The Hydrodist and the basic Tellurometer system utilize cw signals 

 radiated from tlie master stations. The signal is modulated l:)y a pattern fre- 

 quency, received at the remote station, and reradiated as a similar wave with 

 more complex modulation. 



The return waves are received at the master stations and their phase 

 modulations are compared with the outgoing phase. Tlie phase is indicated on a 

 cathode-ray tube in the form of a circular trace in which a small break marks 

 the phase against a circular scale. A decimal scale with 10 major and 100 

 minor divisions is used and is read to the nearest small division, which has a 

 value of one meter . 



A cursor is attached to the cathode-ray-tube display and is manually 

 rotated to follow the position of the gap as a function of range. The cursor is 

 mechanically connected to a counter which, after being set to the initial range, 

 will automatically follow the range in meters. A useful range of 20-25 miles 

 may be expected with this equipment. 



3. ACCURACY 



Hydrodist operates in the 10-cm band. Because this system uses 

 phase comparison methods to obtain the distances from the master to the remote 

 operating points, ambiguities must be resolved. Pattern switching is used for 

 this purpose. Three pattern or modulation frequencies are provided in Hydrodist. 

 They are manually switched, and digital counter readings are tabulated. Vernier 

 readings of transit time in millimicroseconds is quite practical. 



The A pattern measures 100 meters for a complete rotation of the 

 cathode -ray -tube display. A resolution of 1 meter is practical. The A-C and 

 the A-D patterns have full scale ranges of 1, 000 and 10, 000 meters, respec- 

 tively. No ambiguity resolution above 10, 000 meters is provided, as it is re- 

 garded as certain that the vessel's position will be known within 10, 000 meters 

 (7 miles approximately). Hie automatic pattern-following device on the shore 

 stations provides a rapid check on ambiguities . 



The computed transit time is based on a mean value of 1 . 000325 for 

 the refractive index of the atmosphere . A corrected value may be computed by 

 taking meteorological observations at both the master and remote stations . 

 With these corrections, the resultant accuracy of an individual line measure- 

 ment is almost totally a function of the instrument error plus a scale reading 

 error. After a number of fine readings have been taken and properly averaged 

 it is probable that a measurement accurate to about 1/3 meter can be achieved. 



151 



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