1 Introduction

1.1 DEFINITIONS

Navigation is the determination of the position and velocity of a moving vehicle. The three components of position and the three components of velocity make up a six-component state vector that fully describes the translational motion of the vehicle. Navigation data are usually sent to other on-board subsystems, for example, to the flight control, flight management, engine control, communication control, crew displays, and (if military) weapon-control computers.

Navigation sensors may be located in the vehicle, in another vehicle, on the ground, or in space. When the state vector is measured and calculated on board, the process is called navigation. When it is calculated outside the vehicle, the process is called surveillance or position location. Surveillance information is employed to prevent collisions among aircraft. The humans and computers that direct civil air traffic and most military traffic are located in Air Route Traffic Control Centers on the ground, whereas some military controllers are based in surveillance aircraft or aircraft carriers. Existing traffic control systems observe the position of aircraft using sensors outside the aircraft (e.g., surveillance radars) or reports of position from the aircraft itself. “Automatic dependent surveillance” is a term for the reporting of position, measured by sensors in an aircraft, to a traffic control center.

Traditionally, ship navigation included the art of pilotage: entering and leaving port, making use of wind and tides, and knowing the coasts and sea conditions. However, in modern usage, navigation is confined to the measurement of the state vector. The handling of the vehicle is called guidance; more specifically, it is called conning for ships, flight control for aircraft, and attitude control for spacecraft. This book is concerned only with the navigation of manned and unmanned aircraft. The calculation of the navigation state vector requires the definition of a navigation coordinate frame (as discussed in Chapter 2).

1.2 GUIDANCE VERSUS NAVIGATION

The term “guidance” has two meanings, both of which are different from “navigation”:

1. Steering toward a destination of known position from the aircraft's present position. The steering equations can be derived from a plane triangle for nearby destinations or from a spherical triangle for distant destinations (Chapter 2).
2. Steering toward a destination without explicitly measuring the state vector. A guided vehicle can home on radio, infrared, or visual emissions. Guidance toward a moving target is usually of interest for military tactical missiles in which a steering algorithm ensures impact within the maneuver and fuel constraints of the interceptor. One of several related guidance algorithms, collectively called proportionial navigation, processes sensor data and steers the vehicle to impact. Guidance toward a fixed target involves beam-riding, as in the Instrument Landing System (Chapter 13).

1.3 CATEGORIES OF NAVIGATION

Navigation systems can be categorized as positioning or dead-reckoning. Positioning systems measure the state vector without regard to the path traveled by the vehicle in the past. There are three kinds of positioning systems:

1. Radio systems (Chapters 4 to 6). They consist of a network of transmitters (sometimes also receivers) on the ground, in satellites, or on other vehicles. The airborne navigation set detects the transmissions and computes its position relative to the known positions of the stations in the navigation coordinate frame. The aircraft's velocity is measured from the Doppler shift of the transmissions or from a sequence of position measurements.
2. Celestial systems (Chapter 12). They compute position by measuring the elevation and azimuth of celestial bodies relative to the navigation coordinate frame at precisely known times. Celestial navigation is used in special-purpose high-altitude aircraft in conjunction with an inertial navigator. Manual celestial navigation was practiced at sea for millennia and in aircraft from the 1930s to the 1960s.
3. Mapping navigation systems (Section 2.6). They observe images of the ground, profiles of altitude, or other external features.

Dead-reckoning navigation systems derive their state vector from a continuous series of measurements relative to an initial position. There are two kinds of dead-reckoning measurements:

1. Aircraft heading and either speed or acceleration. For example, heading can be measured with gyroscopes (Chapter 7) or magnetic compasses (Chapter 9), while speed can be measured with air-data sensors (Chapter 8) or Doppler radars (Chapter 10). Vector acceleration is measured with inertial sensors (Chapter 7).
2. Emissions from continuous-wave radio stations. They create ambiguous “lanes” (Chapter 4) that must be counted to keep track of coarse position. Their phase is measured for fine positioning. They must be reinitialized after any gap in radio coverage.

Dead-reckoning systems must be re-initialized as errors accumulate and if electric power is lost.

1.4 THE VEHICLE

1.4.1 Civil Aircraft

The civil aviation industry consists of air carriers and general aviation. Air carriers operate large aircraft used on trunk routes and small aircraft used in commuter service. General aviation ranges from single-place crop dusters to well-equipped four-engine corporate jets.

Most air carriers and general-aviation jet aircraft operate exclusively in developed areas where ground-based radio aids are plentiful. Others operate over oceans and undeveloped areas where, before the Global Positioning System (GPS, Chapter 5), navigation aids were nonexistent. Before the 1970s, such aircraft had astrodomes through which a human navigator took celestial fixes with a bubble-sextant (Chapter 12). From the 1970s to 2000, aircraft flying over oceans and undeveloped areas used unaided inertial systems or Omega (Chapter 4). By the year 2000, most of these aircraft will use GPS alone or in combination with inertial systems (Chapter 7). Beginning in the mid-1980s, the US-FAA allowed overwater flight with a single long-range navigation set and a separate single long-range communication set.

