Tactical Air Navigation (TACAN) System TACAN is a radio navigational set that provides slant range and relative bearing to a transmitting ground (surface) station. It has Distance Measuring Equipment (DME) that provides continuous slant range information. The Bearing Distance Heading Indicator (BDHI) provides a visual indication of the navigational situation for that aircraft. Global Positioning System (GPS) The  Global  Positioning  System  (GPS)  is  a space-based radio position and navigation system designed to provide highly accurate three-dimensional position, velocity, and time data to suitably equipped aircraft anywhere on or near the earth. The Satellite Vehicle (SV) consists of 24 operational satellites in six circular orbits (10,900 nmi) above the earth at an inclination angle of 55E   with a 12-hour period. The satellites are spaced in orbit so that at any given time a minimum of four satellites will be in view to users anywhere in the world. The GPS Navigation Set receives and processes SV signals, combines them with air data information, and then calculates and displays the aircraft position for navigation. The information includes present aircraft position, course information, distance and time to waypoint and desired track, along with other navigation information.  GPS  consists  of  three  independent segments—the satellite segment, ground segment, and the user segment. Navigation Computers A new and complex group of electronic navigational equipment is now in use in naval aviation. This equipment does not use a radio receiver as the basic component. Included in this group are navigational computers, Doppler navigation equipment, and inertial navigation equipment. NAVIGATIONAL COMPUTERS.—One of the navigational aids now in use is a latitude and longitude type of airborne computer system. This system can make the following computations during flight: ! The latitude and longitude of the present position of the aircraft. This information is continually displayed on the pilot's console. !    The aircraft ground track angle, relative to true heading. !    The distance from the present position of the aircraft to a preset target or base, as selected on the control panel. !    The bearing of the preset target or base, as selected, relative to true heading. The computer is an analog-type computer. It includes a group of servomechanisms that receive navigational information and, by solving trigonometric equations, produces output information. Data input consists of the following: !    Compass heading !    True airspeed !    Magnetic variation !    Windspeed !    Base position latitude and longitude (usually the starting position) !    Target position latitude and longitude !    Aircraft's latitude and longitude (if not identical to base) The magnetic compass and the true airspeed transmitter automatically furnish compass heading and true airspeed. The remaining inputs are set manually by control knobs on the counter-control panel. The com- puter sections continuously reposition the POSITION- LATITUDE and LONGITUDE counters to show the aircraft's present position and/or the intended target's position. DOPPLER NAVIGATION EQUIPMENT.— Doppler navigation is based on a radar wave trans- mission beamed toward the earth behind the aircraft. This radar does not sense range and bearing (direc- tion) as ordinary search radar does. Instead it uses a continuous wave (CW) transmission to measure the ground-speed and drift angle of the aircraft. The Dop- pler navigation system operates anywhere. It is rela- tively unaffected by weather conditions, and is inde- pendent of ground-based navigation aids. This permits an aircraft crew to compute an aircraft's track. The track is projected on the ground from any known position (usually the position of takeoff) to any position desired. Therefore, long-distance navigation is possible. INERTIAL  NAVIGATION  EQUIPMENT.— An inertial navigation system (INS) is an automatic aid to navigation that is independent of outside references. An INS is a portion of the overall tactical system that provides accurate velocity, attitude, and heading data to a digital data processing system. This overall system permits  accurate  weapons  delivery.  To  function properly, the system must be aligned with reference to initial conditions of altitude, latitude, and longitude. 7-15