Guidance systems

Systems that aid in navigation, that is, in finding and keeping to a route and schedule.

The purpose of guidance systems is to aid in navigation. It is a simple point but one that can easily be lost in the overall complexity of some new guidance systems. Navigation has simple, specific objectives. The navigator should select a route and a schedule. There should be a continuous succession of points against which the navigator can check the progress of the voyage. Next, the planned movement is executed; that is, the craft is kept to the route or course set. Guidance systems enable these simple but important tasks to be accomplished accurately.

External Observation Guidance Systems

Guidance systems comprise many parts, including instrument landing systems (ILS), air traffic control (ATC) systems, radar and database systems, and voice communication controls. Satellite landing systems are increasingly important in providing landing guidance. From the earliest days of air flight to the present, there have been consistent improvements in guidance systems.

The constant monitoring and correction of position is termed a closed loop. Finding the aircraft’s position is achieved by measuring distance or direction or both. Additionally, guidance systems need to measure altitude. The transmission of sound and light waves, as well as other electromagnetic waves, is used in this process.

There are guidance systems to aid in speed measurement, altitude, and every other possible variable for flight. High-speed computers aid in the process, warning pilots and navigators when an aspect of the flight requires attention. The Kalman filtering system weights each datum according to its expected quality. It aids in the process of dead reckoning, speed, and direction, as well as continuously updating the craft’s position. It also determines the speed of the plane, its heading, rate of climb or descent, and how each of these must be maintained or adjusted to stick to the flight plan.

Air traffic controllers keep a dead reckoning check on each aircraft, using strips that show the height, speed, and timing of each plane. The strips break down the flight plan of each aircraft. Radio navigation uses signals in a true beam system. Narrow beams about 3 feet long are used for landing, even in near-zero visibility. Improved microwave systems allow for even narrower beams and aid instrument landing systems.

Laser guidance systems provide pilots with a visual navigation flight path from as far as 20 miles from the runway, with the precision of an advanced instrument landing system. Best of all, the installation of laser guidance and cold cathode technologies to replace or enhance conventional landing light systems requires no additional aircraft equipment, and is cheaper to maintain than conventional lighting. For example, the lifetime cost of cold cathode lights is only 20 percent of that of incandescent lights. The combination of enhanced vision technologies with the latest ground proximity warning systems dramatically reduces the number of controlled-flight-into-terrain accidents.

Inertial Guidance Systems

Inertial guidance is a method of navigation used to guide rockets and airplanes, submarines, and other vehicles. Unlike other methods of navigation, inertial guidance does not rely on observations of land or the stars, on radio or radar signals, or on any other information from outside the vehicle. Instead, a device called the inertial navigator provides the guidance information. An inertial navigator consists of gyroscopes, which indicate direction, and accelerometers, which measure changes in speed and direction.

The principles of inertial guidance have been known since the early 1900’s. Gyroscopes have been used as compasses on ships since that time. They can be set so that they point constantly in one direction, such as toward the North Star. Unlike magnetic compasses, these gyrocompasses always indicate true north and are not affected by steel. In 1923, the German engineer Max Schuler described a method for establishing a vertical line that would not tilt when a vehicle changes speed or direction. If the line tilts, it cannot be used to measure distance. Schuler’s theory is used to build electronic systems that prevent tilting of the vertical line. During World War II, German scientists built an inertial guidance system that guided their V-2 rockets against England. In the late 1940’s and early 1950’s, Charles S. Draper and other scientists at the Massachusetts Institute of Technology built the first highly accurate inertial guidance systems. Space shuttles and other spacecraft are also equipped with inertial navigators. Inertial guidance systems are required on U.S. commercial overseas flights.

The advantages of inertial guidance can be explained by the example of an airplane flight. To reach its destination, an airplane must both fly in the correct direction and cover the correct amount of distance. Without inertial guidance, a pilot has to rely on compasses or on signals from radio beacons at known positions on the ground to be sure the airplane is flying in the right direction. With inertial guidance, pilots need only consult the navigation equipment inside the airplane. They can find their way in spite of poor visibility, faulty communications, and the absence of landmarks. In time of war, enemies cannot jam an inertial guidance navigation system with false or confusing information.

