Navigation Systems PDF

Navigation Systems / 45 One of the !rst scheduled airline flights in the United States occurred just prior to World War I. The St. Petersburg–Tampa Airboat Lines was established to pro- vide regular passenger service between the two Florida cities. For three months, during the winter of 1914, t...

Navigation Systems / 45 One of the !rst scheduled airline flights in the United States occurred just prior to World War I. The St. Petersburg–Tampa Airboat Lines was established to pro- vide regular passenger service between the two Florida cities. For three months, during the winter of 1914, the airline flourished. But when spring arrived, the tourists departed north, and with the lack of passengers, the airline folded. No other serious attempts at starting airline service were made during World War I. At the conclusion of the war, the federal government disposed of many of its military aircraft, selling them to private individuals as surplus property. This enormous influx of inexpensive aircraft helped establish the aviation industry in the United States. Some airline companies were formed after the war using these surplus aircraft, but they proved to be as short-lived as the St. Petersburg– Tampa line. The available war surplus aircraft were expensive to operate and maintain, forcing the airlines to charge passengers high fares. Only the wealthy could afford to fly at these high prices, and they were accustomed to travel- ing in luxury, not in war surplus aircraft. Trying to lure passengers using these aircraft thus proved to be nearly impossible, and most of the fledgling airline companies folded. In 1916, in the midst of World War I, Congress had authorized the Post Of!ce Department to institute the nation’s !rst of!cial airmail service. The war delayed the implementation of this policy until 1918. The !rst flight, from New York City to Washington, D.C., was !nally conducted on May 15 of that year, using U.S. Army aircraft. Airmail service soon proved to be commercially suc- cessful, and within three months the Post Of!ce Department began to transport the mail using its own aircraft and pilots. Additional routes were soon added, and the Post Of!ce Department eventually came to provide airmail service from coast to coast. Within a few years, in an attempt to stabilize the fledgling airline industry, the Post Of!ce Department began to contract airmail routes to the few remain- ing airline companies still struggling to survive. Airmail contracts proved to be a lifesaver to these airlines, since they could now transport mail while conducting passenger flights and use the airmail payments as a subsidy to reduce fares and attract more passengers. The resultant increase in revenue permitted the airlines to dispose of their war surplus aircraft and invest in larger and more luxurious aircraft speci!cally designed to carry passengers. But this merging of passenger and airmail service complicated airline scheduling and operations. When car- rying only airmail, airlines could delay flights because of poor weather con- ditions or darkness. But delays were unacceptable when carrying fare-paying passengers. Passengers demanded that the airlines fly consistent schedules with as few delays as possible. If the airlines hoped to lure passengers away from their main competitor, the railroads, they would have to offer fast, timely flights with few or no delays. Methods that would permit flying during poor weather or at night would have to be developed if the airlines were to survive and prosper. 46 / CHAPTER 2 Visual Navigation Initially, because they lacked flight instruments or navigation systems, airline pilots were limited to daylight flying during good weather conditions. The pilots were forced to use outside visual references to control their aircraft’s attitude, relying on the natural horizon as a reference. They would note any changes in the flight attitude of their aircraft and make the necessary control adjustments that would keep their aircraft in level flight. Pilotage Pilots navigated from airport to airport using either pilotage or deduced reck- oning (commonly called dead reckoning). Pilotage required that the pilot use a map of the surrounding area as a reference. The pilot would draw a line on the map, extending from the departure to the destination airport, and note any prominent landmarks that would be passed while in flight. As the aircraft passed these landmarks, the pilot would note any deviation from the planned flight path and adjust the aircraft’s heading to return to the preplanned course. Since the winds at the aircraft’s cruising altitude usually caused the air- craft to drift either left or right of course, the pilot was forced to constantly alter the aircraft’s heading to counteract these crosswinds. This change in head- ing is known as the crosswind correction angle or wind correction angle. The resultant path in which the aircraft flies over the ground is known as the ground track or the course. Aeronautical The maps used by pilots in the early 1920s were common road maps available Charts at automobile service stations. These maps were unsuitable for aerial naviga- tion since they lacked the necessary landmark information needed to accurately navigate from one airport to the next. It soon became apparent that pilots needed a specialized chart expressly designed for use in aeronautical naviga- tion. The U.S. government then developed and began to print such air naviga- tion charts, known as sectional charts. Sectional charts are aeronautical charts scaled 1:500,000 or about 8 statute miles to the inch. Sectional charts are still used today and depict the relevant information needed by pilots to navigate accurately and safely. This information includes cities, highways, railroads, airport loca- tions, terrain features, and distinctive objects (see Figure 2–1). Sectional charts also depict navigation aids, federal airways, and air traf!c control facilities. With very little change over the years, sectional charts are still being printed by the National Ocean Service (part of the U.S. Department of Commerce) and are primarily used by pilots flying under VFR rules (see Figure 2–2). In addition, pilots flying IFR usually carry appropriate sectional charts in case of navigational equipment failure. If IFR pilots should encounter any electronic navigation problems during flight, they may be able to continue under VFR conditions using sectional charts for visual navigation.  Navigation Systems / 47 Figure 2–1. An example of a legend for a sectional chart. 48 / CHAPTER 2 Figure 2–2. Sample sectional chart.  Navigation Systems / 49 Some pilots carry world aeronautical charts (WACs) instead of section- als during IFR flights (see Figure 2–3). WACs are similar to sectionals but are scaled 1:1,000,000 or about 16 miles to the inch. They present less-detailed information to the pilot but cover a larger area than a sectional chart. Dead When flying using VFR rules, most pilots use dead reckoning, in combination Reckoning with pilotage, to navigate to their destination. With dead reckoning, the pilot uses the forecast winds at the planned cruising altitude and applies trigonom- etry to deduce the proper heading that the aircraft should fly to counteract the crosswind. Properly calculated, this method of navigation is very accurate; however, it is hampered by the fact that the winds-aloft information is a fore- cast not a reflection of the actual winds. To verify that dead reckoning has calculated the proper heading, the pilot must still visually check the accuracy of the deduced heading by using a sectional chart. Flight The !rst step in planning a flight using both dead reckoning and pilotage is to Planning determine the true course that will lead the aircraft to the destination airport. This is accomplished by drawing a line from the departure airport to the desti- nation on the sectional chart. The pilot then determines the angle of this course in reference to true north, using a device called a plotter. The pilot obtains the forecast wind speed and direction at the chosen cruising altitude and, using either a mechanical or an electronic computer, calculates the true heading that the aircraft must fly. The deduced true heading