American universities investigated very early on the possibility of operating the radar principle passively!
Anyone who has ever seen his TV picture with a shadow or even doubled up in a very unfavorable reception position with poor TV reception may have unconsciously already seen the principle of passive detection. The receiver receives both the direct signal and, slightly delayed, a reflected signal. From the difference in propagation time and the known positions of the radio transmitter and receiver, information about the position of the reflecting object can be calculated.
Unfortunately, at that time the computing technology was not yet so advanced that a functioning passive bistatic radar
could be developed from this ingenious principle.
So this technical solution has almost fallen into oblivion again.
But not completely:
“Ramona”, “Tamara”, and “Vera”
The Czech company “TESLA” developed the passive radar “Ramona” and its successor version “Tamara” MCS-93 already in the eighties under the greatest secrecy. Several devices were also in the inventory of the former NVA of the GDR. Therefore some “Tamara” could be taken over into the Army of the united Germany. Rumour has it that even two of them are still in operation, the rest or at least parts of them are lying around somewhere unrecognized as museum scrap.
“Tamara” is a purely passive reconnaissance device and was further developed to “Vera” by Czech companies. The working principle is quite simple: Every modern airplane sends out typical spectra of electromagnetic waves, which are stored like a “fingerprint” in a database in a powerful computer. This spectrum is received from three different locations and by measuring and comparing the different propagation times, the current location of the aircraft is calculated. Likewise, the aircraft can be illuminated by foreign radar equipment or radio and television stations. These reflections can also be received and improve the accuracy of the location.
Is it radar or not?
“Vera” is not a real radar. However, the time of flight measurement which is so characteristic of radar sets is also used here. For secondary radar sets, this term is also used because of this transit time measurement and especially because the secondary radar set can locate airplanes. For the rotating beacon TACAN, for example, the term RADAR is not used, although it also uses the run-time measurement, Tacan as a navigation device does not locate airplanes.
The term “RADAR” can be applied to “Vera”, even if “Vera” itself does not need any transmitters. “Vera” locates airplanes also with the help of the run time measurement of electromagnetic waves! (The company claims to be able to measure time delays with an accuracy of 100 ns, and this in a frequency range of optionally 0.1 to 40 GHz!