AIM

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7. Pilots must be aware of the vertical height between the aircraft's glide slope antenna and the main gear in the landing configuration and, at the DH, plan to adjust the descent angle accordingly if the published TCH indicates the wheel crossing height over the runway threshold may not be satisfactory. Tests indicate a comfortable wheel crossing height is approximately 20 to 30 feet, depending on the type of aircraft. NOTE- The TCH for a runway is established based on several factors including the largest aircraft category that normally uses the runway, how airport layout effects the glide slope antenna placement, and terrain. A higher than optimum TCH, with the same glide path angle, may cause the aircraft to touch down further from the threshold if the trajectory of the approach is maintained until the flare. Pilots should consider the effect of a high TCH on the runway available for stopping the aircraft. e. Distance Measuring Equipment (DME) 1. When installed with the ILS and specified in the approach procedure, DME may be used: (a) In lieu of the OM; (b) As a back course (BC) final approach fix (FAF); and (c) To establish other fixes on the localizer course. 2. In some cases, DME from a separate facility may be used within Terminal Instrument Procedures (TERPS) limitations: (a) To provide ARC initial approach seg- ments; (b) As a FAF for BC approaches; and (c) As a substitute for the OM. f. Marker Beacon 1. ILS marker beacons have a rated power output of 3 watts or less and an antenna array designed to produce an elliptical pattern with dimensions, at 1,000 feet above the antenna, of approximately 2,400_feet in width and 4,200 feet in length. Airborne marker beacon receivers with a selective sensitivity feature should always be operated in the “low” sensitivity position for proper reception of ILS marker beacons. 2. Ordinarily, there are two marker beacons associated with an ILS, the OM and MM. Locations with a Category II ILS also have an Inner Marker_(IM). When an aircraft passes over a marker, the pilot will receive the indications shown in TBL 1-1-3.

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(a) The OM normally indicates a position at which an aircraft at the appropriate altitude on the localizer course will intercept the ILS glide path. (b) The MM indicates a position approxi- mately 3,500 feet from the landing threshold. This is also the position where an aircraft on the glide path will be at an altitude of approximately 200 feet above the elevation of the touchdown zone. (c) The IM will indicate a point at which an aircraft is at a designated decision height (DH) on the glide path between the MM and landing threshold. TBL 1-1-3 Marker Passage Indications Marker Code Light OM _ _ _ BLUE MM _ _ _ _ AMBER IM _ _ _ _ WHITE BC _ _ _ _ WHITE AIM 2/14/08 1-1-10 Navigation Aids 3. A back course marker normally indicates the ILS back course final approach fix where approach descent is commenced. g. Compass Locator 1. Compass locator transmitters are often situated at the MM and OM sites. The transmitters have a power of less than 25 watts, a range of at least 15_miles and operate between 190 and 535 kHz. At some locations, higher powered radio beacons, up to 400 watts, are used as OM compass locators. These generally carry Transcribed Weather Broadcast (TWEB) information. 