GPS and Pitot-static systems are used to determine the position of aircraft and vehicles. GPS has a position error of up to 328 feet for civilian systems and 72 feet for military systems, while Pitot-static systems have static or variable errors that can be corrected with tables in Aircraft Flight Manuals.
Position error is the distance between the actual location of an aircraft or vehicle and the location reported by its internal sensing devices. Many new vehicles come equipped with a global positioning system (GPS) and portable GPS devices are readily available. Aircraft may use a GPS system or may rely on ground-based radar and its internal Pitot-static systems. Pitot-static systems provide several types of data, but latitude and longitude are not among them.
The United States Department of Defense (DoD) sets requirements for the minimum accuracy of all GPS systems. The DoD introduces random error into the output of civilian systems, so no amount of work will allow civilian receivers to read consistently with all accuracy. Civilian GPS position error in the horizontal plane, that is, longitude and latitude, can be up to 328 feet (100 meters) from the actual position. A GPS device for a civil aircraft will measure altitude to within 512 feet (156 meters) of truth.
Military uses of GPS have a smaller position error. The mandate for the military is an error on the ground of no more than 72 feet (22 meters). This distance is not accurate enough to rely on in close combat, so more precise targeting systems are used on all combat vehicles. Altitude position error can be up to 91 feet (27.7 meters), which is adequate in non-combat situations.
Pitot-static systems, used by most aircraft, consist of a Pitot tube and a static port. The pitot tube is aligned with the direction of flight and is usually mounted on the leading edge of the aircraft’s wing. The static port is placed in an area of relatively quiet airflow, such as on the side of the fuselage. Comparison of the pressure readings from the Pitot tube and the static port generates the system information. Airspeed, altitude, and rate of climb are three of the common readings.
The position error for Pitot-static systems is either static or variable. Static errors, a set of values that are common to all aircraft of a particular type, have correction tables in Aircraft Flight Manuals for each aircraft type. Causes of variable errors include skin panels that warp or warp under the stress of maneuvering, ice, and foreign objects entering the system. The pitot tube and static vents contain heaters to prevent icing, but the heaters can fail. Position errors can be altered by changes in airspeed, angle of attack, aircraft weight, or acceleration.
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