Simple general-aviation aircraft (including helicopters) operate over short routes, have two or fewer engines, and are flown by one or two pilots. They are used for water drops on fires, search-and-rescue, ferrying crews to offshore oil platforms, police patrols, interplant shuttles, crop dusting, and carrying logs from forests, for example. Each usage has its own navigation requirements. The simplest aircraft navigate visually or with Loran or GPS sets; the more complex aircraft use the same navigation equipment as do air carriers. In 1996 civil helicopters used VOR/DME (see Chapter 4) in developed areas. They landed visually because their approach paths were too steep for the instrument landing system (ILS, Chapter 13). Many will adopt GPS for instrument approaches.

Civil aircraft fly in a benign environment; the major electrical stresses on avionic equipment are caused by lightning and electric-power transients; the major mechanical stresses are caused by air turbulence, hard landings, and abusive handling by maintenance technicians. Figure 1.1 shows the antenna farm and avionics bays on an advanced transport that is outfitted for civil and military usage. The avionics bay is below the cockpit in the space between the radome and nose wheel well (in many civil aircraft, the avionics bay is aft of the nosewheel). Avionics and air-data sensors are located in the bay. Access is beneath the aircraft.

Figure 1.1 Avionics placement on multi-purpose transport (Courtesy of McDonnell Douglas, modified by author).

Trans-Pacific hypersonic aircraft may be developed in the twenty-first century that will navigate as does the Space Shuttle: inertial boost, GPS or celestial midcourse, and GPS or other radio approach. They will compete with electronic mail and teleconferences.

1.4.2 Military Aircraft

Fixed-wing military aircraft can be divided as follows:

1. Interceptors and combat air patrols. These small, high-climb-rate aircraft protect the homeland, a naval fleet, or an invasion force by seeking and destroying invading bombers, cruise missiles, and aircraft that carry contraband. Interceptors are vectored to their targets by ground-based, ship-borne, or airborne command posts. Interceptors carry on-board air-to-air radar (Chapter 11) to close on their targets. They use inertial navigation (Chapter 7) or Tacan (Chapter 4) to return to their bases. GPS may replace Tacan for returning to fixed bases leaving Tacan for returning to aircraft carriers.
2. Close-air support. These medium-sized aircraft deliver weapons in support of land armies. They may attack troops, tanks, convoys, or command centers. They have inertial navigators or GPS to locate the approximate position of targets and have sensors (e.g., optics and moving-target-indicating (MTI) radar) to locate the precise position of the targets. They carry communication systems that keep them in contact with local troop commanders and airborne command posts. These aircraft have relied on inertial navigation and Tacan to return to their bases. Close-air support aircraft carry inertial navigators as precise attitude references for optical sensors and as velocity references for releasing weapons.
3. Interdiction. These medium-sized and large aircraft strike behind enemy lines to attack strategic targets such as factories, power plants, and military installations. Nuclear strategic bombers and fighter bombers are included in this category. These aircraft carry the most precise navigation systems, based on inertial, GPS, and celestial sensors. They may obtain en-route position fixes with optical or radar image comparators or terrain matching (Section 2.6). Inertial navigators provide precise attitude and velocity references for pointing terminal-area optic sensors and for releasing weapons. Interdiction aircraft often have sensors that find tanker aircraft and allow formation flight in instrument weather conditions. Flying at treetop level to avoid enemy radars complicates the task of navigation.
4. Cargo carriers. These aircraft have the same navigation requirements as do civil aircraft; in addition, they drop cargos by parachute and refuel from tankers. Cargo drops require flight along a predetermined path and release at predetermined positions. Cargo aircraft are also sometimes outfitted as refueling-tankers and mobile hospitals. Tankers are equipped with radar beacons to aid in rendezvous. They may be asked to make Category III landings at third-world airports.
5. Reconnaissance aircraft. These aircraft collect photographic and electronic-signals data. They navigate precisely in order to annotate the data and fly close to hostile borders. They measure velocity precisely in order to compensate cameras and synthetic-aperture radars for vehicle motion.
6. Helicopter and short-takeoff-and-landing (STOL) vehicles. Military helicopters often support troops, for example, to attack tanks, to suppress artillery and small-arms fire, and to transport soldiers and casualties. They search for and destroy submarines from their bases on large ships. They measure position, so they can locate targets in the coordinate frames established by command posts. Most of them navigate by visual pilotage. Some Navy and Army helicopters dead-reckon with Doppler radar and compass (Chapters 9 and 10) using Tacan to return to their ships or land bases. They measure velocity precisely, so they can hover, handover targets, launch weapons, and transition from vertical to horizontal flight. Airspeed is difficult to measure due to the downwash from the rotors. Doppler radar can establish a coordinate frame fixed in the moving ocean surface that is useful when working with submarine-detecting sonobuoys. Search-and-rescue helicopters carry receivers that detect and direction-find emergency locator transmitters [8]. Complex helicopter weapon-platforms carry inertial navigators and optical imagers for locating targets and for landing.
7. Unmanned air vehicles (once called “remotely piloted vehicles”). They range in size from model airplanes to ten thousand kilograms. They are used as target drones, reconnaissance vehicles, and strategic bombers. They attack high-risk targets (radiation-emitting antennas and artillery) without endangering the lives of a crew. Some have elaborate inertial, map-matching, and acoustic sensors; some carry Doppler radars. They often navigate inertially until their on-board optical or infrared sensors acquire the target, then guide themselves to impact with submeter accuracy.

