Spectra Precision Survey Pro v4.5 Ranger Reference Manual - Manual (Page 460)

Survey Pro Reference Manual
R-452
prism eliminating the possibility of the total station tracking
a different reflector that comes into view.
Using Active Prism: Check this option when using an active prism.
(This option is only available when using a total station that supports
active prisms.)
Note: In order to perform automatic repetitions when using active
prisms, active prisms must be set up over both the backsight and
foresight.