be checked at least once a week by determination of the ship's position 

 by visual means or Shoran; experience has shown that changes in the 

 calibration factor on the order of several microseconds can be expected 

 from time to time. The system also is sensitive to land masses along 

 the transmission path, which can cause extremely erratic readings. 



c. Lambda and Two-Range Decca 



These systems are a development of the standard navigational 

 Decca by the British corporation, Decca Navigator, Ltd. and are 

 fundamentally the same. The Lambda has a range of about 250 nautical 

 miles and an accuracy of 100 to 400 feet. In both, the master station 

 is the mobile user, and two shore stations are the slaves. Measure- 

 ment is always along the base line. Only one vessel can use these 

 systems at a time, except by time- sharing with others. Each of the 

 two master- slave pairs radiates on a different frequency, since, if 

 all three transmitters radiate on the same frequency, phase comparison 

 is impossible. However, the frequencies are related so that a common 

 harmonic can be generated -within the receiver in order to compare 

 the phases. The fundamental frequency, f, lies between 14 and 15 kilo- 

 cycles per second. In both systems, the red slave transmits at 8f and 

 the green slave at 9f. The master station uses 12f, and the comparison 

 frequencies are 24f and 36f. The principal difference between the 

 Lambda system and the Two-Range Decca System is that the Two- 

 Range Decca System has the same lane ambiguity as Lorac, whereas 

 the Lambda System provides a means of lane identification. 



Both the Lambda and Two-Range Decca Systems require calibra- 

 tion before use. This can be done by moving the mobile master station 

 (the ship) into visual range of one of the shore stations, positioning 

 the ship with three simultaneous theodolite fixes, computing the posi- 

 tion and then the distance from shore, and using the distance to deter- 

 mine the calibration constant for that pair of master and shore stations. 

 A more satisfactory technique is being developed in which a telluro- 

 meter will be used to measure the distance directly with a considerable 

 saving of time and effort, especially in areas of fog and poor visibility. 

 The Lambda and Two-Range Decca shore stations must be located 

 within 1,000 feet of the coast to avoid signal distortion resulting 

 from overland transmission; any appreciable land mass along the 

 transmission path will create a "shadow" area of poor positioning. 



d. DM Raydist 



This is a system very similar in principle to the British 

 Lambda system, and it has the same number of component stations. 



XV- 11 



