25 July 2016

STANAG-4538, LDL complete session

15945.0 ---: Unid 0918 UTC USB, STANAG-4538 Fast Link setup (FLSU) bursts followed by Low rate Data Link (LDL) forward transmissions and ACK bursts (BW3 and BW4 bursts). The transfer session end is signaled by the LDL EOM burst, similar to a normal BW4 acknowledgement but sent by the source ie in the data forward direction (Fig. 1).

Since the presence of the HARRIS Citadel encryption pattern in  the forward bursts (fig. 3), most likely the used equipment is the HARRIS Falcon II family RF-5800H.
fig.2 - LDL BW3 autocorrelation
fig. 3 - BW3 frame (32 symbols) and Citadel encryption pattern
The presence of the Fast Link Setup (FLSU) burst at the beginning of the transmission (fig. 4) say that this is not a MIL 188-141B/C waveform since FLSU is not defined in such standard but only Robust Link Setup (RLSU). RLSU is hence the only link setup mode providing interoperability between STANAG 4538 and 141B/C.
fig. 4 - FLSU burst

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