is the direction that the aircraft must be aimed in order to track to the desired destination. If there is not wind at the aircraft’s cruising altitude, the true heading and the true course will be exactly the same. However if the pilot encounters a crosswind, he or she must angle the aircraft into the wind to remain on course. The angular difference between the aircraft’s heading and the true course is the crosswind correction or wind correction angle. Aircraft Instrumentation Magnetic Aeronautical charts cannot be properly used by pilots unless they have accurate Compass aircraft heading information. All of these charts are oriented with respect to true north. Unfortunately, the only instrument aboard most aircraft that actu- ally indicates heading is a magnetic compass, which usually points toward mag- netic north (see Figure 2–4). The angular difference between true north and magnetic north is known as variation (see Figure 2–5). The variation depends on the aircraft’s current location. In different areas of the United States, the variation may range from 0° to as much as 20°. To properly use the magnetic compass when navigating, the pilot must add the variation to or subtract it from the aircraft’s true heading 50 / CHAPTER 2 Figure 2–3. Sample world aeronautical chart.  Navigation Systems / 51 Beech Aircraft Corporation Figure 2–4. The magnetic compass. Easterly variation Westerly variation 20∞ 15∞ 10∞ 5∞ 0∞5∞10∞ 15∞ 20∞ 20∞ 20∞ 15∞ 10∞ 15∞ 5∞ Agonic line 10∞ 5∞ 0∞ Figure 2–5. Variation chart. to determine the magnetic heading that must be flown. The pilot may then fly this heading using the aircraft’s magnetic compass. Although the magnetic compass is a relatively reliable instrument, it is subject to various inaccuracies. One of these inaccuracies is known as devia- tion. Deviation is caused by the stray magnetic !elds of electrical equipment or metallic structures within the aircraft. Since all aircraft contain some stray mag- netic !elds, every plane is required to be equipped with a compass deviation 52 / CHAPTER 2 card that lists the inaccuracies and the correction that must be applied when interpreting the magnetic compass. A few other conditions can cause the magnetic compass to indicate inaccu- rately. During changes in airspeed or while the aircraft is turning, the magnetic compass will not indicate correctly. These particular inaccuracies are known as acceleration and turning errors. In general, the only time that the magnetic compass can be accurately interpreted is when the aircraft is in straight and level, unaccelerated flight. In addition, the placement of a metal or magnetized object (such as a flashlight, clipboard, or screwdriver) near the compass will alter the local magnetic !eld and cause magnetic compass errors. Heading Many of the problems inherent in the magnetic compass can be alleviated by Indicator using a heading indicator (see Figure 2–6). Because the heading indicator is a gyroscopic instrument, it is not subject to the same problems that affect the magnetic compass. The heading indicator is initially set by the pilot while on the ground. When properly set, it accurately reflects the aircraft’s magnetic heading during flight. As the aircraft turns, the heading indicator rotates, con- stantly displaying the correct heading. The heading indicator is not subject to acceleration or turning errors, and it is immune to stray magnetic !elds. It has, however, a few inherent problems. Since it is unable to sense magnetic !elds, it must be properly adjusted by the pilot before being used. If the pilot sets the indicator incorrectly, it will not accurately reflect the aircraft’s magnetic heading. In addition, since the heading indicator is subject to internal bearing friction and will slowly drift and begin to indicate inaccurately, the pilot must constantly check its accuracy and reset it as necessary during the flight. It is also possible, though highly unlikely, that the heading indicator will fail mechanically, not indicating the proper heading even when properly set. The heading indicator is also subject to precession and should be periodically reset during the flight. Beech Aircraft Corporation Figure 2–6. A heading indicator.  Navigation Systems / 53 VFR Navigation In theory, using dead reckoning alone, the pilot should be able to fly the com- puted heading and arrive over the airport at the calculated time. But in real- ity, because of imprecise winds-aloft forecasts, most pilots use a combination of pilotage and dead reckoning. The proper heading and time must still be deduced and used, but en route navigation checkpoints are established and marked on the appropriate sectional charts. As the pilot flies toward the des- tination, he or she makes periodic checks to determine whether the aircraft is still on course. If it has deviated from the planned route, the pilot will adjust the aircraft’s heading to return to and remain on the desired flight path. As archaic and old fashioned as this may seem, it is still the primary method of navigation for most VFR pilots today. Although visual navigation works quite well during daylight hours, at night or in marginal weather conditions it is almost impossible for pilots to see objects on the ground and make an accurate determination of their air- craft’s position. Sparsely populated areas of the country may not offer suf!cient ground references to permit the pilot to determine the aircraft’s location. If and when the pilot !nally arrives at the destination airport, he or she may !nd it dif!cult to actually locate the runway in the dark and land the aircraft. The solution, of course, is to have both airport and airway lighting. In the 1920s, airports were illuminated through the use of airport bound- ary lighting, which consisted of steady-burning 40-watt lights on wooden stakes every 300 feet around the perimeter of the airport. Eventually, these lights were equipped with lenses to concentrate the light beam and were mounted on orange-colored steel cones so that they could also be clearly seen during daylight hours. With the outline of the airport now quite visible, the pilots were able to safely land and take off at night. As noted in Chapter 1, the !rst airway lighting was also instituted in the 1920s. At equal intervals along the airway, rotating beacon lights were installed that delineated the airway’s center line (see Figure 2–7). These rotating beacons were installed on steel towers and consisted of 1,000-watt electric lamps that produced a white light of approximately 1,000,000 candlepower. Each lamp was housed in a rotating drum assembly equipped with 36-inch-diameter lenses at each end. One lens was clear while the other lens was colored. The beacon rotated at a speed of about six revolutions per minute. These rotating beacons were installed along the airway at 15-mile intervals. As the pilots flew along the airway, the beacons would appear as flashes of light visible from a distance of over 40 miles. To visually navigate along the airway, all the pilot needed to do was to fly from one beacon to the next. Each rotating beacon was equipped with a colored lens that uniquely identi!ed that particular beacon and enabled pilots to accurately determine their position. Each airport along the airway was also equipped with a rotating beacon having one clear and one green lens. These beacons were designed to help pilots determine the airport’s exact location. The green and white rotating 54 / CHAPTER 2 Michael Nolan Figure 2–7. A rotating beacon. beacons are still used at civilian airports today. Other color combinations are used to differentiate other types of airports. The assigned colors for rotating beacons are as follows: White and green Land airport Green and green* Land airport White and yellow Water airport Yellow and yellow* Water airport Green, white, and white Military airport Green, yellow, and white Lighted heliport * Green or yellow rotating beacons are used to prevent confusion when another airport with a similarly colored rotating beacon is located nearby.  