Abstract: A portable device for measuring aspects of a moving projectile (such as a golf ball) in a sporting activity is provided. The device produces calculations from events it detects. The device comprises a set of detectors in a spaced relationship for detecting movement of the projectile across the device; a display; a set of input keys; a microprocessor controlling operation of the detectors, the display and the input keys; and software operating on the microprocessor for controlling operation of the microprocessor and for producing the calculations based on data received from the detectors and the input keys. In the software, the calculations include: a calculation producing an estimated distance travelled by the projectile, a calculation producing an estimated force imparted on the projectile and a calibration of the estimated distance using a measured distance of the projectile compared against the estimated distance.