sentences:  When speaking to air traffic control police helicopters and fixed-wing aircraft use the callsign "NPAS" followed by a two digit number representing the base of the aircraft.At NPAS London, where three helicopters are based, the first helicopter to lift uses the first callsign, and any others to follow occupy the other.In situations where a patient is critically injured, the police helicopter can be called upon to provide air transport of a patient - directly to the best hospital.