时间：2011-04-18 00:47来源：蓝天飞行翻译 作者：航空 点击：次
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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 AIM 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. Navigation Aids 1.1.19 FIG 1.1.13 The LORAN Pulse and Pulse Group 1.1.20 Navigation Aids FIG 1.1.14 Northeast U.S. LORAN Chain Navigation Aids 1.1.21 FIG 1.1.15 West Coast U.S. LORAN Chain 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.of. arrival 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. 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. AIM 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. |
