Navigation Systems PDF

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...

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 !y 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 !y a longer distance than necessary. It also creates congestion in the air traffic control system, since every aircraft operat- ing under an IFR !ight 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 first 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 re!ected 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 re!ects 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 !y 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 !ight, 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 sufficient number of waypoints along the planned route of !ight to permit a straight course to be !own. 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 !ight, 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 sufficient 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 Pacific 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 first line of position (LOP), the pilot repeats this procedure using a second pair of stations. The second LOP will intersect with the first one, defining the aircraft’s exact location. LORAN-A was never designed to be used by high-speed aircraft. Since a significant amount of time can elapse between the plotting of the first 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 five 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 identified 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, identifies 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 five 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 confirm its initial position determination. As the aircraft continues along its !ight, 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 !own, 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 beneficial 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 first 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 re!ect 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. In addition to knowing the distance to a satellite, a receiver needs to know the satellite’s exact position in space; this is known as its ephemeris. Each satel- lite transmits ephemeris information about its exact orbital location. The GPS receiver uses this information to precisely establish the position of the satellite. Using the calculated pseudo range and the position information supplied by the satellite, the GPS receiver mathematically determines its position by triangula- tion (see Figure 2–37). Figure 2–37. Satellite triangulation as used by the GPS system. 86 / CHAPTER 2 The GPS receiver needs at least three satellites with timing corrections from a fourth satellite to yield an unaided, unique, and true three-dimensional position (latitude, longitude, and altitude). The GPS receiver can then com- pute navigational values such as distance and bearing to a waypoint, ground speed, estimated time en route, estimated time of arrival, and winds aloft. It does this by using the aircraft’s known latitude/longitude, measuring rela- tive movement, and referencing these to a database built into the receiver. The receiver uses data from the best four satellites, automatically adding sig- nals from new ones as it drops signals from others to continually calculate its position. Receiver The receiver (see Figure 2–38) verifies the integrity of the signals received Autonomous from the GPS constellation through receiver autonomous integrity monitor- Integrity ing (RAIM). RAIM is an independent means to determine whether a satellite Monitoring is providing corrupted information. At least one satellite, in addition to those required for navigation, must be in view for the receiver to perform the RAIM function; therefore, for RAIM to work correctly, five satellites must be in view of the receiver. RAIM performs consistency checks between position solutions obtained with various subsets of the visible satellites. The receiver provides an alert to the pilot if the consistency checks fail. Garmin, Inc. Figure 2–38. Example of a high-end, panel-mounted GPS unit.  Navigation Systems / 87 GNSS GNSS signals provide sufficient accuracy for en route and two-dimensional Augment- navigation, but they do not provide acceptable vertical or lateral landing guid- ation ance. The standard GNSS signal needs to be augmented to provide this capabil- ity. This can be accomplished by using either a Ground-Based Augmentation System (GBAS) or Satellite-Based Augmentation System (SBAS). Augmentation will provide more accurate lateral guidance during the approach and departure phases of flight and might be used in some en route environments as well. Augmentation will also provide approach with vertical guidance (APV), which offers pilots a positive and stabilized vertical guidance flight path for approach procedures where no current guidance exists. Augmentation will likely provide APV performance levels similar to that of today’s Category I ILS standard (200’ ceiling and ½-mile visibility). In the future, if security concerns can be minimized and signal integrity can be main- tained, Category II (100’ ceiling and ¼-mile visibility) and Category III (zero/ zero autoland) approaches might be provided as well. SBASs comprise a network of ground reference stations that col- lect the satellite signals and send them to one or more ground processing centers. The centers compare the overall signal inaccuracy from each sta- tion and compute a differential correction. This correction is sent to one or more geostationary satellites that transmit the augmentation message to each aircraft. There are multiple SBASs being developed and/or in operation. One SBAS currently operational is the U.S.