3. EQUIPMENT AND INSTRUMENTS 



59 



ground station and increases nearly linearly 

 with distance up to about the refractive line- 

 of-sight. After this the correction increases 

 nearly exponentially until the signal disap- 

 pears. A discussion of sources of Shoran er- 

 rors is contained in Coast and Geodetic Sur- 

 vey Journal, Vol. 6, pages 23-31. 



Theoretically, the range of Shoran is the 

 line-of-sight distance betvi^een antennas and 

 is expressed in the formula : 



D=K{^/H+^/h) 



where D= Distance in statute mile^^ 



H^ Elevation of r .. an- 



tenna in feet 

 h= Elevation oi ship antenna in feet 

 K= a constant 



For line-of-sight computations X^l.32; 

 however, certain effects of refraction due to 

 atmospheric conditions permit increasing the 

 constant to 1.42 as being a practical value. 

 It is possible to receive usable pulses at 

 much greater distances than those computed 

 by this formula, and the constant in such 

 cases approaches the value 2.0. Shoran 

 should seldom, if ever, be used beyond the 

 limits imposed by K=1A2 because correc- 

 tions to Shoran distance measurements be- 

 come uncertain at greater ranges. It should 

 be noted that the elevations of the respec- 

 tive antennas have added effects. When rela- 

 tively low antennas are used, such as those 

 on launches, the maximum value of K de- 

 creases from 2.0 to about 1.7. 



3-38 Shoran calibrations. — As previously 

 stated, the errors in observed Shoran dis- 

 tances vary with distance and signal inten- 

 sity. The amount of these errors must be 

 determined by comparison of observed 

 distances with known distances. Each ship 

 and launch equipment must be calibrated 

 with each ground station used in a survey (see 

 5-44). When an APN-3 has been calibrated 

 aboard ship and later transferred to a launch 

 the equipment must be recalibrated. When 

 a ground station is moved to a new location, 

 the old calibration values shall not be used and 

 recalibration is necessary. 



An adequate number of calibrations care- 



fully made are essential to avoid complica- 

 tions in plotting and verifying the smooth 

 sheet. 



Before making a correction calibration, 

 the following preparatory procedures shall 

 be followed : 



(a) Allow all equipments in the system 

 to warm up for about one hour prior to 

 calibration. 



(b) Accomplish a calibration adjustment, 

 which is a comparison and adjustment of 

 the =•'"-■■ .ystal oscillator frequency with the 

 ...ore standard frequency of the CPN-2 

 ground station crystal oscillators. 



(c) Adjust the line voltage to the proper 

 value. (See Manuals). 



(d) Make a zero check and record the 

 value for reference. 



(e) Notify the ground stations that cor- 

 rection calibrations are to be made and state 

 the direction antennas are to be pointed 

 if reflectors are used. 



(f) Adjust the receiver gain to a value 

 where additional gain does not produce a 

 change in the shape or position of the ground 

 station signal. 



When notified that a correction calibra- 

 tion is to be made, the ground station oper- 

 ator shall proceed as follows : 



(a) Point the antenna as directed. 



(b) Adjust the line voltage to the proper 

 value. 



(c) Standardize the ground station delay. 



(d) Adjust the receiver gain to the 

 proper value consistent with the ship signal 

 strength. 



3-39 Methods of calibrating Shoran. — 



The most commonly used method of calibrat- 

 ing Shoran is to compare Shoran distances 

 with distances determined by a three-point 

 fix. When this method is used, the fix must 

 be very strong, using triangulation stations 

 where possible, and a check angle should be 

 observed. The comparison is made by plot- 

 ting the fix on a metal-mounted sheet on 

 which the signals and Shoran circles have 

 been plotted. The correct distances to the 

 Shoran stations as determined by the sex- 

 tant fix are scaled off with an Odessey pro- 

 tractor and the differences between these 



