What is Position Error?

Ron Davis

Position error is the distance between the true 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 may be readily purchased. Aircraft may use a GPS system or they may rely on ground radar and their internal Pitot-static systems. Pitot-static systems provide several types of data, but latitude and longitude are not among them.

When a reconnaissance aircraft such as the U-2 is lost over foreign territory, 'position error' is often blamed to avoid diplomatic fallout.
When a reconnaissance aircraft such as the U-2 is lost over foreign territory, 'position error' is often blamed to avoid diplomatic fallout.

The US Department of Defense (DoD) sets requirements for the minimum accuracy of all GPS systems. The DoD introduces random error into the output to civilian systems, so no amount of work will enable civilian receivers to consistently read completely accurately. The civilian GPS position error in the horizontal plane, i.e., longitude and latitude, may be up to 328 feet (100 meters) from the true position. A GPS device for a civilian aircraft will measure altitude within 512 feet (156 meters) of true.

Military uses of GPS have a smaller position error. The mandate for the military is error on the ground of not more than 72 feet (22 meters). This distance is not sufficiently accurate to rely upon on in close combat, so more accurate targeting systems are used in all combat vehicles. The 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 usually mounted on the leading edge of the wing of the aircraft. The static port is placed in an area of relatively undisturbed air flow such as on the side of the fuselage. Comparison of the pressure readings of the Pitot tube and the static port generate the system information. Airspeed, altitude, and rate of climb are three of the common readouts.

Position error for Pitot-static systems are static or variable. Static errors, a set of values that are common to all aircraft of a particular type, have correction charts in the Aircraft Flight Manuals for each type of aircraft. Causes of variable errors include skin panels that are deformed or become deformed under the stresses of maneuver, icing, and foreign objects getting into the system. The Pitot tube and static vents contain heaters to prevent icing, but heaters can fail. Position errors may be altered by changes in airspeed, angle of attack, aircraft weight, or acceleration.

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