-controlled wide area augmentation system (WAAS). The European geostationary navigation overlay service (EGNOS) is another SBAS, but it is still in an early state of operation with the hope of becoming operational by 2010. In the United States, the FAA commissioned WAAS for instrument flight use in July 2003, providing approach and en route navigation across the entire county. Avionics that utilize WAAS to provide vertical guidance during an instrument approach are now available. WAAS-based instrument approaches can now be performed to many airport runways. Wide Area WAAS uses a network of precisely located ground reference stations that moni- Augment- tor transmitted GPS satellite signals. These stations are located throughout the ation System continental United States, Hawaii, Puerto Rico, and Alaska, with additional (WAAS) stations installed in Canada and Mexico. Ground reference stations collect and process GPS information and send it to the WAAS master station. The master station develops a correction message that is sent to users via satellite. The WAAS message improves the accuracy, availability, and safety of GPS-derived position information. Using WAAS, GPS signal accuracy is improved from about plus or minus 20 meters to approximately 2 meters both horizontally and vertically (see Figure 2–39). Ground- Based Aircraft using GBAS receive augmentation information directly from a local Augment- ground-based transmitter. GBAS is similar to SBAS with the exception of the ation System 88 / CHAPTER 2 Figure 2–39. Wide area augmentation system. system error being measured in only one local geographic area, thereby making the augmentation differential calculation very accurate. The augmentation message is sent only to aircraft in the local area, usually by some form of domestic radio communication. GBAS can easily provide Category I ILS performance and will eventually provide Category II and Category III performance if service continuity and integrity problems can be resolved. The FAA program for providing GBAS is the local area augmentation system (LAAS). LAAS augments GPS within an approximate 20- to 30-mile radius of the receiver, which is typically placed at or near an airport. LAAS broadcasts its correction message via a VHF radio data link from a ground-based transmitter. LAAS can yield the extremely high accuracy, availability, and integrity necessary for Category I, II, and III pre- cision approaches as well as ultimately providing flexible, curved approach paths. LAAS-demonstrated accuracy is better than 1 meter both horizontally and vertically. LAAS is currently still in the research and development stage. The FAA is working with industry in anticipation of the certification of the first prototype LAAS ground station to be located in Memphis, Tennessee.  Navigation Systems / 89 Inertial The inertial navigation system (INS) is similar to Doppler radar systems in Navigation that it precisely measures any change in an aircraft’s direction of flight and System uses this information to determine position, ground speed, and the course to be flown to the destination airport. An INS contains accelerometers that can measure the slightest change in an aircraft’s speed or direction of flight. At the beginning of each flight, the pilot is required to program the aircraft’s exact location into the INS computer (see Figure 2–40). Using the informa- tion obtained from the accelerometers, the INS computer on board the air- craft determines the aircraft’s speed and direction of flight. Using this information, the INS can calculate the course to be flown and the estimated time of arrival. This information is then displayed to the pilot or directed to the aircraft’s autopilot. When used correctly, the INS is highly accurate. INS information may be accurate to within !25 miles after a transoceanic !ight in excess of 14 hours. Since the INS is independent of ground-based radio navigation stations, it can be used by aircraft anywhere around the world. But as with Doppler radar, pilots must be careful to correctly enter the aircraft’s initial starting position into the navigation computer prior to depature. Since every subsequent position determination will be made based on this initial programming, any input errors will render all subsequent navigation information invalid. Delco Systems Operation, Delco Electronics Corporation Figure 2–40. An inertial navigation system. 