OCEAN ELECTRONIC NAVIGATIONAL AIDS 27 



which is necessary, according to the various purposes of the stations. For 

 the same reason the primary stations are restricted to a few widely separated 

 points of strategic importance to navigation, which are valuable as landfall 

 stations or for long distance approach. 



Local, low-power radiobeacons have been placed on inside waterways, such 

 as Strait of Juan de Fuca, Long Island Sound, and Chesapeake Bay, but the 

 greater number of radiobeacons are of intermediate power, and are located 

 and spaced to meet the usual requirements, both for coastwise and lake 

 navigation, and for approaching entrances. These signals are now sufficient 

 in number so that a vessel near the coasts of continental United States or on 

 the Great Lakes will always be within range of one of these signals and 

 usually two or more of them. 



In general, radiobeacons are located at all important entrances and at 

 outstanding intermediate points along the coast. There are only a few 

 that have not been placed at established lighthouses. This is advantageous 

 because such positions are shown on the charts and are well known to mari- 

 ners and because this is the most economical arrangement, both as to installa- 

 tion and operation of radiobeacons. (See figs. 2-6, 2-7, and 2-8.) 



Lightships have been found to be the most valuable and convenient sta- 

 tions for radiobeacons. They are in the positions of greatest importance to 

 the navigator, and they may be steered for directly and passed on either side. 

 All lightships have radiobeacons. 



In this country, during periods of good visibility, radiobeacon signals are 

 sent out for 1 minute out of each 3 minutes, for one or two 10 -minute periods 

 each hour. During fog and low visibility they are operated continuously. 



It would be convenient to the navigator to have long, continuous operating 

 periods, or even to have the radiobeacons send continuously without any 

 silences, thus making these aids to navigation always available, as are light- 

 houses and buoys. The system that is in use is a compromise, adopted to 

 lessen interference. Masters of vessels who understand the necessity of the 

 simple plan of operation are satisfied with the system and many letters have 

 been received by the Coast Guard confirming this statement. 



Vessel operators may request and obtain the continuous operation of a 

 radiobeacon for purposes of calibration of ship radio direction finder equip- 

 ment, providing calibration is undertaken during the station's clear weather 

 operating schedule and providing no other radiobeacon station in the same 

 frequency-sequence group is observed in operation at the time. 



Accurate timing of radiobeacon signals is accomplished by a signal timer 

 which in turn is controlled by a primary clock. The latter is checked fre- 

 quently against U. S. Naval Observatory time signals, thus making possible the 

 grouping of stations with a minimum of interference. In addition, the signal 

 timer at the light station controls all timed aids-to-navigation signals and 

 the starting and stopping of the equipment necessary to make these signals. 

 The signals and equipment controlled include distance finding sound signals, 

 main light, radiobeacon, engine generator starting, and warming transmitters, 

 all controlled in their proper sequence. 



FREQUENCY BAND RESERVED FOR RADIOBEACONS 



The International Radiotelegraph Conference at Washington in 1927, and 

 the regulations attached to the convention, provided that radiobeacons "shall 

 use waves of 285 to 315 kilocycles per second (1050 to 950 meters)", and that 

 continuous or modulated continuous waves would be used. This frequency 

 allocation has remained the same until 1947, when the conference of the 

 International Telecommunication Union, Atlantic City, made a change. The 

 regulations of that conference allocated the frequency band 285 to 325 kilo- 

 cycles per second for marine radiobeacon operation in ITO region 2. The 

 entire United States radiobeacon system is within the area defined by the 

 Atlantic City conference as region 2. 



FREQUENCY SEPARATION AND SYNCHRONIZATION OF SENDING 



PERIODS OF MARINE RADIOBEACONS 



Interference is a problem affecting the navigational use of radiobeacons as 

 it does other uses of radio. To minimize interference, the following steps 

 have been taken. Frequencies of adjacent stations or groups of stations have 

 been separated; adjacent stations in a group have been synchronized so as 

 to send for different minutes; and the power of the signals has been limited 

 or reduced. With the operating schedule of 1 minute on and 2 minutes silent, 



