S31 was a military test station operated by the Czech Republic that sent test counts played from a tape. Although it was given an Enigma ID in January 2000, it was active long before then.  It was first spotted around the 1970’s, and possibly earlier.  Last observed in 2000.

There were two different tests that S31 sent.  The first test was transmitted on 1122 kHz which counted from 50 to 90 every hour of the day during minutes 10-12 and again 40-42.

The tests on frequency 2136 kHz began with 9 tones that sounded like a telephone ringing . Then, a short pause and a message was sent, followed by counting from 50-59.  The numbers of cycles that S31 counted from 50-59 varied from 9-19 times.  This test began daily at 18:45 CEST.  Counts also did not always start at 50 or end at 90.

Station Summary

Nickname: None
Inactive, since 2000
Emission mode: AM
1122, 2136 kHz
Voice Summary:
Believed Country of Origin:
Hradešín, Czechia


“Czech Station 2136.” Nineteenth Edition of the N&O Column / Spooks Newsletter (Archived)
“S31 New Designation.” ENIGMA Newsletter 18 (January 2000), Page 15
Československý DX klub – CD-4 Zvuky v éteru (Archived)