Pioneer ND-PS1 Manual page 5

LIMITATIONS
Fig 1
Fig 2
Fig 1: The display may not be the same as the real condition due to
the sensor position, obstacle, shape, reflection condition, and so on.
If an object is too high
If there is a very small object
When backing up downhill
Fig 2: Objects hard to be detected in the following conditions.
Smooth slope
Smooth round objects
Objects that absorb waves (e.g. cotton)