Guidance

First, let us attempt a definition: Guidance is the logic that issues steering com­mands to the vehicle to accomplish certain flight objectives. For a missile the objective may be to hit a target. Given the vehicle and target states—position, velocity, and angles—the guidance algorithm generates the autopilot commands that steer the missile to the target.

For an aircraft on final approach, the objective is the descent on a glide slope leading to the touchdown point on the runway. If the pilot is at the controls, he issues the steering commands directly to the fin actuators, based on the situational awareness presented to him by the cockpit instruments. He fulfills the function of the guidance logic and the autopilot. Alternatively, if a stability augmentation system is engaged, he sends via his stick guidance signals to the autopilot. In the case of an automatic landing system, the electronics take over completely and provide the guidance logic for a hands-off touchdown.

Dealing with unmanned vehicles, like missiles and projectiles, the guidance logic is the most important function for ensuring mission success. Therefore, it is imbued with particular stature and is called the guidance law. You have probably heard about the proportional navigation law, which more appropriately should be called proportional guidance law. Some of its prefixes indicate performance improvements, like augmented, higher order, etc. Other guidance laws have names like line guidance, parabolic guidance, squint angle guidance, and others.

The open literature does not cover all aspects of guidance because some of the tricks of this trade are either classified or proprietary to industry. However, a few good references have been published just recently. Advances in Missile Guidance Theory by Ben-Asher13 addresses guidance from a modern control as­pect. An easier text to read is the third edition by Zarchan Tactical and Strategic Missile Guidance.14 Practical guidance aspects are provided in Modem Naviga­tion Guidance, and Control Processing by Lin.15 Also many of us owe much insight into optimal guidance to Bryson and Ho and their classic Applied Optimal Control.16

For our simplified five-DoF representation of missiles and aircraft, I will limit the discussion to the basic, but all-pervasive proportional navigation law for missiles, and to line guidance, suitable for way-point guidance or landing approaches. Some of the more advanced schemes are introduced in conjunction with the full six-DoF simulations (see Chapter 10).

Although we live in this chapter in the pseudo-five-DoF world, with its special autopilot provisions, the guidance loop (outer feedback loop) is affected little by these simplifications. This fact is useful in two ways. During the conceptual phase of a vehicle design, detailed guidance studies can be conducted without detailed knowledge of aerodynamic and autopilot specifications. Alternately, if a full six-DoF simulation must be simplified, e. g., shortening run time by simplifying aerodynamics and reducing the autopilot bandwidth, the guidance loop can be transferred directly to the pseudo-five-DoF implementation.