Ch. 4— Technologies for Exploring the Exclusive Economic Zone • 163 



placed on the seafloor) would enable one to return 

 to within visual range to photograph or take sam- 

 ples. '"^ 



Gravity surveys and seismic reflection surveys 

 do present demanding navigational requirements. 

 For detailed gravity surveys, the velocity of the 

 measuring instrument must be known with uncer- 

 tainties less than 0.05 meter/second. For seismic 

 work, the quality of the data is directly related to 

 the positioning accuracy of the sequence of shots 

 and the streamer hydrophones. Three-dimensional 

 seismic surveys for exploration geophysics require 

 positioning precision on the order of 10 centimeters 

 over a survey area of about 100 square kilome- 

 ters.'"* In some instances (e.g., determining rela- 

 tive motion of oceanic plates) accuracy on the or- 

 der of 1 centimeter is important, but exploration 

 technologies generally do not require this high de- 

 gree of precision. 



Precise positioning and tracking of remote sys- 

 tems, such as towed "fish" or ROVs, is also con- 

 sidered challenging. Positioning is usually done by 

 acoustic rather than electromagnetic systems. Long 

 baseline systems employ three or more fixed-bottom 

 or structure-mounted reference points (e.g., acous- 

 tic transponders), while short baseline systems em- 

 ploy three or more ship-mounted transducers that 

 receive an acoustic pulse from a subsea acoustic 

 source.'"^ 



Accurate marine charting requires precise navi- 

 gational control relative to global coordinates. Al- 

 though requirements are stringent, the state-of-the- 

 art is sufficient for producing high-quality bathy- 

 metric charts. The National Ocean Survey (NOS) 

 has established a "circular error of position" stand- 

 ard of 50 meters (164 feet) or better (in compliance 

 with international standards for charting). This is 

 about the average for survey ships operating be- 

 yond the range at which navigation technologies 

 can be frequently calibrated. Accuracies of 5 to 10 

 meters are typical with calibrated equipment. '"^ 



'"'National Research Council, Seafloor Referenced Positioning: 

 Needs and Opportunities (Washington, DC; National Academy Press, 

 1983), p. 6. 



'"Ibid., pp. 8-10. 



"■'Frank Busby, Undersea Vehicles Directory — 1985 (Arlington, 

 VA: Busby Associates, Inc., 1985), pp. 426-430. 



""Perry, "Mapping the Exclusive Economic Zone." 



NOS, for example, uses ARGO and Raydist sys- 

 tems for charting work within about 120 miles of 

 the coast, where these systems may achieve hori- 

 zontal position accuracies of 5 to 10 meters. They 

 are cumbersome to use, however, because they re- 

 quire special onshore stations to be set up and must 

 be calibrated by a more precise system, such as a 

 line-of-sight system like Mini- Ranger."" Beyond 

 about 120 miles of the coast, these systems are un- 

 able to reliably meet NOAA's 50-meter standard. 

 Far offshore, only the Global Positioning System 

 (GPS) is capable of meeting the desired accuracy 

 for charting. 



LORAN-C is a commonly used ground-based 

 navigation system. LORAN-C coverage is avail- 

 able within most of the U.S. EEZ, and it is accurate 

 relative to global coordinates to within 460 meters. 

 Users who want to return to a site whose coordi- 

 nates have been measured with LORAN-C can ex- 

 pect to return to within 18 to 90 meters (60 to 295 

 feet) of the site using LORAN-C navigation; 18 

 to 90 meters is thus the system's repeatable ac- 

 curacy. LORAN-C is expected to be phased out 

 once the GPS is fully operational. However, this 

 is not expected to occur before 2000. Once GPS 

 is fully operational, plans call for a 15-year transi- 

 tion period during which both LORAN-C and GPS 

 will be available. A satellite system available for ci- 

 vilian use is TRANSIT. This system is often used 

 to correct for certain types errors generated by 

 LORAN-C. 



GPS is a satellite navigation system intended for 

 worldwide, continuous coverage. When fully de- 

 ployed, the system will consist of 18 satellites and 

 three orbiting spares. Only six R&D satellites are 

 operating now, and, due to the interruption in the 

 space shuttle launch schedule, deployment of the 

 operational satellites has been delayed about 2 

 years. The system is now scheduled to be fully de- 

 ployed by 1991. Some of the current R&D satel- 

 lites may also be used in the operational system. 

 Costs to use the GPS are expected to be less than 

 costs to use current systems. 



GPS is designed for two levels of accuracy. The 

 Precise Positioning Service, limited to the military 

 and to users with special permits (NOS, for in- 



"Ibid., p. 1192. 



