Block Diagram for Navigation, Guidance and Control

Guidance, Navigation and Control

Guidance, navigation and control (abbreviated GNC, GN&C, or G&C) is a branch of engineering dealing with the design of systems to control the movement of vehicles, especially, automobiles, ships, aircraft, and spacecraft. In many cases these functions can be performed by trained humans. However, because of the speed of, for example, a rocket's dynamics, human reaction time is too slow to control this movement. Therefore, systems—now almost exclusively based around digital electronics—are used for such control. Even in cases where humans can perform these functions, it is often the case that GNC systems provide benefits such as alleviating operator work load, smoothing turbulence, fuel savings, etc. In addition, sophisticated applications of GNC enable automatic or remote control.

• Guidance refers to the determination of the desired path of travel (the "trajectory") from the vehicle's current location to a designated target, as well as desired changes in velocity, rotation and acceleration for following that path.
• Navigation refers to the determination, at a given time, of the vehicle's location and velocity (the "state vector") as well as its attitude.
• Control refers to the manipulation of the forces, by way of steering controls, thrusters, etc., needed to track guidance commands while maintaining vehicle stability.

Guidance, navigation and control software command the GN&C system to effect vehicle control and to provide the sensor and controller data needed to compute these commands. The process involves three steps: guidance equipment and software first compute the orbiter location required to satisfy mission requirements, navigation then tracks the vehicle's actual location, and flight control then transports theorbiter to the required location.

A redundant set of four orbiter general-purpose computers forms the primary avionics software system; a fifth GPC is used as the backup flight system.

The GPCs interface with the various systems through the orbiter's flight forward and flight aft multiplexers/demultiplexers.
 The data buses serve as a conduit for signals going to and from the various sensors that provide velocity and attitude information as well as for signals traveling to and from the orbiter propulsion systems, orbiter aerodynamic control surfaces, and displays and controls.


The GN&C system consists of two operational modes: auto and manual (control stick steering). In the automatic mode, the primary avionics software system essentially allows the GPCs to fly the vehicle; the flight crew simply selects the various operational sequences. The flight crew may control the vehicle in the control stick steering mode using hand controls, such as the rotational hand controller, translational hand controller, speed brake/thrust controller and rudder pedals. The translational hand controller is available only for the commander, but both the commander and pilot have a rotational hand controller.


In the control stick steering mode, flight crew commands must still pass through and be issued by the GPCs. There are no direct mechanical links between the flight crew and the orbiter's various propulsion systems or aerodynamic surfaces; the orbiter is an entirely digitally controlled, fly-by-wire vehicle.


During launch and ascent, most of the GN&C commands are directed to gimbal the three space shuttle main engines and solid rocket boosters to maintain thrust vector control through the vehicle's center of gravity at a time when the amount of consumables is changing rapidly. In addition, the GN&C controls SSME throttling for maximum aerodynamic loading of the vehicle during ascent-referred to as max q-and to maintain an acceleration of no greater than 3 g's during the ascent phase. To circularize the orbit and perform on-orbit and deorbit maneuvers, the GN&C commands the orbital maneuvering system engines. At external tank separation, on orbit and during portions of entry, GN&C controls commands to the reaction control system. In atmospheric flight, GN&C controls the orbiter aerodynamic flight control surfaces.


Functions of GN&C software include flight control, guidance, navigation, hardware data processing and flight crew display. Specific function tasks and their associated GN&C hardware vary with each mission phase.
Vehicle control is maintained and in-flight trajectory changes are made during powered flight by firing and gimbaling engines. During atmospheric flight, these functions are performed by deflecting aerosurfaces. Flight control computes and issues the engine fire and gimbal commands and aerosurface deflection commands.
Flight control includes attitude processing, steering, thrust vector control and digital autopilots. Flight control receives vehicle dynamics commands (attitudes, rates and accelerations) from guidance software or flight crew controllers and processes them for conversion to effector commands (engine fire, gimbal or aerosurface). Flight control output commands are based on errors for stability augmentation. The errors are the difference between the commanded attitude, aerosurface position, body rate or body acceleration and the actual attitude, position, rate or acceleration.