90 / CHAPTER 2 In an attempt to reduce the risk of pilots erroneously programming the INS computer, most manufacturers have designed their inertial navigation systems to interconnect with other navigation systems on board the aircraft (such as LORAN or VORTAC). Using the information available from these systems, the INS can continuously examine its own calculations and deter- mine their validity. If a gross discrepancy is noted by the INS, the pilot will be alerted. The INS is certified by the FAA as a primary means of en route naviga- tion. The INS is fairly expensive and is normally found only on large, expensive commercial aircraft or business jets. Performance- In 1983, ICAO formed the Future Air Navigation System (FANS) committee Based to develop a strategy that would include new concepts of aircraft communica- Navigation tion, navigation, surveillance, and air traffic management (CNS/ATM). One of the strategies that came out of this group was performance-based naviga- tion (PBN). PBN is a framework for defining navigation performance requirements to be applied to an air traffic route, instrument procedure, or defined airspace. PBN includes both Area Navigation (RNAV) and Required Navigation Perfor- mance (RNP). With PBN, once the required performance level is established, the aircraft’s own capability determines whether it can safely achieve the speci- fied performance and qualify for the operation. PBN is not a navigation system but a framework for defining a navigation performance specification within which aircraft must comply with specified operational performance requirements. Unlike other navigation specifications, PBN is not equipment specific but rather establishes required performance on the basis of defined operational needs. It is the aircraft’s own capability that determines whether the pilot can achieve the specified performance and qualify for the specific operation. The FAA and industry have defined PBN specifications that can be sat- isfied by a range of navigation systems. PBN simply specifies aircraft system performance requirements in terms of accuracy, integrity, availability, continu- ity, and functionality needed for the proposed operations. It represents a shift from sensor-based to performance-based navigation. Performance require- ments are identified in navigation specifications, which also identify the choice of navigation sensors and equipment that may be used to meet the performance requirements. In the future, the FAA will describe navigation requirements in terms of required performance instead of specific onboard navigation systems such as VOR, GPS, etc. In the future, the international community will also likely establish minimum performance capabilities in the areas of required communi- cations performance (RCP) and surveillance performance (RSP). Required Navigation RNP provides specifications based on demonstrated levels of navigation Performance performance and capabilities rather than a required set of specific  Navigation Systems / 91 navigation equipment. ICAO defines RNP as “a statement of the navigation performance accuracy necessary for operation within a defined airspace.” This navigation performance accuracy is quantified with two values: a dis- tance in nautical miles (known as the RNP type) and a probability level (usually 95%). For example, an airplane will be certified to operate on an RNP-4 airway if the performance of the navigation system will result in the airplane being within 4 nautical miles of its indicated position at least 95%of the time. The RNP capability of an aircraft varies depending upon the equip- ment installed on the aircraft as well as the navigation infrastructure. Generally, aircraft will be equipped with multi mode receivers (MMR) that automatically select the most accurate system available and display that information to the pilot. The aircraft can then use procedures for which the aircraft’s navigation systems qualify. For example, an aircraft may be equipped and certified for RNP 1.0 but may not be capable of RNP 1.0 operations if during flight the aircraft’s navigation system detects transmitter or receiver problems or limited navaid coverage. The onboard MMR will automatically select from a GPS, WAAS, VOR, TACAN, ILS, or DME navigation signal to provide the pilot with the most accurate solution set. The best solution will be graphically presented to the pilot for navigation use as will the RNP accuracy level. Different airspace, routes, or procedures will have specified minimum RNP level requirements for use. ICAO has already defined standard minimum RNP values for the four possible navigation phases of flight: oceanic, en route, terminal, and approach. The required RNP value is expressed as a distance in nautical miles from the intended centerline of a pro- cedure, route, or path. The FAA has developed conforming standards, which are specified in Table 2–2. In special circumstances, U.S. RNP levels for specific routes and proce- dures might be based on the use of a specific navigational system such as GPS or VORTAC, but generally the aircraft MMR will choose the most accurate system. Table 2–2. U.S. Standard RNP Levels RNP Level Application 0.3 nm LNAV approaches 