Guidance and Control

On episode 95, I ended up in the proverbial “1v0” trying to describe the differences between surface-to-air missile guidance and control. To be fair, the subjects are closely related. So, when our former B-52 Stratofortress guest, Ken Katz, emailed a short summary articulating the difference between the two, I suggested a Musing might be appropriate. After all, if a former fighter pilot struggles to fully comprehend this, odds are some listeners do too.

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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:

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

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

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

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.

The seeker is the primary sensor. It might be an active radar, a semi-active radar, an infrared sensor, or something else. Whatever physical principle it uses, it measures the angle from the missile’s longitudinal axis to the target. The missile also has another sensor: a rate gyro. We’ll get to its purpose later.

The seeker supplies the guidance function with the target angle. In a typical missile, the guidance and control section combines those two functions. The guidance function calculates the desired target angle to achieve an intercept, and what motion the missile should make to change the current target angle as measured by the seeker to the desired target angle. If using proportional guidance, the missile will attempt to attain a target angle rate of zero rather than any specific target angle, which will put it on a collision course with the target.

The output of the guidance function to the control function is the missile rate command; in other words, the desired rate that the missile should turn to attain the desired target angle. You might think that this missile rate command would be the maximum rate the missile can turn but recall the missile is a physical object with inertia. Too high of a missile rate command will cause the missile to overshoot the desired target angle or angle rate. The guidance function instead calculates the missile rate that will point the missile as fast as possible in the desired direction without overshooting.

The control function calculates how the fins will be commanded to achieve the missile rate command. Again, the challenge is to achieve the desired missile rate without overshooting. The rate gyro measures the missile rate and feeds it back to the control function, which compares the error between desired and actual. That error is used to calculate the fin command. But again the fins, their actuator, and the missile body are physical objects with inertia, so the fin command needs to be calculated to include the effects of inertia. The fin command is sent to the actuator, the actuator moves the fins, and the fins create aerodynamic forces which turn the missile.

The missile turns but the target is also moving. The target may be maneuvering to evade the missile. The missile seeker measures the target angle again, and the entire process repeats, many times each second.

…You can see why all this can be difficult for Jell-O to wrap his head around with his public education (his words, not mine). In summary, sensors, navigation, guidance, and control combine to make a vehicle move in a way that achieves mission objectives—whether that mission be a recreational flight, an airliner flying an instrument approach to landing, or a missile intercepting a target. Together, they are one of the core disciplines within aerospace engineering.