In this case, the reception central frequency must be shifted by 12 KHz, i.e the frequency on the USB receiver must be adjusted to 136.963 MHz (136963 KHz instead 136975 KHz). However, a standard USB receiver can be used (both solutions are equivalent from a performance point of view).The I/Q PSK demodulation will be done by Multipsk, so as the decoding. Or adjust the SDR frequency on 136863 kHz and the waterfall frequency on 12 kHz (making it 136.875 MHz). Adjust the SDR frequency on 136963 kHz and the waterfall frequency on 12 kHz (making it 136.975 MHz). The simplest way to decode this mode is to directly demodulate 8PSK frames by MultiPSK, with a SDR receiver (FUNcube Dongle, for example) connected to MultiPSK and the SDR interface started (on the Configuration screen).It is a short distance (400 km maximum) service proposed through a network of VHF ground stations (on 136.975 MHz mainly but also 136.875 MHz in Europe).
The VDL2 (for "VHF Data Link mode 2") mode permits automatic transmissions between planes and ground stations, with an exchange of different pieces of information (and especially positions) through AVLC frames (I, UI, RR, XID.) very close to Packet frames.
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