On Wednesday night, we had some of the biggest ground stations in the world listening in on Mars Express, along with collaborative stations from the JIVE network. The professionals weren't the only people listening to our signal though - amateur radio operator Bertrand Pinel (F5PL) from near Castelnaudary, France, used his 3.5m dish throughout the flyby to listen to the signal from Mars Express.
Bertrand sent us a recording from his dish of the tone of the Mars Express signal, exactly what we would have heard if our ears were sensitive to the X-Band radio used by Mars Express! Hidden in this tone are tiny variations caused by the presence of Phobos and the MEX scientists are working hard right now to extract these from the recordings from the professional stations (Thanks, NASA!), which have the listening power necessary to detect these very fine variations.
Listen here to the whistle of Mars Express as it hurtles towards Phobos and consider that this signal was generated on Mars Express by a transmitter only slightly more powerful than a light bulb. This tiny signal travelled over 100 million kilometres and was picked up by Bertrand with his 3.5m dish - a truly remarkable achievement! Take a look after the jump for more details on Bertrand's station and the recording he made. -- Thomas
The recording of the tone of Mars Express shifts from high to low because, at the time of the recording, Mars Express was travelling away from the Earth with a speed of 39 000 km/hr - accelerating as it went - causing the change in tone. This is due to the Doppler effect and is exactly the same reason the sound of an ambulance siren changes as it passes you.
In the graph above, there is a sharp zig-zag at the start of the recording (the time starts at the bottom and goes to the top) - this is the effect of Mars Express catching onto the "sweep" of NASA's 70m ground station. This is a technique where the station on Earth varies its frequency to find one where Mars Express is listening. Then the Mars Express transmitter 'follows' this frequency from the ground, which is a much more stable frequency source than the one on board the spacecraft. This extreme stability is necessary to find the minute variations in the signal due to Phobos.
In the audio, the pitch of the Mars Express tone sometimes jumps from low to high: this is caused by Bertrand shifting the receiver to stay locked on the Mars Express frequency as it drifts due to the Doppler effect - exactly like re-tuning a station on your radio at home.
An 'Amateur' Deep Space Station
Bertrand's station in France (see him with it in this picture) was built through his own work and investment - but the equipment he operates is similar to the set-up of ESA's professional ESTRACK ground stations. There is community of dedicated amateurs around the world that build such stations to listen in on spacecraft at the farthest reaches of our Solar System. While these dishes aren't big enough to catch data from the missions, they can hear the faint tone of a spacecraft transmitting far away from Earth.
Bertrand's set-up is typical of one of these stations; he wrote to us some technical details of his station:
"The antenna is the key to success, along with much work and many hours to improve the signal quality!"
"The 3.5m dish is from an old CNES ground station near Toulouse that was decommissioned many years ago, it used to provide a 12Ghz satellite link for France Telecom. Much of the equipment attached to it is from discarded old hardware, such as an HP network analyzer and spectrum analyzer and 2 Rubidium atomic clocks."
As well as this Bertrand has all the equipment we are used to seeing on the ESTRACK ground stations, including a corrugated feed horn, polarizer and low-noise amplifier. All of this has been built, bought or given to Bertrand to make this station.
He's not the only one! In fact another amateur DSN member Wolfgang (DJ3QD), from Wöllstadt in Germany, also listened in on Mars Express with his 1.75m dish, but unfortunately lost the signal as we approached Phobos. For more details on amateur DSN activities and successes, check out their website: UHF-Satcom Amateur DSN.
The "Control Room" of Bertrand Pinel's 3.5m station during an earlier tracking of Mars Express.