OCEAN ELECTRONIC NAVIGATIONAL AIDS 29 



characteristics of radiobeacon marker stations (low power) are varied in 

 order to facilitate identification where installations become congested. 



DISTANCE FINDING STATIONS 



A large number of radiobeacon stations are now equipped so that a vessel 

 in the vicinity provided with radio may, by a single observation, determine 

 its distance from the station. If the vessel has a radio direction finder and 

 takes a radio bearing at the same time, its position at once is determined by 

 the distance and the bearing. Without the radio bearing, the distance obser- 

 vation locates the vessel somewhere on a circle of that radius from the radio- 

 beacon. As the method is dependent on the hearing of a sound-in-air signal, 

 it is subject to the same uncertainties as affect such fog signals, and the dis- 

 tance to which it may be used is, limited by the range of audibility of the 

 sound signal. 



The signals consist of blasts from a sound-producing device (fog signal) 

 synchronized with radio-tone signals. Since the radio signals arrive prac- 

 tically instantaneously (speed 186,000 mi./sec), the later arrival of the sound 

 signal (speed approximately 1,100 ft./sec.) gives an indication of the distance 

 traversed by the latter, therefore of the vessel's distance from the station. 

 At distance finding stations, transmission of the characteristic radiobeacon 

 signal is curtailed 8 seconds before the end of the operating minute, and a 

 1-second dot followed by a 5-second dash are transmitted simultaneously with 

 blasts of corresponding lengths from the sound signal device. An observa- 

 tion consists of noting the time difference with reference to any distinctive 

 part of the signals — for example, the end of the long radio dash and the end 

 of the long sound blast. Dividing the time in seconds by 5.5 gives the dis- 

 tance in nautical miles with an error which should not exceed ±10 percent. 

 The distance finding signals are transmitted only in thick or foggy weather 

 when the fog signal is operating. 



The method is applicable to stations equipped with sound-in-air fog signals 

 capable of being brought to full power of sound in a very brief time. 



All distance finding stations and their method of operation are shown in 

 Coast Guard Light Lists covering areas where radiobeacons are located. 



EQUIPMENT OF A RADIOBEACON STATION 



The equipment of a radiobeacon station consists of a transmitter, primary 

 clock, signal timer, warning device and accessories. All apparatus, so far as 

 practicable, is installed in duplicate with convenient means for switching 

 from one transmitter, generator, or signal timer to another in case of trouble, 

 so as to insure continuity of service. 



TRANSMITTER. — The transmitter is selected on the basis of output power 

 which, in turn, is dependent on the desired range. Transmitters and power 

 amplifiers are available for 5, 25, 150, 750, and 1,500 watts output. The radio- 

 beacon transmitter is in most respects similar to a communications trans- 

 mitter. 



MASTER CLOCK. — Two types of primary clocks are employed to exercise con- 

 trol over timed functions at the radiobeacon stations. One is a jewelled, 

 weight-driven, pendulum clock capable of maintaining an accuracy within 2 

 seconds in 24 hours. These clocks are installed at shore radiobeacons where 

 vibration is not excessive. The second type is a jewelled, temperature- 

 compensated, marine-escapement clock which has an accuracy within 5 sec- 

 onds in 24 hours. This latter type is used at all lightship radiobeacons, and 

 at shore stations where vibration is excessive. The function of either type 

 clock is to make an electrical contact once each minute to furnish correcting 

 impulses to the time, which, in turn, regulates the functions of the radio- 

 beacon station. 



SIGNAL TiMER.—The heart of the control system of the radiobeacon station 

 is the timer mechanism. The timer is a mechanical device having a series 

 of cams accurately rotated with respect to standard time. The cams actuate 

 contacts to which are connected the various circuits of the radiobeacon and 

 auxiliary equipment, controlling them in desired sequence and at predeter- 

 mined intervals accurately based on standard time. As stated above the 

 timer cams are kept in synchronism with standard time by means of the im- 

 pulses supplied through the clock contacts. The timer controls any or all of 

 the following functions at the radiobeacon station characteristics; code tone 

 transmitter on and off, clear weather or fog schedule of transmission (1 min- 



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