Navigation Systems / 55 Instrument Flying Lighting the airports and airways proved to be a tremendous advance in night- time navigation, but it still required that pilots fly in weather conditions that would permit them to see the rotating beacons. If a pilot flew in or above a cloud layer, or if the flight visibility diminished to less than 15 miles, he or she would be unable to see the rotating beacons and navigate to the destination airport. As advances were made in aircraft design and instrumentation, it soon became possible for pilots to control their aircraft using just cockpit instrumen- tation, flying their aircraft without visual reference to the natural horizon. The new cockpit instruments were based on gyroscopic principles and included the artificial horizon (now called the attitude indicator), the heading indicator, and the turn and bank indicator (now called the turn coordinator). The attitude indicator (Figure 2–8) mimics the movements of the natural horizon, providing the pilot with accurate aircraft attitude information. Using the attitude indicator, pilots can determine whether their aircraft is banked and whether its nose is pointed up or down. This allows them to adjust their aircraft’s flight attitude and keep the aircraft upright and under control. The heading indicator, as described previously in this chapter, provides a more stable and accurate indication of the aircraft’s flight direction than does the magnetic compass. The turn coordinator (Figure 2–9) is used by the pilot to indicate the direction and the rate of turn. These instruments, used in conjunction with the already existing altimeter (Figure 2–10) and airspeed indicator (Figure 2–11), made it possible for pilots to accurately control their aircraft without reference to the natural horizon. Unfortunately, the federal airway system had not kept pace with these instrumentation developments and, until the late 1920s, was still based on sec- tional charts and rotating beacons, which required that pilots have at least 15 to 20 miles of visibility to navigate at night. There was no provision for naviga- tion when the visibility dropped below these values. Beech Aircraft Corporation Figure 2–8. The attitude Figure 2–9. The turn Figure 2–10. The Figure 2–11. The indicator. coordinator. altimeter. airspeed indicator. 56 / CHAPTER 2 Electronic Navigation Four-Course In an early attempt to remedy this situation, the federal government began to Radio Range install the four-course radio range in the late 1920s. This new radio device was placed at intervals along each federal airway and permitted the pilot to navi- gate without using outside visual references. Four-course radio ranges soon became the U.S. and international standard for aviation navigation and were widely used in the United States until the 1950s. The four-course radio range used a 1,500-watt transmitter that oper- ated on a frequency between 190 and 565 kHz. The transmitting antenna con- sisted of two single-wire vertical loops strung out on !ve wooden masts. These wires were attached to the masts to form two !gure-eight patterns (see Figure 2–12). This arrangement produced two separate radio transmission patterns g O le n- e co rs ur ou s c e n- le O g N Transmitter A A N g O le n- e co urs ur co se n- le O g Figure 2–12. Field pattern of four-course radio range.  Navigation Systems / 57 that overlapped slightly. One loop constantly transmitted the Morse code for the letter A (dot-dash); the other transmitted the Morse code for the letter N (dash-dot). Any pilot wishing to use a four-course range simply tuned the receiver on the aircraft to the proper frequency and listened to the transmitted signal through earphones. If the aircraft was located somewhere within the A sector, the pilot would hear Morse code for the letter A (dot-dash) constantly being repeated. If the aircraft was in the N sector, the pilot would hear the Morse code for the letter N (dash-dot). If the aircraft was located where the two transmissions overlapped (the on-course line of position), the dot-dash and the dash-dot would be of equal strength and would produce a constant tone in the pilot’s headset. When navigating to a four-course radio range, all the pilot needed to do was to head the aircraft toward one of these on-course “legs” and then proceed along it to the radio range. If the aircraft drifted off course, the pilot would begin to hear the individual A or N Morse code becoming dominant, requir- ing an adjustment in the aircraft’s heading until he or she could again hear the constant on-course tone. The wire loop antennas of the four-course range could be constructed in such a manner as to “aim” the on-course legs toward other radio ranges. Although it permitted navigation during periods of low visibility, the four-course (or A–N) range still had a number of de!ciencies that limited its usefulness. For example, disoriented pilots found it very dif!cult to accurately determine their position using the A–N range. Although pilots could easily determine whether their aircraft was located on one of the on-course legs, they often found it impossible to determine which of the four legs they were on. Through trial and error, a pilot could eventually determine which head- ing would keep the aircraft on course and lead to the station. If the pilot was between on-course legs (totally within either an A or an N sector), it was time consuming and dif!cult to pinpoint the aircraft’s location and determine the proper heading that would lead to an on-course leg. In addition, since the A–N ranges operated in the 190 to 565 kHz band (just below the present AM radio band), the transmitted signal could easily be distorted by obstructions or dis- rupted by lightning-induced static. In mountainous areas, it was possible for the radio transmission of the four-course range to bounce off nearby terrain and produce false on-course signals. During thunderstorms, when pilots desperately needed the course guidance of the radio range, lightning-induced static could overwhelm the relatively weak signal transmitted by the radio range, leaving the pilot with only static emanating from the receiver. Certainly, the A–N range was a tre- mendous advancement in instrument navigation, but these de!ciencies limited its overall use. Introduction The A–N range provided the pilot with only bearing and course information. of Marker It did not provide any information concerning distance to the station. To mini- Beacons mize this problem, the CAA began installing marker beacons along the on- course legs. These low-powered radio beacons were designed to transmit a 58 / CHAPTER 2 distinctive tone and code that could be received by the aircraft as it passed directly overhead. The pilots could use the code to identify which beacon was crossed and use this information to accurately determine their aircraft’s posi- tion along the on-course leg. But whenever the aircraft was between marker beacons or no longer on one of the on-course legs, the marker beacons were useless in helping determine the aircraft’s location. Nondirec- While the CAA was developing and installing A–N radio ranges, the nondirec- tional tional radio beacon (NDB) was also being developed. The NDB transmits a Beacons uniform signal omnidirectionally from the transmitter, using the low- and medium-frequency band (190–540 kHz). The receiver on the aircraft (known as a direction finder or DF) was originally equipped with a looptype antenna that the pilot rotated manually. When the antenna was rotated so that the plane of the loop was perpendicular to the transmitted signal, the “null” position was reached, and the pilot would be able to hear the transmitted signal. Using the magnetic compass and the NDB receiver, the pilot could then determine the aircraft’s bearing from the nondirectional beacon. This bearing could be plot- ted on a chart as line of position. Plotting lines of position from two NDBs permitted the pilot to pinpoint the aircraft’s exact location. If the pilot wished to fly toward the NDB, he or she would turn until the NDB station was located directly ahead of the aircraft. If the winds aloft caused the aircraft to drift off course, the pilot would readjust the aircraft’s heading, keeping the NDB directly ahead of the aircraft. This method of navigation is called homing. Automatic Trying to manually manipulate the DF antenna while flying the aircraft proved Direction to be a cumbersome method of navigation and usually provided the pilot with Finder relatively inaccurate position information. As advances were made in aircraft electronics, the manually operated NDB receiver was soon replaced by the automatic direction finder (ADF), which could electronically determine the bearing to the NDB and display this information to the pilot (see Figure 2–13). Using ADF equipment in conjunction with the aircraft’s heading indicator, the Beech Aircraft Corporation Figure 2–13. ADF receiver and indicator.  