VDL Mode 2

Specs

VDL-2 stands for VHF Data Link - Mode 2. It is the 'next-gen' version of ACARS that is designed to be a higher-throughput communications link supporting a wider variety of messages (e.g. ATN, IP packets).

VDL-M2's Magic Numbers

Property	Value
Modulation	D8PSK
Symbol rate	10500
Bit rate	31500
Filter	Raised cosine (α = 0.6 (nominal excess bandwidth)
Constellation	Gray code, 22.5 degree rotation
Detector lock	16 symbol synchronisation sequence

References

• PDF Summary from Wavecom
• PDF describing technical aspects of VDL 2, 3 and 4.

Demodulation

A series of transmissions visible on the waterfall:

A DPSK demodulator flow graph in GRC that will print out a transmission's initial raw symbols to the console once the received (AM) signal power exceeds a threshold (simple squelch):

Initial results

The beginning of a series of transmissions are recorded.

You can see the sync sequence appearing soon after the squelch has opened (following three zeros as the transmitter ramps up). There are some bit errors too (optimise demodulator parameters?). The final zero is the 'reserved symbol'.

0045423000002430053004757020717774013045147502324403113674664551207303324143134717422433410225705742... [468 symbols]
3714211710000026304534167571206305744131250311707376473211422744512073033241635367174224334102661261... [132 symbols]
3631140711167000042634013404757020460760213021510502033116326553022164044614664011402645246340465355... [1763 symbols]
00413672720023746770000426340174047570200455542030250311507375053210422147 [74 symbols]
6231354774030404020747000042634017004755021021554003065031150735605721042230712237003324063174715422... [132 symbols]
4266304447134176517604677556375270000026300534047571201455702031651675123245 [76 symbols]
4106466441600714543221075635650065714071030000662641710415754414055164716 [73 symbols]
5243657703612360706140163002700442222720300004263045340675702053055411302503101254164732114225605526... [131 symbols]

Note: there are three bits per symbols (digits range from 0 to 7). Here the bit order for each symbol is reversed (in case you're wondering why it doesn't match the spec).