2. Compass locators transmit two letter identifi- cation groups. The outer locator transmits the first two letters of the localizer identification group, and the middle locator transmits the last two letters of the localizer identification group. h. ILS Frequency (See TBL 1-1-4.) TBL 1-1-4 Frequency Pairs Allocated for ILS Localizer MHz Glide Slope 108.10 334.70 108.15 334.55 108.3 334.10 108.35 333.95 108.5 329.90 108.55 329.75 108.7 330.50 108.75 330.35 108.9 329.30 108.95 329.15 109.1 331.40 109.15 331.25 109.3 332.00 109.35 331.85 109.50 332.60 109.55 332.45 109.70 333.20 109.75 333.05 109.90 333.80 109.95 333.65 110.1 334.40 110.15 334.25 110.3 335.00 110.35 334.85 110.5 329.60 110.55 329.45 Localizer MHz Glide Slope 110.70 330.20 110.75 330.05 110.90 330.80 110.95 330.65 111.10 331.70 111.15 331.55 111.30 332.30 111.35 332.15 111.50 332.9 111.55 332.75 111.70 333.5 111.75 333.35 111.90 331.1 111.95 330.95 i. ILS Minimums 1. The lowest authorized ILS minimums, with all required ground and airborne systems components operative, are:

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(a) Category I. Decision Height (DH) 200_feet and Runway Visual Range (RVR) 2,400 feet (with touchdown zone and centerline lighting, RVR 1,800 feet); (b) Category II. DH 100 feet and RVR 1,200_feet; (c) Category IIIa. No DH or DH below 100_feet and RVR not less than 700 feet; (d) Category IIIb. No DH or DH below 50_feet and RVR less than 700 feet but not less than 150_feet; and (e) Category IIIc. No DH and no RVR limitation. NOTE- Special authorization and equipment required for Categories II and III. j. Inoperative ILS Components 1. Inoperative localizer. When the localizer fails, an ILS approach is not authorized. 2. Inoperative glide slope. When the glide slope fails, the ILS reverts to a nonprecision localizer approach. REFERENCE- See the inoperative component table in the U.S. Government Terminal Procedures Publication (TPP), for adjustments to minimums due to inoperative airborne or ground system equipment. AIM 2/14/08 1-1-11 Navigation Aids k. ILS Course Distortion 1. All pilots should be aware that disturbances to ILS localizer and glide slope courses may occur when surface vehicles or aircraft are operated near the localizer or glide slope antennas. Most ILS installations are subject to signal interference by either surface vehicles, aircraft or both. ILS CRITICAL AREAS are established near each localizer and glide slope antenna. 2. ATC issues control instructions to avoid interfering operations within ILS critical areas at controlled airports during the hours the Airport Traffic Control Tower (ATCT) is in operation as follows: (a) Weather Conditions. Less than ceiling 800 feet and/or visibility 2 miles. (1) Localizer Critical Area. Except for aircraft that land, exit a runway, depart or miss approach, vehicles and aircraft are not authorized in or over the critical area when an arriving aircraft is between the ILS final approach fix and the airport. Additionally, when the ceiling is less than 200 feet and/or the visibility is RVR 2,000 or less, vehicle and aircraft operations in or over the area are not authorized when an arriving aircraft is inside the ILS MM. (2) Glide Slope Critical Area. Vehicles and aircraft are not authorized in the area when an arriving aircraft is between the ILS final approach fix and the airport unless the aircraft has reported the airport in sight and is circling or side stepping to land on a runway other than the ILS runway. (b) Weather Conditions. At or above ceil- ing 800 feet and/or visibility 2 miles. (1) No critical area protective action is provided under these conditions. (2) A flight crew, under these conditions, should advise the tower that it will conduct an AUTOLAND or COUPLED approach to ensure that the ILS critical areas are protected when the aircraft is inside the ILS MM. EXAMPLE- Glide slope signal not protected.

