At the beginning of last month we posted about an update to JAERO which allows us to now listen to AERO C-Channel voice audio. AERO is a satellite based communications service used by modern aircraft, and it's possible to easily receive the signals with an RTL-SDR, L-band patch antenna and LNA. The C-Channel conversations are typically about Medlink which is a support line for medical emergencies, but other conversations may be heard too.
While it is possible to listen to these conversations, due to legal reasons regarding patents it is necessary to compile the audio decoder manually from source, and this can be quite an involved multi-step process on Windows. Fortunately, YouTuber Corrosive, who has been making SDR related videos for some time now has put up a three part video series on the process.
JAERO was recently updated by programmer Jonti, and it now supports the decoding of AERO C-Channels which are voice audio channels that exist on both the L-Band and C-Band frequencies of AERO. AERO is a satellite based communications service used by modern aircraft. The information transferred are normally things like aircraft telemetry, short crew messages, weather reports and flight plans. It is similar information to what is found on VHF/HF ACARS.
Jonti notes that these C-Channel voice signals are very weak as they are spot beams, so a good antenna system is required to receive them. Over on Jonti's JAERO website there is now some information about these C-Channels (scroll all the way down to the C-Channel heading and read to the end of the page), as well as a frequency list. An excerpt of the information is pasted below:
Inmarsat C and in particular AERO C channels provide circuit switched telephony services to aircraft. The channels of interest are those that carry AMBE compressed audio at a channel rate 8400 bps and voice rate of 4800bps. There is also an older speech codec still in use, LPC at a voice rate of 9600 bps and an overall channel rate of 21000bps.
Telephone channels are two-way duplex. In the from-aircraft direction transmissions are roughly in the 1646 to 1652 Mhz range. The satellite up-converts these transmissions to C band, similar to T and R channel burst transmissions. So it is possible to receive the from-aircraft transmissions although it is significantly more difficult than those in the to-aircraft direction on the L band. So for those who want to get started receiving these transmissions the L band is by far the easiest place to start.
Another aspect of the C channels is that they most often use spot beams rather than global beams which makes it more difficult to receive transmissions for aircraft using a spot beam that is aimed at another region. However if you are inside the spot beam the transmissions are relatively easily received on L band. A 60 cm dish with an LHCP helical and L band LNA will provide excellent results but even with a patch antenna it can be done.
Decoding these channels to audio in JAERO takes a little effort to setup. Due to the uncertain legal status of the digital audio AMBE codec, the codec code needs to be compiled manually first, and then placed into the JAERO directory. Jontio has uploaded the AERO AMBE codec source code at https://github.com/jontio/libaeroambe. Since JAERO is a Windows program, compilation of libaeroambe involves using MSYS2.
Once fully set up with the audio codec, the audio will come out of default soundcard set in Windows audio properties, so ensure that any Virtual Audio Cables are not set as the default device.
On the L-band link you can get conversations from the ground to the plane. The C-band link would get you the plane to ground side of the conversation too, but that is a challenging signal that would require a large dish and Jonti doesn't know of anyone who has managed to receive that before. Typically the conversation topics are things like Medlink which is a multilingual medical support line that can provide backup to doctors or aircrew handling medical emergencies in the air. In Europe the USAF also apparently use C-Channel.
In a post uploaded last month we noted that Outernet was selling off some of their old L-Band satellite antennas cheaply. Nils Schiffhauser (DK8OK) decided to take advantage of the sale and bought one. Now Nils has created a blog post that shows how he's been able able to decode 12 L-Band AERO channels simultaneously with the Outernet L-band antenna, an Airspy R2 and SDR-Console V3. AERO is the satellite based version of aircraft ACARS, and it's L-band signals contain short ground to air messages like weather reports and flight plans. Multiple channels are often in use at any one time.
To achieve this Nils uses the multi-channel tuning capabilities of SDR-Console V3, which allows him to open up 12-channels, each tuned to a different AERO frequency. He then opens up 12 instances of the AERO decoder known as JAERO, and then uses VB-Cable to pipe the audio from each channel into a JAERO instance. Nils writes that the key to making JAERO run with multiple instances is to install JAERO into different folders on your PC, and give each JAERO.exe a unique file name like JAERO_1.exe.
He collects all the data into a program called Display Launcher and Nils notes that the whole set up has been stable digesting 54,000 messages over the last 24 hours.
AERO is essentially the satellite based version of aircraft ACARS. AERO's L-band signals contains short ground to air messages with things like weather reports and flight plans. The C-band signals are the air to ground portion of AERO and more difficult to receive as they require an LNB and large dish. However they are much more interesting as they contain flight position data, like ADS-B.
Over on YouTube Tomasz Haddad has uploaded a video of C-band AERO being received from the Inmarsat 3 F2 (Atlantic Ocean Region – East (AOR-E) 15W satellite. He uses a 1.80m motorized satellite dish with Kaonsat KS-N201G C-band LNB, a Prof 7301 PCI satellite card (to power the LNB) and an RTL-SDR V3. The C-band LNB translates the high C-band frequencies down to L-band which is receivable with an RTL-SDR. He notes that the LNB drifts quite a lot as it is not frequency stabilized.