Navigation Systems / 59 pilot could easily determine the aircraft’s relative bearing from the station and use this information to determine the proper heading that would lead to the beacon. The development of the ADF hastened pilot acceptance of the NDB as a navigation aid. The !rst NDB was installed in the United States in 1924. By 1964, 272 high-powered NDBs had been installed throughout the country. A series of federal airways using NDBs for en route navigation were soon developed. Because these airways were designated by a color and a number (for example, RED-64 or GREEN-32), they were soon referred to simply as colored airways. Compass In addition to their role as en route navigation aids, NDBs were located at Locators airports or along instrument-approach paths to assist pilots who were conduct- ing such approaches. NDBs along the !nal approach are known as compass locators. In 1965, the federal government began to decommission the high- powered NDBs used for en route navigation. Due to their extremely low cost and ease of installation, however, low-power units continued to serve smaller airports as instrument-approach aids. Visual Aural In 1937, the Radio Development Section of the Bureau of Air Commerce dem- Range onstrated an improved radio range at its research center in Indianapolis. This new radio range, called the visual aural range (VAR), was an improvement over the old A–N range in two major areas. The VAR was designed to operate in the very high frequency (VHF) band located around 63 mHz. This frequency band was chosen since transmitters operating on VHF frequencies are rarely affected by static caused by lightning. VHF transmissions are also line of sight, which means that they do not follow the curvature of the earth. One signi!cant advan- tage of using VHF frequencies is that although they can easily be blocked by terrain and obstructions, they are seldom reflected by them. The use of VHF frequencies would thus minimize the reflection problem that plagued the A–N ranges. The VAR also solved the orientation problem inherent with the A–N range by transmitting four radio signals instead of two. While retaining the Morse-coded A and N signals, the VAR also transmitted overlapping “blue” and “yellow” signals perpendicular to the A and N signals (see Figure 2–14). An instrument on board the aircraft would indicate whether it was in the blue or the yellow sector. The pilot was still required to listen to the VAR to determine whether the aircraft was within the A or the N sector, however. The addition of the overlapping color signals gave each sector a unique identi!cation that enabled pilots to accurately determine their aircraft’s location. The !rst operational VAR was installed at Matawan, New Jersey, in 1944. By 1948 a total of sixty-eight VARs had been commissioned by the CAA and were located along federal airways. The VAR never gained wide acceptance, however, since it was soon replaced by an improved radio range that emitted an in!nite number of courses instead of just four. This new navigation aid was 60 / CHAPTER 2 g O le n- se co ur urs co e n- le O g N Yellow Transmitter sector A A N Blue sector g O le n- se c ou r ou rs c e n- le O g Figure 2–14. VAR operation. called the VHF omnidirectional range (VOR). In the early 1950s, as VORs were being installed around the country, the CAA began to decommission the VARs, with the last being retired from service in 1960. VHF Omni- Research on a radio range that would offer pilots more than four courses and directional transmit in the static-free VHF radio spectrum had started in 1937. The Range (VOR) Washington Institute of Technology delivered the !rst operable VHF omnidi- rectional range (VOR) to the CAA in 1944. This experimental VOR operated on a frequency of 125 mHz. After extensive testing and development by the CAA, three prototype VORs were installed at Patuxent River, Maryland; Philipsburg, Pennsylvania; and Ogden, Utah. After operational testing at these three sites, the CAA adopted the VOR as the national civil navigation standard in 1946. The  Navigation Systems / 61 Wadsworth, Inc., photo by Paul Bowen Photography, Inc. Figure 2–15. VOR ground station. VOR was also selected as the international civil navigation standard in 1949 by the International Civil Aviation Organization (see Figure 2–15). VOR Operation The VOR offered a number of improvements over the old A–N and VAR methods. The VOR transmits an in!nite number of navigation courses, selectable by the pilot, instead of just four. The VOR is also relatively immune to the reflections and static inherent in the operation of the A–N ranges. Each VOR is assigned a frequency between 108.10 and 117.90 mHz. The VOR transmission is modulated with two signals: a reference-phase signal that is constant in all directions and a variable-phase signal whose phase varies with azimuth. The variable-phase signal is modulated so that at magnetic north the reference and variable signals are precisely in phase with each other. In any other direction, the VOR is designed so that the two signals are no longer in phase. The VOR receiver on board the aircraft measures the phase difference between the two signals to determine the azimuth angle of the aircraft in relation 62 / CHAPTER 2 R V R 360∞ R Radial 315∞ 45∞ V Radial V Radial R R VOR 270∞ 90∞ Radial Radial V V R R 225∞ 135∞ Radial Radial 180∞ Radial V V V R = Reference signal V = Variable signal Figure 2–16. VOR operation. to the VOR transmitter. When the aircraft is directly east of the VOR, the vari- able signal will lag the reference signal by 90°. An aircraft located directly east of the VOR is said to be on the 90° radial of the VOR (see Figure 2–16). An aircraft directly south of the VOR will receive the variable signal lagging the reference signal by 180° and will be on the 180° radial. An aircraft located on the 359° radial (north of the VOR) will receive the variable signal lagging the reference signal by 359°. The radial to be flown by the pilot is selected on the aircraft’s VOR indica- tor (see Figure 2–17) using the omni bearing selector (OBS). After selecting the appropriate VOR frequency, the indicator in the cockpit will inform the pilot whether the selected course will lead to the station or away from it (known as the To–From flag). The VOR indicator will also display any lateral deviation from the selected course, using a vertical pointer known as the course deviation  Navigation Systems / 63 Bendix/King Division of Allied Signal Aerospace Company Figure 2–17. VOR receiver and indicator. indicator (CDI). If the aircraft is to the right of the selected course, the CDI will be to the left of center, advising the pilot to alter the aircraft’s course to the left in order to return to the selected radial. If the aircraft is left of course, the CDI will be right of center. If the aircraft is precisely located on the radial selected by the pilot, the CDI will be centered. VORs used for en route navigation have an output power of 200 watts and are assigned a frequency between 112.00 and 117.90 mHz. This signal permits en route VOR reception up to a distance of 200 miles. Terminal VORs (used solely for instrument approaches) have an effective radiated power of 50 watts and are assigned a frequency between 108.10 and 111.80 mHz. Terminal VORs can be received up to a distance of about 25 nautical miles. Since VOR transmissions are line of sight, these reception distances vary depending on the receiving aircraft’s altitude (see Figure 2–18). VOR Categories A number of dif!culties were encountered as soon as the CAA began to install VORs along the federal airways. Since VHF transmissions are line of sight, low-flying aircraft were unable to receive the VOR signal if they were “below