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16.2.7 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.
16.2.8 Each individual LORAN chain provides navigation-quality signal coverage over an identified area as shown in FIG ENR 4.1.10 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.

FIG ENR 4.1.8
The LORAN Pulse and Pulse Group

Federal Aviation Administration Twentieth Edition
FIG ENR 4.1.9
Northeast U.S. LORAN Chain

FIG ENR 4.1.10
West Coast U.S. LORAN Chain

Federal Aviation Administration Twentieth Edition
16.3 The LORAN Receiver
16.3.1 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.
16.3.2 The basic measurements made by certified LORAN receivers are the differences in time.of.ar-rival 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.
16.3.3 An aircraft’s LORAN receiver must recog-nize three signal conditions:
16.3.3.1 Usable signals;
16.3.3.2 Absence of signals, and
16.3.3.3 Signal blink.
16.3.4 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.
16.3.5 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.
16.3.6 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.
16.3.7 Careful installation of antennas, good metal. to.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.
　
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