Virtually every military aircraft carries an instrument-landing system (ILS) receiver (Chapter 13). In the past, some relied on a “ground-controlled approach” in which a human, watching a radar display on the ground or on an aircraft carrier, radioed steering commands to the pilot. The special problems of landing on an aircraft carrier are discussed in Chapter 13. Military aircraft often engage in high-speed, high-g, low-altitude maneuvers that challenge the mechanical design of on-board avionics. Guns and rocket launchers impose shock and vibration loads. By the year 2000, most military aircraft will carry GPS receivers.

1.5 PHASES OF FLIGHT

1.5.1 Takeoff

The takeoff phase begins upon taxiing onto the runway and ends when climb-out is established on the projected runway centerline. The aircraft is guided along the centerline by hand-flying or a coupled autopilot based on steering signals (from an 1LS localizer since 1945). Two important speed measurements are made on the runway. The highest ground speed at which an aborted takeoff is possible is precomputed and compared, during the takeoff run, to the actual ground speed as displayed by the navigation system. The airspeed at which the nose is lifted (“:rotation”) is precalculated and compared to the actual airspeed as displayed by the air-data system. Barometric altitude rate or GPS-derived altitude rate (inertially smoothed) is measured and monitored.

1.5.2 Terminal Area

The terminal phase consists of departure and approach subphases. Departure begins when the aircraft maneuvers away from the projected runway centerline and ends when it leaves the terminal-control area (by which time it is established on an airway). Approach begins when the aircraft enters the terminal area (by which time it has left the airway) and ends when it intercepts the landing aid at an approach fix. In 1996, vertical navigation was based on barometric altitude, and heading vectors were assigned by traffic controllers. Major airports have standard approach and departure routes unique to each runway. In the United States, the desired terminal-area navigation accuracy is 1.7 nmi, 2-sigma per Advisory Circular 20-130. Further details are in Chapter 14.

1.5.3 En Route

The en-route phase leads from the origin to the destination and alternate destinations (an alternate destination is required of civil aircraft operating under instrument flight rules). From the 1930s to the 1990s, airways were defined by navigation aids over land and by latitude-longitude fixes over water. The width of airways and their lateral separation depended on the quality of the defining navaids and the distance between them. The introduction of inertial navigation systems and DME in the 1970s caused aviation authorities to create “area-navigation” airways (RNAV) that do not always interconnect VOR navaids [7: AC-90-45A] (see Section 2.7.4).

Beginning in the 1990s, GPS has allowed precise navigation anywhere, not just on airways. Given the extensive use of on-board collision-avoidance equipment and the trend toward reducing government budgets, “free-flight” is being introduced in controlled airspace. Each aircraft would agree on a route before takeoff and then be free to change the route, after interaircraft communication verified that the risk of collision is sufficiently low. En-route surveillance by independent ground-based radars may disappear or be replaced by position fixes and reports via com-nav satellites. Busy terminal areas and airport surfaces are likely to remain under central, positive control.

In the United States in 1996, the en-route navigation error must be less than 2.8 nmi over land and 12 nmi over oceans (2-sigma) [7: AC-20-130]. As regional maps become available in digital form with aeronautical annotation (e.g., minimum en-route altitude), aircraft in undeveloped areas will use GPS for en-route navigation and nonprecision approaches.

1.5.4 Approach

The approach phase begins at acquisition of the landing aid and continues until the airport is in sight or the aircraft is on the runway, depending on the capabilities of the landing aid (Chapter 13).

During an approach, the decision height (DH) is the altitude above the runway at which the approach must be aborted if the runway is not in sight. The better the landing aids, the lower the decision height. Decision heights are published for each runway at each airport (Chapter 13). The decision height for a Category III landing is 100 ft or less. By law, an approach may not even be attempted unless the horizontal visibility, measured by a runway visual range (RVR) instrument, exceeds a threshold that ranges from zero (Category IIIC, not approved anywhere in 1996) to 800 meters (Category I). A nonprecision approach has electronic guidance only in the horizontal direction. An aircraft executing a nonprecision approach must abort if the runway is not visible at the minimum descent altitude, which is typically 700 ft above the runway.

In 1996, civil aircraft outside the ex-Soviet bloc used the Instrument Landing System for low-ceiling, low-visibility approaches. A Microwave Landing System had been approved by the International Civil Aviation Organization for precision approaches and was being installed at major international airports, especially in Europe. In the United States, Loran and GPS had been approved for nonprecision approaches at many airports. (Landing aids are discussed in Chapter 13.)