the horizon.” This limitation forced the CAA to place the VORs no farther than 80 miles from each other to ensure adequate recep- tion for aircraft operating at low altitudes. Because only a limited number 64 / CHAPTER 2 A and B received A received No B received signal VOR A VOR B Approximate coverage Maximum distance in Altitude of statute miles from VOR aircraft 45 1,000 100 5,000 150 10,000 200 20,000 Figure 2–18. VOR reception distances. of frequencies can be assigned to VORs, some would have to be assigned the same operating frequency, which could cause interference problems for aircraft operating at very high altitudes, as they might receive the signals being broad- cast from two or more VORs operating on the same frequency. The resulting interference would render the navigation signal unusable. The CAA responded by designating every VOR as a terminal, low-, or high-altitude VOR. Terminal VORs (TVORs) are low powered and are usable up to a distance of 2.5 nautical miles. TVORs are not to be used for en route navigation but are reserved for local navigation and instrument approaches. Low-altitude VORs guarantee interference-free reception to aircraft operating up to 40 nautical miles away. This interference-free zone is guaranteed only at or below 18,000 feet. Low-altitude VORs cannot be used by aircraft operating above 18,000 feet or farther than 40 miles away, as there is no guarantee that another VOR operating on the same frequency will not cause interference. High- altitude VORs are used by aircraft operating between 18,000 and 60,000 feet, at ranges up to 200 nautical miles. These limitations imposed upon VORs are known as service volumes (see Figures 2–19 and 2–20).  Navigation Systems / 65 100 n mi 60,000 ft. 40 n mi 130 n mi 18,000 ft. 45,000 ft. 18,000 ft. 1,000 ft. 14,500 ft. Low-altitude 40 n mi 1,000 ft. VOR High-altitude 25 n mi VOR 12,000 ft. 1,000 ft. TVOR Figure 2–19. VOR service volumes. Unusable Radials Testing of the VOR found that a clear zone of several thou- sand square feet around the VOR was necessary for proper operation. Any obstruction within this area could blank out or reflect some of the signal from the VOR and cause incorrect course information to be transmitted to the air- craft. Tall buildings located thousands of feet from the VOR transmitter could even distort the transmitted signal. In an attempt to solve this problem, the CAA developed the Doppler VOR (DVOR). Although the operating principles of this VOR differ radically from a conventional VOR, the information available to the pilot is exactly the same. The VOR receiver on board the aircraft is unable to differentiate between Doppler or conventional VOR transmissions. Doppler VOR is less sensitive to reflections from buildings or terrain. In locations unsuitable for conventional VOR installation, a DVOR might be necessary (see Figure 2–21). 66 / CHAPTER 2 Figure 2–20. VOR service volume chart. If the DVOR fails to correct the reflections or blanking, the affected radi- als must be listed as unusable, which means that although the pilot may be able to receive these radials, they are not accurate and should not be used. Unusable VOR radials are published in the Airport Facility Directory (see Figure 2–22). Airway The CAA faced an enormous task when trying to determine where VORs Altitudes should be located. CAA planners had to consider potential obstructions, ter- rain, and the position of other VORs operating on the same frequency to deter- mine that a suitable interference-free signal could be received by any aircraft operating along a VOR-equipped airway. The airways must be constantly flight checked. After these checks, a minimum en route altitude (MEA) is designated for each airway. Aircraft operating at or above the MEA are guaranteed clear- ance above any obstruction located along or near the airway.  Navigation Systems / 67 FAA Figure 2–21. A Doppler VOR station. Along some airways, if they differ from the MEA, minimum obstruction clearance altitudes (MOCAs) are also designated (see Figure 2–23). MOCAs are lower than MEAs and are designed to provide obstacle clearance only. In case of an emergency, the pilot may safely descend to the MOCA and will still be guaranteed obstacle clearance. Pilots flying at the MOCA altitude are also guaranteed proper VOR reception as long as they are within 22 nautical miles of the VOR. Maximum authorized attitudes (MAA) are sometimes assigned to certain high attitude airways. An MAA is the maximum usable altitude at which an interference free ground-based radio reception signal is assured. MAA’s are designated for route segments where interference from another navaid operat- ing on the same frequency is possible. Airway When VOR airways are designated, their identifying numbers are pre- Designators ceded by the letter V if they are low-altitude airways, or the letter J if they are high-altitude airways. Aircraft Positioning Methods The VOR provides only bearing information to the pilot (known as rho), not distance from the station (known as theta). There are only two ways for a pilot using the VOR to accurately determine an aircraft’s position: using either rho–rho or rho–theta position determination. Rho–rho position determination requires that the pilot obtain bearing information from two different VORs. Using airborne VOR equipment, the pilot can plot a line of position from each VOR. These two lines of position (or radials) are then plotted on a navigation chart, with the aircraft being located at the intersection of the two radials (see Figure 2–24). 68 / CHAPTER 2 Figure 2–22. Unusable radials listed in the Airport Facility Director (gray screen).  Navigation Systems / 69 MEA MOCA Re n Re n ce o ce tio p pti p ep t lim tion ece it lim tion c it R lim it Re limi No reception at MOCA Figure 2–23. Minimum en route and minimum obstruction clearance altitudes. 360∞ Radial 270∞ Radial VOR VOR Figure 2–24. Plotting aircraft position using two VORs. 70 / CHAPTER 2 The rho–rho method of position determination requires that the aircraft be within the service volume of both VOR transmitters. These two stations should also be at approximately right angles to each other. Since the VOR receiver on the aircraft can legally have an accuracy of !6°, this in effect makes each radial 12° wide. The aircraft’s location will be somewhere within the area de!ned by the limits of the VOR receivers’ accuracy. If the two radials do not bisect each other at approximately right angles, the area de!ned by the two radials becomes much larger, thereby making the position determination less accurate (see Figure 2–25). DME Position If a pilot wishes to determine an aircraft’s location using just one station, Determination rho–theta position determination techniques must be used. The pilot must 360∞ Radial 270∞ Radial VOR Aircraft’s probable location VOR Figure 2–25. Actual location of an aircraft using two VORs for position determination.  Navigation Systems / 71 determine on which radial the aircraft is located (rho) and then use distance measuring equipment (DME) to determine the aircraft’s distance (theta) from the VOR transmitter. Rho–theta position determination requires specialized DME equipment both on the aircraft and at the VOR transmitter. The DME system uses the principle of elapsed time measurement as the basis for distance measurement. The DME system consists of an interrogator located on board the aircraft and a transponder located at the ground station. At regularly spaced intervals, the interrogator transmits a coded pulse on a frequency of around 1,000 mHz (see Figure 2–26). When the ground-based DME transponder receives this pulse, it triggers a coded reply that is transmitted on a different frequency. When the interroga- tor receives this pulse, the elapsed range time is electronically calculated. Range time is the interval of time between the transmission of an interrogation and the receipt of the reply to that interrogation. The approximate range time for a signal to travel 1 nautical mile and return is 12.36 microseconds. The DME equipment on board the aircraft measures the elapsed time between interro- gator transmission and reception of that signal. This time is divided by 12.36 microseconds, providing the distance the aircraft is from the ground station. This determination is known as the line of sight or slant range distance. Slant range is the actual distance between the aircraft and the ground- based DME transponder. As the aircraft’s altitude increases, the difference between slant range and ground distance increases. For instance, if an aircraft Interrogator Transponder Figure 2–26. DME operation. 