Actual attitude is derived from inertial measurement unit angles, aerosurface position is provided by feedback transducers in the aerosurface servoamplifiers, body rates are sensed by rate gyro assemblies, and accelerations are sensed by accelerometer assemblies. In atmospheric flight, flight control adjusts control sensitivity based on air data parameters derived from local pressures sensed by air data probes and performs turn coordination using body attitude angles derived from IMU angles. Thus, GN&C hardware required to support flight control is a function of the mission phase.


The guidance steering commands used by the flight control software are augmented by the guidance software or are manually commanded by the hand controller or speed brake/thrust controller. When flight control software uses the steering commands computed by guidance software, it is termed automatic guidance; when the flight crew is controlling the vehicle by hand, it is called control stick steering. The commands computed by guidance are those required to get from the current state (position and velocity) to a desired state (specified by target conditions, attitude, airspeed and runway centerline). The steering commands consist of translational and rotational angles, rates and accelerations. Guidance receives the current state from navigation software. The desired state or targets are part of the initialized software load and some may be changed manually in flight.
The navigation system maintains an accurate estimate of vehicle position and velocity, referred to as a state vector. From position, attitude and velocity, other parameters (acceleration, angle of attack) are calculated for use in guidance and for display to the crew. The current state vector is mathematically determined from the previous state vector by integrating the equations of motion using vehicle acceleration as sensed by the IMUs and/or computed from gravity and drag models. The alignment of the IMU and, hence, the accuracy of the resulting state vector deteriorate as a function of time. Celestial navigation instruments (star trackers and crewman optical alignment sight) are used to maintain IMU alignment in orbit. For entry, the accuracy of the IMU-derived state vector is, however, insufficient for either guidance or the flight crew to bring the spacecraft to a pinpoint landing. Therefore, data from other navigation sensors-air data system, tactical air navigation, microwave scan beam landing system and radar altimeter-is blended into the state vector at different phases of entry to provide the necessary accuracy. The three IMUs maintain an inertial reference and provide velocity changes until the microwave scan beam landing system is acquired. Navigation-derived air data are needed during entry as inputs to guidance, flight control and flight crew dedicated displays. Such data are provided by tactical air navigation, which supplies range and bearing measurements beginning at 160,000 feet; the air data system provides information at about Mach 3. Tactical air navigation is used until the microwave scan beam landing system is acquired or an altitude of 1,500 feet is reached if MSBLS is not available.
During rendezvous and proximity operations, the onboard navigation system maintains the state vectors of both the orbiter and target vehicle. During close operations (separation of less than 15 miles), these two state vectors must be very accurate in order to maintain an accurate relative state vector. Rendezvous radar measurements (range and range rate) are used for a separation of about 15 miles to 100 feet to provide the necessary relative state vector accuracy. When two vehicles are separated by less than 100 feet, the flight crew relies primarily on visual monitoring (aft and overhead windows and closed-circuit television).


In summary, GN&C hardware sensors used by navigation include IMUs, star trackers, the crewman optical alignment sight, tactical air navigation, air data system, microwave scan beam landing system, radar altimeter and rendezvous radar. The GN&C hardware sensors used by the flight control system are accelerometer assemblies, orbiter rate gyro assemblies, solid rocket booster rate gyro assemblies, controllers and aerosurface servoamplifiers.

Modern Navigation Methods

Dead reckoning

 or DR, in which one advances a prior position using the ship's course and speed. The new position is called a DR position. It is generally accepted that only course and speed determine the DR position. Correcting the DR position for leeway, current effects, and steering error result in an estimated position or EP. An inertial navigator develops an extremely accurate EP.

Pilotage

 involves navigating in restricted waters with frequent determination of position relative to geographic and hydrographic features.

Celestial navigation

 involves reducing celestial measurements to lines of position using tables, spherical trigonometry, and almanacs.

Electronic navigation 

Radio navigation

 uses radio waves to determine position by either radio direction finding systems or hyperbolic systems, such asDecca, Omega and LORAN-C.

