Outernet is a satellite based file delivery service. Currently they’re beta testing their service and they are using RTL-SDR’s as the receiver. In previous posts we’ve seen that they’re now regularly transmitting weather updates, wikipedia files and more files like images and books. Over time the service is becoming more and more useful. If you’re interested in receiving their service we have a tutorial available here.
While most of the Outernet software is open sourced, the signal protocol itself is closed source, which ties you into needing to use the official Outernet software. Over on his blog, Daniel Estévez has been working on reverse engineering the Outernet signal with the goal of publishing the results and building a fully open source receiver.
So far he’s managed to fully reverse engineer the modulation, coding and framing. He’s also been able to build a GNU Radio program that receives the Outernet frames and a Python program called free-outernet which does the decoding. His post goes into greater details on how he reverse engineered the signal and what his finding are.
Outernet is a new L-band satellite services which aims to be a “library in the sky”. Their satellite signal can be received from almost anywhere in the world, and they aim to constantly transmit data like news, weather updates, books, images/videos and other data files. The service is free and can be received with an RTL-SDR, LNA and patch antenna. We have a full tutorial on receiving their service available here.
The “rxOS” decoder, file management system and web interface GUI has recently been updated to version 3.0. This new version has several new features:
Downloaded files are automatically decompressed after downloading, so they can be viewed directly in the Outernet web interface.
An hourly transmission of APRS data which comes from the repeater on board the international space station. APRS messages can now be relayed across the world via the ISS and Outernet.
This Monday they will begin transmitting NOAA weather data (we are unsure if this entails images or text data yet)
Soon they should begin transmitting news data too.
More details on the update can be found on their forum post. To update the service on a CHIP or Pi 3, download the .pkg file from the links on the forum and choose this file in the Update Firmware section of the Outernet settings menu.
An example of some received APRS messages from the Outernet.APRS messages
At this year’s hacker themed Eleventh Hope conference, Stefan “Sec” Zehl and Schneider gave a talk which discusses their latest work on decoding data from Iridium satellites using SDR’s. Iridium is a truly global satellite service which provides various services such as global paging, satellite phones, tracking and fleet management services, as well as services for emergency, aircraft, maritime and covert operations too. There are currently 72 operational satellites operating.
In their talk they discuss how Iridium security is moderate to relaxed, pointing out that Iridium claims that the majority of ‘security’ comes from the complexity of the system, rather than actual security implementations. They then go on to discuss how the Iridium system works, how to receive it with an RTL-SDR or HackRF/Rad1o, how the gr-iridium decoder implementation works, and how to use it to actually decode the data. Later in the presentation they show some interesting examples such as an intercepted Iridium satellite phone call to a C-37 aircraft.
A few days ago we posted a review on the Outernet LNA which can can be used to help receive their new L-band service signal. Their LNA uses a filter which restricts the frequency range from 1525 – 1559 MHz as this is the range in which the Outernet signals are located.
Additional Note Regarding the Downconverter: Also, it appears that the Outernet downconverter prototype that we posted about back in May has unfortunately been discontinued indefinitely and will not enter mass production. For now the LNA is the best option for receiving their signal.
Back in April we posted about Philip Hahn and Paul Breed’s experiments to use an RTL-SDR for GPS logging on their high powered small rockets. As GPS is owned by the US military, a standard GPS module cannot be used on a rocket like this, as they are designed to fail if the GPS device breaches the COCOM limit, which is when it calculates that it is moving faster than 1,900 kmph/1,200 mph and/or higher than 18,000 m/59,000 ft. The idea is that this makes it harder for GPS to be used in non-USA or home made intercontinental missiles. As SDR GPS decoders are usually programmed in open source software, there is no need for the programmers to add in these artificial limits.
In their last tests they managed to gather lots of GPS data with an RTL-SDR, but were only able to decode a small amount of it with the GNSS-SDR software. In this post Philip discoversa flaw in the way the GNSS-SDR performs acquisition and retrackingthat GNSS-SDR decodes in such a way that makes it difficult to obtain a location solution with noisy high-acceleration data. By using a different GPS implementation coded in MATLAB, he was able to get decoded GPS data from almost the entire ascent up until the parachutes deploy. Once the parachutes deploy the GPS has a tough time keeping a lock as it sways around. His post clearly explains the differences in the way the code is implemented in GNSS-SDR and in the MATLAB solution and shows why the GNSS-SDR implementation may not be suitable for high powered rockets.
In addition, they write that while the flight was just under the artificial COCOM GPS fail limits for speed and height, the commercial GPS solution they also had on board failed to collect data for most of the flight too. With the raw GPS data from the RTL-SDR + some smart processing of it, they were able to decode GPS data where the commercial solution failed.
GPS data acquired from the RTL-SDR on the rocket (blue line shows solution from MATLAB code, yellow shows GNSS-SDR solution, and red shows commercial GPS receiver solution).
Over the last few weeks Adam 9A4QV has been testing L-Band Inmarsat reception with his LNA4ALL low noise amplifiers. In a previous post he tested reception with two LNA4ALL and found that he got an improved SNR ratio over using just one LNA4ALL. In his latest video he tests Inmarsat reception with three LNA4ALL’s and two L-band filters. His results show that the SNR is improved over using two LNA4ALL’s, and can almost match the results obtained by a commercial L-band front end which he also demonstrated in a previous video.