1.0 nm Arrival or departure routes 2.0 nm En route airways 4.0 nm Oceanic/remote areas where 30 nm lateral separation is currently required 10.0 nm Oceanic/remote areas where 50 nm lateral separation is currently required 92 / CHAPTER 2 Special A requirement of RNP is the aircraft navigation system’s ability to continuously Aircraft and monitor current navigation performance and inform the pilot if the minimum Aircrew requirements cannot be met during any specific operation. This onboard moni- Authorization toring and alerting capability enhances the pilot’s situational awareness and Required can enable reduced obstacle clearance and closer route spacing without inter- vention by air traffic control. Some RNP operations might require additional procedures and equip- ment and/or specialized flight crew training before the FAA will permit their use. This might include the addition of advanced features on the onboard navigation system or additional approved flight training and crew procedures. These specific instrument flight operations require FAA approval before they can be utilized and are known as special aircraft and aircrew authorization required (SAAAR) procedures. Instrument Approach Procedures The navigation systems discussed to this point are those primarily utilized for en route navigation between airports. If, upon arrival at the destination airport, the pilot can see the airport and safely perform an approach to the runway and land, the pilot may use either a visual approach or a contact approach. The specific differences between these two approach procedures will be covered in Chapter 6. In general, a visual approach can be conducted if the visibility is greater than 3 miles. Visual approaches can be initiated by either the pilot or the controller. Contact approaches may be conducted whenever the visibil- ity is greater than 1 mile. Only the pilot can initiate this type of approach. In this chapter, both types of approaches will be generically referred to as visual approaches. During a visual approach, the pilot accepts the responsibility for navigat- ing to the airport and avoiding any obstacles within the local area. When visual approaches are being conducted, air traffic controllers are still responsible for separating aircraft that are using them from aircraft operating on IFR !ight plans; Only the navigation is left to the pilot. If the weather conditions at the destination airport are such that the pilot is unable to, or chooses not to, conduct a visual approach, he or she must conduct an instrument approach procedure (IAP). During the conduct of an instrument approach, the pilot must follow a specified procedure that provides course guidance and obstacle clearance. This procedure guides the pilot to the destination airport where he or she can then make a safe landing. Instrument approach procedures are designed and published by the U.S. government and are made available to pilots and private corporations. When requested by a sponsoring agency, specially trained FAA personnel accurately determine the routes and altitudes that aircraft will !y when approaching the airport under marginal weather conditions. These specialists use the procedures contained in the terminal instrument approach procedures (TERPS) manual  Navigation Systems / 93 published by the FAA. The TERPS manual specifies the criteria that must be met before the FAA can certify an instrument approach procedure. TERPS specialists ensure that pilots complying with a published instrument approach procedure will avoid every obstacle in the vicinity of the approach path and will still be able to safely land at the completion of the approach. When the FAA specialists have finished designing an instrument approach procedure, specially trained FAA pilots conduct flight checks in specially instru- mented aircraft to ensure that the approach procedure actually meets TERPS criteria. After this !ight check, the FAA publishes the instrument approach and permits pilots to use these procedures (see Figure 2–41). These instrument approach procedures are actually considered Federal Aviation Regulations (FARs), and FAR 91 mandates that pilots comply with these procedures when conducting an instrument approach. The National Ocean Service (NOS) and Jeppessen Incorporated (a privately owned company) use the TERPS information to publish instrument approach procedure charts (sometimes called approach plates) that graphically depict the transition from the airway structure to the actual instrument approach proce- dure. Each publisher uses the same information when designing its charts but presents this information differently. NOS charts are primarily used by the FAA, Department of Defense, and general aviation pilots. Jeppessen (or JEPP) charts are primarily used by airline, corporate, and some general aviation pilots. Segments An instrument approach procedure essentially consists of four components: the of an initial approach, intermediate approach, final approach, and missed approach Instrument segments. A detailed description of each segment is provided in the TERPS Approach manual, available from the