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3. Aircraft holding below 5,000 feet between the outer marker and the airport may cause localizer signal variations for aircraft conducting the ILS approach. Accordingly, such holding is not authorized when weather or visibility conditions are less than ceiling 800 feet and/or visibility 2 miles. 4. Pilots are cautioned that vehicular traffic not subject to ATC may cause momentary deviation to ILS course or glide slope signals. Also, critical areas are not protected at uncontrolled airports or at airports with an operating control tower when weather or visibility conditions are above those requiring protective measures. Aircraft conducting coupled or autoland operations should be especially alert in monitoring automatic flight control systems. (See FIG 1-1-7.) NOTE- Unless otherwise coordinated through Flight Standards, ILS signals to Category I runways are not flight inspected below 100 feet AGL. Guidance signal anomalies may be encountered below this altitude. 1-1-10. Simplified Directional Facility (SDF) a. The SDF provides a final approach course similar to that of the ILS localizer. It does not provide glide slope information. A clear understanding of the ILS localizer and the additional factors listed below completely describe the operational characteristics and use of the SDF. b. The SDF transmits signals within the range of 108.10 to 111.95 MHz. c. The approach techniques and procedures used in an SDF instrument approach are essentially the same as those employed in executing a standard localizer approach except the SDF course may not be aligned with the runway and the course may be wider, resulting in less precision. d. Usable off-course indications are limited to 35_degrees either side of the course centerline. Instrument indications received beyond 35 degrees should be disregarded. e. The SDF antenna may be offset from the runway centerline. Because of this, the angle of convergence between the final approach course and the runway bearing should be determined by reference to the instrument approach procedure chart. This angle is generally not more than 3 degrees. However, it should be noted that inasmuch as the approach course originates at the antenna site, an approach which is continued beyond the runway threshold will lead the aircraft to the SDF offset position rather than along the runway centerline. AIM 2/14/08 1-1-12 Navigation Aids FIG 1-1-7 FAA Instrument Landing Systems AIM 2/14/08 1-1-13 Navigation Aids

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f. The SDF signal is fixed at either 6 degrees or 12_degrees as necessary to provide maximum flyability and optimum course quality. g. Identification consists of a three-letter identifi- er transmitted in Morse Code on the SDF frequency. The appropriate instrument approach chart will indicate the identifier used at a particular airport. 1-1-11. Microwave Landing System (MLS) a. General 1. The MLS provides precision navigation guidance for exact alignment and descent of aircraft on approach to a runway. It provides azimuth, elevation, and distance. 2. Both lateral and vertical guidance may be displayed on conventional course deviation indica- tors or incorporated into multipurpose cockpit displays. Range information can be displayed by conventional DME indicators and also incorporated into multipurpose displays. 3. The MLS supplements the ILS as the standard landing system in the U.S. for civil, military, and international civil aviation. At international airports, ILS service is protected to 2010. 4. The system may be divided into five functions: (a) Approach azimuth; (b) Back azimuth; (c) Approach elevation; (d) Range; and (e) Data communications. 5. The standard configuration of MLS ground equipment includes: (a) An azimuth station to perform functions (a) and (e) above. In addition to providing azimuth navigation guidance, the station transmits basic data which consists of information associated directly with the operation of the landing system, as well as advisory data on the performance of the ground equipment. (b) An elevation station to perform function_(c). (c) Distance Measuring Equipment (DME) to perform range guidance, both standard DME (DME/N) and precision DME (DME/P). 6. MLS Expansion Capabilities. The stan- dard configuration can be expanded by adding one or more of the following functions or characteristics. (a) Back azimuth. Provides lateral guidance for missed approach and departure navigation. (b) Auxiliary data transmissions. Provides additional data, including refined airborne position- ing, meteorological information, runway status, and other supplementary information. (c) Expanded Service Volume (ESV) propor- tional guidance to 60 degrees. 7. MLS identification is a four-letter designa- tion starting with the letter M. It is transmitted in International Morse Code at least six times per minute by the approach azimuth (and back azimuth) ground equipment. b. Approach Azimuth Guidance 1. The azimuth station transmits MLS angle and data on one of 200 channels within the frequency range of 5031 to 5091 MHz. 2. The equipment is normally located about 1,000 feet beyond the stop end of the runway, but there is considerable flexibility in selecting sites. For example, for heliport operations the azimuth transmitter can be collocated with the elevation transmitter.

