Last week we posted how programmer Jonti had successfully implemented a C-Band AERO decoder into his JAERO software. C-band AERO signals are the earth downlink portion of AERO. Planes transmit data upwards towards the satellites and then the Inmarsat C-band transmitter re-transmits the information back to a basestation on earth. This is different to the L-band AERO signals which are signals transmitted from the satellites to the aircraft. C-band signals are interesting because they contain plane position info, and so can be used to track aircraft much like what is done with ADS-B reception, but over a much larger area. However, C-Band signals are much more difficult to receive as they are at 3.616 GHz and require a 1.8m or larger satellite dish.
Over on YouTube user AceBlaggard has uploaded a video showing an example of C-Band signals being received with an Airspy SDR and being decoded with the new version of JAERO. About the hardware used AceBlaggard writes:
Hardware is a 1.8M PF dish and Titanium Satellite C1 PLL LNB feeding a Prof-Tuner 7301 sat card which loops out to an Airspy SDR.
The two most important things to pay attention to when receiving AERO signals are signal SNR and frequency stability. In order to lock on to the signal, the signal’s frequency must remain relatively stable over a short period of time. For the stability test Jonti writes the following, referencing the image posted below:
You can see the old RTL dongle moves almost 3kHz within a couple minutes after being turned on, this speed is so rapid that JAERO can’t keep up with the frequency changed during this period of time. What’s odd is the old RTL dongle does some fairly crazy stuff around 20 minutes in that lasts for about 15 minutes, JAERO also can’t cope with some of that. The other thing to notice in the old RTL’s spectrograph are vertical lines, these lines I believe are caused by interference entering the dongle between the RTL dongle’s tuner and ADC (analog-to-digital converter).
The frequency stability of the new RTL dongle can only be described as amazing!!! There is not much more than 100 Hz change during the whole test.
The range of frequencies for the SDRPlay is similar to that of the old RTL dongle of about 3kHz. The difference being the transition from the lowest frequency to the highest frequency is slow. Any demodulator should not have any issue tracking this slow and steady change. The only problem you will encounter here is when you are trying to tune into a particular frequency your frequencies will be slightly different depending on the temperature of the SDRPlay.
Jonti also found that in terms of sensitivity the SDRplay was the best at receiving when a non active antenna (an active antenna is an antenna with a built in LNA) was used. The RTL-SDR dongles could not receive well at all when a non active antenna was used. When an active GPS antenna was used the SDRplay was only about 1dB more sensitive than the RTL-SDR dongles.
In his article Jonti expressed concern that the SDRplay did not see much improvement in SNR over the RTL-SDRs when an active antenna was used. Our thoughts on the sensitivity findings are that the SDRplay does not see much improvement with an active antenna because the noise figure of the system is not reduced any further by adding an additional front end LNA (the noise figure in a RF system is almost entirely determined by the first LNA in a RF chain). Adding an extra LNA could even potentially make reception worse by reducing the overall linearity of the system. An external LNA would only be beneficial if a long run of coax was used between the feed and SDR, and in Jonti’s connections he connected the feed and SDRplay with a very short cable. The RTL-SDR only works well with an active antenna because its raw sensitivity at 1.5 GHz isn’t great, and it needs the extra boost from the LNA.
The JAERO decoder for AERO signals on Inmarsat satellites has recently been updated to version 1.03. This new version supports the decoding of 10.5k Aero-H and Aero-H+ signals. The author of JAERO Jonti writes that on these channels he’s seeing significantly more traffic than on the narrowband signals and that he was suprised to see that other non-aircraft messages such news was broadcast on this 10.5k signal. Jonti writes about his experience in developing the 10.5k decoder and his experience with receiving the messages in this post.
Back in August of this year we showed how it was possible to use an RTL-SDR dongle, satellite antenna, LNA and decoding software to receive and decode STD-C EGC signals from Inmarsat satellites. We also showed how it was possible to modify a low cost GPS antenna to use as a satellite antenna.
JAERO is a program that demodulates and decodes Classic Aero ACARS (Aircraft Communications Addressing and Reporting System) messages sent from satellites to Aeroplanes (SatCom ACARS) commonly used when Aeroplanes are beyond VHF range. Demodulation is performed using the soundcard. Such signals are typically around 1.5Ghz and can be received with a simple low gain antenna that can be home brewed in a few hours in conjunction with a cheap RTL-SDR dongle.
In the advent of MH370, Classic Aero has become a well-known name. A quick search on the net using “Classic Aero MH370” will produce thousands of results. The Classic Aero signals sent from satellites to the Aeroplanes are what JAERO demodulates and decodes.
Unlike the usual VHF ACARS, with SatCom ACARS you can not receive signals from the Aeroplane only the people on the ground talking to the people in the Aeroplane. This means you do not get the airplanes reporting their position. Instead you tend to get weather reports, flight plans, and that sort of stuff. Just like VHF ACARS they usually use cryptic shorthand notation. For example “METAR YSSY 040400Z 08012KT 9999 FEW040 SCT048 23/09 Q1024 FM0500 05012KT CAVOK=” is the weather report for Sydney Airport in Australia in a format called METAR. It tells you the time, when the report was issued, the wind direction and speed, visibility, clouds, temperature, due point and air pressure. Then it says from 5 AM UTC the wind direction and speed and that the weather will be nice. There are sites such as Flight Utilities that can decode such information and display it in a more understandable format.
In his post Jonti also shows how he uses a modified GPS antenna to receive the AERO signals.
We gave JAERO a test and found that it decoded AERO signals easily, even with low signal strength. To use JAERO tune to an Inmarsat AERO signal in SDR# or a similar program using USB mode. JAERO will listen to the audio from the sound card or from a virtual audio pipe. We recommend setting the AFC (Automatic Frequency Control) setting on on if you find that your RTL-SDR drifts too much.
AERO signals can be found at around 1545 MHz. They only use about 800 Hz in bandwidth. See UHF satcoms page for a list of AERO frequencies.
Remember that some R820T/2 RTL-SDR dongles can have problems when receiving this high, especially when they heat up. If you find that your dongle gets deaf at these L-band frequencies try cooling the R820T/2 chip with a heatsink or fan. The Airspy or SDRplay RSP software defined radios are better choices for decoding signals this high, but the RTL-SDR will work fine if your signal strength is decent and the R820T/2 chip is kept cool.
If you are interested in VHF ACARS as well, then we have a tutorial about decoding that here.