Greetings again to all Phobos and Mars aficionados!

Christian Andreas Doppler (29 November 1803 – 17 March 1853)

The closest-ever flyby of Phobos to date will be dedicated to an experimental method called 'radio sounding'. The way radio sounding works is that we place the object (think a celestial body) we want to investigate close to the trajectory of a moving vehicle (think a spacecraft) equipped with a very stable and precise radio transmitter. We also need a very sensitive receiver equipped with measurement devices to record the received signal (think ground stations). The transmitter  on the moving vehicle sends out a continuous unmodulated signal (meaning no actual data will be transmitted - just an 'empty' carrier signal). The receiver receives the signal and then sends that signal to the measurement equipment, which will record the famous Doppler shift of the received signal's frequency (access more details under the 'Full story' link below) -- Hannes.

As the transmitter on the moving vehicle moves past our object of interest, its velocity will be influenced by the object's gravity. This influence can be picked up by the measurement equipment on the receiving end as a minute variation in the Doppler shift as compared to an undisturbed trajectory. By calculating the expected Doppler shift and comparing this with that actually recorded, the effect due to gravity of the object on the moving vehicle can be isolated and interpreted.

The resulting velocity profile lets scientists compute the 'shape' of the gravitational field of the object, which in turn yields information about the interior mass distribution of the object. Combining this information with spectrometer readings, subsurface sounding radar data and visual images will provide clues to the object's origin and formation process.

The object of interest in this case is - you already guessed right - Phobos. Our moving vehicle is Mars Express - equipped with a precision transmitter - and on the receiving end we have the finest receiver that the world has to offer: NASA's mighty 70m deep space tracking station in Madrid (read Daniel's previous post here about DSN stations). To enhance the signal stability of the transmitter on board Mars Express, we will send an even more precise signal up to the spacecraft, which it locks on to it. To isolate other influences on the signal propagation, we will have Mars Express transmit in two different radio bands. So what we are doing with this closest-ever flyby is actually setting up an experiment that ranges across the inner Solar System and spans a whopping 117.5 million kilometres!

To do that, we have to manoeuvre the spacecraft into a position close to the moon Phobos - the closer the better - and record the spacecraft's signal starting from several hours before the encounter to several hours after. This is necessary to precisely know the trajectory before and after the encounter.

The first orbit manoeuvre to place Mars Express onto the correct trajectory was completed successfully a few days ago (see my earlier blog post). Two days ago, I finished the final planning for the second manoeuvre to be conducted next week and my Mars Express team mates in charge of implementing these are checking everything as I write these lines. Once my colleague who is working on the mission planning computer system right now lets me have it back, I will start preparing the final plans for the actual flyby and related events.

In preparation for closest approach, two proficiency tests have been scheduled for this coming Monday and Wednesday (22 & 24 February). These tests will give ground station personnel and spacecraft controllers the opportunity to rehearse the appropriate steps and settings that will have to be made to properly record the spacecraft's radio signal. We will, in effect, simulate the flyby as if it were for real using Mars Express, with the only exception being that Phobos will actually not pass by.

Stay tuned as we buckle up for the second orbit manoeuvre and get in shape to unlock yet another clue to the secret of Solar System formation! -- Hannes

PS: Excellent NASA article on Doppler
http://imagine.gsfc.nasa.gov/YBA/cyg-X1-mass/Doppler.html