The inertial navigator automatically measures changes in a vehicle’s speed and direction, and sends the information to the computer. The computer calculates the effect of all the changes and keeps track of how far and in what direction the vehicle has moved from its starting point. Three gyroscopes inside the inertial navigator spin in different directions on axles. The axles are placed so that they form 90-degree angles with each other, like three edges of a box meeting at a corner. The axles keep their directions as long as the gyroscopes continue to spin. Each gyroscope is supported by gimbals (movable frames) so that it stays in position as the vehicle rolls, pitches, or turns. Together, the gyroscopes establish an inertial reference system (a stable set of lines). The accelerometers detect changes in the vehicle’s motion in reference to the stable lines defined by the gyroscopes. The inertial navigator measures how far a vehicle has traveled by recording the changes in the position of a vertical line. This line indicates the direction to the center of the earth. Vertical lines from any two points on the earth meet at the center of the earth. The angle between the lines indicates the distance between the points. Each minute (one-sixtieth of a degree) of angle indicates a surface distance of one nautical mile (6,076.1 feet, or 1,852 meters). New York City is 3,006 nautical miles from London. Therefore, a pilot flying from New York City to London knows the airplane has gone far enough when the vertical line of the inertial navigator has moved through an angle of 3,006 minutes (50 degrees, 6 minutes).

Inertial guidance systems are subject to errors that grow over time. In some systems, a computer periodically combines the system’s outputs with an independent source of position, such as a radio beacon. This procedure helps minimize the size of navigation errors.


Gyroscopes are essential in the working of inertial guidance systems. The gyroscope functions as a compass when the gyroscope is considered to be mounted at the equator of the earth. The spinning axis lies in the east-west plane; the gyroscope continues to point along the east-west line as the earth rotates. Laser gyros provide guidance in the most advanced aircraft systems. These gyros are not really inertial devices. Instead, they measure changes in counter-rotating beams of laser light, caused by changes in the aircraft’s direction. The electrically suspended gyro, another advanced system, uses a hollow beryllium sphere suspended in a magnetic cradle. There are also fiber-optic systems in the works to aid in navigation.

The gyroscope also aids in the automatic pilot program of a plane through detecting and correcting variations in its selected flight plan, and it supplies corrective signals to the ailerons, elevator, and rudder. There are, in fact, a number of gyroscopes to detect changes in altitude, barometric pressure, and other factors. These gyroscopes transmit electrical signals to a computer, which combines and amplifies them, and then transmits these corrective signals to servomotors attached to the control surfaces of the aircraft. The pilot is thus able to use an autopilot to make corrections and to combine navigation and radio aids, such as inertial navigation systems, Doppler radar navigation systems, and radio navigation beacons. The autopilot can also couple beams of instrumental landing systems used in airport runways.


  • Biezad, Daniel J. Integrated Navigation and Guidance Systems. Reston, Va.: American Institute of Aeronautics & Astronautics, 1999. A navigation textbook, with excellent coverage of Global Positioning Systems (GPS) and inertial navigation systems.
  • Clausing, Donald J. Aviator’s Guide to Navigation. 3d ed. McGraw-Hill, 1997. An advanced guide to air navigation, covering all types of systems that an aviator can encounter in modern aircraft.
  • Kayton, Myron, ed. Avionics Navigation Systems. 2d ed, New York: John Wiley & Sons, 1997. A systematic overview of modern navigation and sensing systems, written for engineers and professional navigators. Very thorough, but requires some familiarity with the systems to begin with.

Air traffic control






Doppler radar

Flight plans


Landing procedures



Takeoff procedures

Guidance system technology helps pilots safely take off and land their planes in bad weather, when visibility is low.

(Rockwell Collins)