1.5.5 Landing

The landing phase begins at the decision height (when the runway is in sight) and ends when the aircraft exits the runway. Navigation during flare and decrab may be visual or the navigation set's electrical output may be coupled to an autopilot. A radio altimeter measures the height of the main landing gear above the runway for guiding the flare. The rollout is guided by the landing aid (e.g., the ILS localizer). Landing navigation is described in Chapter 13.

1.5.6 Missed Approach

A missed approach is initiated at the pilot's option or at the traffic controller's request, typically because of poor visibility, poor alignment with the runway, equipment failure, or conflicting traffic. The flight path and altitude profile for a missed approach are published on the approach plates. The missed approach consists of a climb to a predetermined holding fix at which the aircraft awaits further instructions. Terminal area navigation aids are used.

1.5.7 Surface

Aircraft movement from the runway to gates, hangars, or revetments is a major limit on airport capacity in instrument meteorological conditions. Surface navigation is visual on the part of the crew, whereas the ground controllers observe aircraft visually or with a surface surveillance radar. No matter how good the surface navigation, collision avoidance among aircraft and ground vehicles requires central guidance, typically provided by a human controller with computer assistance. Position reports (e.g., via GPS) from aircraft that are concealed in radar shadows reduce the risk of collision and help keep unwanted aircraft off active runways.

1.5.8 Weather

Instrument meteorological conditions (IMC) are weather conditions in which visibility is restricted, typically less than 3 miles as defined by law. Aircraft operating in IMC are supposed to fly under instrument flight rules (IFR), defined by law in each country (Chapters 13 and 14).

1.6 DESIGN TRADE-OFFS

The navigation-system designer conducts trade-offs for each aircraft and mission to determine which navigation systems to use. Trade-offs consider the following attributes:

1. Cost. Included are the construction and maintenance of transmitter stations and the purchase of on-board hardware and software. Users are concerned only with the cost of on-board hardware and software. In the past, governments have paid to operate radio-navigation transmitters. In the future, combined com-nav aids may be operated privately and funded by user charges.
2. Accuracy of position and velocity. This is specified in terms of the statistical distribution of errors as observed on a large number of flights [4]. The accuracy of military systems is often characterized by circular error probable (CEP, in meters or nautical miles; Chapter 2). The maximum allowable CEP is frequently established by the kill radius of the weapons that are released from the aircraft. For civil air carriers, the allowable en-route navigation error is based on the calculated risk of collision. In the 1990s, each subsystem was allocated a safety-related failure probability of 10⁻⁹ per hour [4]. The accuracy of the navigation systems is often defined as “twice the distance root mean square” (2drms), which encompasses 95% to 98% of the errors (Section 2.8.1). The allowable landing error depends on runway width, aircraft handling characteristics, and flying weather.
3. Autonomy. This is the extent to which the vehicle determines its own position and velocity without external aids. Autonomy is important to certain military vehicles and to civil vehicles operating in areas of inadequate radio-navigation coverage. Autonomy can be subdivided into five classes:
   • Passive self-contained systems that neither receive nor transmit electromagnetic signals. They emit no radiation that would betray their presence and require no external stations. Failures are detected and corrected on board. They include dead-reckoning systems such as inertial navigators.
   • Active self-contained systems that radiate but do not receive externally generated signals. Examples are radars and sonars. They do not depend on the existence of navigation stations.
   • Receivers of natural radiation. These systems measure naturally emitted electromagnetic radiation. Examples are magnetic compasses, star trackers, and passive map correlators. Some unmanned military weapons guide themselves toward acoustic emissions. These systems do not announce their presence by emitting, nor do they need navigation stations.
   • Receivers of artificial radiation. These systems measure electromagnetic radiation from navaids (Earth based or space based) but do not themselves transmit. Examples are Loran, Omega, VOR (Chapter 4), and GPS (Chapter 5). They require external cooperating stations but do not betray their own presence.
   • Active radio navaids that exchange signals with navigation stations. These include DME, JTIDS, PLRS, and collision-avoidance systems (Chapters 4, 6, and 14). The vehicle betrays its presence by emitting and requires cooperative external stations. These are the least autonomous of navigation systems.
4. Time delay in calculating position and velocity, caused by computational and sensor delays. Time delay (also called latency) can be caused by computer-processing delays, scanning by a radar beam, or gaps in satellite coverage, for example. Forty years ago, it took five minutes to plot a fix manually on an on-board aeronautical chart. Today, navigation calculations are completed in tens of milliseconds by a digital computer.
5. Geographic coverage. Terrestrial radio systems operating below approximately 100 KHz can be received beyond line of sight on Earth; those operating above approximately 100 MHz are confined to line of sight. Each satellite can cover millions of square miles of Earth, while a constellation of satellites can cover the entire Earth.
6. Automation. The aircraft's crew receives a direct reading of position, velocity, and equipment status, usually without human intervention, as described in Section 1.9. In years past, navigation sets were operated by skilled people, to the extent of manipulating wave forms on cathode-ray tubes.
7. Availability. This is the fraction of time that the system is usable for navigation. Downtime is caused by scheduled maintenance, by unscheduled outage (usually due to equipment failure), and by radio-propagation problems that cause excessive errors.
8. System capacity. This is the number of aircraft that the system can accommodate simultaneously. It applies to two-way ranging systems.
9. Ambiguity. This is the identification, by the navigation system, of two or more possible positions of the aircraft, with no indication of which is correct. Ambiguities are characteristic of ranging and hyperbolic systems when too few stations are received.
10. Integrity. This is the ability of the system to provide timely warnings to aircraft when its errors are excessive. For en-route navigation in 1996, an alarm must be generated within 30 seconds of the time a computed position exceeds its specified error. For a nonprecision landing aid, an alarm must be generated within ten seconds. For a precision landing aid, an alarm must be generated within two seconds. Integrity is an important issue for GPS, especially when it is used as a landing aid in the differential mode (Chapters 5 and 13). Any sensor that is the sole means of navigation must have high integrity.