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3. The azimuth coverage extends: (See FIG 1-1-8.) (a) Laterally, at least 40 degrees on either side of the runway centerline in a standard configuration, (b) In elevation, up to an angle of 15 degrees and to at least 20,000 feet, and (c) In range, to at least 20 NM. AIM 2/14/08 1-1-14 Navigation Aids FIG 1-1-8 Coverage Volume Azimuth APPROACH AZIMUTH AZIMUTH -40 40 20 NM ESV ESV 14 NM 60 MAXIMUM LIMIT 14 NM -60 c. Elevation Guidance 1. The elevation station transmits signals on the same frequency as the azimuth station. A single frequency is time-shared between angle and data functions. 2. The elevation transmitter is normally located about 400 feet from the side of the runway between runway threshold and the touchdown zone. 3. Elevation coverage is provided in the same airspace as the azimuth guidance signals: (a) In elevation, to at least +15 degrees; (b) Laterally, to fill the Azimuth lateral coverage; and (c) In range, to at least 20 NM. (See FIG 1-1-9.) FIG 1-1-9 Coverage Volumes Elevation ELEVATION GLIDE NORMAL PATH MAXIMUM LIMIT 20,000’ 20 NM 30 3 15 o o o o d. Range Guidance 1. The MLS Precision Distance Measuring Equipment (DME/P) functions the same as the navigation DME described in paragraph 1-1-7, Distance Measuring Equipment (DME), but there are some technical differences. The beacon transponder operates in the frequency band 962 to 1105 MHz and responds to an aircraft interrogator. The MLS DME/P accuracy is improved to be consistent with the accuracy provided by the MLS azimuth and elevation stations. 2. A DME/P channel is paired with the azimuth and elevation channel. A complete listing of the 200_paired channels of the DME/P with the angle functions is contained in FAA Standard 022 (MLS Interoperability and Performance Requirements). 3. The DME/N or DME/P is an integral part of the MLS and is installed at all MLS facilities unless a waiver is obtained. This occurs infrequently and only at outlying, low density airports where marker beacons or compass locators are already in place. e. Data Communications 1. The data transmission can include both the basic and auxiliary data words. All MLS facilities transmit basic data. Where needed, auxiliary data can be transmitted. 2. Coverage limits. MLS data are transmitted throughout the azimuth (and back azimuth when provided) coverage sectors.

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3. Basic data content. Representative data include: (a) Station identification; AIM 2/14/08 1-1-15 Navigation Aids (b) Exact locations of azimuth, elevation and DME/P stations (for MLS receiver processing functions); (c) Ground equipment performance level; and (d) DME/P channel and status. 4. Auxiliary data content: Representative data include: (a) 3-D locations of MLS equipment; (b) Waypoint coordinates; (c) Runway conditions; and (d) Weather (e.g., RVR, ceiling, altimeter setting, wind, wake vortex, wind shear). f. Operational Flexibility 1. The MLS has the capability to fulfill a variety of needs in the approach, landing, missed approach and departure phases of flight. For example: (a) Curved and segmented approaches; (b) Selectable glide path angles; (c) Accurate 3-D positioning of the aircraft in space; and (d) The establishment of boundaries to ensure clearance from obstructions in the terminal area. 2. While many of these capabilities are available to any MLS-equipped aircraft, the more sophisticated capabilities (such as curved and segmented approaches) are dependent upon the particular capabilities of the airborne equipment. g. Summary 1. Accuracy. The MLS provides precision three-dimensional navigation guidance accurate enough for all approach and landing maneuvers. 2. Coverage. Accuracy is consistent through- out the coverage volumes. (See FIG 1-1-10.) FIG 1-1-10 Coverage Volumes 3-D Representation 3. Environment. The system has low suscepti- bility to interference from weather conditions and airport ground traffic. 4. Channels. MLS has 200 channels- enough for any foreseeable need. 5. Data. The MLS transmits ground-air data messages associated with the systems operation. 6. Range information. Continuous range in- formation is provided with an accuracy of about 100_feet. 1-1-12. NAVAID Identifier Removal During Maintenance During periods of routine or emergency maintenance, coded identification (or code and voice, where applicable) is removed from certain FAA NAVAIDs. Removal of identification serves as a warning to pilots that the facility is officially off the air for tune-up or repair and may be unreliable even though intermittent or constant signals are received. NOTE- During periods of maintenance VHF ranges may radiate a T-E-S-T code (-_______ⴀ⤀⸀ AIM 2/14/08 1-1-16 Navigation Aids NOTE- DO NOT attempt to fly a procedure that is NOTAMed out of service even if the identification is present. In certain cases, the identification may be transmitted for short periods as part of the testing. 1-1-13. NAVAIDs with Voice

