A slight modification..
.. might help discourage hacking this system.
The interference pattern collected presumably depends on the frequency of the source, and if I understand the intent, they currently propose / use a constant RF frequency.
That being the case, then perhaps a long-period pseudo-random sequence producing a (possibly very wide ) range of radio frequencies could produce a series of interferograms that would be quite hard to predict / spoof if you didn't know the sequence of RF frequencies.
Provided you stay away from the worst water / atmospheric absorbers in the RF spectrum - tens of GHz up to THz I suppose - , you might also gain insights into the surface materials in the area being monitored - the drums / structure/ marker antennae will all have an RF absorption spectrum that will affect the interferogram for each frequency.
That might well make tampering / substitution just a bit harder.
I'm sure there are lots more variations on this theme; an interesting and unusual piece of work by the folks at the MPI.