Guidance System - Guidance Systems

Guidance Systems

Guidance systems consist of 3 essential parts: navigation which tracks current location, guidance which leverages navigation data and target information to direct flight control "where to go", and control which accepts guidance commands to effect change in aerodynamic and/or engine controls.

Navigation is the art of determining where you are, a science that has seen tremendous focus in 1711 with the Longitude prize. Navigation aids either measure position from a fixed point of reference (ex. landmark, north star, LORAN Beacon), relative position to a target (ex. radar, infra-red, ...) or track movement from a known position/starting point (e.g. IMU). Today's complex systems use multiple approaches to determine current position. For example, today's most advanced navigation systems are embodied within the Anti-ballistic missile, the RIM-161 Standard Missile 3 leverages GPS, IMU and ground segment data in the boost phase and relative position data for intercept targeting. Complex systems typically have multiple redundancy to address drift, improve accuracy (ex. relative to a target) and address isolated system failure. Navigation systems therefore take multiple inputs from many different sensors, both internal to the system and/or external (ex. ground based update). Kalman filter provides the most common approach to combining navigation data (from multiple sensors) to resolve current position. Example navigation approaches:

• Celestial navigation is a position fixing technique that was devised to help sailors cross the featureless oceans without having to rely on dead reckoning to enable them to strike land. Celestial navigation uses angular measurements (sights) between the horizon and a common celestial object. The Sun is most often measured. Skilled navigators can use the Moon, planets or one of 57 navigational stars whose coordinates are tabulated in nautical almanacs. Historical tools include a sextant, watch and ephemeris data. Today's space shuttle, and most interplanetary spacecraft, use optical systems to calibrate inertial navigation systems: Crewman Optical Alignment Sight (COAS), Star Tracker.
• Long-range Navigation (LORAN) : This was the predecessor of GPS and was (and to an extent still is) used primarily in commercial sea transportation. The system works by triangulating the ship's position based on directional reference to known transmitters.
• Global Positioning System (GPS) : GPS was designed by the US military with the primary purpose of addressing "drift" within the inertial navigation of Submarine-launched ballistic missile(SLBMs) prior to launch. GPS transmits 2 signal types: military and a commercial. The accuracy of the military signal is classified but can be assumed to be well under 0.5 meters. GPS is a system of 24 satellites orbiting in unique planes 10.9-14.4 Nautical miles above the earth. The Satellites are in well defined orbits and transmit highly accurate time information which can be used to triangulate position.
• Inertial Measurement Units (IMUs) are the primary inertial system for maintaining current position (navigation) and orientation in missiles and aircraft. They are complex machines with one or more rotating Gyroscopes that can rotate freely in 3 degrees of motion within a complex gimbal system. IMUs are "spun up" and calibrated prior to launch. A minimum of 3 separate IMUs are in place within most complex systems. In addition to relative position, the IMUs contain accelerometers which can measure acceleration in all axis. The position data, combined with acceleration data provide the necessary inputs to "track" motion of a vehicle. IMUs have a tendency to "drift", due to friction and accuracy. Error correction to address this drift can be provided via ground link telemetry, GPS, radar, optical celestial navigation and other navigation aids. When targeting another (moving) vehicle, relative vectors become paramount. In this situation, navigation aids which provide updates of position relative to the target are more important. In addition to the current position, inertial navigation systems also typically estimate a predicted position for future computing cycles. See also Inertial navigation system.

• Radar/Infrared/Laser : This form of navigation provides information to guidance relative to a known target, it has both civilian (ex rendezvous) and military applications.
  • active (employs own radar to illuminate the target),
  • passive (detects target’s radar emissions),
  • semiactive radar homing,
  • Infrared homing : This form of guidance is used exclusively for military munitions, specifically air-to-air and surface-to-air missiles. The missile’s seeker head homes in on the infrared (heat) signature from the target’s engines (hence the term “heat-seeking missile”),
  • Ultraviolet homing, used in FIM-92 Stinger - more resistive to countermeasures, than IR homing system
  • Laser designation : A laser designator device calculates relative position to a highlighted target. Most are familiar with the military uses of the technology on Laser-guided bomb. The space shuttle crew leverages a hand held device to feed information into rendezvous planning. The primary limitation on this device is that it requires a line of sight between the target and the designator.
  • Terrain contour matching (TERCOM). Uses a ground scanning radar to "match" topography against digital map data to fix current position. Used by cruise missiles such as the BGM-109 Tomahawk.

Guidance is the "driver" of a vehicle. It takes input from the navigation system (where am I) and uses targeting information (where do I want to go) to send signals to the flight control system that will allow the vehicle to reach its destination (within the operating constraints of the vehicle). The "targets" for guidance systems are one or more state vectors (position and velocity) and can be inertial or relative. During powered flight, guidance is continually calculating steering directions for flight control. For example the space shuttle targets an altitude, velocity vector, and gamma to drive main engine cut off. Similarly, an Intercontinental ballistic missile also targets a vector. The target vectors are developed to fulfill the mission and can be preplanned or dynamically created.

Control. Flight control is accomplished either aerodynamically or through powered controls such as engines. Guidance sends signals to flight control. A Digital Autopilot (DAP) is the interface between guidance and control. Guidance and the DAP are responsible for calculating the precise instruction for each flight control. The DAP provides feedback to guidance on the state of flight controls.