WSPR: Protocol

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.[2]

• 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).