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aircraft track to obtain the new position. Track and speed are sampled and present position is updated
many times per second. Waypoint navigation is a simple addition to the navigation computer.
16.8.2. A database of coordinates can be added to the system to determine distance to go and estimated
time of arrival. If the aircraft changes speed, the ETA is automatically updated using the new ground
speed.
16.9. Decision Algorithm. Simple navigation systems determine position as described above. The
operator updates the position for errors that will eventually occur. More complex systems have
additional problems. When a system has a variety of sources that provides redundant information, how
does the computer decide which source to use? What if the sensors are subject to errors? What if the
operator inputs an inaccurate update to the system? How can we get a computer to make simple
decisions once left to the navigator? Can we program a computer to analyze and correct for the
predictable and unpredictable errors in sensor data? Bias in the accuracy and variability of data are two
types of error that navigation systems actually experience and can be solved with the use of statistical
software called decision algorithms.
16.9.1. To compensate for these predictable and unpredictable errors in sensor data, we can include
statistical measuring software that will weigh the accuracy of each data source and the accuracy of the
data itself. These programs will determine the most likely value for track and velocity in order to
compute the most likely present position.
16.9.2. One type of program used to determine the most likely sensor values is called a Kalman filter.
Kalman filters are used extensively in computer controlled communications, electronics, and equipment.
When used as part of a navigation system, a Kalman filter computes the most likely position of the
aircraft and updates the weighing factors with each new position update. The Kalman filter compares the
actual sensor data used prior to the update with the data from the update. By comparing the first position
with the second position, actual distance and heading can be determined. It then determines the amount
of error in the original data and estimates a correction to the data for the next time period. The Kalman
filter is an iterative program requiring several updates prior to achieving completely reliable data. If
used, the Kalman filter will also be used to evaluate the reliability of operator inputs and weigh how
330 AFPAM11-216 1 MARCH 2001
much of each position update to accept. Kalman filtering provides increased reliability in navigation
systems so an operator can trust that the information used is valid. Kalman filters protect the operator
from inaccurate sensor data and even operator error.
Section 16C— Inertial Navigation System (INS)
16.10. Basics. Inertial navigation is accepted as an ideal navigation system because it meets all the
criteria of an ideal system. INS provides worldwide ground plot information regardless of flight path and
aircraft performance. An INS can measure GS independently of wind and independently of the operating
environment. INS is completely independent of ground transmissions, immune to enemy
countermeasures, and passive in operation. It is self-contained and portable; most units weigh less than
100 pounds. Some ring laser gyro systems weigh as little as 20 pounds. The need for a system with these
properties has spurred development to the point where INS is superior to almost every other navigation
system. INS provides accurate velocity information instantaneously for all maneuvers, as well as an
accurate attitude and heading reference. INS accuracy decreases as the time between position updates
increases. INS will maintain its accuracy for short sorties without position updates; however, longer
sorties may require periodic in-flight updates.
16.11. Types of Inertial Systems. In the last several years inertial technology has taken several leaps
forward. Early inertials were bulky devices weighing several hundred pounds, whose installation had to
be precise and whose operation had to be planned in great detail. Today there are compact systems that
fit in a briefcase and can be bolted to an aircraft in any space available. While some inertial systems still
have mechanical gyroscopes, pendulous linear accelerometers, and space stable platforms, most have
evolved to keep pace with the advances in technology. Acoustic gyros, ring laser gyros, and
electronically suspended gyros have replaced the gimbaled gyroscope. Laser and acoustic
accelerometers are replacing the pendulous linear accelerometer. Highly accurate computers and
precision sensors have led to modifications of the space stable platform so the INS housing does not
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