1.7 EVOLUTION OF AIR NAVIGATION

The earliest aircraft were navigated visually. Pilots had an anemometer for air-speed, a barometer for altitude, and a magnetic compass for heading. Artificial horizons and turn-and-bank indicators allowed pilots to hold attitude and heading in clouds, hence motivating the installation of navigation aids. Lighted beacons were installed across the United States in the 1920s to mark airmail routes. Starting in 1929, four-course radio beacons were also added to the lighted airways to guide aircraft. Four-course beacons were installed in France, South America, and North Africa. In the 1930s, aircraft were equipped with medium-frequency and high-frequency direction finders (MF/DF and HF/DF) that measured the bearing of broadcast stations relative to the axis of the aircraft. A fix was obtained by plotting the direction toward two or more stations. Beacons near an airport allowed aircraft to fly a “nonprecision approach” to the runway (Chapter 13). Vertical beacons at 75 MHz, called z-beacons or marker beacons, were installed along the four-course airways and along approaches to runways to give a positive indication of position (Chapters 13 and 14).

Air-traffic control was procedural, following the precedent of railroad “block” clearances. Overland airways connected radio beacons; overwater airways were defined on a map, hundreds of miles apart. The airways were divided into longitudinal blocks of 20- to 30-minutes flying time. The air-traffic controller relied on the pilots' report of position, allowed only one aircraft at a time to enter a block, and kept the block free of other traffic until the pilot reported leaving. The size of the block was commensurate with the uncertainties in navigation at the time.

During World War II, meteorologists learned to route aircraft to take advantage of the cyclonic winds that circle around high- and low-pressure regions at mid- and high-latitudes. Bellamy [12] states that the transatlantic flying time was reduced an average of 10% compared to a great-circle track, with occasional savings exceeding 25%, by taking advantage of cyclonic tail winds. These pressure-pattern routes were plotted graphically in the 1940s–1960s but are now computed routinely in airline and military dispatch offices.

Crosswinds cause an aircraft to “drift” perpendicularly to its longitudinal axis. From the 1930s to the 1960s, drift angle was measured in flight with a downward-looking telescope that observed the direction of movement of the ground, when it was visible. From the 1940s to the 1960s, drift was estimated over oceans by observing trends in the difference, D, between the readings of the radio altimeter and pressure altimeter. Bellamy showed that in cyclonic winds, drift is proportional to the horizontal gradient of D [12]. The introduction of Doppler and inertial navigators in the 1960s and 1970s allowed drift to be observed directly. The Doppler navigator measures the direction of the ground-speed vector relative to the aircraft's centerline. The inertial navigator subtracts in-flight-measured airspeed from the measured ground velocity to calculate wind, hence lateral drift.

After World War II, VOR stations (Chapter 4) and Instrument Landing Systems (ILS, Chapter 13) were installed. VOR/DME and ILS have been the basis of navigation in western countries ever since. During the 1960s, air-traffic controllers came to rely on surveillance radar in densely populated airspace (Chapter 14). The controller identified the aircraft on his screen, hence eliminating the need for a position report from the crew. Radar surveillance of air traffic is called “positive control,” which, in 1996, existed in the United States, most of Canada, western Europe, and Japan. In the late 1990s, the automatic reporting of on-board-derived position began to supplement (perhaps eventually to replace) radar surveillance.

The former Soviet republics have ICAO navigation aids and ILS at about 50 international airports and on corridors connecting them to the borders. Overflying western aircraft navigate inertially and with Omega, GPS, and nondirectional beacons. Since the late 1960s, domestic civil and military aircraft have used an L-band range-angle system known by its Russian acronym, RSBN, and not standardized by ICAO. It has 176 channels between 873 and 1000 MHz. Domestic airports guide landing aircraft with ground-based precision approach radar (PAR) using verbal commands to the crew. At international airports, PARs monitor aircraft on ILS approaches. In the 1990s, the former Soviet republics were purchasing western avionics equipment.

The People's Republic of China depended on imported Russian nondirectional beacons and PARs until the late 1970s, when it began to install western radars, ILS, VOR, and DME. In the 1990s, China installed VHF air-to-ground radio relays throughout most of the nation [13]. In 1996, western air-traffic control and navigation equipment was being installed throughout Southeast Asia and Indonesia.