72 / CHAPTER 2 is 5.0 ground miles from the DME station, at an altitude of 6,000 feet, the DME indicator on board the aircraft will indicate approximately 5.1 nautical miles from the station. But if the aircraft is directly over the DME station, at an altitude of 30,000 feet, the DME indicator will also indicate about 5.1 nautical miles (see Figure 2–27). The difference between slant range and ground distance is most pro- nounced when aircraft are operating at high altitudes fairly close to the DME ground station. This difference has been taken into consideration by the FAA when determining holding-pattern sizes, intersection locations, and airway positioning. Tactical Air The VOR-DME system has de!ciencies that make it unusable for certain mili- Navigation tary operations. A conventional VOR transmitter is fairly large and needs an (TACAN) extensive clear zone around it to minimize reflections. In addition, since all of the DME interrogators on board aircraft transmit at the same frequency when interrogating a station, a DME ground station can become saturated from too many aircraft within its vicinity interrogating at the same time. If this happens, the interrogator signals may interfere with one another and cause inaccurate DME distances to be displayed in the cockpit. After an extensive evaluation of the civilian VOR-DME system, the Department of Defense chose to develop an alternative navigation system known as tactical air navigation (TACAN). TACAN is a polar coordinate–based 5 5.1 n mi Sla.1 n m Slant range nt r i ang e 5.0 mi DME station Figure 2–27. DME slant range measurement.  Navigation Systems / 73 Table 2 –1. Radio Frequency Allocation Name Abbreviation Frequency Uses Very low frequency VLF 3–30 kHz Naval communication Low frequency LF 30–300 kHz LORAN, NDB Medium frequency MF 300–3,000 kHz NDB High frequency HF 3–30 mHz Long-range communications Very high frequency VHF 30–300 mHz VOR, localizers, marker beacons, civil communi- cations Ultra high frequency UHF 300–3,000 mHz DME, TACAN, MLS, glide slope, military communications, GPS Super high frequency SHF 3–30 gHz Radar Extremely high frequency EHF 30–300 gHz navigation system that provides both bearing and distance (rho–theta) informa- tion to the pilot using a single transmitter located on the ground. This ground- based TACAN equipment operates within the ultra high frequency (UHF) band between 960 and 1,215 mHz (see Table 2-1). Operation in this frequency range permits both the interrogator and the transponder to be much smaller than conventional VOR-DME equipment. UHF frequencies are line of sight but are not as susceptible to reflection as those in the VHF band, which reduces the siting problems inherent in the VOR. These advantages make TACAN ideal for use on aircraft carriers or in mobile, land-based equipment. Because of its smaller size and ease of installation, a TACAN station is far easier to move than a VOR station, which makes it ideal for use in hostile areas or in tempo- rary air!elds (see Figure 2–28). TACAN is seldom used by civilian aircraft. TACAN does not use a passive transmitter on the ground like the VOR but instead operates in much the same way as the DME system. During opera- tion, the TACAN equipment on the aircraft (the interrogator) transmits a coded signal to the TACAN station on the ground (the transponder). On receipt of the interrogator signal, the transponder transmits a properly coded reply. The interrogator on board the aircraft measures the elapsed time and calculates the distance between the aircraft and the TACAN transmitter. (This is done in the same manner as with civilian DME equipment.) The interrogator on board the aircraft also decodes the signal and determines the aircraft’s azimuth from the TACAN ground station. The airborne equipment can then display both bearing and distance information to the pilot, using a display system similar to civilian VOR-DME indicators. VORTAC While the military was developing TACAN, the CAA was developing and implementing the civilian VOR-DME system. Congress expressed concern over 74 / CHAPTER 2 E-Systems, Montek Division Figure 2–28. A mobile TACAN ground station. the increased expense of developing, operating, and maintaining two separate navigation systems when both would provide pilots with the same navigational information. The CAA recommended adoption of VOR-DME as the civil navi- gation standard, since system implementation had already begun and VOR-DME receivers were readily available at a lower cost than TACAN equipment. In addition, the CAA believed that the VOR-DME system was more flexible, since VOR and DME equipment could be purchased separately. The CAA preferred a system that would permit the pilot to purchase just VOR equipment; DME equipment could be installed in each aircraft at a later date if the pilot felt that the expense was justi!ed. In addition, since the CAA had previously recom- mended that pilots install VOR equipment and many pilots had already made this expensive investment, the CAA felt that it would be unfair to require air- craft owners to remove their VOR equipment and install even more expensive TACAN receivers. The Department of Defense, however, believed that TACAN was better suited to military operations because of its smaller size and portability. After years of negotiations, the CAA and the Department of Defense eventually  Navigation Systems / 75 agreed that civilian aircraft would be permitted to use ground-based TACAN transponders to provide distance information while still using VOR ground sta- tions for azimuth information. Military aircraft, however, would be equipped solely with TACAN equipment and would be dependent on it for both azimuth and distance information. The military and the CAA agreed to place VORs and TACANs at the same locations using common physical structures. This combined navigation aid would henceforth be known as VORTAC. The VORTAC system was chosen by Congress to become the nation’s new en route navigation standard, providing both distance and bearing infor- mation to military and civilian aircraft. TACAN frequencies would be paired with the appropriate VOR frequencies to simplify pilot operation. To use VORTAC, all that civilian pilots needed to do was select the appropriate VOR frequency, and the DME interrogator would automatically tune itself to the proper TACAN UHF frequency. Military pilots using TACAN were required to select an appropriate channel number, and their receiver automatically tuned itself to the proper frequency. Most of the VORs across the United States were soon colocated with TACANs and became VORTACs (see Figure 2–29). In locations where the military had no need for TACAN but civilian aircraft still needed some form of navigation, a VOR with civilian DME equipment was installed. (These VOR-DME facilities cannot be used by military aircraft unless they are VOR equipped.) In addition, some locations justify installation of a VOR station but not a DME station. In this case, a VOR is installed without associated DME or TACAN equipment. Such facilities can be used for azimuth information by aircraft equipped with VOR. Area Navigation To navigate the airways using the VORTAC system, pilots are required to fly from VORTAC to VORTAC until they reach the destination airport. Because of airport locations and VORTAC placement restrictions, it is seldom possible to navigate in a straight line from the departure to the destination airport. This navigation restriction forces pilots to fly a longer distance than necessary. It also creates congestion in the air traf!c control system, since every aircraft operat- ing under an IFR flight plan is forced to navigate along a limited number of air- ways. In an attempt to alleviate this congestion, a number of systems have been developed to permit pilots to bypass the airway system and navigate directly