Inmarsat L Band on a shoe string

Transcription

1 Inmarsat L Band on a shoe string 13:10: AES:ABBxxx GES:90.NxxxNW! RA W 41 TP AF Z 1.AFW.* ALERT * HAZ:TSTMS SOURCE:SATELLITE AREA:1N000E 07N001E 04N006E 1N004E CVRG:SCT TOPS:FL450 TREND:DEC MVG FROM:CELLS 0800 INFO:RISK ICE CRYSTAL ICING-B757 ACTION RQRD 3.VALID 18109Z/181500Z 4.CANCEL AF19 ALONG ROUTE ENTER 18/18Z ARISO EXIT 18/1301Z DNMM A couple of weeks back a group member mentioned that we might look at setting up a separate input in PP for Inmarsat L band. It's well over three years since we started our Inmarsat satellite project. Initially we checked L band in some detail to ascertain whether or not any of the data contained position reports, bearing in mind L band carries the uplink information from the GES (Ground Stations] only. At the time we came to the conclusion that only C band contained "downlinks" from the aircraft and focussed our attention on that aspect. Prompted by the recent comments I decided to re-visit the world of Inmarsat L band to see if anything had changed. L band reception is much easier to set up than C band, the latter requiring satellite dish equipment. The kit for L band is as basic as a wideband amplifier and antenna. The antenna can be an old helical, such as the one pictured below which was lying in the back of the shed, having been used

2 nearly twenty years ago for weather sat reception. The amplifier, again old kit from the weather sat imaging era. Timestep amplifier... The amplifier is the key. Although the TimeStep device is no longer available, there are a

3 number of wideband board amplifiers available for under 0. Search your favourite online store for "1500mhz satellite amplifier" The results, using this set up are excellent. The signal is fed to a standard RTL dongle which in turn plugs into your PC/Laptop. AES:AE04B S! A4 X /PIKCLYA.FS1/FSM EGGX RCH888 RCL RECEIVED IF NO CLEARANCE WITHIN 15 MINUTES REVERT TO VOICE PROCEDURES END OF MESSAGEF030 Software is either SDR# or SDR Console plus JAERO, both programmes are available free of charge although both software writers would welcome donations to help fund their research. There is plenty of information about setting up SDR in the PlanePlotter wiki. You'll need a standard RTL dongle and I'm assuming that if you are reading this, you already have one to hand and will be aware that you will need to have run Zadig to allow your PC to recognise it. The sequence of components is: Laptop / PC running SDR# or SDR Console : RTLDongle in USB port : cable from dongle to wide band amp board : cable from amp to antenna. With SDR running and set to 1545 mz, upper side band, point the antenna in the direction of your prefered Inmarsat. At present [ September 018 ]for the UK that is 5 degs east Alphasat or 15 degrees west 3F, elevation at about 5 degrees. For CONUS point at 97 degrees west 4F3, for Australia 143 degrees east 4F1. Check PlanePlotter for the best satellite for your area. No need to be too accurate at this stage as the L band downlink is extremely strong. You should start to see something like this...

4 The frequencies appear to be common to each satellite and polarisation is not critical. Next, bring JAERO into the game, initially set to 600bps. Don't forget to select the correct soundcard input from SDR, in JAERO settings. You should now start to see some activity in JAERO as above. Most of the L band messages are log-on requests sent by the ground stations to the aircraft.

5 15:16: 15:16:3 15:16:3 15:16:30 15:16:33 15:16:35 AES:AE0946 AES:A8C88F AES:A47903 AES:A47903 AES:393A6E AES:A8B9B N665UA.N388AA.N388AA.F-GOTO.N661UA ! H1 Y G Y Z Z A interspersed with ground to air requests, weather and airfield information. / CAPTAIN, WE HAVE NOT RECEIVED A POSITION REPORT WITH FUEL FOR OVER 80 MINUTES. A VALID POSITION REPORT IS REQUIRED AT INTERVALS NOT TO EXCEED 90 MINUTES. PLEASE SEND A POSITION REPORT WITH FUEL AT OR ABEAM THE NEXT OR LAST WAYPOINT. FTWDP AUTOMATED MESSAGE END Although non of this can be fed into PP, if received in conjuction with monitoring the Oceanic Traffic on PlanePlotter it can provide a wealth of background info. However, here's the best bit! You don't need an expensive/large outdoor antenna to receive L band. In fact the antenna doesn't even need to be outside. The image below shows an Inmarsat L band capture and decode using a home made wire antenna pointing very roughly at Inmarsat 5 degrees east [ Alphasat]... through a double glazed window! Just for fun, I made the antenna out of a short length of satellite coax cable. The inner core is very roughly shaped into

6 a "dipole". It took less than five minutes. It is impossible to get anything more simple than the above! But here's the rub. It works! From this point on, you can make your antenna as refined as you wish knowing that provided you follow the basic principles, the signal strength should get better and better. I also tried an old military dipole, given to me twenty years ago which was lying in the "bits" box...pictured left, below. It worked a treat!

7 So, L band reception isn't difficult, it isn't expensive to set up and provides an excellent monitoring companion to watching Oceanic traffic on PlanePlotter. The L band signals don't provide any positional data which can be used in PlanePlotter but do open up a new aspect of the hobby and who knows, having mastered L band you may be tempted take up the challenge and give C band a go. Satcom September 018