The radar you’re talking about is a pulse-doppler radar. It doesn’t see things the same way your eyes (or a synthetic-aperture radar for example) do and it’s not down to filters. A P-D radar observes and measures the Doppler effect which only occurs when the object is moving and determines the contact’s velocity, heading and altitude based on that. A jet flying vertically at 0 GS would probably be detectable, although this would be difficult since, as you said, it’s not moving directly towards or away from the radar.
The radar you’re talking about is a pulse-doppler radar. It doesn’t see things the same way your eyes (or a synthetic-aperture radar for example) do and it’s not down to filters. A P-D radar observes and measures the Doppler effect which only occurs when the object is moving and determines the contact’s velocity, heading and altitude based on that. A jet flying vertically at 0 GS would probably be detectable, although this would be difficult since, as you said, it’s not moving directly towards or away from the radar.