FAA Navigation Systems PDF

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...

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 !ying 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 defined by the limits of the VOR receivers’ accuracy. If the two radials do not bisect each other at approximately right angles, the area defined 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 deficiencies 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 re!ections. 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 re!ection 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 airfields (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 !exible, 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 justified. 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