MART Vol. II MO/MP - NESA - Civil Air Patrol

displayed, typically in miles and tenths of miles, on a dial or digital indicator on theinstrument panel. When DME is used in combination with VOR, a pilot can tell ata glance the direction and distance to a tuned station.Slant rangeGround RangeFigure 5-9DME measures straight-line distance, or slant range, so there is always analtitude component within the displayed distance. If you fly toward a station at analtitude of 6,000 feet over the station elevation, the DME will never read zero. Itwill continuously decrease until it stops at one mile. That mile represents theaircraft’s altitude above the station. The distance readout will then begin toincrease on the other side of the station. Under most circumstances the altitudecomponent of slant range can be ignored, but when reporting position using DME,especially to air traffic controllers, it is customary to report distances in "DME", notnautical miles, e.g., "Holly Springs 099° radial at 76 DME."Some DME equipment can also compute and display the actual ground speedof the aircraft, provided that the aircraft is tracking directly to or from the groundstation (and usually only after the aircraft has been stabilized on this track for oneor two minutes). In all other circumstances, the ground speed information is notaccurate and should be ignored.5.5.4 Global Positioning System (GPS)Initially developed by the Department of Defense for military users, the GlobalPositioning System has become the most accurate navigational system availableto civilian aircraft operators. Certified systems will eventually replace many of thenavigational systems already discussed, as they already have replaced the ADF.The system relies on a chain of 24 satellite transmitters in polar orbits aboutthe earth. The speed and direction of each satellite, as well as each satellite'saltitude is precisely maintained so that each satellite remains in a highly accurateand predictable path over the earth's surface at all times.GPS receivers process signals transmitted by these satellites and triangulatethe receiver's position, which the user again can read directly in latitude andlongitude coordinates from a digital display. Similar additional features as thosediscussed in LORAN are available and vary depending upon the design andmanufacturer. The system is substantially more accurate than LORAN, VOR,DME, or ADF and has several advantages.81