Hello,

I have a basic idea of the Doppler effect/Doppler shift between a source (emitting electromagnetic waves) and a receiver (measuring the certain. However, as I look up some reference, I find that most books on this subject seem to discuss only two scenarios:

1)the source is moving and the receiver is stationary;

2)the source is stationary and the receiver is moving.

(Note: let "stationary" and "moving" be with respect to the ground/Earth), and I know that there are two different formulas, one for each of the above two scenarios (although if the relative velocity between the two nodes is much smaller compared with the speed of light c, both formulas yield approximately the same result). What about the scenario where both the source and receiver are moving? What would be the Doppler shift then? Somehow, I haven't been able to find that in the literature that I've read so far.

Also, in the industry, I'm wondering if anyone can provide me with some information about how accurately measuring the velocity of, say a car, can be achieved by the idea of Doppler shift using today's technology. For example, when two vehicles are 50[m] apart and the relative speed is 5[m/s], would today's technology be able to achieve a precision of less than 1% of error (i.e., 4.95--5.05[m/s])? Does anyone have any knowledge about that?

Thanks.