Outside the developed world, major cities and some airways had VOR/DME-based procedural traffic control, so aircraft filing flight plans could be separated from each other by human controllers. Polar areas, the South Atlantic Ocean, and much of the Pacific and Indian Oceans had no navaids and no control whatsoever. Most of the rest of the world was divided into Flight Information Regions that advised crews of weather conditions and the status of airports and navigation aids but did not separate traffic. Position reports over oceans and in remote areas are mostly by HF radio but, beginning in the 1990s, were being made via satellite (e.g., North Pacific and Atlantic Oceans). In 1996, a few airlines were transmitting GPS-inertial position over digital data links via geostationary communication satellites over the Pacific Ocean, a system called Automatic Dependent Surveillance, the first step in the Future Air Navigation System (FANS, Chapter 14). Outside the United States and Canada, most aircraft pay directly for traffic control services.

Until the 1970s, precise absolute time could not economically be measured on a vehicle. Hence, radio navigation aids were built that measured the difference in time of arrival of radio signals from ground stations. The earliest (hyperbolic Loran and Decca, some military systems) date from the 1940s. As airborne clocks became more stable in the 1970s, “passive” or “one-way” ranging systems could solve for position and the absolute clock offset by processing precisely timed signals from several stations. Direct-ranging Loran and Omega (as distinguished from hyperbolic Loran and Omega, all discussed in Chapter 4), GPS and GLONASS (Chapter 5), and JTIDS (Chapter 6) are examples of such one-way ranging systems. As airborne clocks become more accurate in the twenty-first century, absolute time of arrival will be directly measurable and clock offsets will become negligible.

GPS and GLONASS are based on one-way passive range measurements to several stations, most of which are spacecraft (Figure 1.2). A few stations are ground-based pseudolites whose transmissions mimic those of spacecraft. Chapter 5 describes the GPS and GLONASS systems. The receiver in the airplane computes position, velocity, the offset in the airborne clock, and, in some receivers, the ionospheric delay (Chapter 5). In 1996, the military modes of GPS achieved 20-meter (2drms) accuracy anywhere in the world, while the civil mode could achieve 40-meter accuracy but was intentionally degraded to 100-meters, a handicap to civil navigation that may be discontinued before the year 2000. The United States and Russia have announced that GPS and GLONASS will be available worldwide, free of charge, for a least 15 years and thereafter with 6 years' warning of the end of service. Nevertheless, worldwide civil authorities are reluctant to rely on military-controlled navigation aids that might be switched off or degraded during hostilities. The advent of continuous GPS allows the use of AHRS-quality inertial/attitude-reference systems (Chapters 7 and 9) in all but the most demanding military applications. The undetected loss of a navigation signal or the failure of a receiver could be catastrophic, especially during a landing at low decision height.

Figure 1.2 Global Positioning System Spacecraft. Block IIF (courtesy of Rockwell). L-band antenna array, S-band control antenna, and solar array are visible.

A widespread method of improving GPS accuracy and monitoring the signals is to install a ground station that receives GPS signals and transmits position errors or ranging errors and satellite failure status on a radio link to nearby aircraft. This differential GPS (DGPS) can achieve centimeter accuracy for fixed observers and 1- to 5-meter accuracy on aircraft that can solve at tens of iterations per second or whose velocity calculations are smoothed by an inertial navigator. The United States was experimenting with a nationwide DGPS system (Wide Area Augmentation System, WAAS; Chapter 5) that could eventually replace the network of VORTACs.

The GPS and GLONASS systems are expensive to operate, each costing nearly a billion dollars per year for the replacement of satellites and the maintenance of the ground-control and monitoring network. The cost of collecting user charges (e.g., by selling encryption keys or taxing receivers) would exceed the revenue that could be extracted from navigation-only users. Hence, in the next generation, GPS transmitters will be installed on low-cost communication satellites as a way to augment the GPS network or as a low-cost replacement for dedicated GPS satellites. Governments would still maintain the control and monitoring stations that calculate the orbits and uplink data for rebroadcast. Taxpayer support is more likely if GPS becomes widely used in automobiles.

If present trends continue toward fee for service, taxpayer-funded navigation aids may cease to exist circa 2020, when commercial com-nav satellites will have superimposed ranging codes on their communication signals. Communication and navigation would then be available on a per-call basis, forcing aircraft again to rely on precise dead reckoning between intermittent fixes, probably from self-contained panel-mounted micro-machined inertial instruments (Chapters 7 and 9).

JTIDS and PLRS (Chapter 6) are military com-nav systems that constitute a battlefield-sized network whose terminals are in command centers and vehicles. PLRS terminals can be backpacked by soldiers.

Since airborne digital computers became available in the 1960s, algorithms have been invented and perfected that combine the measurements of diverse navigation sensors to create a “best estimate of position and velocity”. They are used in “hybrid” navigation systems. From 1970 to the end of the century, various forms of Kalman filters were favored for combining data from diverse navigation sensors (Chapter 3).