Radar navigation

 uses radar to determine the distance from or bearing of objects whose position is known. This process is separate from radar’s use as a collision avoidance system

Satellite navigation

 uses artificial earth satellite systems, such as GPS, to determine position

Modern Techniques of Navigation

Most modern navigation relies primarily on positions determined electronically by receivers collecting information from satellites. Most other modern techniques rely on crossing lines of position or LOP. A line of position can refer to two different things: a line on a chart and a line between the observer and an object in real life. A bearing is a measure of the direction to an object.^[18] If the navigator measures the direction in real life, the angle can then be drawn on a nautical chartand the navigator will be on that line on the chart.

In addition to bearings, navigators also often measure distances to objects.On the chart, a distance produces a circle or arc of position. Circles, arcs, and hyperbolae of positions are often referred to as lines of position.

If the navigator draws two lines of position, and they intersect he must be at that position. A fix is the intersection of two or more LOPs.

If only one line of position is available, this may be evaluated against the Dead reckoning position to establish an estimated position.

Lines (or circles) of position can be derived from a variety of sources:

• celestial observation (a short segment of the circle of equal altitude, but generally represented as a line),
• terrestrial range (natural or man made) when two charted points are observed to be in line with each other,
• compass bearing to a charted object,
• radar range to a charted object,
• on certain coastlines, a depth sounding from echo sounder or hand lead line.

There are some methods seldom used today such as "dipping a light" to calculate the geographic range from observer to lighthouse.

Methods of navigation have changed through history. Each new method has enhanced the mariner’s ability to complete his voyage. One of the most important judgments the navigator must make is the best method to use. Some types of navigation are depicted in the table.

Basic concepts

Latitude

Roughly, the latitude of a place on Earth is its angular distance north or south of the equator. Latitude is usually expressed in degrees (marked with °) ranging from 0° at the Equator to 90° at the North and South poles. The latitude of the North Pole is 90° N, and the latitude of the South Pole is 90° S.^[10] Mariners calculated latitude in the Northern Hemisphere by sighting the North Star Polaris with a sextant and sight reduction tables to correct for height of eye and atmospheric refraction. The height of Polaris in degrees above the horizon is the latitude of the observer, within a degree or so.

Longitude

Similar to latitude, the longitude of a place on Earth is the angular distance east or west of the prime meridian or Greenwich meridian.^[10] Longitude is usually expressed in degrees (marked with °) ranging from 0° at the Greenwich meridian to 180°east and west. Sydney, for example, has a longitude of about 151° east. New York City has a longitude of 74° west. For most of history, mariners struggled to determine longitude. Longitude can be calculated if the precise time of a sighting is known. Lacking that, one can use a sextant to take a lunar distance (also called the lunar observation, or lunar for short) that, with a nautical almanac, can be used to calculate the time at zero longitude (see Greenwich Mean Time). Reliablemarine chronometers were unavailable until the late 18th century and not affordable until the 19th century.For about a hundred years, from about 1767 until about 1850, mariners lacking a chronometer used the method of lunar distances to determine Greenwich time to find their longitude. A mariner with a chronometer could check its reading using a lunar determination of Greenwich time.

Etymology

1530s, from L. navigationem (nom. navigatio), from navigatus, pp. of navigare "to sail, sail over, go by sea, steer a ship," from navis "ship" and the root of agere "to drive". Also, From Middle English navigate, from Latin navigo, from nāvis (“ship”) + agō (“do”), from Proto-Indo-European *nau- (boat), it is in fact known to have originated from the Sanskrit word "navgati" which means science of sailing, 'Nav' means ship and 'Gati' means pace or speed in Sanskrit.

History

In the European medieval period, navigation was considered part of the set of seven mechanical arts, none of which were used for long voyages across open ocean. Polynesian navigation is probably the earliest form of open ocean navigation, though it was based on memory and observation rather than on scientific methods or instruments. Early Pacific Polynesians used the motion of stars, weather, the position of certain wildlife species, or the size of waves to find the path from one island to another.

