between the eye and the target. latency or slew rate – how much lag there is between the helmet and the cue. field of regard – the angular range over which the sight can still produce a suitably accurate measurement. weight and balance – total helmet weight and its center of gravity, which are particularly important under high "g" maneuvers. Weight is the largest problem faced by fighter aircraft HMD designers. This is much less a concern for helicopter applications, making elaborate helicopter HMDs common. safety and flightdeck compatibility, including ejection seat compatibility. optical characteristics – calibration, sharpness, distant focus (or 'Collimation', a technique used to present the images at a distant focus, which improves the readability of images), monocular vs. binocular imagery, eye dominance, and binocular rivalry. durability and ability to handle day-to-day wear and tear. cost, including integration and training. fit and interfacing the aviator's head to the aircraft – head anthropometry and facial anatomy make helmet-fitting a crucial factor in the aviator's ability to interface with the aircraft systems. Misalignment or helmet shift can cause an inaccurate picture. Head tracking HMD designs must sense the orientation (elevation, azimuth and roll) and in some cases the position (x, y, and z) of the pilot's head relative to the airframe with sufficient precision even under high "g", vibration, and during rapid head movement. Five basic methods are used in current HMD technology – inertial, optical, electromagnetic, sonic, and hybrid. Hybrid trackers use a combination of sensors such as inertial and optical to improve tracking accuracy, update rate, and latency. Hybrid inertial optical Hybrid inertial tracking systems employ a sensitive Inertial Measurement Unit (IMU) and an optical sensor to provide reference to the aircraft. MEMS based IMUs benefit from high update rates such as 1,000 Hz but suffer from precession and drift over time, so they cannot be used alone. In this class of tracker, the optical sensor is used to constrain IMU drift. As a result, hybrid inertial/optical trackers feature low latency and high accuracy. The Thales Scorpion® HMCS and HMIT HMDs utilize a tracker made by InterSense called the Hybrid Optical-based Inertial Tracker (HObIT). Optical Optical systems employ infrared emitters on the helmet (or flightdeck) infrared detectors in the flightdeck (or helmet), to measure the pilot's head position. The main limitations are restricted fields of regard and sensitivity to sunlight or other heat sources. The MiG-29/AA-11 Archer system