vrijdag 22 februari 2013

Listening to satellites

The sound of satellites can not be heard by our ears. However using complicated (or less complicated) electronics, we are capable of converting the signals into sound and pictures. How does the sound of a satellite looks like?

I think it is beautiful. A perfect frequency shift, due to the motion of the satellite with respect to the receiving station. This figure is in the time-frequency domain, after I did a Fourier transformation on the .wav signal. Time runs from bottom to top and the frequency scale is inverted, just to make life easy.

We did not know if the satellite would still be alive when we set up the equipment. But when it crossed the horizon, we heard the signal emerge from the noise. There it was, a piece of space junk that was launched a few years ago, still transmitting its daily data. A coded beeping sound, sending data about the status of its tiny solar cells and other components.

We tried to tune another radio such that we could actually listen to the signal:


Everyone who sees this movie comments that it sounds like the old dial-up tone in the early days of the internet. My girlfriend calls it noise..., but beautiful noise... This beebing sound is the satellite trying to downlink its data. You can clearly notice the Doppler effect, the drop of the frequency. Just like an ambulance. You experience this by hearing the sound becomes lower and lower. Halfway the short-movie we shift the radio such that the signal has a high pitch sound. We did this because the sound became so low, it was not hearable anymore.

Some students are now trying to filter and process this data. They will convert this data into range-rate observations. Range-rate means the relative velocity of the satellite with respect to the ground station. Think about it, just by listening to the signal of satellites, you are able to tell where it is and how fast the satellite is orbiting around Earth.

I like science...