AAUSAT-II Downlink decoder demo.

This demo is a decoder for AX.25 packets from AAUSAT-II built by 
students at Aalborg University, Denmark.

Currently there is no demodulation software included, but the datasheet 
for the FFSK/MSK modem is (CMX469AD3.pdf). 

The demo compiles on both linux (gcc) and windows (DEVCPP) and it 
use our AX.25 library from the groundstation to decode the packets
and check for checksum errors. 

Demo outline:
Data is read from the file 50 bytes at a time and thrown in a ring buffer.
The check_ax25_buffer() function is used to check for complete packets and
if one is found it is returned. The packet is then decoded and the raw 
payload is hex-dumped to screen. 

Every packet from the satellite has the RSSI from the last uplinked packet 
to the satellite and also the frequency error on the uplink signal and these
are also extracted. The numbers on the packets in this demo are somewhat off
but that is bacause the converion functions here are for the FM and the data 
was recorded from the EM. 

The beacon contents are decoded and written to the file beacon.txt.

Raw AX.25 data are included in the file raw_beacon_data.bin

