17 October 2019

async FLSU call followed by STANAG-4197 (3G-HF "circuit mode")

This transmission was logged and recorded by my friend DK8OK Nils on 11228.0 KHz/USB, and refers to op-comms between  "INY" Trapani-Birgi airport and "DHN66" Neuteveren/Geilenkirchen NATO air base (INY provides technical-operational and logistical support to the AWACS of the E-3A Component, based in Geilenkirchen). Nils kindly sent me the file for its analysis.
The sample is an example of a STANAG-4538 3G-HF FLSU (Fast Link Setup) asynchronous call followed by traffic in "circuit mode" (data continuous, not packed); short voice comm is in the middle. Although synchronous calls are the preferred mode in 3G networks, async calls might be used if the called (or the caller) station may not have achieved net synchronisation. 
The BW5 burst waveform used by FLSU is recognizable in the initial PSK-8 segment from its duration and from the conveyed tribit symbols (2432), as it results from the cross correlation function and the demodulated stream (Fig. 1).

Fig. 1 - CCF/ACF and demodulated stream
According to Annex C to STANAG-4538, the async call of FLSU protocol begins with the LBT (listen before transmit) for at least one dwell period, followed by the transmission of 1.35N (nearest integer value) Async Request PDUs on the requested link frequency, where N is the number of channels in the scan list, and 1.35 is the duration of each dwell period in seconds. The async call procedure ends with a single LFSU Request PDU (Fig. 2).

Fig. 2 - async FLSU PDUs
Looking at the 50-bit payloads in Fig. 2, type 3 (011) PDUs are sent 10 times and are followed by a single type 0 (000) PDU: since PDUs type 3 indicate the Async_FLSU_Req PDU, and type 0 indicates the FLSU_Request PDU, the sample exactly matches the async call procedure as above. By the way, it's worth noting that since up to 10 Async_FLSU_Request PDUs are used, 7 are the allocated channels for this network.


001 00 0000101000 0000001010 1 0 011 111111 010010 11011011
001 00 0000101000 0000001010 1 0 011 111111 010010 11011011
001 00 0000101000 0000001010 1 0 011 111111 010010 11011011
001 00 0000101000 0000001010 1 0 011 111111 010010 11011011
001 00 0000101000 0000001010 1 0 011 111111 010010 11011011
001 00 0000101000 0000001010 1 0 011 111111 010010 11011011
001 00 0000101000 0000001010 1 0 011 111111 010010 11011011
001 00 0000101000 0000001010 1 0 011 111111 010010 11011011
001 00 0000101000 0000001010 1 0 011 111111 010010 11011011
001 00 0000101000 0000001010 1 0 011 111111 010010 11011011
001 00 0000101000 0000001010 1 0 000 111111 010010 01001101

The STANAG-4197 waveform following the call is most likely used in a ANDVT modem in order to achieve secured voice transmission.  
Notice the apparent lack of the fourth doppler tone at 2812.5 KHz: indeed, it's seems just barely visible in the bottom sonagram of Fig.3: probably a defect/malfunction of the HF modem. 

Fig. 3 - STANAG-4197 segment

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