U.S. Government Printing Office. Procedure Initial Approach Segment The initial approach segment is designed to transi- tion the aircraft from the en route airway structure to the intermediate approach segment. The initial approach segment begins at one of the initial approach fixes (IAFs) located along the federal airways. This segment is usually defined as a heading or a radial to !y from the IAF to the intermediate approach segment. The initial approach segment specifies the minimum allowable altitude that may be !own along that route. There is usually one initial approach segment for every airway that pilots might be using as they approach the airport. The initial approach segment terminates when it joins the intermediate approach segment. Intermediate Approach Segment The intermediate approach segment is designed to permit the pilot to descend to an intermediate altitude and align the aircraft in order to make an easy transition to the final approach segment. The intermediate approach segment terminates at the final approach fix (FAF), which is designated on the approach chart with a maltese cross for nonpreci- sion approaches and a lightning bolt for precision approaches. There is usually only one intermediate approach segment for every approach. It is not ordinar- ily identified as such on an approach chart. The intermediate segment may simply consist of a course to !y that leads to the final approach fix, or it may be part of a procedure turn. Figure 2–41. FAA form 8260: written description of a standard instrument approach procedure (NDB runway Indiana).  Navigation Systems / 95 45∞ Turn 45∞ Turn Runway Final approach fix 1 min. 180∞ Turn Figure 2–42. Procedure turn. Procedure turns are necessary whenever the heading of the initial approach segment is nearly opposite that of the intermediate segment. A procedure turn is a maneuver performed in a designated area of airspace where the pilot turns the aircraft around and tracks inbound on the intermediate approach segment (see Figure 2–42). Typically, the airspace reserved for a procedure turn includes all of the airspace on one side of the approach course within a distance of about 10 nautical miles from the final approach fix. The pilot is authorized to use all of this airspace when reversing course from an initial to intermediate approach segment. Final Approach Segment The final approach segment is used to navigate the aircraft to the runway and properly position it to permit a safe landing. This segment begins at the final approach fix and ends at the missed approach point (MAP). The final approach segment guides the aircraft to the desired runway using a navigation aid located either at the airport or nearby. The navigation aid can be one of two general types: precision or nonprecision. A precision approach aid provides the pilot with both lateral and vertical course guidance to the approach end of the runway. A nonprecision approach aid provides only lateral guidance to the pilot. Nonprecision Approach During a nonprecision approach, upon reaching the final approach fix the pilot descends to a predetermined minimum descent alti- tude (MDA) published on the instrument approach chart (see Figure 2–43). The pilot maintains this altitude while tracking along the final approach segment toward the missed approach point. If the runway or runway environment is sighted prior to reaching the MAP and the pilot feels that a safe landing can be made, he or she is legally authorized to continue the approach and land. If the 96 / CHAPTER 2 Final approach fix Missed approach Intermediate point approach segment Missed Minimum approach descent segment altitude Runway Figure 2–43. Nonprecision approach procedure using a minimum descent altitude. runway is not in sight prior to reaching the MAP, or if a safe landing cannot be accomplished, the pilot must transition to the missed approach segment, which usually leads back to an initial approach fix. This is called missed approach procedure. Precision Approach During the conduct of a precision approach, the pilot descends while tracking along the final approach segment. The precision approach aid provides an electronic descent path for the pilot known as a glide path. When the designated altitude (known as the decision height or DH) has been reached, the pilot must determine whether a safe landing can be made (see Figure 2–44). If, in the pilot’s opinion, it is safe to land, he or she is legally authorized to continue the descent and land. However, if the pilot determines that it is not safe to continue, a transition to the missed approach segment must be made, and the missed approach procedure must be conducted. Because of the accuracy of precision approach aids, the pilot is usually authorized to descend to a lower altitude before making a decision about land- ing. This makes a precision approach much more valuable to the pilot during periods of marginal weather. Since precision approach aids are usually more expensive to purchase, install, and operate than nonprecision aids, they are normally reserved for use at airports that experience a significant amount of