens ofWSPR signals can fit into a tiny 200-Hz segment of each amateur band. The WSPR protocol iseffective at signal-to-noise ratios as low as –28 dB in a 2500 Hz bandwidth, some 10 to 15 dB
below the threshold of audibility. On most bands, typical WSPR power levels are 5 W or less —sometimes a lot less. You will be amazed to discover where your QRP signals are copied, in
distant corners of the world.

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Page 2 of 9WSPR OPERATIONWSPR can be freely downloaded from www.physics.princeton.edu/pulsar/K1JT/. Packagedinstallation files are available for Windows and Linux; the program can also be compiled forMacintosh, FreeBSD, and other operating systems. WSPR is ―open source‖ software, and its
source code is maintained in a public repository at developer.berlios.de/projects/wsjt/.
Like all soundcard modes, WSPR requires audio connections between your computer andradio transceiver. Briefly stated, soundcard audio out goes to the transceiver’s audio in, and theradio’s audio out goes to soundcard in. You can use VOX control for T/R switching; if you
prefer hard-keyed switching you’ll need a serial port or USB-to-serial adapter. A serial
connection can also provide handy CAT control of most modern transceivers. If you use otherdata modes such as PSK31, you probably have the necessary connections already in place. YourSSB transceiver should be set to use upper sideband.
WSPR operation is largely automated. Time-synchronized transmissions last for slightly lessthan two minutes, nominally starting one second into an even UTC minute. Reception andtransmission intervals alternate in a pseudo-random fashion such that on average, a specified
percentage (typically 20 to 25%) of two-minute intervals are used for transmitting. It’s
important for your computer’s clock to be accurate to within a second or so. Conventional
operating frequencies for WSPR are summarized in Table 1. Many additional details of WSPR
operation, including step-by-step startup instructions, are given in the WSPR 2.0 User’s Guide,
which — thanks to a number of bi-lingual users — is now available in English, French, German,
Italian, Japanese, Polish, Portuguese, and Russian atwww.physics.princeton.edu/pulsar/K1JT/wspr.html.
In normal operation the main WSPR screen looks something like Figure 1. At the end ofeach two-minute reception interval the software decoder looks for all detectable WSPR signals in
a 200 Hz passband and displays the results in a waterfall spectrogram, a scrolling text window,
and a scrolling Band Map. The spectrogram covers a frequency range of about 220 Hz; the last
three digits of the received frequency, in Hz, are displayed on the vertical scale at right. Timeruns from left to right in the spectrogram, the full width spanning about half an hour. On atypical computer screen each two-minute interval corresponds to a strip about 1 cm wide in thespectrogram. The times of your own transmissions are denoted by thin green vertical lines. Forexample, at the time Figure 1 was made, transmissions had been made at 22:04, 22:16, and22:24 UTC.

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Each decoded WSPR signal produces text showing the UTC, signal-to-noise ratio in dB (in a2500 Hz reference bandwidth), time offset DT in seconds, frequency in MHz, drift rate inHz/minute, and the decoded message. Time offsets greater than about ±2 seconds indicate asignificant clock error at transmitter or receiver, or possibly both. Apparent frequency drifts
greater than ±1 Hz per minute can usually be traced to the transmitter, and should be corrected ifpossible. (Of course, receiver drift can also contribute to measured drifts, but this condition iseasily recognized because nearly all signals will appear to drift by the same amount.) Good
frequency stability is essential to WSPR’s remarkable sensitivity, because the software filters
used for decoding are only about 1.5 Hz wide.
WSPRnetThe WSPRnet.org web site is written and maintained by Bruce, W1BW. It provides a central
repository for WSPR reception reports (―spots‖) and offers a simple user interface for querying
the database, a mapping facility, and many other handy features. By default, the world-wide
map shows all WSPR stations reporting or decoded over the past hour, and illustrates the open
propagation paths between them. The map can be zoomed and panned, and you can set various
criteria to determine exactly which spots are included. The WSPRnet site also offers band-byband counts of stations reporting in the past hour, a chat facility for brief communications
between operators, an interface to the historical database back to March 2008, and a number ofstatistical summaries of the data. An example of the WSPRnet home page is shown in Figure 2.
This particular screen capture, taken in August 2010, mentions that the WSPR database contains
over 32 million spots. Recently an average of 300 to 500 stations, scattered around the world,
have been submitting roughly 50,000 to 100,000 WSPR reports each day.
Figure 3 is a typical example of the WSPRnet world map, in this case for the 30-meter band.
You can specify selection criteria that limit the map to a particular band, a longer or shorter timeinterval, or spots involving a particular call sign. You can click on a callsign to see what otherstations are hearing and being heard by that station. Red labels on the map indicate stations (orSWLs) operating in receive-only mode.
WSPR Protocol and Software
The WSPR protocol was originally named MEPT_JT, which stood for ―Manned
Experimental Propagation Tests, by K1JT.‖ The ―Manned‖ part of the name was a reminder that

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under FCC rules a transmitting station (with a few very specific exceptions) must always beattended.

In current practice, everybody just calls the mode WSPR.