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a. Voice equipped en route radio navigational aids are under the operational control of either an FAA Automated Flight Service Station (AFSS) or an approach control facility. The voice communication is available on some facilities. Hazardous Inflight Weather Advisory Service (HIWAS) broadcast capability is available on selected VOR sites throughout the conterminous U.S. and does not provide two-way voice communication. The avail- ability of two-way voice communication and HIWAS is indicated in the A/FD and aeronautical charts. b. Unless otherwise noted on the chart, all radio navigation aids operate continuously except during shutdowns for maintenance. Hours of operation of facilities not operating continuously are annotated on charts and in the A/FD. 1-1-14. User Reports on NAVAID Performance a. Users of the National Airspace System (NAS) can render valuable assistance in the early correction of NAVAID malfunctions by reporting their observations of undesirable NAVAID performance. Although NAVAIDs are monitored by electronic detectors, adverse effects of electronic interference, new obstructions or changes in terrain near the NAVAID can exist without detection by the ground monitors. Some of the characteristics of malfunction or deteriorating performance which should be reported are: erratic course or bearing indications; intermittent, or full, flag alarm; garbled, missing or obviously improper coded identification; poor quality communications reception; or, in the case of frequency interference, an audible hum or tone accompanying radio communications or NAVAID identification. b. Reporters should identify the NAVAID, loca- tion of the aircraft, time of the observation, type of aircraft and describe the condition observed; the type of receivers in use is also useful information. Reports can be made in any of the following ways: 1. Immediate report by direct radio communica- tion to the controlling Air Route Traffic Control Center (ARTCC), Control Tower, or FSS. This method provides the quickest result. 2. By telephone to the nearest FAA facility. 3. By FAA Form 8000-7, Safety Improvement Report, a postage-paid card designed for this purpose. These cards may be obtained at FAA FSSs, Flight Standards District Offices, and General Aviation Fixed Base Operations. c. In aircraft that have more than one receiver, there are many combinations of possible interference between units. This can cause either erroneous navigation indications or, complete or partial blanking out of the communications. Pilots should be familiar enough with the radio installation of the particular airplanes they fly to recognize this type of interference. 1-1-15. LORAN a. Introduction 1. The LOng RAnge Navigation-C (LORAN) system is a hyperbolic, terrestrial-based navigation system operating in the 90-110 kHz frequency band. LORAN, operated by the U.S. Coast Guard (USCG), has been in service for over 50 years and is used for navigation by the various transportation modes, as well as, for precise time and frequency applications. The system is configured to provide reliable, all weather navigation for marine users along the U.S._coasts and in the Great Lakes. 2. In the 1980's, responding to aviation user and industry requests, the USCG and FAA expanded LORAN coverage to include the entire continental U.S. This work was completed in late 1990, but the LORAN system failed to gain significant user acceptance and primarily due to transmitter and user equipment performance limitations, attempts to obtain FAA certification of nonprecision approach capable receivers were unsuccessful. More recently, concern regarding the vulnerability of Global Positioning System (GPS) and the consequences of losing GPS on the critical U.S. infrastructure (e.g.,_NAS) has renewed and refocused attention on LORAN. 3. LORAN is also supported in the Canadian airspace system. Currently, LORAN receivers are only certified for en route navigation. AIM 2/14/08 1-1-17 Navigation Aids 4. Additional information can be found_in_the_“LORAN-C User Handbook,” COMDT PUB-P16562.6, or the website http://www.navcen.uscg.gov. b. LORAN Chain 1. The locations of the U.S. and Canadian LORAN transmitters and monitor sites are illustrated in FIG 1-1-11. Station operations are organized into subgroups of four to six stations called “chains.” One station in the chain is designated the “Master” and the others are “secondary” stations. The resulting chain based coverage is seen in FIG 1-1-12. FIG 1-1-11 U.S. and Canadian LORAN System Architecture FIG 1-1-12 LORAN Chain Based Coverage AIM 2/14/08 1-1-18 Navigation Aids 2. The LORAN navigation signal is a carefully structured sequence of brief radio frequency pulses centered at 100 kHz. The sequence of signal transmissions consists of a pulse group from the Master (M) station followed at precise time intervals by groups from the secondary stations, which are designated by the U.S. Coast Guard with the letters V, W, X, Y and Z. All secondary stations radiate pulses in groups of eight, but for identification the Master signal has an additional ninth pulse. (See FIG 1-1-13.) The timing of the LORAN system is tightly controlled and synchronized to Coordinated Universal Time (UTC). Like the GPS, this is a Stratum_1 timing standard.

