A friend of mine wanted to monitor aircraft ACARS data transmissions from the Inmarsat “Alphasat” 4a-F4 satellite.
He asked me to build a DIY antenna for his proposed ground station.
The test equipment we were using was a simple sdr radio dongle, a windows laptop running JAERO software and some antenna hardware which will be covered in this article.
Inmarsat “Alphasat” 4a-f4.
Amongst other things, this satellite handles aircraft ACARS data messages , It is very far away..it is in a geostationary orbit around 35,770 kilometres (22,200 miles) above the earth. Also its big as far as satellites go..Nicknamed “the A380 of space” It has a mass of 6,649 kg (6.5 tons) and it was launched on board an Ariane 5 Rocket in 2013 from Kourou Spaceport French Guiana by EASA.
This satellite sends and receives messages in L-Band of the microwave radio spectrum, typically around 1.54 ghz.
The footprint is centred over Africa, coverage is all of Europe, most of Africa, the Middle East and parts of Asia.
Here is a photo of 4A-F4, the “business bit” in the middle…it is a huge lump of a thing !
And an image of what Inmarsat 4A-F4 “Alphasat ” looks like in space with its solar arrays and reflector extended..Considerably bigger !
(Photo credits European Space Agency ESA)
I had tried satellite acars using JAERO software on a raspberry pi 3 computer a few months ago using the excellent sdr play RSP1A with a GPS type active magmount antenna with integral amplifier powered by 5v bias tee (antenna photo below), the same antenna is sold by sdr-kits here in the UK.
The results were not exactly brilliant. Perhaps it was my impatience or using a bare bones budget computer or lack of knowledge in this area of radio or a combination of all three but it was a frustrating experience. No matter how much I tried, I could not get reliable decoding. Just parts of messages.
Improved Antenna Build
Anyway on to making an improved antenna for receiving ACARS data signals.
I found a video on youtube from fellow radio amateur Adam (9A4QV) in Croatia (Adam also constructs & sells the very popular LNA4ALL in line amplifier products). He had made a DIY patch antenna from 2 metal sheets and he found it worked well for L-Band alphasat reception.
I decided to have a try at replicating this but after watching the video several times over, things were not clear as to the materials Adam used (In particular the metals used) and I anticipated much difficulty in soldering a connector on to the antenna front plate so I decided to get in touch with him via email. Luckily Adam responded very quickly and gave me some much needed guidance.
He had used tin plate for the front patch to aid soldering and aluminium plate for the larger rear reflector.
Adam 9A4QV had suggested it would be feasible to use brass plate for the front patch and this was easier to obtain locally so I went for that option.
Here is an image of the dimensions.
Note that the pair of 6mm diameter holes are positioned in the exact centre of each plate. The 2 plates are joined and electrically shorted together by an M6 x 12 stainless steel machine screw , washers or a spacer and a locknut. There is a constant spacing/ air gap of 7mm between the 2 plates and washers or ideally a spacer is used as a sleeve on the machine screw and between the plates to maintain the 7mm air gap.
The 10mm hole in the aluminium plate is to accept a BNC connector, the exact centre of the 10mm hole should align perfectly with the exact centre of the 2mm hole in the brass plate.
The 2mm hole in the brass plate is for the ferrule tube of the BNC connector, the tube is quite long and will protrude through the brass element front plate.
Here is a photo of the construction, here the plates are cut and correctly spaced, note the bnc ferrule is poking out of the brass element.
I had to mount a few 10mm diameter stainless steel washers behind the BNC connector to reduce the protrusion of the tube ferrule.
Some gentle use of a smooth file brought the BNC tube ferrule flush with the plate, then its just a case of soldering and making things tidy.
Around the back, something different.
I found a 40mm diameter aluminium furniture leg from ebay, I fixed this to the back of the reflector . Why ? I hear you ask..
Well..Now for some inspiration, I looked long and hard at those images from Inmarsat/EASA and noticed that the Inmarsat boffins have used a large reflector on the satellite and below it there is what looks like a patch antenna…similar to the type used on top of an aircraft fuselage.
See circled area inset below.
Okay..Lets DIY copy their design !…. Enter a 90cm solid glass fibre satellite dish and tripod found on a local auction site for £15 GBP/$20 USD.
The dish LNB clamp accepts standard 40mm LNB collars so the ebay furniture leg is the perfect size, this gives me the ability to obtain the best signal possible by adjusting the patch antenna forwards and backwards in the clamp to find the perfect focus point. The patch antenna has to be orientated for Left Hand Circular Polarisation (LHCP), so the antenna is turned 90 degrees.
The best/easiest way to explain LHCP and RHCP: The signals transmitted from the satellite are “wound up” in a helix pattern and the helix has right hand turns (clockwise), when we try to intercept the signals we need an antenna with (anti-clockwise) left hand turns to “unwind” the signal and into the radio .
Here is what the patch antenna looks like when its facing the dish
Here you can see the 7mm gap between the plates, the plate spacer nut and the washers on the back of the BNC connector.
Now to employ some signal amplification,powered by 5v bias tee, here we are using a purpose made unit by Nooelec, the Sawbird IO.
Inside that nice aluminium casing, the unit look like this.
Next I used dish pointer https://www.dishpointer.com/ program to line up the dish and patch with the Inmarsat “Alphasat” satellite at 24.8 degrees east.
This neat website shows you a direct line to the satellite from your location, just move the green dish icon to the exact site of your dish and the webpage does everything for you.
There is a really nice app on the ipad & iphone for this too called “dish align “, this gives an audible and visual signal when you have the ipad in the correct position for the satellite. Excuse the poor quality video but I wanted to show you just how cool this app is, and its free.
Here I have selected Inmarsat @ 25 degrees east from the drop down list in the app and I placed a sample fixed dish site location into the map page, watch how dish align uses the ipad on board compass and positioning data, also it finds the correct elevation and the ipad/iphone speaker emits a “lock good” tone..how cool is that !
Technology is Marvellous isnt it ? The stuff of dreams not too long ago !
So now we have the dish aligned correctly here is a summary diagram of the setup.
Now we start the Jaero program and see if any signals are coming through.
Success ! ..3 solid green lights and some data coming through.
A message for Alitalia A330 -200 (I-EJGB) !
A sample aircraft list with message window..Gulf Air a321 A9C-CB is quite some distance away from my location in the UK ..she is over 4500 miles away working route Bahrain-Chennai this evening as flight GF68. As you can see in the message window below , this flight is receiving on route sector weather reports for Al Maktoum Airport Dubai (OMDW) , Abu Dhabi Airport (OMAA), and Muscat airport Oman (OOMS).
My dish and antenna can see the satellite and so can the flight crew of GF68 , curvature of the earth is not an issue so we both see the data at the same time.
For me, This is a totally new branch of the great hobby of radio monitoring.
I have learned so much in the making of this little patch antenna I cannot begin to explain.
It is working very reliably and is much better reception wise than the small powered active GPS type antenna I tried months before.
It really was some kind of achievement for me to take 2 bits of metal plate and make an antenna system to receive signals from a 6 metre long box floating around in space over 22,000 miles away.
It worked out much better than I ever expected.
The data signals can be fed to Planeplotter very easily.
I have to send thanks to Adam 9A4QV for the support and guidance on antenna construction.
Also thanks to local satellite aficionado John Locker (Planeplotter Support) http://satcomuk.yolasite.com/ for some excellent advice on all things Inmarsat, and of course finally many thanks to the author of the JAERO decoding software “Jonti” (Johnathan Olds from Wellington, New Zealand)