range is 25 to 40 km and they can achieve an accuracy of 1 to 3 m. The major 

 components are a master station aboard the survey vessel and two onshore tran- 

 sponder stations set up at known locations. Basically, the system determines 

 horizontal position by measuring the round- trip time for signals to travel 

 between the transponders and the master station, and then converting this data 

 to linear distance. Position is calculated using the range distances and the 

 length of the baseline between the two transponders. 



69. Microwave systems have a few limitations. They are restricted to 

 line-of-site situations. The transponders must be located near the shore or 

 at a sufficiently high elevation to eliminate any obstruction to the signals. 

 Onshore personnel are required to periodically move the transducers from place 

 to place as the survey progresses and to protect them from vandalism. The 

 cost of these systems is considerably more than required for Loran C 

 receivers . 



70. Laser- type positioning systems provide distance and angle measure- 

 ments from a shore station that can be converted to x-y coordinate data. They 

 are highly accurate systems that can locate a vessel within an error of 



0.1 ft/mile.* Because their range is limited to about 4 km, laser systems are 

 not suitable for a general borrow site exploration program although they could 

 be used for detailed site surveys of specific deposits that were within their 

 range . 



71. Satellite navigation and positioning systems are a new, space-age 

 technology that may replace both the Loran C and microwave systems in the 

 future. The Navstar satellite system under development by the Department of 

 Defense is expected to provide continuous horizontal two-dimensional coverage 

 positioning by 1991. The satellite systems should provide satisfactory accu- 

 racy and they cost no more than microwave systems. 



Fathometers 



72. Fathometers are instruments that use acoustic pulses to measure 

 water depths in streams, lakes, and oceanic areas. There are a variety of 

 Fathometers on the market ranging from the small, direct-readout models, 



* A table of factors for converting non-SI units of measurement to SI 

 (metric) units is presented on page 4. 



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