Maritime navigation using scientific instruments such as the mariner's astrolabe first occurred in the Mediterranean during the Middle Ages. Although land astrolabes were invented in the Hellenistic period and existed in classical antiquity and the Islamic Golden Age, the oldest record of a sea astrolabe is that of Majorcan astronomer Ramon Llull dating from 1295. The perfectioning of this navigation instrument is attributed to Portuguese navigators during early Portuguese discoveries in the Age of Discovery.The earliest known description of how to make and use a sea astrolabe comes from Spanish cosmographer Melvin Mel Pros Cespedes's  Arte de Navegar (The Art of Navigation) published in 1551,based on the principle of the archipendulum used in constructing the Egyptian pyramids.

Open-seas navigation using the astrolabe and the compass started during the Age of Discovery in the 15th century. The Portuguese began systematically exploring the Atlanticcoast of Africa from 1418, under the sponsorship of Prince Henry. In 1488 Bartolomeu Dias reached the Indian Ocean by this route. In 1492 the Spanish monarchs fundedChristopher Columbus's expedition to sail west to reach the Indies by crossing the Atlantic, which resulted in the Discovery of America. In 1498, a Portuguese expedition commanded by Vasco da Gama reached India by sailing around Africa, opening up direct trade with Asia. Soon, the Portuguese sailed further eastward, to the Spice Islands in 1512, landing in China one year later.

The first circumnavigation of the earth was completed in 1522 with the Magellan-Elcano expedition, a Spanish voyage of discovery led by Portuguese explorer Ferdinand Magellanand completed by Spanish navigator Juan Sebastián Elcano after the former's death in the Philippines in 1521. The fleet of seven ships sailed from Sanlúcar de Barrameda in Southern Spain in 1519, crossed the Atlantic Ocean and after several stopovers rounded the southern tip of South America. Some ships were lost, but the remaining fleet continued across the Pacific making a number of discoveries including Guam and the Philippines. By then, only two galleons were left from the original seven. The Victoria led by Elcano sailed across the Indian Ocean and north along the coast of Africa, to finally arrive in Spain in 1522, three years after its departure. The Trinidad sailed east from the Philippines, trying to find a maritime path back to the Americas, but was unsuccessful. The eastward route across the Pacific, also known as the tornaviaje (return trip) was only discovered forty years later, when Spanish cosmographer Andrés de Urdaneta sailed from the Philippines, north to parallel 39º, and hit the eastward Kuroshio Current which took its galleon across the Pacific. He arrived in Acapulco on October 8, 1565.

Guidance, Navigation, & Control Systems

The guidance, navigation, and control subsystem controlled the orientation of the orbiter (the direction in which it was pointed) as it traveled through space and maintained knowledge of where celestial bodies were located (for example, Earth and the Sun). This knowledge was critical for the spacecraft to perform the correct maneuvers to get to Mars, to keep its solar arrays pointed toward the Sun to produce power, and to keep its antenna pointed toward the Earth to maintain communications.

While the spacecraft is in orbit around Mars, this subsystem continues to maintain constant knowledge of where the spacecraft is in its orbit, and is used to point the science cameras very accurately (within about 1/20th of one degree). That ability is critical to enable science instruments to take images of desired targets on the surface.

Together, the guidance, navigation, and control subsystem and structures subsystem were designed to provide the smoothest ride possible. Preventing even tiny vibrations (from, for example, movement of the solar arrays) is crucial, as the shaking motion could otherwise cause the science images to become blurred.

To perform its functions, the guidance, navigation and control subsystem uses several types of:

	sensors:	for determining where the spacecraft is pointed, how fast it is turning, and how its speed is changing.
	control devices:	for changing the spacecraft's pointing direction, rate of turning, and speed. Courtesy: Jet Propulsion Laboratory (California Institute of Technology)

What is Navigation ?

Navigation is a field of study that focuses on the process of monitoring and controlling the movement of a craft or vehicle from one place to another.The field of navigation includes four general categories: land navigation, marine navigation, aeronautic navigation, and space navigation.It is also the term of art used for the specialized knowledge used by Navigators to perform navigation tasks. All navigational techniques involve locating the navigator's position compared to known locations or patterns.