The Bulgarian diplomatic service is active on HF almost daily.
The mode used is RFSM, emitted on the upper sideband. RFSM is a proprietary derivative of the MIL-STD-188-110 standard, formerly developed and sold by the Russian IDE Group. The Bulgarian diplomatic service uses it with the built-in data masking option enabled, rendering the transmitted data unusable without a copy of the key file.
The network is operational every day from Monday to Friday inclusive, and follows a fixed schedule according to Eastern European Time (UTC+2 in winter, UTC+3 in summer). An 1800 Hz tone may be heard shortly before the start time. The missions usually transmit on a frequency different from Sofia, noted here as QSX.
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