1090 MHz Aviation Transponders & The USRP

Illustration 1: Sample Mode S Transponder Squawk received on 1090 MHz with 3.2MHz sample rate.

Equipment used:

All from Ettus Research (http://www.ettus.com/custom.html):

USRP With DBSRX 800 MHz – 2.4 GHz Daughterboard
LP0926 Log Periodic Antenna

The sampling rate was selected at 3.2MHz (USRP decimation 20) to avoid DBSRX or computer generated frequency spurs that appear at ±2 MHz relative to 1090 MHz. Aviation transponders use 0.5 µS pulses; our selected bandwidth is thus sufficient to catch each individual pulse.

MODE S Data Example:

The data from the illustration above is sliced at a level of about 250 A.U. and then processed into a stream of 0.5 µS pulses. After the 8 µS Mode S header we have a stream of data bits encoded as 10 for a '1' bit and 01 for a '0' bit; the data bit rate is 1 megabit / second. With received data highlighted in yellow the above example becomes:

1010000101000000

MODE S Header:

Pulses at 0, 1, 3.5, and 4.5 µS

0110011010

0 1 0 1 1

Format Number: DF = 11;

(ALL CALL REPLY)

100110

1 0 1

Capability

100110010110011010100110100110100110010110101001

1 0 1 0 0 1 0 1 1 1 0 1 1 0 1 1 0 1 0 0 1 1 1 0

MODE S Address: Hex A5DB4E

(or 51355516 octal)

101010100110011010101010011010100110010101010101

1 1 1 1 0 1 0 1 1 1 1 1 0 1 1 1 0 1 0 0 0 0 0 0

Parity

Downlink format (DF) 11 does not include altitude information. However, just with the MODE S address we may go to web pages such as http://www.airframes.org/ and learn more about the plane such as registration number N477CA, type, owner, et cetera.

Receivable 1090 MHz Traffic:

MODE C	Traditional aircraft squawk codes
MODE S	Traditional aircraft squawk codes
ADS-B	Enhanced data including geographical coordinates (in implementation phase)
TCAS	Aircraft collision avoidance system data

Military modes also use 1090 MHz and may be received from time to time.

No Need To Reinvent The Wheel:

USRP MODE S support has been added to GNU Radio not too long ago (~ middle of 2007). The usrp_mode_s_logfile.py program can be used to satisfy all those special MODE S data logging urges.

Commercial Hobbyist Aviation Transponder Receivers:

If you're really serious about the aviation monitoring hobby you may consider commercial products targeted for the enthusiast such as the SBS-1 (http://www.kinetic-avionics.co.uk/sbs-1.php).