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)
Radio Receiver Options
There are quite a few SDR radios suitable for this application including Airspy Mini and Airspy R2 and the excellent range of radios by SDRPlay.
The signals will need to be amplified so It is important that the receiver has the facility to supply 4.5-4.8 volts DC (bias tee voltage) along the coaxial to power an in-line amplifier or antenna with integral amplifier.
One cheaper option I found was the Nooelec NESDR Smartee v2 SDR Dongle, This is a nice little radio , great for experimenting. Small profile and Aluminium casing. At the time of writing (January 2020), this unit is around $25 USD.
The Nooelec smartee does a fine job with no fuss, the bias tee voltage is always on and not dependent on software tick boxes. The driver is very easy to install on your computer. The dongle comes with an info card and directs you to a set up guide..easy !.
Please Make sure you order the correct model “Nooelec nesdr smartee v2″ with integrated bias tee.. and NOT the very similar looking Nooelec nesdr smart dongle which has no bias tee.
You can buy direct from Nooelec or try Amazon in your region, Nooelec have stock held by Amazon warehouses around the world. I purchased mine from Amazon UK and my order took 2 days to arrive. Not bad at all !.
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 Nooelec smartee dongle worked fine with this as did the SDRPlay RSP1A which is one fantastic radio.The results though 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.
Installation of JAERO software on Raspberry Pi
You will need to run an older desktop version of rasperian system for Jaero to work, new versions of Pi OS will not work. (See update Below)
You will need a Pi image Jessie, this was before the release of Stretch version, most likely (but not definitely) the Jessie version dated 2017-07-05 in the list below.
You will need to install the desktop version of the OS.
Plus you will need GQRX sdr radio package installed on your pi before installing Jaero.
The only version of Jaero I could get to work properly on raspberry pi was V188.8.131.52-7.1armv7hl.rpm
Link to download this file from my google drive
Set up the pi with standard raspberian desktop version.
Create a new folder to work with on the pi graphical desktop, name the folder Jaero
And unzip the 7.1 rpm to that folder, click on the Jaero icon and the decoder window should open and it should work.
News Update 27/1/2020 : A user from France has reported to me that he got Jaero v184.108.40.206-7.1 working well with Buster version of pi OS.
He downloaded the rpm Jaero version 7.1 from my link above then used the following commands.
sudo apt-get install pulseaudio qtmultimedia5-dev libqt5multimedia5-plugins rpm2cpio -y
rpm2cpio jaero-220.127.116.11–7.1.armv7hl.rpm | cpio -idmv
mv usr JAERO
cd JAERO/bin && ./JAERO
When you have Jaero installed successfully..
Play a test file on your pi system locally to test if the decoder is working and your audio routing is correct
Here is a test file (again hosted on my Google Drive) of satellite acars to download and play on your system.
Audio with Windows installs
You will need a way of piping the audio output from the sdr radio or dongle to the Jaero program for decoding.
A program is available from Evgenii Muzychenko and that program has up to 256 virtual audio cables, there is a trial and paid versions…great work Evgeni !
Evgenii is an audio software expert “par excellence” !
This is the original VAC software which all others try to copy and imitate. Developed by Evgeni way back in 1998 and first sold in 2001 on the internet. a single licence is $30 usd which is very reasonable if you are considering this part of the radio hobby seriously.
There have been many developments since the first release and there is a comprehensive user manual and online/email support direct from the developer.
There is a program called VB Cable Virtual Audio Device, the free version only gives 1 channel but the paid one will give up to 8 virtual cables so you could (providing you have the pc spec to do it) open several instances of sdr# by Youseff Touil, sdr console by Simon Brown or sdr uno by the SDRplay team on different frequencies and open several instances of Jaero and each frequency could be piped through simultaneously to each Jaero. I state again, check each program for their respective audio output options and set the output to VB cable which will link to Jaero. Following the standard VB audio cable install program, you will find the folder VB in program files, there is a small graphical representation of the control panel if you click on the icon for it.
VB Audio cable can be downloaded from here
Before starting out to decode signals It is important to set your windows default sound device as VB-Audio Cable .
Next open Jaero and open “Settings” from the top menu , at the bottom of the settings window you will see “soundcard” make sure the Cable Output VB-Audio cable is chosen.
Again you can download the ACARS Satellite test ACARS audio file hosted on my Google Drive and play this locally on your computer when the Jaero window is open
If you have your settings correct.
You should see a green light for volume, this indicates that the sound is being piped through correctly by the virtual audio cable settings and a blue line will be jumping around indicating that activity is happening.
For starting out , Next you need to find some acars channels and move sdr sharp, sdr uno, or sdr console to these frequencies, you will see the peaks in your sdr waterfall (around 1.545.029 is a good start), set jaero to 600bps or 1200bps at the bottom of the Jaero window to start with as these are the easiest to decode then later move on to 10500bps.
Successful dish/antenna alignment and freq chosen will result in the signal and data indicators showing green, red on the volume indicator means that too much volume is set in your sdr radio program..turn it down a little, conversely white in the volume indicator means not enough or no volume of audio is being received by jaero, make adjustments to vb cable settings or check that the sdr radio program you are using isn’t muted..
Observe the constellation window with the yellow dots, Ideally you should get 4 clusters of yellow dots, one in each of the 4 squares on the chart, this indicates an excellent antenna alignment.
Okay Lets fire up the radio and open Jaero I have set the bit rate to 10500 for this ambitious example !
Success ! ..3 solid green lights and some data coming through, and 4 clusters of dots in the centre of the 4 squares.
Some data is 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 Satcoms..John you are a star !, and of course finally many thanks to the author of the JAERO decoding software “Jonti” (Johnathan Olds from Wellington, New Zealand)