L-band reception frequencies and techniques
L-Band downlinks mostly consist of traffic which is delivered to mobile users; there are exceptions to this of course. At L-band you can receive Inmarsat and Iridium digital telephony / data, Digital weather pictures, and many streams of telemetry type traffic. There is about a 50/50 split in the traffic as to its origin, with some coming from Geostationary satellites, other traffic from orbiting satellites. One particularly interesting set of LEO L-Band satellites are those that form the COSPAS-SARSAT search and rescue satellite radio-location system.
In order to successfully receive L-band signals, you will need a good preamplifier, the 13cm/23cm VK5EME preamp is recommended for this as it provides a low noise figure and high associated gain. There are a number of other cheap LNA's including the LNA4ALL, and numerous SPF5043 type amplifiers. Coupled with a decant preamp, a suitable antenna must also be used. For feeding either an offset or prime focus dish, a 3.5 turn helical can be used; these antennas are simple to make and tollerate dimensional sloppyness to some extent. Few L-band down-links are linearly polarised, the exception was Meteosat at 0W which uses horizontal polarisation, everything else is pretty much RHCP. The frequency range to define L-band as far as satellite downlinks is usually from 1.3GHz to 1.7GHz. For Inmarsat-C reception it is possible to use an amplified / filtered patch antenna such as a modified GPS type.
The pictures below show an assembled VK5EME dual stage LNA board. N sockets are used as they are low-loss at 1.5GHz. This particular LNA is suitable for Inmarsat Reception due to its low noise and high associated gain. The custom milled enclosure was chosen mainly for mechanical rigidity reasons.
A simple LHCP helical antenna centred on 1550MHz is described below; this antenna can be used with prime focus or offset dishes. This antenna is ideal for Inmarsat reception when combined with a 60cm to 90cm dish and suitable LNA board such as the ones mentioned above.
The dimensions are not overly critical, but should be within a couple of MM of those quoted. The ground plane can be made from PCB or aluminium, and its recommended that an SMA or N-Type connector is used. The helix element can be made of 1.5mm or 2mm wire, the prototype used 2mm silver plated copper wire. The helix element can be made self supporting, but it is better practice to make a supporting rod from nylon or plastic. The first quarter turn of the helical should be bent to be parallel with the ground plane, leave a 1mm gap here for impedance transformation purposes (140ohm to 50ohm).
The Inmarsat L-Band transponder spectrum plots for 15.5W can be found below:
(above) Inmarsat 3-F2 Before STD-A switch off
(above) Inmarsat 3-F2 After STD-A switch off
(above) Inmarsat 3-F2 downlink spectrum - 5th January 2009.
(above) Inmarsat 3-F2 downlink spectrum - 2nd January 2008.
(above) Inmarsat 3-F4 downlink spectrum - 7th January 2009.
(above) Inmarsat 3-F5 downlink spectrum - 5th January 2009.
(above) Inmarsat 3-F5 downlink spectrum - 2nd January 2008.
(above) Inmarsat 4-F2 downlink spectrum - 5th January 2009.
(above) Inmarsat 4-F2 downlink spectrum - 2nd January 2008.
Inmarsat-C channels are analysed with the Inmarsat-C demo decoder available at InmarsatDecoder.com - no message content except broadcast messages like SafetyNet / EGC's are shown in the demo, but CES capabilities, frequency info etc is shown as per the screen shots below;
There is a gallery of pictures received via the L-Band 1691MHz analogue APT downlink here
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.)
EO-Satellites with SARSAT payloads:
|Satellite Name||121.5 MHz||243 MHz||406 MHz|
|Cosmos 2471 Glonass-K||no||no||yes|
|GPS DASS Transponder S-Band||no||no||yes|
LEO-Satellites with SARSAT payloads:
|Satellite Name||121.5 MHz||243 MHz||406 MHz|
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)