DEVELOPMENTS IN TIDAL & TIDAL CURRENT MEASUREMENTS 773 



The Roberts meter lends itself to economical operation for obser- 

 vation at one or two stations simultaneously with large-scale surveying 

 activities in the same general area. Activities of this nature are usually 

 carried on by vessels maintaining continuous radio watches. Radio per- 

 sonnel can monitor and receive the meter transmissions without greatly 

 interrupting their normal routine. Since the advent of electronic ship- 

 positioning methods, it is sometimes desirable to have electronic shore- 

 station personnel receive meter returns from isolated current stations 

 within range. This method is not as desirable as the ship receiving 

 method unless electronic shore-station personnel are increased sufficient- 

 ly to permit the usual "on the spot" tabulations. 



For short-period observations, it has been found desirable on oc- 

 casion to use the Roberts meter without the radio link. In practice a 

 small vessel, or launch, is anchored on station and the meter hoisted 

 in and out in the manner normally employed when using non-record- 

 ing meters. The meter is connected electrically to a tape chronograph, 

 but without the radio receiver and amplifier. 



Revised Field Methods 



Earlier methods have been revised and new components introduced 

 which have made it possible to more than double the former rate of 

 receiving. In the early days each meter required a separate radio fre- 

 quency and transmitter. Monitoring, receiving, and tabulating records 

 from six or seven transmitters were about all the receiving-station per- 

 sonnel could manage in one 30-minute observation period. Last year 

 a timing device, known as the "Sequence Switch," was designed and 

 developed in the Bureau. Since then it has become standard practice 

 to suspend three meters from one buoy. By inserting the sequence 

 switch in the meter circuits returns from each meter are coded and 

 transmitted over one frequency. It is now possible to receive returns 

 from 18 meters in one 30-minute period. This switch is illustrated and 

 described briefly in the 1952 revised operating manual. An article 

 concerning the switch appeared in the Surveying and Mapping News 

 section of the 1952 September-October issue of the Military Engineer. 



With the development of the three-meter suspension method it be- 

 came necessary to discard the leaders. The added weight of meters 

 and connections plus a 30- to 50-pound lead, or cast iron "sinker" below 

 the meters prevents any great amount of lift in moderate velocities. 

 Care must be exercised in attaching the two extra meters to prevent 

 "twist" in electrical cables— otherwise they may not stream freely with 

 the current. If this does occur, it can first be detected in the direction 

 data, as, obviously any prolonged indication of directions normal to 