to the destination airport. These various systems are collectively referred to as area navigation or RNAV. Doppler One of the !rst area-navigation systems adopted for use was Doppler radar. Radar The Doppler radar system is composed of a radar transmitter, a receiver, a sig- nal processor, and display unit, all installed on board the aircraft. The Doppler 76 / CHAPTER 2 E-Systems, Montek Division Figure 2–29. A VORTAC ground station. system constantly transmits a radar signal straight down from the aircraft at a precise frequency. After the radar signal has reflected off the ground back to the receiver, the signal processor compares the frequency of the transmitted signal with that returned to the aircraft. If the aircraft were not moving at all, no detectable change would be noticed in the frequency of the transmitted radar signal. But when the aircraft  Navigation Systems / 77 is moving in any direction, either longitudinally or laterally, the radar frequency will change as it reflects off the earth’s surface. This phenomenon is called the Doppler effect, and the change in frequency is known as a frequency shift. The signal processor on board the aircraft measures the frequency shift and uses this information to calculate the aircraft’s ground speed and true course. This information is then displayed in a manner that permits pilots to navigate to their destination. The Doppler radar system measures only the aircraft’s relative motion over the earth’s surface; it cannot actually determine an aircraft’s location. For the system to operate correctly, the pilot must input the starting position of the aircraft into the Doppler system before takeoff. Any error in this input will cause the system to inaccurately calculate the current position of the aircraft. This is the primary disadvantage of the Doppler radar system. Since the Doppler system is self-contained within the aircraft, it oper- ates without using any ground-based navigation stations (such as VORTAC or NDB) and can be used where navigational aids are sparse or nonexistent. This characteristic makes Doppler radar ideal over long stretches of desert or ocean. Doppler radar is no longer one of the most accurate RNAV sys- tems, however, and is rapidly being replaced for primary navigation by the systems described next; if installed, it is usually used as a backup navigational system. Course-Line The course-line computer (CLC) was developed to permit pilots to use existing Computers VORTAC stations to fly directly from one airport to another. Using rho–theta navigation principles, the course-line computer can determine the aircraft’s position using any VORTAC or VOR-DME station. Upon receiving the azi- muth and distance information from a VORTAC station, the CLC mathemati- cally calculates the bearing and distance from the aircraft to any desired location and produces navigation instructions that lead the pilot to that point. The CLC accomplishes this task by electronically creating a phantom VORTAC station (known as a waypoint at the desired destination and then providing bearing and distance information to that station using the aircraft’s VOR and DME indicators (see Figure 2–30). During flight, the pilot selects an appropriate VORTAC station and electronically “moves” it to the desired location. The CLC then constantly obtains position information from the VORTAC, calculates the bearing and distance to the waypoint, and displays the course guidance information to the pilot using the course deviation indica- tor, (CDI) on the aircraft. Distance to the waypoint is constantly displayed on the DME indicator. The primary limitation to CLC-based area navigation is that the way- point must be located within the service volume of an actual VORTAC station. If the aircraft is not in a position to receive an accurate navigation signal from an existing VORTAC, the CLC cannot determine the aircraft’s present location or compute the bearing and distance to the waypoint. This limitation forces the pilot to electronically create a suf!cient number of waypoints along the planned route of flight to permit a straight course to be flown. 78 / CHAPTER 2 ay rw Ai Ai VORTAC rw ay Airway Airway D m VOR fro D m DM ial fro d Ra E Waypoint 2 Ra dia l fr ay Phantom om Airw VORTAC #2 DME arc from D B Waypoint 1 Radial from A VOR Rad ial fr Phantom om C ay VORTAC #1 ay rw Airw Ai VOR ay rw Direct RNAV route Ai Indirect airway route Figure 2–30. CLS(course-line computer)-based RNAV. During the entire flight, the aircraft must be within reception distance of one of the selected VORTACs, and the pilot must locate waypoints within each VORTAC’s service volume. If the aircraft strays outside the service volume, the CLC will be unable to receive suf!cient information to provide course guid- ance to the waypoint. This reduces the CLC’s effectiveness over sparsely settled terrain. CLC-based RNAV can be used over most of the continental United States, however. CLC-based RNAV can also be used as a navigational aid when approach- ing airports. Upon arriving within the vicinity of the destination airport, the pilot can electronically move a VORTAC and place it at the center of the des- tination airport, simplifying instrument approach procedures. VFR pilots can  Navigation Systems / 79 Bendix/King Division of Allied Signal Aerospace Company Figure 2–31. A typical light aircraft RNAV system. use CLCs to assist in navigating to airports that are not served by VORs or NDBs. The course-line computer is one of the most common area-navigation systems in use today and is typically called RNAV (see Figure 2–31). LORAN The long-range navigation (LORAN) system was initially developed as a mari- time navigation system. Since LORAN stations provide coverage primarily over the Atlantic and Paci!c Oceans, where aviation navigation aids are virtu- ally nonexistent, LORAN was eventually adapted for aviation use. LORAN differs from most aviation navigation systems in that it is a hyperbolic naviga- tion system, rather than a rho–theta navigation system such as VORTAC. When using LORAN, the pilot plots multiple hyperbolic lines of position to determine the aircraft’s position. 80 / CHAPTER 2 The LORAN-A system consists of a master station and a slave station installed about 500 nautical miles apart. At precise intervals, the master station transmits a coded pulse in the 1700 to 2000 kHz band. When the slave station receives this pulse, it transmits another coded pulse on the same frequency. The LORAN receiver on board the aircraft measures the time delay and displays this information on an indicator. The pilot can then use this time-delay infor- mation to plot a line of position on which the aircraft is located. After plotting the !rst line of position (LOP), the pilot repeats this procedure using a second pair of stations. The second LOP will intersect with the !rst one, de!ning the aircraft’s exact location. LORAN-A was never designed to be used by high-speed aircraft. Since a signi!cant amount of time can elapse between the plotting of the !rst and second LOPs, there were always inherent inaccuracies whenever an aircraft’s position was determined using LORAN-A. LORAN-C LORAN-A was operated by the U.S. Coast Guard and was decommissioned in the 1990s. LORAN-B was a replacement system that was developed but never made operational. LORAN-D is a short-range military version used for pinpoint navigation. LORAN-C is the current civilian version of LORAN and was again designed to be used primarily for maritime navigation. LORAN-C operates on the same general principles as LORAN-A but uses a computer to quickly and accurately plot multiple lines of position (see Figure 2–32). Since Master Baseline Slave 1 Baseline Aircraft Location Lin ion e o sit 1 ba se of po p e of slav d on sit ion e Slave 2 Lin on sla sed ve ba 2 Figure 2–32. LORAN-C line of position plotting.  