operation and construction, but it is new and has not been fully eval- 

 uated as yet. The microdist consists of two units and operates on the 

 interrogator-responder principle, but the responder transmits a return 

 signal with a different modulation. Either instrument may be used as 

 an interrogator or responder. Each unit weighs 35 pounds. The system 

 has a maximum range of about 50 nautical miles and a stated accuracy 

 of three parts per million ±1 inch. It needs a 12- or 24- volt battery 

 for power. The crystal calibration is automatic, a feature not found 

 in the earlier models of the tellurometer. Certain modifications and 

 more testing remain to be done before this instrument is ready for 

 actual survey use. It appears to be easier to operate than the telluro- 

 meter, but is bulkier and more difficult to transport in rough country. 

 Manufactured by the Cubic Corporation of San Diego, California, a 

 complete system costs about $12,000. 



d. Miscellaneous instruments 



The Motorola Corporation and W. & L. E. Gurley Co. both 

 have instruments similar to the tellurometer and microdist that are 

 not yet available. The Motorola model is completed and undergoing 

 tests at present. The amount of modification and retesting necessary 

 before it is ready for use is not known. The Gurley model is not as 

 yet ready for testing. ERDL has an extensive development program 

 underway which includes airborne tellurometer s, automatic tracking 

 theodolites, and electronic computers. 



B. ELECTRONIC POSITIONING SYSTEMS (NAVAIDS) 



1. Introduction 



A number of different electronic positioning systems are treated 

 separately from the other electronic measuring devices usedin survey- 

 ing, because these systems are designed to determine a position in 

 space rather than to measure an angle or a distance, although they 

 also may make one or the other of these measurements. As in all 

 electronic systems, they are based on the principle of measurement 

 of travel times of electromagnetic waves. 



Since the propagation velocity of radio waves is, nominally at 

 least, a constant, the several systems differ among themselves in the 

 frequency used, the type of transmission used (continuous wave or 

 pulsed wave), and the types of lines of position produced. The electronic 

 positioning systems may be divided into four general types on this 

 last basis: hyperbolic systems, such as Lorac and Loran; ranging 

 systems, such as Shoran and Lambda; azimuthal systems, such as 



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