1.8 INTEGRATED AVIONICS

1.8.1 All Aircraft

“Navigation” is one of several electronic subsystems, collectively called avionics. The other subsystems are as follows:

1. Communication. An airplane's communication system consists of an intercom among the crew members and one or more external two-way voice and data links.
2. Flight control. This consists of stability augmentation and autopilot. The former points the airframe and controls its oscillations, while the latter provides such functions as attitude-hold, heading-hold, and altitude-hold. Flaps, slats, and spoilers are often controlled electronically in addition to rudder, elevator, and ailerons.
3. Engine control. This is the electronic control of engine thrust, often called throttle management. Afterburner and thrust reversers may be controlled manually, perhaps via a thrust-by-wire control system.
4. Flight management. This subsystem stores the coordinates of en-route waypoints and calculates the steering signals to fly toward them. It calculates climb and descent profiles that may be followed with or without constraints on the time at which designated fixes and altitudes are crossed. Crossing fixes at predetermined times and altitudes is sometimes called four-dimensional navigation; it requires that the flight management subsystem control engine thrust. In 1996 all flight management subsystems stored waypoints in digital form. By the year 2000 many will store digital maps of the en-route airspace, standard approaches (called STARs in the United States), standard departures (called SIDs in the United States), approach plates, and checklists (see Chapter 14).
5. Subsystem monitoring and control. Faults in all subsystems are displayed, as are recommended actions to be taken. This subsystem includes wired logic and software for the automatic reconfiguration of faults in time-critical subsystems (e.g., flight control, where a fault can destroy the aircraft in less than three seconds). Quick-responses to safety-critical faults were automated in flight-control systems by the 1980s. In the 1990s, the trend was to automate the responses to slower-acting faults, thus reducing the workload in one-pilot and two-pilot aircraft. The failure-monitoring subsystem may include an on-board maintenance recorder, the radio transmission of faults to reduce repair time, and an accident recorder whose data survive a crash (required by law on many aircraft).
6. Collision avoidance. This subsystem predicts impending collisions with other aircraft or the ground and recommends an avoidance maneuver (Chapter 14).
7. Weather detection. This subsystem observes weather ahead of the aircraft so that the route of flight can be altered to avoid thunderstorms and areas of high wind-shear. The sensors are usually radars (Chapter 11) and lasers (Chapter 8).
8. Emergency locator transmitter (ELT). This subsystem is triggered automatically on high-g impact or manually. In 1996, ELTs emit distinctive tones on 121.5, 243, and 406 MHz [8]. These frequencies (and perhaps soon 1.6 GHz) are monitored by search-and-rescue aircraft and by SARSAT-COSPAS satellites.

1.8.2 Military Avionics

The avionics often cost 40% of the value of a military aircraft. In addition to the navigation subsystem and the subsystems described in Section 1.8.1, military avionics consist of

1. Radar, infrared, and other target sensors. These may have their own displays and controls or may share multipurpose devices.
2. Weapon management
   • Fire control. Calculates lead angle for aiming guns and unguided rockets at other aircraft and at ground targets.
   • Stores management, that initializes and launches guided weapons: missiles and bombs.
3. Electronic countermeasures. This subsystem detects, locates, and identifies enemy emitters of electromagnetic radiation. It may also generate jamming signals. In 1996, electronic countermeasures were often so complex that they were installed in an externally carried pod on specially equipped aircraft.
4. Mission planning. Pre-flight mission planning is usually done at the air-base by a computer that prepares coordinated flight plans for an entire squadron. On-board software replans routes through enemy defenses based on en-route observations. En-route replanning requires on-board digital maps of the terrain and the real-time detection of enemy radars.
5. Formation flight. This subsystem maintains formation flight in instrument meteorological conditions. It once consisted of beacons, transponders, and communication links but is being replaced by relative GPS.

1.8.3 Architecture

Before the 1960s, electrical and electronic systems on aircraft consisted of independent subsystems, each with its own sensors, analog computers, displays, and controls. The appearance of airborne digital computers in the 1960s created the first integrated avionic systems. The interconnectivity of airborne electronics is called architecture. It involves six aspects:

