Receiving Aalto-1 satellite with RTL-SDR

I recently bought an RTL-SDR dongle mostly to try to receive weather satellitse (see e.g. this guide for guide on how, and this nice software for more info), but also to experiment with receiving other kinds of radio communications.

While browsing the net for information about different satellites, antennas, etc I ended up to a Finnish page about receiving Aalto-1 satellite. The satellite is built by students at a Finnish Aalto university, hence the name. The satellite information is freely available and some parts of the communication is easy to receive.

The easiest part, the only part I got, is a morse code of the satellite callsign, AALTO1. It is transmitted every 3 minutes at 437.220 MHz frequency and it lasts for 6 seconds. The modulation is constant wave.

I used a simple V-dipole antenna which I mainly use for weather satellites at 137...138 MHz. I pointed it to south (since from there the satellite crosses the sky at my location) with as clear as possible view to sky. The antenna is most likely not optimal, but seemed to work somehow.

 

I used gpredict to see when the Aalto-1 satellite would cross the sky, but it is possible to track it online, too. When the time was, I tuned the RTL-SDR dongle to 437.220 MHz, and set it to record the baseband. About when the satellite was directly above, I got the above signal! (The vertical dashed line in the middle). Success! Looks very similar to the picture in the other blog, but not as clear due to lower signal level.

A bit later I also received next beacon with better signal quality, but unfortunately I was not recording at this time. Picture of the second beacon (beginning was already off the screen):

After recording was done, I reloaded the IQ wav file back to RTL-SDR using the FilePlayer plugin and searched for the signal. After finding it, I set CW modulation with 500 Hz bandwidth and centered the tuner to the signal. The signal frequency was a bit off, partly due to the low quality oscillator of the RTL-SDR dongle, partly due to doppler effect as the satellite flies so fast over the sky. I tuned about to the center of the beacon.

I played the demodulated signal to Audacity via a virtual audio cable. The signal indeed sounds like morse code an parsing the first 5 characters, they are .- .- .-.. - ---, which stands for AALTO. The last character in this recording is bad, but from the screenshot of second beacon, it can be barely seen that it is .---- which stands for 1.

Here is a link to the CW demodulated audio signal from the first recording. The doppler shift can clearly be heard in the signal!