Outernet are a startup company that hope to revolutionize the way people in regions with no, poor or censored internet connectivity receive information. Their service is downlink only, and runs on C and L-band satellite signals, beaming up to date news as well as other information like books, educational videos and files daily. To receive it you will need one of their official or homemade versions of the Lighthouse or Lantern receivers (the latter of which is still to be released), or an RTL-SDR or similar SDR. Recently they began test broadcasts of their new 5 kHz 1539.8725 MHz L-band signal on Inmarsat I4F3 located at 98W (covers the Americas), and they hope to begin broadcasts in more regions soon too.
The typical RTL-SDR is known to often have poor or failing performance above 1.5 GHz (though this can be fixed to some extent), so Outernet have been working on an L-band downconverter. A downconverter works by receiving signals, and shifting them down to a lower frequency. This is advantageous because the RTL-SDR is more sensitive and does not fail at lower frequencies, and if used close to the antenna, the lower frequency allows longer runs of cheap coax cable to be used without significant signal loss.
Earlier this week we received in the mail a prototype of their downconverter. The downconverter uses a 1.750 GHz LO signal, so any signal input into it will be subtracted from this frequency. For example the STD-C frequency of 1.541450 GHz will be reduced to 1750 MHz – 1541.450 MHz = 208.55 MHz. This also means that the spectrum will appear reversed, but this can be corrected by selecting “Swap I & Q” in SDR#. The downconverter also amplifies the signal with an LNA, and has a filter to remove interfering out of band signals.
The prototype Outernet downconverter circuit board.Specsheet for the downconverter.
We tested the downconverter using their patch antenna which they had sent to us at an earlier date (the patch antenna is used and shown in this Inmarsat STD-C reception tutorial). Our testing found that overall the downconverter works extremely well, giving us much better signal levels. Previously, we had used the patch + LNA4ALL and were able to get reception good enough to decode STD-C and AERO signals, but with the requirement that the patch be carefully pointed at the satellite for maximum signal. With the downconverter the signals come in much stronger, and accurate pointing of the patch is no longer required to get a signal strong enough to decode STD-C or AERO.
The downconverter can be powered by a bias tee connection, and this works well with our bias tee enabled RTL-SDR dongles. We also tested with the bias tee on the Airspy R2 and Mini and had no problems. It can also be powered with a direct 5V connection to a header, and they note that the header will be replaced by a USB connector in the production version.
The release date and exact price that these will be sold at is not confirmed, but we believe that it will be priced similarly to upconverters at around $50 USD or less. A good low cost downconverter should help RTL-SDR and other SDR users receive not only the Outernet signal better, but also other satellite signals such as STD-C and AERO. Although the input is filtered and the RF frequency is specified at 1525 to 1559 MHz, we had no trouble receiving signals up to GPS frequencies of 1575 MHz, and even up to Iridium signals at 1.626 GHz, though reception was much weaker up that high.
Below are some screenshots of reception. Here we used the Outernet patch antenna sitting in a windowsill with the downconverter directly after the antenna, and then 10 meters of RG6 coax cable to the PC and bias tee enabled RTL-SDR. We found that with the downconverted ~200 MHz signal the loss in the RG6 coax was negligible. Better reception could be obtained by putting the patch outdoors. In some screenshots we used Vasilli’s R820T driver with the decimation feature, which allows you to zoom into narrowband signals much more clearly.
Some AERO Signals Zoomed in with the Decimation feature in SDR#. Received with the Outernet downconverter and patch antenna.Some AERO and other Signals Zoomed in with the Decimation feature in SDR#. Received with the Outernet downconverter and patch antenna.Signals zoomed out. Received with the Outernet downconverter and patch antenna.
To show that a specialized antenna is not required to receive L-band Inmarsat AERO satellite signals, YouTube user SkyWatcher has uploaded a video showing how he was able to receive these signals with a cheap DVB-T antenna. SkyWatcher writes:
I’ve recently upgraded from my RTL-SDR sticks (E4000, R820T2) to an Airspy Mini.
I did some testing during the last week and found it very interesting that I was able to receive Inmarsat L-Band signals indoors, with just a DVB-T antenna and amplifier behind the window, no downconverter, no special antenna, no super low-noise amplifier. The window is facing south, with a few degrees to the east and the satellite I’ve received was Inmarsat 15.43W. So, angle antenna to satellite should be estimated 20 degrees.
I’ve used a 18dB DVB-T/Satellite-TV inline amplifier as a ‘LNA’ (noise < 5dB) and a VHF/UHF DVB-T antenna which seems to be a stacked dipole, and therefore should be quite wideband and should make a reasonable general purpose antenna. Anyway, I did not expect it to work on 1.5GHZ at all. Also, I want to mention that the inline amplifier is rated 5 to 18V, but it works just fine with the 4.5V from the Airspy Mini.
It seems that with 10dB S/N, Aero reception is possible and with about 12dB S/N, it is getting reliable.
In general, I am very satisfied with the upgrade to the Airspy Mini. It has a much lower noisfloor and a much cleaner spectrum, compared to my old RTL SDRs. Also, I am very happy with the CPU-usage which is only about 12% on my i5-3210M when using 2.4MHz bandwith, and 18-20% with a bandwith of 4.8MHz.
Together with the ability to use SpectrumSpy and the very useful decimation-feature, the Airspy Mini is the best option to upgrade from a RTL-SDR for me at the moment. Anyway, of course this is just my very personal opinion… 😉
AERO is essentially the satellite based version of ACARS, and the L-band signals contains short ground to air messages with things like weather reports and flight plans intended to be transmitted to aircraft. To decode it with an SDR, the JAERO software can be used.
