I think it depends on the sensor: some are optimized for speed in line
with the beam, some for speed perpendicular to the beam (cross wind
measurement, like for bullets). Perhaps there are ones that can read the
vector wind?


If so, you would just need a single sensor facing forward. You still
would have the problem of getting it perfectly horizontal, though.


I have no idea how much crossways speed the various sensors
can tolerate.


I am betting that these sensors can only measure speed directly toward
or away from the sensor. Sort of like a radar gun. So I am not
understanding how they could measure a glider's sink rate.