Programmer Andres has recently been working on creating a toolset for receiving AX.25 packets (FSK 9600) from satellites with an RTL-SDR or other software defined radio. The AX.25 protocol is commonly used for APRS packet radio or telemetry in amateur radio satellites. Andres’ programs focus on using a true UNIX philosophy of piping data between different programs. The toolset consists of doppler correction and demodulation tools and the piping philosophy is demonstrated in the following example:
rtl_sdr | doppler | demod | multimon-ng
rtl_sdr receives raw IQ data from satellites which is then piped to “doppler” which corrects doppler offset. Zero centered baseband signal is piped to “demod” which outputs demodulated audio suitable for multimon-ng to do actual AX.25 packet decoding.
Such pipeline is intended for resource constrained embedded platforms like RaspberryPi or BeagleBoneBlack where running full blown SDR software would be too much.
The doppler corrector tool works by using the same libraries for calculating satellite positions as those used in Gpredict and the demod tool uses the liquid-dsp library to demodulate the IQ stream.
More information about Andres’ project can be found in these three blog posts that he has written.
Jan first writes how the Funcube Dongle was originally invented as a low cost means of receiving the FUNcube satellite, but now there are the even lower cost RTL-SDR dongles. Jan’s post then goes over how to receive the FUNcube at a frequency of 145.935 MHz using software such as SDR-Radio or SDR# and how to decode the telemetry data using the FUNcube dashboard. He also explains a bit about the FUNcubes operating modes which change the satellites transmission strength depending whether or not its solar panels are in sunlight or not.
Marco, a reader of RTL-SDR.com and user of our sister site sigidwiki.com has been developing some Windows software to display the sigidwiki.com database in an easier to access format. The software is called Artemis and can be downloaded from http://markslab.tk/project-artemis/.
Artemis allows for various example signals to be quickly viewed with the corresponding example waterfall image, frequency, bandwidth and other information. There is also a filtering function that allows you to search by frequency and type of signal.
Marco writes that he would love to hear any user requests for new features such as more filters, improvements, or anything else as well as any bug reports. We also note that data such as frequencies and bandwidths provided in the sigidwiki.com database may not yet be 100% correct since the wiki is relatively new and is yet to mature.
During several hacker and security themed conferences (Shmoocon, Bsides, Derbycon, Defcon, etc) organizers from Wireless Village have been setting up competitive WiFi and SDR themed capture the flag (CTF) games. In the competition the organizers broadcast a signal and the competitors are required to complete various tasks (capturing flags) such as determining the centre frequency of the transmission, demodulating the signal and finding any meta data contained within the signal such as codec flags in DVB-T signals and RDS data in FM signals. The team which captures the most flags wins a prize. The process of capturing flags often requires the use of some sort of software defined radio like the RTL-SDR, HackRF or BladeRF.
Recently, Russell one of the CTF organizers wrote in to let us know about a SDR CTF training resource that he has put together. The site contains various exercises/tutorials that allow participants to practice the skills needed to compete in the competition. Most exercises involve using a Raspberry Pi together with PiFM for transmitting a simulated competition signal, then receiving and demodulating the signal with a SDR. The exercises include running rtl_power, setting the PPM offset, decoding morse code, AFSK, RDS, ASK/OOK, DVB-T, POCSAG, MotoTRBO, SSTV and decoding numbers stations.
Happysat, a reader of RTL-SDR.com has written in to remind us that the International Space Station (ISS) is currently transmitting slow scan television (SSTV) images out of respect of the 80th birthday of Russian cosmonaut and first man to go to space Yuri Gagarin. The images will be transmitted continuously until 24 February 21.30 UTC.
SSTV is a type of radio protocol that is used to transmit low resolution images over radio. A RTL-SDR dongle and satellite antenna (QFH, turnstile, even terrestrial antennas like random wire antennas and monopoles have been reported to work) can be used to receive and decode these images. Happysat writes that it is expected that the ISS will continuously transmit 12 images at a frequency of 145.800 MHz FM using the SSTV mode PD180, with 3 minute off periods between each image.
The Radio Spectrum Processor (RSP) by SDRplay is a receive only software defined radio with a 100 kHz to 2 GHz range (with a small gap at 380 MHz to 430 MHz), a 12-bit analogue to digital converter (ADC) (~10.4 ENOB), 8 MHz bandwidth and a bank of several switched front end filters. It currently costs $299 USD and with these specs and price range we consider the RSP to be a competitor to the Airpsy and Funcube Dongle software defined radio offerings.
In a previous post we talked about the SatNOGS project which aims to provide low cost satellite ground stations (where one critical component is currently an RTL-SDR dongle) along with free networking software in order to create a crowd sourced satellite coverage network. The SatNOGS project was also recently the grand prize winner of the Hackaday prize which saw them take almost $200k US dollars of prize money.
The new versions fixes some minor errors, brings back their ‘spectrum viewer’ software and also comes with a ‘DAB mini’ receiver which is simply a smaller windowed version of the regular DAB receiver. The new version also now supports the sdrplay and Airspy software defined radios.
The goal of Ilias’ project was to be able to use the RTL-SDR and MATLAB to uncover the details of a 433 MHz transmitter he bought on Ebay. He wanted to see if he could determine the protocol and recover the data before even looking at the transmitter’s library code.
To do this he first used SDR# to record the data sent at 433 MHz. Then by looking at the waveform in the Audacity audio editor he was able to determine that the signal was on-off-key (OOK) modulated and from this knowledge he was able to manually recover the binary string. Next he used MATLAB to create a program that can automatically decode the received OOK signal. His post goes into further detail about the signal processing steps he took in MATLAB.
In the article Jan discusses the antennas required to receive satellites, the satellite tracking software gpredict and he introduces some amateur radio satellites that have strong transmitters and are thus easy to receive. He also shows waterfall screenshots of several amateur radio satellites that he has received.