2574 HYDROGRAPHIC MANUAL PaGE 144 



the R.A.R. method. Sono-radio buoys placed 12 or 15 miles apart will usually furnish 

 adequate control in an average area, but they must be spaced at closer intervals where 

 the R.A.R. method is limited to short acoustic distances. 



Other sono-radio buoys are anchored offshore from the traverse line and these are 

 located by acoustic distances from the sono-radio buoys in the traverse (see 2533) . It 

 is important to place these stations close enough to the traverse line to be well within 

 the limits of reliable returns, and each should be anchored at unequal distances from 

 the traverse stations from which distances are to be measured, to prevent the returns 

 from two stations being recorded simultaneously on the chronograph tape and being 

 unidentifiable (see 6814). 



The time intervals are usually measured with the source of sound located at the 

 stations whose positions are to be determined, but where the buoys to be located are 

 sono-radio buoys, the source of sound may be located at any ordinary buoy station in 

 the traverse. It is frequently possible and desirable to measure the time intervals in 

 both directions; first when the offshore stations are anchored, and subsequently just 

 before the sono-radio buoys in the traverse line are weighed, or at any time the vessel 

 is in the vicinity of a station at one end of a line whose length is to be determined. 

 It is advisable to measure the time intervals between two stations as often as it is con- 

 venient to do so without consuming too much time, for each additional measurement 

 increases the strength of the position determination, if reliable data are obtained 

 (see 2533). 



In an extensive area it is frequently necessary to establish R.A.R. stations still 

 farther offshore, and where the R.A.R. method operates efficiently the positions of these 

 may be located by acoustic distances from the stations first so located. If the acoustic 

 distances are not sufficiently reliable for this, the positions may be materially strength- 

 ened by measuring, with taut wire, the offshore distance from a station in the traverse 

 line. In figure 40, the distance from buoy F, in the traverse line, to buoy L and the 

 distance from buoy L to buoy P. the offshore sono-radio buoy, would be measured by 

 taut wire. These measurements will serve to strengthen the entire scheme for they 

 will provide comparatively accurate offshore distances of buoys L and P. In an area 

 controlled by R.A.R. methods a dependable offshore distance has an additional value, 

 for it may be used for an experimental determination of the velocity of sound (see 6352). 



The adjusted positions of the stations in the traverse are determined by the 

 regular computations for a traverse (see 2525 and 2561). The positions of the stations 

 located by acoustic distances are detennined from a graphic plot on an aluminum- 

 mounted sheet of sufficient size to include the required traverse stations on a scale 

 slightly larger than the smooth sheet. If aluminum-mounted topographic sheets are 

 used, two or more may be recpiired, or a grained aluminum sheet of sufficient size may 

 be used (see 7131). The computed positions of only the required traverse stations are 

 plotted by dms. and dps. and the acoustic distances to each station are plotted with a 

 beam compass, the required positions being determined as explained in 2533. The 

 taut-wire distances are also plotted with a beam compass but they are accepted as 

 correct, and the positions of the respective stations must be accepted on the arcs repre- 

 senting taut-wire distances. 



2574. Subaqueous Quadrilaterals 



111 offshore areas where the general depths permit anchoring buoys but are not so 

 shoal as to require closely spaced lines, a system, of control by R.A.R. methods may 

 I)e located entirely by subaqueous sound ranging. The method is limited to localities 



