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Guidance and Control

Guidance and control are the engineering disciplines that point a vehicle in the desired direction, traveling at the desired velocity, towards the desired destination. Let’s start with some definitions: Sensor — a device that measures a physical quantity and converts the measurement into a signal that can be processed and used. An operator’s eyeball is a sensor, for example. Radar is a sensor. A gyroscope is a sensor…. You get the idea. Navigation — the estimation of position and velocity using sensor data. Fundamentally, there are only two methods of navigation: position-fixing and dead reckoning. The former—what sailors call ‘pilotage’—is the estimation of position using external references. A pilot comparing his position relative to landmarks on an aeronautical map is position fixing. ILS, TACAN, and VOR are traditional forms of aviation radio-navigation and all of them are forms of position fixing. GPS is another type of position fixing. When a fighter or a missile uses a radar to detect and track a target, that is also position fixing. Dead reckoning, on the other hand, is what engineers define as integrating the velocity vector over time. In the simplest case, a pilot who flies a course of 090 degrees at an airspeed of 120 knots for 10 minutes can estimate that his position is 20 nautical miles east from of he started (this, of course, completely ignores wind). Modern military aircraft have inertial navigation systems that essentially do the same thing. Guidance — the calculation of the path from where a vehicle is now to its destination, including the direction and magnitude of the velocity vector that will put the vehicle on that path and keep it there. Guidance is dependent on navigation because the position and velocity of the vehicle and destination must be known in order to guide the vehicle. A simple guidance law pilots use is manipulating the heading of an aircraft to maintain a direct course between two points with an unknown crosswind. Usually the pilot starts by flying a heading that is the same as the course. As the aircraft drifts left or right of course, the pilot turns 10 degrees to return to the desired course. If the drift continues, the pilot turns an additional 10 degrees. Once the heading returns the aircraft to the desired course, the pilot halves the heading correction in an attempt to maintain that course. The pilot continues to make smaller and smaller heading changes as needed until the aircraft is on course and the exact heading to correct for the crosswind has been determined. Missiles following a moving target may use command-to-line-of-sight, pursuit, or proportional guidance. For command-to-line-of-sight guidance, the missile attempts to follow the line from a guidance station to the target. When using the pursuit guidance, the missile flies straight to the target. Proportional guidance is the same thing as constant bearing / decreasing range, in which the missile flies a collision course with the target. Ballistic missiles, space launch vehicles, and satellites use different kinds of guidance laws. Control — the manipulation of vehicle attitude and other parameters so that the lift, drag, and thrust vectors will direct the vehicle as desired to implement the guidance law. A vehicle is a physical object, so it doesn’t just go where desired, it must be made to go where desired. A control system consists of the following:

  1. Inceptors — the things that the pilot uses such as a stick and rudder pedals

  2. Computers — might be the pilot’s brain or digital computers such as in the F/A-18 Super Hornet

  3. Sensors — perhaps only the pilot’s eyes, inner ears and the feeling in the seat of his pants, or multiple air data computers and inertial measurement units in a modern airliner or military aircraft, and

  4. Actuators — move the effectors (e.g. elevators, ailerons, rudders) that exert forces on the vehicle To continue our previous example of a pilot trying to maintain a course, recall that the pilot has estimated the position and velocity of the airplane and destination (navigation), plotted a straight course between the departure point and the destination (guidance), and is varying the heading to compensate for the crosswind (also guidance). Now the pilot needs to control the aircraft to attain that heading. The pilot rolls the aircraft until it is at a desired bank angle, and that bank angle corresponds with a rate of change of heading. The rate of change of heading is maintained until the aircraft is at the desired heading, then the pilot rolls the aircraft out of the bank to stop changing the heading. But the control problem is more complicated because the airplane is a physical object with inertia and does not instantaneously roll to the desired bank angle. So the pilot needs to make control inputs that take into account the need to anticipate the finite roll rate of the aircraft; furthermore the pilot uses his eyes to measure its roll rate and bank angle, the directional gyroscope to measure heading, and the sense of feel in his hands and arms to determine how much force to apply to the control yoke or stick. So even in this simplest of examples, all the elements of sensors, navigation, guidance, and control are present. Let’s consider a missile attempting to intercept an aircraft. We’ll simplify the example by considering two dimensions and neglecting gravity.

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