Page 641 radio acoustic ranging 6823 



Unexpected current, or a sudden change in current, may occur to affect the accu- 

 racy of a position based on one R.A.R. distance. All such positions must be considered 

 tentative until after they have been verified by dead reckoning between well-fixed 

 positions before and after them. 



6823. Positions Near a Bisectrix 



Where it is practicable to do so, the hydrographer should avoid obtaining an 

 R.A.R. position near the bisectrix between two stations (see 6814). In such a case it is 

 difficult for the clu-onograph attendant to identify the returns from the two stations 

 and an erroneous identification may be made, resulting in some cases in an erroneously 

 plotted position and even causing the hydrographer to change course on a false 

 assumption. 



Where a position occurs near a bisectrix, the distance arc, if only one return is 

 identifiable on the tape, should be plotted from both of the stations in an effort to 

 decide from which of the two stations the return was received. Sometimes the station 

 may be identified from a knowledge of the dead-reckoning position, but where the 

 position is nearly on a bisectrix the identification is always doubtful, to say the least. 



It is sometimes necessary to anticipate such a situation and change the ship's 

 course away from a bisectrix so as to be sure which one of two stations will be received 

 first. 



6824. Relative Strength of Positions 



The strength of a position located by two or more R.A.R. distances depends 

 theoretically on the angle of intersection of the distance arcs. Assuming that a correct 

 velocity of sound has been used or that the time intervals have been converted into 

 accurate horizontal distances, the strongest positions are those where the distance 

 arcs intersect approximately at right angles, and the more acute the angle of intersection 

 becomes, the weaker the position is. Where the distance arcs are nearly parallel, the 

 position is generally located very accurately in a direction perpendicular to the arcs, 

 but very weakly in a direction parallel to the arcs. Such intersections generally 

 occur at extreme distances from the R.A.R. stations and are to be expected near 

 the offshore limits of offshore surveys. To fix positions satisfactorily in such areas, 

 the R.A.R. distances must be supplemented by dead-reckoning data. 



In R.A.R. surveys, distance arcs are occasionally plotted with data received from 

 two stations which are nearly on range (in the same direction, or in opposite directions) 

 with the position, so that the arcs plot nearly parallel to each other. If the arcs coin- 

 cide at the position, it is an indication that the correct velocity of sound has been 

 used, and another distance arc intersecting these two approximately at right angles 

 will give a strong position. In boat-sheet plotting, the arcs from stations on range 

 with the position seldom do coincide because a preliminary value of the velocity of 

 sound is used, and generally, the position must be assumed to be between the two arcs, 

 on a third intersecting arc or a^ a position indicated by the dead reckoning. 



Sound is transmitted through sea water along a path which may vary for different 

 distances and different depths of the water (see section 62). If one of the distances 

 of an R.A.R. position differs considerably from the others, it can seldom be com- 

 bined satisfactorily with them if the same velocity is used for all the distances unless 

 the elapsed times are first corrected for the paths of the sound waves (see 636) . A 

 similar discrepancy may be expected if the path of the sound wave to one R.A.R. 



