UHF-Satcom.com - SARSAT monitoring   (xBr, blh, pjm and nils)                                                    (23/04/2011 08:08:09 +0100)

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.

Update : since 1st February 2009, 121.5 and 243.0 are no longer supported by the Cospas-Sarsat system. Onboard receivers has been disabled and the 1544,5 downlink now only carries the 406 MHz transponder.

GEO-Satellites with SARSAT payloads: GOES 9 (160° E), GOES-East (75° W), GOES-West (135° W), INSAT 3A (93.5° E), MSG-1 (9.5° W), MSG-2 (0°)

The Satellites mentioned above carry an linear transponder which is centred on 406 MHz and downlinks the whole spectrum of approximately +- 100KHz around the centre frequency on 1544,500. Note, that these signals are weak. Used dishes on MSG were mainly greater than 1.8m in diameter.

Much unintentional relayed voice traffic like traffic from FM repeaters, operating around 406 MHz. can be heard by tuning to one of the following (most active) frequencies in FM (from MSG-1/2):

0/9,5E                  1544.406            FM Voice
MSG-1/2             1544.443            FM Voice
                            1544.468.5         FM Voice
                            1544.481            FM Voice
                            1544.531            FM Voice
                            1544.565            FM Voice *very active
                            1544.579            FM Voice
                            1544.593            FM Voice  

(Reception used for creating this list/sound sample was 1,8m dish + 3,5 turn RHCP Helical + L-Band preamp EME-103 B.)

MEO-Satellites with SARSAT payloads:

LEO-Satellites with SARSAT payloads:

Official updates of the COSPAS-SARSAT system can be found 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 46KHz wide*
170KHz sub carrier carrying 406MHz 80KHz wide

*no longer in use

BCN #0 REC PLY 46 SHR OK  07:42:62.23 406.070541 000010101100001
BCN #0 REC RTM 10 LNG OK  09:43:54.35 406.011455 111100110001110
BCN #0 OTH RTM 34 SHR OK  08:10:06.41 405.985964 011001100010100
PSE #2 REC PLY 32 LNG OK  19:35:15.18 406.049532 110101110110010
BCN #3 OTH RTM 26 SHR OK  26:37:26.20 406.075293 011011100100001
PSE #0 REC RTM 10 LNG OK  21:54:29.25 406.004550 110011001010111
PSE #2 REC PLY 56 SHR OK  10:46:59.01 406.023901 001001110000001
BCN #0 OTH RTM 54 LNG OK  31:35:19.38 406.003841 111001011110100
BCN #0 OTH PLY 49 LNG OK  00:45:16.04 406.000757 110100000011010
PSE #3 REC RTM 20 SHR OK  05:27:32.63 406.043145 010001110111001
BCN #2 OTH RTM 26 LNG OK  20:30:43.12 406.069457 101011100011101

The data above shows the typical output from the 2.4Kbps low rate data transmitted by the LEOSAR satellites. The data includes demodulated and decoded 406MHz EPIRB data, along with Hex ID and positioning data where available. (The decoder was r00ts SARSAT decoder).

(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)