With the signals received by his setup he's able to use the JAERO decoding software together with Virtual Radar Server to plot aircraft positional data using Virtual Radar Server. The plotted aircraft are mostly all in the middle of the ocean or in remote areas, which is where C-band AERO is normally used due to the lack of ground ADS-B stations.
Inmarsat 3 F2 15W C Band AERO Reception Using Jaero And Virtual Radar
In the past the Outernet project operated on L-band frequencies, and for the service they manufactured a number of active L-band active ceramic patch antennas for use with RTL-SDR dongles. Outernet has since moved on to faster Ku-band delivery, and hence their old L-band antennas can no longer be used for their service. There are a few of these patch antennas left over in Outernet's stock and they are currently selling them on eBay for US $29 + shipping.
Although no longer useful for Outernet, these antennas are still very useful for receiving other L-band services such as STD-C SafetyNET and AERO. SafetyNET is a text broadcast intended for sailors at sea, but contains many interesting and potentially useful messages for others too. Often they transmit data like military sea live firing warnings, reports of marine pirate activity, search and rescue reports, scientific vessel reports as well as weather reports. AERO is the satellite version of ACARS, and is used by aircraft to communicate with text messages to and from ground stations. L-Band AERO signals only contain information from the ground station up to the aircraft. For air to ground you'll need a C-band receiver set up. AERO is the satellite communications protocol that was so heavily centered on during the MH370 flight disappearance investigation.
In the past we've reviewed the Outernet L-band ceramic patch and found it to work very well. Certainly STD-C and AERO signals are easy to receive with the antenna if you point it at the satellite. The antenna requires bias tee power and can easily be used in combination with the bias tee on our RTL-SDR V3 dongles. The onboard filter helps reduce problems from interfering signals, but restricts reception to 1525 - 1559 MHz, so Iridium signals cannot be received with this antenna.
Thanks to Manuel a.k.a. Tysonpower for submitting his latest YouTube video tutorial about building an 1550 MHz L-band LHCP helical antenna for receiving satellite signals such as Inmarsat, AERO and HRPT.
Manuel's design is based on a 3D printed part which is used to accurately form the helical winding. The winding then mounts onto an aluminum plate and a satellite dish arm using a custom 3D printed adapter for the dish arm. In the video he uses the helical feed with an 80cm satellite dish and a standard 40mm LNB mount on the dish arm. Attached to the feed are two LNAs in series which help to lower the noise figure and reduce losses in the coax cable.
With this setup he writes that he was able to get very good AERO and Outernet reception from Alphasat (25E geostationary). He also writes that he's had good results using it for HRPT reception as well.
The 3D printing STL files and list of parts required are available on Thingiverse, and the companion video is shown below. Note that the video is narrated in German, but English subtitles are available.
[EN subs] LHCP Helix L-Band Feed - 3D Druck für eine genaue Helix
Recently the Outernet project transitioned from using RTL-SDR dongles and C.H.I.P single board computers to using their Dreamcatcher board, which is an RTL-SDR and computing board all in one. In between the transition they also produced a number of ‘SDRx’ dongles. These were custom RTL-SDR dongles with a built in L-band LNA and filter. As they no longer need the SDRx they have them on clearance at their store.
The clearance price is $15 USD which is an excellent deal. Remember though, that the SDRx is limited in frequency range – it is designed for receiving L-band satellites between 1525 – 1559 MHz and the filter will cut off all other frequencies.
Just add a simple L-band tuned antenna to the port and you should be able to receive Inmarsat and a signal like STD-C, AERO or the Outernet signal. A suitable antenna might be a homebrew patch, helix, cooking pot antenna or even a small tuned V-dipole antenna can work for the stronger AERO signals.
We also see that the price of their L-band Outernet active ceramic patch antenna has been dropped down slightly to $25 USD. This antenna is bias tee powered and can be used with a V3 dongle or their Dreamcatcher hardware. The Dreamcatcher itself is also now reduced in price to $59 USD.
We have a review of the Dreamcatcher and active ceramic patch antenna available here.
We also now list Outernet products in our store. These are commission sales so we receive a little bit per purchase which supports the blog, and the items are shipped by Outernet within the USA.
If you were unaware, Outernet is a free L-band based satellite service that provides content such as news, weather data, APRS repeats and more. Currently you can get about 20MB of data a day. Outernet receivers are also all based around the RTL-SDR, allowing for very cheap receivers to be built
Over on his YouTube channel Adam 9A4QV has uploaded a new video showing reception of L-band signals with a bias tee powered LNA4ALL and a small patch antenna. The video seems to show a new miniature bias tee powered LNA4ALL device that Adam might be working on. The LNA4ALL is a low noise amplifier that works well with our bias tee capable RTL-SDR dongles.
The patch antenna is made out of a single piece of PCB board which was made by etching out the patch pattern with masking tape. While the patch antenna is not optimal, and tested indoors, Adam is still able to receive some AERO signals.
Later in the video he compares the PCB patch against a GPS patch antenna which gets no reception. He also compares the results when two LNA4ALL’s are used in series. Using two LNA’s improves reception slightly.