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navigation software, aerial delivery software, and diagnostic programs. These computers can
mathematically manipulate data in any way imaginable because they deal strictly with digital
information. The output from digital computers may need to be converted into an analog format for most
efficient use by the navigator, however; the digital computer cannot do that. It can display the digital
data in an approximation of analog data. While a digital computer can perform any mathematical
function, it must first be programmed for that function. In-flight reprogramming is not generally
possible.
16.7. Sensors. Many types of sensors are used for inputs to navigation systems.
16.7.1. Astrotracker. The use of astrotrackers has decreased; however, they are still excellent sources
of position information. They automatically track celestial bodies and compute position information
using celestial techniques. They are passive but require clear skies.
16.7.2. Doppler. The doppler radar measures GS and drift. These two data inputs can be put to several
uses in the computer system. Doppler GS is used to determine distance to update the aircraft position.
Drift can be used to compute winds and aircraft track. Doppler outputs can be used in platform leveling
and verifying inertial GS in an inertial navigation system. Doppler radar is an essential part of many
navigation computer systems.
16.7.3. Heading System. The gyro-stabilized magnetic heading source is corrected to true heading with
the local magnetic variation. This can be applied manually or automatically from a database in the
computer. Magnetic or true course can be calculated by applying doppler or inertial drift.
16.7.4. NAVAIDS. NAVAIDS are easily added to a computer system. VOR or TACAN bearings and
DME provide the same information as a radar fix. The computer needs the location and frequency of the
transmitter, which can be programmed into the computer before the mission begins. Some corrections
must be applied to bearing data. The computer must correct for magnetic variation and slant range from
the station to the aircraft.
16.7.5. Pressure Altimeter. Pressure altimeter data is an input to the true airspeed computations.
Additionally it can be used with temperature data to compute true altitude.
16.7.6. Radar. When a ground mapping radar is incorporated into the navigation system, present
position can be corrected based on the measurements to surveyed radar returns. The operator identifies
radar returns on his radar scope and measures the range and bearing to the return. The operator
determines the aircraft position relative to the return and updates the aircraft position. Automatic
systems allows the operator to pre-load the coordinates of radar returns in a database, place a movable
electronic cursor (or crosshairs) on the return, and push a button to update the system. The computer
determines the distance and bearing from the aircraft to the set coordinates. The computer then generates
the cursor on the radarscope at the calculated range and bearing. If there is any error in the navigation
system position, the cursor will not fall on the radar return. The operator adjusts the cursor or crosshairs
onto the radar return. The operator pushes a button to automatically update the system.
16.7.7. Temperature Sensors. The air data computer uses the information collected by temperature
sensors. Temperature gradients can be used with pressure altimeter data to compute true altitude.
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16.7.8. True Airspeed. True airspeed can be calculated from indicated airspeed, temperature, and
pressure. True airspeed and winds can be used as a backup for cross-checking ground speed.
16.7.9. Independent Systems. INS and GPS can also act as sensors for a navigation system. They are
discussed in greater detail later in this chapter.
16.8. Determining Position. The ever-present problem facing the navigator is determining aircraft
position. With a navigation system this problem is solved because the computer converts input data into
a constantly updated present position for the aircraft. Advanced systems provide altitude, attitude,
heading, and velocity information.
16.8.1. The mathematics of navigation over the surface of a sphere has been known for several centuries.
Starting from an initial position the computer determines the distance and direction traveled since
starting navigation. Aircraft direction, or track, may be supplied by INS, GPS, or the heading reference
system in combination with doppler drift. Ground speed may come from INS, GPS, or doppler ground
speed, or may be determined from any NAVAID capable of range and bearing fixes. The computer
multiplies speed against time interval to determine distance traveled. Distance is projected along the
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