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3. The time interval between the reoccurrence of the Master pulse group is called the Group Repetition Interval (GRI). The GRI is the same for all stations in a chain and each LORAN chain has a unique GRI. Since all stations in a particular chain operate on the same radio frequency, the GRI is the key by which a LORAN receiver can identify and isolate signal groups from a specific chain. EXAMPLE- Transmitters in the Northeast U.S. chain (FIG 1-1-14) operate with a GRI of 99,600 microseconds which is shortened to 9960 for convenience. The master station (M) at Seneca, New York, controls secondary stations (W) at Caribou, Maine; (X) at Nantucket, Massachusetts; (Y) at Carolina Beach, North Carolina, and (Z) at Dana, Indiana. In order to keep chain operations precise, monitor receivers are located at Cape Elizabeth, ME; Sandy Hook, NJ; Dunbar Forest, MI, and Plumbrook, OH. Monitor receivers continuously measure various aspects of the quality (e.g., pulse shape) and accuracy (e.g., timing) of LORAN signals and report system status to a control station. 4. The line between the Master and each secondary station is the “baseline” for a pair of stations. Typical baselines are from 600 to 1,000_nautical miles in length. The continuation of the baseline in either direction is a “baseline extension.” 5. At the LORAN transmitter stations there are cesium oscillators, transmitter time and control equipment, a transmitter, primary power (e.g.,_com- mercial or generator) and auxiliary power equipment (e.g., uninterruptible power supplies and generators), and a transmitting antenna (configurations may either have 1 or 4 towers) with the tower heights ranging from 700 to 1350 feet tall. Depending on the coverage area requirements a LORAN station transmits from 400 to 1,600 kilowatts of peak signal power. 6. The USCG operates the LORAN transmitter stations under a reduced staffing structure that is made possible by the remote control and monitoring of the critical station and signal parameters. The actual control of the transmitting station is accomplished remotely at Coast Guard Navigation Center (NAVCEN) located in Alexandria, Virginia. East Coast and Midwest stations are controlled by the NAVCEN. Stations on the West Coast and in Alaska are controlled by the NAVCEN Detachment (Det), located in Petaluma, California. In the event of a problem at one of these two 24 hour-a-day staffed sites, monitoring and control of the entire LORAN system can be done at either location. If both NACEN and NAVCEN Det are down or if there is an equipment problem at a specific station, local station personnel are available to operate and perform repairs at each LORAN station. 7. The transmitted signal is also monitored in the service areas (i.e., area of published LORAN coverage) and its status provided to NAVCEN and NAVCEN Det. The System Area Monitor (SAM) is a single site used to observe the transmitted signal (signal strength, time difference, and pulse shape). If an out-of-tolerance situation that could affect navigation accuracy is detected, an alert signal called “Blink” is activated. Blink is a distinctive change in the group of eight pulses that can be recognized automatically by a receiver so the user is notified instantly that the LORAN system should not be used for navigation. Out-of-tolerance situations which only the local station can detect are also monitored. These situations when detected cause signal transmissions from a station to be halted. 