The WSPR protocol is designed to do just one thing, and do it very well. Messages normallyconsist of a standard callsign, a 4-character grid locator, and the power level in dBm (decibels
relative to 1 milliwatt). This information is compressed into 50 binary digits and then encodedusing a convolutional code with constraint length K = 32 and rate r = 1/2. Each of the resulting
162 bits is used as the most significant bit of a two-bit ―channel symbol‖ to be transmitted using
4-tone frequency-shift keying at 1.46 baud. The least significant bit is defined by a pseudorandom sequence known to the software at both transmitter and receiver and used to establish
accurate synchronization of time and frequency.
Convolutional codes with long constraint lengths have the important advantage that
undetected decoding errors are rare. These codes are too complex to be decoded with the wellknown and highly efficient Viterbi algorithm, so the WSPR decoder uses the so-called―sequential‖ algorithm, instead. Full details of the WSPR protocol and its implementation in theWSPR program will be published elsewhere. WSPR is licensed under the GNU General Public
License and its source code is freely available to anyone.
Propagation Studies
The WSPRnet database represents a rich source of experimental data for propagation studies.
To provide a simple example, we queried the database to give us all spots of K1JT, WB3ANQ,and W1BW posted by VK6DI on the 30-meter band. It happens that all four of these stations
were running WSPR more or less around the clock between March 20 and April 12, 2009.
VK6DI was a receive-only station; K1JT mostly ran 5 W, WB3ANQ 1 W, and W1BW 100 mW.
All stations used simple dipole antennas. Figure 5 shows the signal-to-noise ratios reported byVK6DI for each US station, sorted by time of day (in 15-minute intervals) and then averaged
over the three week period. As expected, the 5 W signals from K1JT typically start a littlesooner and are somewhat stronger than the lower power signals from WB3ANQ and W1BW, butotherwise the data for all three stations are remarkably consistent. At this low point of thesunspot cycle, each station enjoyed both short-path and long-path propagation on 30 m from
northeast US to Western Australia, on most days — even at the 100 mW power level. In fact,
WSPR signals from both WB3ANQ and W1BW of less than 10mW were also decoded by
VK6DI, nearly halfway around the world. You can surely think of many other fascinating waysto explore propagation phenomena by using the WSPRnet database.

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Page 5 of 9Conclusion
Radio Amateurs keep finding new ways to challenge the frontiers of wirelesscommunication, exploring the wonders of the electromagnetic spectrum and the extraordinarily
wide range of interactions between electromagnetic waves and the terrestrial environment.
Conceived with just-for-fun, hobbyist motivations, WSPR has helped to bring some recent
technical advances from the professional and scientific world into amateur radio, thereby
providing educational benefits to the nation and the world as well as many hours of enjoyment
for technically minded experimenters. We hope you’ll enjoy playing with WSPR as much as wehave, and at the same time will add to your knowledge and understanding of radio propagationand modern communication techniques.
Many people have contributed to the development of WSPR and WSPRnet — indeed, too many
to list here — but we especially wish to thank VA3DB and G4KLA, who have worked tirelessly
to help ensure portability of WSPR to the GNU/Linux, FreeBSD, and OS X operating systems,and G4ZOD and OZ1PIF, who have spent countless hours helping us to identify and eliminate
bugs in the software.
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 then has been Professor of Physics at Princeton
University. He was awarded the Nobel Prize in Physics in 1993 for discovery of the firstorbiting pulsar. He chases DX from 160 meters through the microwave bands.
Bruce Walker has been licensed since 1991 and has held the call signs N1IKV, WT1M, andW1BW. He holds a degree in physics from MIT, and has spent most of his career involved inhigh performance scientific computing, currently with the Broad Institute in Cambridge, MA.
His primary radio interests are currently very low power (QRPp) operation on HF and softwaredefined radios (SDRs).

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Page 6 of 9Table 1
Conventional frequencies for WSPR activity
Band(m)Dial Freq(MHz)

Tx Freq(MHz)160804030201715121061.836 600
3.592 6007.038 600
10.138 700
14.095 600
18.104 600
21.094 600
24.924 600
28.124 600
50.293 000

1.838 000 – 1.838 200
3.594 000 – 3.594 200
7.040 000 – 7.040 200
10.140 100 – 10.140 300
14.097 000 – 14.097 200
18.106 000 – 18.106 200
21.096 000 – 21.926 200
24.926 000 – 24.926 200
28.126 000 – 28.126 200
50.294 400 – 50.294 600_Fig 1 — Typical appearance of the main screen during WSPR operation.

Taylor & Walker / Whispering Around the WorldFig 2 — The WSPRnet home page.

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Fig 3 — The WSPRnet global map of spots posted on 30 m over a typical one-hour period.

Fig 4 — Seven-day moving average of the number of stations participating, per day, fromearly 2008 through mid-2010.

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Fig 5 — Average S/N reported by VK6DI (Western Australia) for K1JT, WB3ANQ, andW1BW (all in northeast USA) during March and April, 2009, on the 30 m band, plotted vs.
time of day.