WSPR (pro­nounced “whis­per”) stands for “Weak Sig­nal Prop­a­ga­tion Reporter”. It is a pro­to­col, imple­ment­ed in a com­put­er pro­gram, used for weak-sig­nal radio com­mu­ni­ca­tion between ama­teur radio oper­a­tors. The pro­to­col was designed, and a pro­gram writ­ten ini­tial­ly, by Joe Tay­lor, K1JT. Soft­ware is now open source and is devel­oped by a small team. The pro­gram is designed for send­ing and receiv­ing low-pow­er trans­mis­sions to test prop­a­ga­tion paths on the MF and HF bands.

WSPR imple­ments a pro­to­col designed for prob­ing poten­tial prop­a­ga­tion paths with low-pow­er trans­mis­sions. Trans­mis­sions car­ry a sta­tion’s call­sign, Maid­en­head grid loca­tor, and trans­mit­ter pow­er in dBm. The pro­gram can decode sig­nals with S/N as low as −28 dB in a 2500 Hz band­width. Sta­tions with inter­net access can auto­mat­i­cal­ly upload their recep­tion reports to a cen­tral data­base called WSPR­net, which includes a map­ping facil­i­ty.

The type of radio emis­sion is “F1D”, fre­quen­cy-shift key­ing. A mes­sage con­tains a sta­tion’s call­sign, Maid­en­head grid loca­tor, and trans­mit­ter pow­er in dBm.  The WSPR pro­to­col com­press­es the infor­ma­tion in the mes­sage into 50 bits (bina­ry dig­its). These are encod­ed using a con­vo­lu­tion­al code with con­straint length K = 32 and a rate of r = ​1⁄2.  The long con­straint length makes unde­tect­ed decod­ing errors less prob­a­ble at the cost, that the high­ly effi­cient Viter­bi algo­rithm must be replaced by a sim­ple sequen­tial algo­rithm for the decod­ing process.

Pro­to­col spec­i­fi­ca­tion

The stan­dard mes­sage is <call­sign> + <4 dig­it loca­tor> + <dBm trans­mit pow­er>; for exam­ple “K1ABC FN20 37” is a sig­nal from sta­tion K1ABC in Maid­en­head grid cell “FN20”, send­ing 103.7milliwatts, or about 5.0 Watts (legal lim­it for 630 m). Mes­sages with a com­pound call­sign and/or 6 dig­it loca­tor use a two-trans­mis­sion sequence. The first trans­mis­sion car­ries com­pound call­sign and pow­er lev­el, or stan­dard call­sign, 4 dig­it loca­tor, and pow­er lev­el; the sec­ond trans­mis­sion car­ries a hashed call­sign, 6 dig­it loca­tor, and pow­er lev­el. Add-on pre­fix­es can be up to three alphanu­mer­ic char­ac­ters; add-on suf­fix­es can be a sin­gle let­ter or one or two dig­its.

Stan­dard mes­sage com­po­nents after loss­less com­pres­sion:
28 bits for call­sign,
15 bits for loca­tor,
7 bits for pow­er lev­el,
total: 50 bits.
For­ward error cor­rec­tion (FEC):
non-recur­sive con­vo­lu­tion­al code with con­straint length K = 32, rate r = ​1⁄2.
Num­ber of bina­ry chan­nel sym­bols:
nsym = (50 + K − 1) × 2 = 162.
Key­ing rate is ​12000⁄8192 = 1.4648 baud.
Mod­u­la­tion is con­tin­u­ous phase 4 FSK, with 1.4648 Hz tone sep­a­ra­tion.
Occu­pied band­width is about 6 Hz
Syn­chro­niza­tion is via a 162 bit pseu­do-ran­dom sync vec­tor.
Each chan­nel sym­bol con­veys one sync bit (LSB) and one data bit (MSB).
Dura­tion of trans­mis­sion is 162 × ​8192⁄12000 = 110.6 s.
Trans­mis­sions nom­i­nal­ly start one sec­ond into an even UTC minute: e.g., at hh:00:01, hh:02:01, etc.
Min­i­mum S/N for recep­tion is around –28 dB on the WSJT scale (2500 Hz ref­er­ence band­width).

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