While monitoring the UHF band, my friend Kosmod successfully captured and shared a series of military satellite 2400 Bd bursts on 252.2525 and 253.6460 MHz. The signal displays BPSK modulation at 2400 Baud (or 2400 bps, given the 1 bit/symbol efficiency of BPSK).
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| Fig. 1: waveform analysis of the captured signals |
The 253.646 MHz signal falls directly within the downlink range for UHF MILSATCOM. This specific frequency is part of the UFO (UHF Follow-On) and legacy FLTSATCOM constellations operated by the US Navy to provide global coverage for maritime and ground forces.
Technically, this is Channel 14 in the Navy's 'Bandplan Bravo'. While the nominal center is 253.650 MHz, these satellite transponders act as 'bent-pipes', simple relays that repeat the uplink signal across their available bandwidth. Because of this, seeing a signal centered at 253.646 MHz is perfectly normal; it's simply sitting on the lower edge of that 25 kHz wideband assignment.
Based on the location of Kosmod, UFO-11 (USA 174) (1) is the primary "workhorse" for this region. These satellites utilize Global Beams; for instance, a satellite parked at 75°E and at a geostationary altitude of 35,786 km (like UFO-11) covers everything from Central Europe all the way to Western China.
Looking at the differential decoded bitstreams (Figure 2), we are looking at a classic Satellite Military Communication (SATMILCOMM) frame structure. These patterns are highly characteristic of bursts sent over tactical waveforms (like MIL-STD-188-181 or similar UHF SATCOM standards).
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| Fig. 2 : differential decoded bitstreams |
Here is a proposed breakdown of what those specific hex blocks could represent:
This is the Bit Synchronization sequence. Purpose: In binary, AA is 10101010. This alternating pattern of ones and zeros allows the receiver's modem to "lock on" to the clock frequency of the transmission.
2. The Sync Word / Frame Marker
The block starting with 46 8B 87 84... is the Frame Synchronization Word. Notice that this block is identical in all three headers. Once the receiver has the timing (from the AAs), it looks for this specific unique bit string to know exactly where the "data" actually begins. It marks the boundary between the "noise/tuning" and the actual message.
3. The Metadata / Packet Header (The Variable Block)
This is the most interesting part. Notice that this section changes in every capture. This block typically contains the Control Channel information. In military protocols, this often includes: Destination/Source IDs, Initialization Vector (IV), Message Length/Type.
4. The Flush / Transition Pattern (8B 87 84 7B ...)
You see this repeating sequence just before the encrypted payload. This acts as a pad or transition gap. It ensures the receiver's hardware buffer is ready and that the cryptographic resync has occurred before the high-entropy (encrypted) data hits the stream. The fact that it repeats exactly five times suggests a fixed-length buffer requirement for the specific radio hardware being used.
A note on the "Encrypted Data": because the data following these headers is encrypted (likely using AES-256 or a sovereign military grade equivalent), it will appear as completely random noise with no repeating patterns. Without the specific cryptographic key and the correct algorithm, that portion cannot be decoded.
The captured transmission is almost certainly not the classic Fleet Broadcast beacon, but rather a more active DAMA/IW Control Channel:
* Fleet Broadcast (Beacons) are usually found at the "low end" of the UFO band (e.g., 250.350 MHz to 250.650 MHz) and they are constant, 24/7 transmissions
* Tactical Data Links (Your Signal) sits in the "Tactical" portion of the bandplan. This area is reserved for Demand Assigned Multiple Access (DAMA) and Integrated Waveform (IW). These channels don't just broadcast one-way; they manage a "conversation" between the satellite and hundreds of ground terminals.
It is interesting to note the parallels with an HF signal captured previously on 7961 kHz (USB): a 3G-HF STANAG-4538 transmission. The demodulated bitstreams exhibit a series of data blocks characterized by a 32-bit framing structure, primarily due to the fixed length of their headers [1].
https://disk.yandex.com/d/3b9hpOGFoZkp6g
(1) Satellite Profile: UFO-11
NORAD ID: 28117
Current Position (March 2026): It is stationed in a geostationary orbit over the Indian Ocean, currently hovering around 75° East longitude. Coverage: This position provides a massive footprint that covers almost all of Russia (except the extreme Far East), the Middle East, Eastern Europe, and Africa. Inclination: As of 2026, UFO-11 has an orbital inclination of approximately 8.5° to 9°. This means it is no longer perfectly stationary; it appears to drift North and South in the sky over a 24-hour period.
UFO-11 was the final satellite in the UFO series and includes the most advanced digital processing of the bunch.
Communications Payload (The "Bent-Pipe" Hardware)
UHF Channels: 39 Channels (Total), voice and Low-speed data
Wideband: 21 Channels @ 25 kHz used for DAMA/IW
Narrowband: 17 Channels @ 5 kHz used for legacy tactical teletype/secure voice
Anti-Jam: FHSS (Frequency Hopping), protection for the Fleet Broadcast (FLTSAT)
GBS Payload: 4 Transponders (Ka-Band), high-speed Global Broadcast Service (video/maps).
Signal Technical Specifications
Modulation: BPSK / SBPSK
Symbol Rate: 2400 Baud
Protocol Standard: MIL-STD-188-181C The "Integrated Waveform" (IW) standard. It governs how the satellite manages multiple users in a single 25 kHz channel.
Frame Structure: TDMA Burst Time Division Multiple Access. The signal consists of a Preamble, a 160-bit Unique Word, a Control Header, and the 5-fold redundant Trailer
[1] https://i56578-swl.blogspot.com/2018/03/unid-32-bit-secondary-protocol.html
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