What does "A Target with Zero Doppler" Mean? I have seen several references to target at zero Doppler and some simulations that command users to place a target at zero Doppler. I know that when a target is flying tangential to the signal the Doppler is zero (is this even true?) but I have no clues as to what the target being at zero doppler means.
In Pulse Doppler Radar, the Doppler frequency that a target's return signal exhibits can be used to calculate the Radial Velocity of target.  That means, the return frequency of a pulse will be different than the frequency of the pulse that was sent out depending on how fast the target is moving toward the source of the pulse.  If the target's "Radial Velocity" is zero, zero doppler will result, i.e. the resulting doppler caused by the target's Radial Velocity will be 0 Hz.  

Wikipedia shows the calculation of Doppler to be:

$$
F_{Doppler} = \frac{2 * F_{Transmit} * V_{Range} }{c} 
$$

$c$ being the Speed of light.

So, if the target's Radial Velocity is zero (not moving towards or away from the observer), $F_{Doppler}$ will also be zero.

https://en.wikipedia.org/wiki/Pulse-Doppler_radar