Page 151 control and signal building 2616 



Wliere the supporting structures can be placed far enough apart, the length of the 

 antenna should be one-half the wave length according to the formula: 



antenna length =^=^ 



where Z = wave length (in meters). 



c = velocity of radio waves (300,000,000 meters per second). 



/= frequency of radio waves to be transmitted (in cycles per second). 



The antenna should be as high above the ground as the supporting structures will 

 permit, but under no circumstances should this be less than 15 feet. The antenna 

 should be insulated at each end by means of high quality strain insulators. 



The radio set may be connected to the antenna by a transmission line composed 

 of a single wire or several wires spaced by insulating spreaders. The connection may 

 be made to either the end or center of the antenna. 



2616. Radio Frequencies 



The shore station radio transmitters operate on frequencies ranging from 1,700 kc. 

 to 5,000 kc. For distances involved in R.A.R., frequencies with this range will give 

 the most consistent results for day and night operations. The particular frequency 

 within this range is selected by determining in advance which one of the available 

 frequencies is most free, in the project area, from interference from other radio stations. 

 Quartz crystals to fix the frequency of the transmitter should then be requested from 

 the Washington Office. Shore stations should have on hand crystals for the 1,700, 

 2,000, and 4,000 kc. bands, so that the transmitter's frequency may be quickly changed 

 if interference from other radio stations occurs on one frequency. The frequencies 

 available to the Coast and Geodetic Survey for R.A.R. purposes are listed in 6441. 



262. Ship Stations 



When R.A.R,, control was first used on the Atlantic Coast it soon became apparent 

 that shore stations could be used for only limited distances offshore, far less than was 

 required for the extensive shoal areas common on that coast. Before the development 

 of the sono-radio buoy, ship R.A.R. stations were employed to extend the control 

 farther offshore. 



The station ship was, in reality, a mobile shore station that could be anchored near 

 a located marker buoy or at a position where it could be located by sextant fixes. The 

 station circuits were identical with those used at shore stations, but the hydrophone 

 was buoyed and trailed astern of the ship on a cable of sufficient length to remove it 

 from ship noises. On offshore surveys the relation between the hydrophone and the 

 marker buoy was measured at frequent intervals and transmitted to the survey ship ^ 

 so that the correct position of the hydrophone could be plotted on the boat sheet. 



Ship stations have not been used in R.A.R. surveys since the first successful use of 

 sono-radio buoys. 



27. SIGNAL BUILDING 



271. General Statement 



Hydrographic surveying consists essentially in determining the depths of the 

 water at consecutive known positions of the survey vessel. There are two principal 

 methods used to determine positions. The first and older is by the measurement, with 

 sextants on board the vessel, of angles between visible shore objects or buoys, whose 

 geographic positions are known. The second and newer is by Radio Acoustic Ranging, 



