Radars are used to detect the moving objects that are faraway from the radar. That is to say it teledetects the objects. It loos lie seeing the objects by the eye of an observer.These objects are normally in air and their case is made from metal. So, the best way to detect them is by using  radio waves. in order to resolve the objects geometrically one uses microwaves radiators which emits in a relatively narrow angles and therefore., the transmitter radiates microwave, which propagates till meeting the object where it bounces back to the radar station where the receiver reside. So, in the radar station both the transmitter and receiver are present side by side. By receiving the bounced wave one can measure the delay time between the transmitted and the revived reflected wave , from which one can measure the distance between the radar station and the object.

If one exploits the absorption of waves such as in NMR, one would put the receiver behind the object to measure the transmittance for teledetection this would be practically impossible. 

This may shed some light on answering your question.

Many thanks professor zekry, but NMR doesnt work just based on abosrbtion of energy, it actually works when one small nucleus start to resonate and radiate energy. I know that radar basically needs to compute time difference for ranging, but without time difference we still can allocate resonant objects by setting multiple separated receivers and computing the RSS ar each nodes. With some estimation process we can allocate the objects. With this approch we dont need a coherency between send and recieved signals to allocate objects.

If I understand your question correctly, the main challenge with NMR is the small amount of signal and the presence of interference and noise. In an MRI suite, we use a very strong magnet so that we can pick up quite small signals, and we do it in a shielded room so we can minimize noise and interference. 

Dear Behrad,

I would like that you devise a radar system based on NMR and then you can detect if it is feasible or not and under which circumstances could it be successful.

My stand point is that the incident wave from the radar source will be absorbed by the NMR. After this absorption it re-emits radioowave of the same frequency but this radiation is spontaneous and scattered in all space direction. Only avery small portion of it will propagate in the direction of the incident wave with no phase relationship to the original incident wave. In addition as Mohammed noted, it needs a strong permanent  magnetic field to affect the NMR. How would you form permenat magnetic field from so large distances. As i said before it may be very non practical as a distant radar method.

Best wishes