UHF-Satcom.com - SARSAT monitoring   (xBr, blh and pjm)                                                                                 (16/03/2008 19:42:16 -0000)

COSPAS-SARSAT is a series of VHF/UHF to L-Band transponders that fly on a few satellites, with the sole purpose of providing a 'search and rescue' radio location service. The SARSAT satellites and SAR (Search and Rescue) antennas are provided by the USA. The SARSAT SARR and SARP are provided by Canada and France respectively and the COSPAS satellites and SAR payloads are provided by Russia.

The most interesting traffic is that which is unintentionally relayed traffic via the 1.5GHz downlink. It's mostly air to air and air to ground communications. Occasionally a siren type noise is heard which is typical for a distress beacon. AM aircraft traffic can be heard by tuning to the 47KHZ sub carrier which can be demodulated by placing the receiver in WFM mode, and tapping off the base band (i.e. the usual method of demodulating FDM). Another method is to tap the IF output of the receiver, and tune to IF + 47KHz with an external receiver, leaving the main receiver on the 1544.5MHz signal, preferably using the AFC function to track the satellites carrier. There is a nice piece of software available from COAA that decodes the 406MHz EPIRB's - well worth getting and experimenting with.

GEO-Satellites with SARSAT payloads: GOES 9 (160° E), GOES-East (75° W), GOES-West (135° W), INSAT 3A (93.5° E), MSG-1 (3.4° W)

Official updates of the COSPAS-SARSAT system can be found here. An excellent document also exists on the COSPAS-SARSAT pages describing the technical details of the SAR system, including transponder info, here. Each COSPAS-SARSAT satellite has transponders with the following RF characteristics:

RX

121.5MHz
243MHz
406MHz

TX - 1544.5MHz
2.4KHz sub carrier carrying low rate telemetry
47KHz sub carrier carrying 121.5MHz 25KHz wide
94KHz sub carrier carrying 243MHz 25KHz wide
170KHz sub carrier carrying 406MHz 80KHz wide

(above) base band demodulation of NOAA-17 SARSAT transponder, showing the 3 transponders relayed as sub carriers of the 1544.5MHz downlink. This base band reception was done by connecting the discriminator output of an AR5000 receiver to the antenna input of an SDR-IQ which was then used to tune 0 to 190KHz. The advantage of this is that the AR5000 can AFC the 1544.5MHz downlink from the satellite leaving the base band in good condition.

(above) NOAA-15 SARSAT transponder as received off air (top) and overlaid textbook transponder description (bottom)