Low airspeed information is required for two reasons: firstly for test purposes to either ensure that the helicopter is in a true zero-wind hover or to provide accurate low airspeed so that its effect on handling and performance can be investigated; secondly for operational reasons to provide wind speed and direction information for improved weapon delivery. Thus there arises a subtle distinction between a ‘hovermeter’ and a true low airspeed sensor. A hovermeter is simply a device that accurately indicates the zero airspeed condition whilst giving the pilot a general indication of airspeed away from this condition so that he can bring the aircraft into the hover. A low airspeed sensor on the other hand delivers accurate airspeed information throughout the whole low speed envelope of the helicopter.

http://www.google.com/url?sa=t&source=web&cd=19&ved=0CE4QFjA...