Hello, I got Mode S and ADS-B receiving working. I dump the messages to standard output for redirect to a simple log file. Simple programs take the log and decode it into English. I have not decoded all the messages yet like Airborne Velocity.
Tick: Sat Apr 28 08:43:52 2007 Extended Squitter A/F: a6b55a Extend Cap: Unassigned 5 Airborne Position O ln: -70.949388 la: 42.521745 Alt: 6100 All-Call Reply A/F: a6b55a Extend Cap: Unassigned 5 Short Altitude A/F: a6b55a Airborne No Downlink Request Alt: 6100 Tick: Sat Apr 28 08:43:54 2007 Extended Squitter A/F: a6b55a Extend Cap: Unassigned 5 Airborne Velocity I was having problems with GNURadio stopping while trying to receive from the USRP. It seemed like the USB stream would stop. I thought it was the laptop so I setup a Core 2 Duo system. The problem still occurred. I "fixed" the problem by removing the TVRX board from RXB and moving the DVB board from RXA to RXB. Now I can log all day. One neat little program I use makes a kml file out of the positions and I use Google Earth to see how good the range is. I can only receive stuff up to 20 or so miles out. There are a number of Aircraft with ADS-B that use Boston or fly over on the way to Europe. As I am reading technical articles and other information I see the techniques progressed as 1090 MHz got more crowded and more powerful DSP chips became available. Mode S started out using simple receivers with pulse edge timing or centered sampling of pulse amplitudes. A radar site sends interrogations at their Pulse Repetition Frequency so the maximum reply rate was under 450 per second. FRUIT became a problem as more sites came on-line and aircraft started using 1090 MHz for TCAS. FRUIT as in False Replies Uncorrelated In Time. Basically overlapping of different Mode S signals or Mode S with Mode A/C pulses. One interesting article outlines using 10 or 8 MSPS for the sampling rate and using factors such as amplitudes of pulses being within 3 db of each other and the effects of beat frequencies of interfering Mode S signals to determine if a bit is a one or a zero. So this is turning into an interesting DSP and data decoding experiment. I suspect that I am in a high FRUIT environment and my limited range is due to the basic receiver techniques I am using and the Discone Antenna. I am currently implementing a better receiver in code and may put up a 1090 MHz antenna with a preamp. Another interesting project is to also receive the 1030 MHz interrogations and figure out the range and bearing of Mode S aircraft by the timing between the interrogations and replies. 73 Eric _______________________________________________ Discuss-gnuradio mailing list [email protected] http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