Navigation Systems / 81 LORAN-C is primarily a marine system, most of the transmitters are still located along the coasts of the United States and around the Great Lakes. It has been made available to aviation users and has limited approval from the FAA as an aviation navigation system. LORAN-C ground stations consist of one master station (designated as station M) and two to !ve slave stations (designated as stations V, W, X, Y, and Z). This assembly of transmitting stations is known as a chain. Seventeen LORAN-C chains are currently in operation worldwide, with nine of them located within the United States and Canada (see Figure 2–33). At regularly spaced intervals, the master station transmits a coded pulse at a frequency of 100 kHz. Each master station transmits its signal at 100 kHz with a unique time interval between transmissions. This time interval is known as the group repetition interval (GRI). Each chain of stations is identi!ed by a unique GRI. For example, the Great Lakes LORAN-C chain has a GRI of 89,700 microsec- onds and is therefore known as the GRI-8970 chain. As each slave station receives the pulse transmitted by its own master station, it in turn transmits its own coded signal on the same frequency. The LORAN receiver on the aircraft receives these coded signals, identi!es which chain is being received, and measures the time difference between the master and each of the slave-station transmissions. The computer in the receiver uses these time differences to plot multiple lines of position. The LORAN receiver Figure 2–33. Darker areas indicate LORAN-C worldwide coverage. 82 / CHAPTER 2 can then plot up to !ve LOPs from each chain of stations. The LORAN receiver on board the aircraft (Figure 2–34) then electronically determines the inter- section of these LOPs and displays the aircraft’s position to the pilots as lati- tude–longitude coordinates or as a bearing and distance from any preselected location. Since all of the LORAN ground stations operate at the same frequency (100 kHz), the airborne receiver can use the transmissions from other LORAN chains to con!rm its initial position determination. As the aircraft continues along its flight, the LORAN receiver constantly calculates the aircraft’s new position and uses this information to compute the aircraft’s course and ground speed. Using this information, the pilot can program the LORAN-C receiver to guide the aircraft to the desired destination. The LORAN-C receiver displays course guidance and distance informa- tion in a number of different formats, all of which provide the same essential information to the pilot. This information includes ground speed, ground track, course to be flown, distance to the destination airport, and estimated time of arrival. LORAN-C is a fairly accurate navigation system but has a number of important limitations. The radio frequencies used by LORAN are in the low frequency (LF) band and are not line of sight, which makes it possible for an aircraft to receive the LORAN signal at a distance of up to 1,500 miles from the transmitter. This is usually bene!cial but can sometimes prove to be a disadvantage. During certain atmospheric conditions (usually at twilight), II Morrow, Inc. Figure 2–34. A typical LORAN-C receiver.  Navigation Systems / 83 Ionosphere ve wa y Ground wave Sk Surface Figure 2–35. LORAN-C ground wave versus sky wave. an aircraft might receive two or more distinct signals from each master and slave station. The !rst signal is the ground wave, which is the signal the LORAN receiver is designed to utilize. Under certain conditions, the LORAN transmission may also travel into space and reflect off the ionosphere and return to the aircraft. This secondary signal takes longer to reach the aircraft and can confuse the LORAN receiver, since it now receives two pulses from every transmitter (see Figure 2–35). This condition makes it impossible for the LORAN receiver to accurately determine time delays or plot lines of position. Under these circumstances, the receiver is designed to ignore the transmissions from the affected chain and must be switched to an alternate chain of stations. LORAN stations include the transmitting equipment as well as antenna towers, with heights ranging from 700 to 1,350 feet. Depending on the cover- age area requirements, each LORAN station transmits a signal that ranges from 400 to 1,600 kilowatts of peak signal power. The actual control of each transmitting station is accomplished remotely from the Coast Guard Naviga- tion Center located in Alexandria, Virginia. Each transmitted signal is moni- tored, and its status is constantly transmitted to navigation center personnel. If a situation that could affect navigational accuracy is detected, an alert signal, called a blink, is activated. A blink signal is a change in the group of eight transmitted pulses automatically recognized by a LORAN receiver. If a blink signal is activated, the LORAN receiver displays an appropriate warning that the LORAN system should not be used for navigation. It was originally envisioned that satellite navigation would supersede LORAN, and the system might be decommissioned. With increased security concerns since 9/11, the LORAN-C system continues to operate in the United States and might eventually be used as a backup navigation system. 84 / CHAPTER 2 Global The Global Navigation Satellite System (GNSS) is the accepted term for nav- Navigation igation systems that provide ground-based users with global navigation via Satellite space-based satellite systems. GNSS transmitters are typically located on Low System Earth Orbit satellites permitting users with fairly small, inexpensive receivers to determine their location in three dimensions (latitude, longitude, and altitude). As long as the transmitters are within the sight line of a number of satellites, the receivers can determine their location within a few meters or even feet. The United States Global Positioning System (GPS), operated by the U.S. Air Force, is the only fully operational GNSS at this time, although it is expected that the Russian GLONASS system will be restored to full operation by 2010. The European Union is developing its own civilian GNSS system called Galileo, and China is developing a system called Compass. India is currently developing its own system as well. Due to its accuracy and worldwide availability, GNSS has been designated by ICAO as the future navigation system to meet all civil aviation needs, including departure terminal, oceanic, en route, nonprecision approach, precision approach, and surface navigation. Global In 1989, the Department of Defense (DoD) launched the first production series Positioning of GPS satellites, which were declared operational in 1993. The Federal Avia- System tion Administration established the civil operational status of GPS in 1994. Two years later, in 1996, the United States officially reiterated the country’s commitment to continue broadcasting GPS signals on a worldwide basis, free of charge for the foreseeable future. GPS is a space-based positioning, velocity, and time system composed of twenty-four satellites, (twenty-one operational plus three spares) in six orbital planes. The satellites operate in circular orbits arranged so that at any one time users worldwide are able to view a minimum of five satellites (see Figure 2–36). GPS operations are based on the concept of ranging and triangulation from a group of satellites in space that act as precise reference points. Figure 2–36. Global Positioning Satellite system.  Navigation Systems / 85 A GPS receiver measures distance from a satellite using the same travel time as a radio signal. Each satellite transmits a specific code, called a course/acqui- sition (CA) code, that contains information on the satellite’s position, the GPS system time, its clock error, and the health and accuracy of the transmitted data. GPS satellites have highly accurate atomic clocks to calculate signal travel time. The GPS receiver matches each satellite’s CA code with an identical code contained in the receiver’s database. By shifting its copy of the satellite’s code in a matching process, and by comparing this shift with its internal clock, the receiver can calcu- late how long it took the signal to travel from the satellite to the receiver. The distance derived from this method is called a pseudo range because it is not a direct measurement of distance but a measurement based on time. Pseudo range is subject to several errors, such as ionospheric delay or time dis- parities between the atomic clocks in the satellites and the GPS receiver, which the receiver can correct. I