During Phobos flyby on 29 December, @AsteroidEnergy asked a question via Twitter:
— Asteroid Initiatives (@AsteroidEnergy) December 30, 2013
It took some time (due to holidays – Happy New Year!!!), but we now have a reply from ESA’s Olivier Witasse, MEX Project Scientist at ESTEC.
The radar was originally designed solely for the observation of Mars. For safety reasons, the radar software blocks operations when the target is closer than 240 km. In the case of past Phobos flybys, because the distance is sometimes lower than that, the radar was therefore re-configured to operate at close distance by bypassing protections preventing the opening of the receiver before a certain time from transmission had elapsed.
Although not without risk, this procedure was thoroughly tested and successfully used throughout several Phobos flybys. The operational distance is now *** 175 km ***. Therefore, we usually switch on MARSIS when the minimum flyby distance is around 175-200 km.
See you in 2014!