marginal weather conditions. Terminal Arrival Area Criteria Most existing VOR, NDB, or instrument land- ing system (ILS) approach procedures require that the aircraft transition from the en route airway structure to the instrument approach procedure using specified ground tracks defined by ground-based navigation aids. The advent  Navigation Systems / 97 Final approach fix Intermediate approach segment Missed Decision approach height segment Runway Figure 2–44. Precision approach procedure using a decision height. of GNSS, coupled with the concept of free !ight, means that many aircraft will no longer !y these routes. Requiring a transition from free !ight to a fixed route structure as the aircraft nears the airport will reimpose a traffic constraint, nul- lifying any operational advantage offered by GNSS/free !ight. In response to the introduction of GPS navigation systems in the United States, the FAA has begun to establish new, standardized instrument approach configurations for use at each airport. These new criteria are defined in FAA Order 8260, entitled Terminal Arrival Area (TAA) Design Criteria. These stan- dardized approach procedures will be established for all new GPS approach procedures. Most likely, any remaining non-GPS instrument approach proce- dure retained by the FAA will be converted similarly. Instead of defining specific routes and altitudes that the aircraft must use while transitioning to the GNSS-based instrument approach, TAA criteria define one final approach, one missed approach, and three initial approach fixes in addition to three airspace areas (see Figure 2–45). These fixes are arranged in a T-shaped configuration. In the example shown in Figure 2–46, aircraft approaching the airport from the southwest would be located in airspace A and would navigate direct to the initial approach fix (in this example, called Alpha). Aircraft approaching from the southeast would navigate direct to Charlie, whereas aircraft approaching in a clockwise arc from west to east (airspace B), would navigate directly to Bravo. To provide obstacle and terrain clearance, each airspace area extends 30 nautical miles out from its associated initial approach fix. A minimum alti- tude is established for each area that provides a minimum of 1,000 feet of obstacle clearance. If a single altitude cannot be provided for a particular sector, 98 / CHAPTER 2 IAF IF(IAF) IAF FAF MAP Figure 2–45. Basic TAA configuration. Airspace B Straight-In Area Navigating to Bravo Alpha Charlie Bravo Right Base Area Left Base Area Navigating to Alpha Navigating to Charlie Airspace A Airspace C Figure 2–46. Standard TAA with airspace areas. step-down areas will be developed. Instead of being fixed points in space, as they are in current instrument approach procedures, the step-down areas will exist as arcs centered on the appropriate initial approach fix (see Figure 2–47). Aircraft initiate the instrument approach entering via either airspace A or C, proceed direct to the appropriate fix (either Alpha or Charlie), and then turn 90° and !y direct to Bravo. Aircraft entering airspace B would simply proceed direct to Bravo. These three !ight segments are the initial approach segments. The length of these segments will be between 3 and 10 miles, depending on the speed of the aircraft that typically use this approach. Any aircraft required to hold would do so at Bravo. Holding at the final approach fix would no longer be a common procedure. Procedure turns would also become a thing of the  Navigation Systems / 99 180∞ 30 NM 210∞ 6000' 2000' 22 NM 4700' 20 NM 090∞ 270∞ 4100' 3000' 17 NM 17 NM 6000' 6000' 30 NM 30 NM 360∞ Figure 2–47. A sectorized TAA with step-down arcs. past. If any form of course reversal were required, it would be accomplished at the 5-mile holding pattern depicted at Bravo. The intermediate segment exists from Bravo to the final approach fix. An altitude will be established that provides the aircraft with 1,000 feet of vertical clearance from all obstructions during this narrow segment. This segment ter- minates at the final approach fix. At that point, the pilot would !y the approach to the missed approach point just like any other approach. The approach could be a nonprecision GPS approach, or if some form of vertical guidance were provided, the pilot would conduct a precision instrument approach. In most procedures, the missed approach procedure will require the aircraft to turn either right or left, returning to either Alpha or Bravo where the approach will recommence. The plan is that this new configuration will become the new standard for every existing and newly developed instrument approach procedure. This will provide a standardized configuration that permits direct !ight to the beginning of every approach, thereby increasing efficiency and no longer tying aircraft to fixed, in!exible routes and altitudes while still providing safe separation and transition to the instrument approach (see Figure 2–48). 100 / CHAPTER 2 N C-1, 12 F E B 2009 to 12 MA R 2009 N C-1, 12 F E B 2009 to 12 MA R 2009 Figure 2–48. RNAV GPS runway 27R approach at Grand Forks, North Dakota.