used in terrain-following radar allowing combat aircraft to fly at very low height above the terrain. Lidar technology is used to help navigate the helicopter Ingenuity on its record-setting flights over the terrain of Mars by means of a downward-facing Lidar altimeter. Global Positioning System Global Positioning System (GPS) receivers can also determine altitude by trilateration with four or more satellites. In aircraft, altitude determined using autonomous GPS is not reliable enough to supersede the pressure altimeter without using some method of augmentation. In hiking and climbing, it is common to find that the altitude measured by GPS is off by as much as depending on satellite orientation. Other modes of transport The altimeter is an instrument optional in off-road vehicles to aid in navigation. Some high-performance luxury cars that were never intended to leave paved roads, such as the Duesenberg in the 1930s, have also been equipped with altimeters. See also Acronyms and abbreviations in avionics ICAO recommendations on use of the International System of Units Flight instruments Flight level Hypsometer Jason-1 and Ocean Surface Topography Mission (Jason-2) are satellite missions that use altimeters to measure sea surface height Level sensor Lidar Pressure sensor Primary flight display Radar altimeter Satellite altimetry Turkish Airlines Flight 1951, an accident attributed to a malfunctioning radio altimeter United Airlines Flight 389, an accident attributed to misreading of an altimeter Variometer, a gauge measuring the rate of change of altitude References External links