1. Displays. They present information from the avionics to the pilots (Chapters 9 and 15). The information consists of vertical and horizontal navigation data, flight-control data (e.g., speed and angle of attack), and communication data (radio frequencies). The displays show the status of all subsystems including their faults. Displays consist of dedicated gauges, dedicated glass displays, multipurpose glass displays, and the supporting symbol generators. In 1996, flat-panel vertical- and horizontal-situation displays were displacing cathode-ray tubes as “glass displays.” Multipurpose displays of text and block diagrams are flat-panel matrices surrounded by buttons whose labels change as the displays change. On-board digital terrain data, used for mission planning, can be displayed on the horizontal situation display or on a head-up display.
2. Controls. The means of inputting information from the pilots to the avionics. The flight controls traditionally consist of rudder pedals and a control-column or stick. Fly-by-wire aircraft are increasingly using either two-axis hand controllers and rudder pedals or (especially in manned spacecraft) three-axis hand controllers. The subsystem controls consist of panel-mounted buttons and switches. Switches are also mounted on the control column, stick, throttle, and hand-controllers; sometimes 5 buttons per hand. The buttons on the periphery of multipurpose displays control the subsystems.
3. Computation. The method of processing sensor data. Two extreme organizations of computation exist:
   • Centralized. Data from all sensors are collected in a bank of central computers in which software from several subsystems are intermingled. The level of fault tolerance is that of the most critical subsystem, usually flight control. It has the simplest hardware and interconnections. In 1996, central computers were redundant uniprocessors or multiprocessers.
   • Decentralized. Each traditional subsystem retains its integrity. Hence, navigation sensors feed a navigation computer, flight-control sensors feed a flight-control computer that drives flight-control actuators, and so on. This architecture requires complex interconnections but has the advantages that fault-tolerance provisions can differ for each subsystem according to the consequences of failure and that software is created by experts in each subsystem, executes independently of other software, and is easily modified. When designed with suitable intercomputer channels and data-reasonability tests, decentralized systems have more reliable software than do centralized systems.

   Many avionic systems combine features of centralized and decentralized architectures.

4. Data buses. Copper or fiber-optic paths among sensors, computers, actuators, displays, and controls, as discussed in Chapter 15. Some data paths are dedicated and some are multiplexed. Complex aircraft contain parallel buses (one wire, pair of wires, or optical fiber per bit) and serial buses (bits sent sequentially on one wire-pair or fiber). A large aircraft can have a thousand pounds of signal wiring.
5. Safety partitioning. Commercial fly-by-wire aircraft sometimes divide the avionics into a highly redundant safety-critical flight-control system, a dually redundant mission-critical flight-management system, and a nonredundant maintenance system that collects and records data. Military aircraft sometimes partition their avionics for reasons other than safety.
6. Environment. Avionic equipment are subject to aircraft-generated electric-power transients, whose effects are reduced by filtering and batteries. Equipment are also subject to externally generated disturbances from radio transmitters and lightning. The effects of external disturbances (high-intensity radiated fields, HIRF) are reduced by shielding metal wires and by using fiberoptic data buses. Aircraft constructed with a continuous metal skin have an added layer of Faraday shielding. Nevertheless, direct lightning strikes on antennas destroy input circuits and may damage feed cables. A nearby strike may induce enough current to do the same. Composite airframe structures can be transparent to radiation, thus exposing the avionics and power systems to external fields.
7. Standards. The signals in space created by navaids are standardized by the International Civil Aviation Organization (ICAO), Montreal, a United Nations agency [3], These standards are written by committees that consist of representatives of the member governments. Interfaces among airborne subsystems, within the aircraft, are standardized by ARINC (Aeronautical Radio, Inc.), Annapolis, Maryland, a nonprofit organization owned by member airlines [1], Other requirements are imposed on airborne equipment by two nonprofit organizations supported by member entities (mostly airframe and avionics manufacturers and government agencies). In the United States, RTCA, Inc. (Formerly Radio Technical Commission for Aeronautics), Washington D.C., defines the environmental specifications and test procedures for airborne hardware and software, and writes performance specifications for airborne equipment [5]. In Europe, EUROCAE (European Organisation for Civil Aviation Equipment), Paris, produces specifications for airborne equipment, some of which are in conjunction with RTCA [11]. Government agencies in all major nations define rules governing the usage of navigation equipment in flight, weather minimums, traffic separation, ground equipment required, pilot training requirements, and so on [6–10] (see Chapters 13, 14). Some of these rules are standardized internationally by ICAO. U.S. military organizations once issued their own standards for airborne circuit boards but have accepted civil standards since the early 1990s.

1.9 HUMAN NAVIGATOR

Large aircraft often had (and a few still had in 1996) a third crew member, the flight engineer, whose duties were to operate engines and aircraft subsystems such as air conditioning and hydraulics. Aircraft operating over oceans once carried a human navigator who used celestial fixes, whatever radio aids were available, and dead reckoning to plot the aircraft's course on a paper chart (some military aircraft still do). Those navigators were trained in celestial observatories to recognize stars, take fixes, compute position, and plot the fixes.

The navigator's crew station disappeared in civil aircraft in the 1970s, because inertial, Doppler, and radio equipment came into use that automatically selected stations, calculated position, calculated waypoint steering, and accommodated failures. Hence, instead of requiring a skilled navigator on each aircraft, a smaller number of even more skilled engineers were employed to design the automated systems. Since the 1980s, the trend has been to automate large aircraft so that subsystem management and navigation can be done by one or two pilots. Displays and controls are discussed in Chapters 9 and 15.

The key navigation skill in the twenty-first-century airplane is the operation of flight-management, inertial, satellite-navigation, and VOR equipment, each of which has different menus, inputting logic, and displays. The crew must learn to operate them in all modes, respond to failures, and enter waypoints for new routes manually. A new industry was created in the 1990s to produce computer-based trainers (called CBTs) that emulate subsystem software and include replica control panels in order to allow crews to practice scenarios without consuming expensive time on a full-mission simulator.