8. Each individual LORAN chain provides navigation-quality signal coverage over an identified area as shown in FIG 1-1-15 for the West Coast chain, GRI 9940. The chain Master station is at Fallon, Nevada, and secondary stations are at George, Washington; Middletown, California, and Search- light, Nevada. In a signal coverage area the signal strength relative to the normal ambient radio noise must be adequate to assure successful reception. Similar coverage area charts are available for all chains. AIM 2/14/08 1-1-19 Navigation Aids FIG 1-1-13 The LORAN Pulse and Pulse Group AIM 2/14/08 1-1-20 Navigation Aids FIG 1-1-14 Northeast U.S. LORAN Chain AIM 2/14/08 1-1-21 Navigation Aids FIG 1-1-15 West Coast U.S. LORAN Chain AIM 2/14/08 1-1-22 Navigation Aids c. The LORAN Receiver 1. For a currently certified LORAN aviation receiver to provide navigation information for a pilot, it must successfully receive, or “acquire,” signals from three or more stations in a chain. Acquisition involves the time synchronization of the receiver with the chain GRI, identification of the Master station signals from among those checked, identification of secondary station signals, and the proper selection of the tracking point on each signal at which measurements are made. However, a new generation of receivers has been developed that use pulses from all stations that can be received at the pilot's location. Use of “all-in-view” stations by a receiver is made possible due to the synchronization of LORAN stations signals to UTC. This new generation of receivers, along with improvements at the transmit- ting stations and changes in system policy and operations doctrine may allow for LORAN's use in nonprecision approaches. At this time these receivers are available for purchase, but none have been certified for aviation use. 2. The basic measurements made by certified LORAN receivers are the differences in time-ofarrival between the Master signal and the signals from each of the secondary stations of a chain. Each “time difference” (TD) value is measured to a precision of about 0.1 microseconds. As a rule of thumb, 0.1_microsecond is equal to about 100 feet.

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3. An aircraft's LORAN receiver must recog- nize three signal conditions: (a) Usable signals; (b) Absence of signals, and (c) Signal blink. 4. The most critical phase of flight is during the approach to landing at an airport. During the approach phase the receiver must detect a lost signal, or a signal Blink, within 10 seconds of the occurrence and warn the pilot of the event. At this time there are no receivers that are certified for nonprecision approaches. 5. Most certified receivers have various internal tests for estimating the probable accuracy of the current TD values and consequent navigation solutions. Tests may include verification of the timing alignment of the receiver clock with the LORAN pulse, or a continuous measurement of the signal-to-noise ratio (SNR). SNR is the relative strength of the LORAN signals compared to the local ambient noise level. If any of the tests fail, or if the quantities measured are out of the limits set for reliable navigation, then an alarm will be activated to alert the pilot. 6. LORAN signals operate in the low frequency band (90-110 kHz) that has been reserved for marine navigation signals. Adjacent to the band, however, are numerous low frequency communications transmitters. Nearby signals can distort the LORAN signals and must be eliminated by the receiver to assure proper operation. To eliminate interfering signals, LORAN receivers have selective internal filters. These filters, commonly known as “notch filters,” reduce the effect of interfering signals. 7. Careful installation of antennas, good metalto-metal electrical bonding, and provisions for precipitation noise discharge on the aircraft are essential for the successful operation of LORAN receivers. A LORAN antenna should be installed on an aircraft in accordance with the manufacturer's instructions. Corroded bonding straps should be replaced, and static discharge devices installed at points indicated by the aircraft manufacturer. d. LORAN Navigation 1. An airborne LORAN receiver has four major parts: (a) Signal processor; (b) Navigation computer; (c) Control/display, and (d) Antenna.