19_ 



Leica MX41R differential beacon receiver to obtain positioning data at an accuracy of 

 ±3 m in the horizontal control of NAD 83. 



The GPS receiver utilized signals emitted from a constellation of satellites that 

 provides positioning data to an accuracy of ±100 m. In order to increase the accuracy of the 

 raw GPS data, a differential beacon receiver was used to collect and decode corrections from 

 a shore-based station. Signals broadcast from the U.S. Coast Guard differential beacon at 

 Montauk Point, New York (293 kHz) were utilized for satellite corrections due to its 

 geographic position relative to NLDS. When merged with the satellite data, the correctors 

 provide differential GPS positions to an accuracy of ±3 m with an update rate of 1 Hz. 



The bathymetric survey area occupied in September 1997 was centered at 

 41° 16.274' N 72° 04.580' W (NAD 83). This survey was performed to characterize all 

 the bathymetric features within the confines of the disposal site. A total of 85 lanes, 

 oriented east-west with a 25 m lane spacing, were occupied during the September 1997 

 field operations to provide a detailed bathymetric chart of the 4.41 km 2 area (2100 x 

 2100 m; Figure 2-2). No bathymetric data were collected over the project mounds during 

 the 1998 survey effort. 



2.1.3 Bathymetric Data Collection 



An ODOM DF3200 Echotrac® Survey Fathometer with a narrow beam, 208 kHz 

 transducer measured individual depths to a resolution of 3.0 cm (0. 1 ft) as described in 

 DAMOS Contribution No. 48 (SAIC 1985). The fathometer is interfaced directly with the 

 navigation system. Depth soundings were collected along the individual survey lanes, 

 adjusted for transducer depth, and transmitted to INDAS/PINSS at a frequency of 10 Hz. 

 The soundings were averaged by the navigation system, merged with positional and time 

 information, and recorded at a frequency of 1 Hz. Survey vessel speed and course were 

 tightly controlled (2 to 3 meters per second) to ensure adequate numbers of depth values 

 collected along the survey lane. 



A Seabird Instruments, Inc. SEACAT SBE 19-01 Conductivity, Temperature, and 

 Depth (CTD) Probe was used to obtain sound velocity measurements at the start, midpoint, 

 and end of each survey day. The data collected by the CTD were bin-averaged to 1 meter 

 depth bins to account for any pycnoclines (rapid changes in density creating distinct layers 

 within the water column). A mean sound velocity was then calculated using the bin- 

 averaged values. The mean sound velocity was recorded and later used in the post- 

 processing of the bathymetric data. 



Monitoring Cruise at the New London Disposal Site, 1992 - 1998 



