Electronic positioning systems (EPS)

bration is 0.25 mile (USACE EM 1110-2-1003). Therefore, LORAN-C is

not recommended as the sole navigation and positioning system for a cap-

ping project, and its use with other systems (e.g., a taut-moored buoy)

should be thoroughly scrutinized. Some of the earlier less than fully suc-

cessful capping projects conducted by the New England Division, where

the initial cap did not fully cover the contaminated sediments, were due in

part to problems with LORAN-C (SAIC 1995a). High-frequency systems

(particularly UHF and microwave) are more commonly used for position-

ing offshore vessels. In general, operating distances are limited to radio

line of sight, which allows use in riverine, harbor, and coastal locations

(USACE EM 1110-2-1003).

The most accurate positioning system and rapidly becoming the stand-

ard for horizontal positioning is the satellite-based global positioning sys-

tem (GPS). The NAVSTAR GPS is a real-time, passive satellite-based

navigation system operated by the U.S. Department of Defense. The 24 GPS

satellites orbit the earth such that from any place on earth at any time, at

least four (the minimum required by the GPS receiver for positioning) are

visible above the horizon. Standard GPS accuracies (50 to 100 m with

DoD selective availability) are not ideal for capping operations. Increased

accuracies can be obtained with differential GPS (DGPS). DGPS uses the

same NAVSTAR GPS satellite system but requires two receivers with pre-

cise coordinates of one of the receivers known (usually a fixed land-based

receiver). Accuracies of DGPS range from 0.1 to 1.0 m (USACE EM

1110-2-1003) (Hales 1995).

Kinematic DGPS is an additional refinement of DGPS that can provide

accuracies of a few centimeters (USACE EM 1110-2-1003) and thus can

eliminate the vertical datum problem that often occurs in the open ocean.

Kinematic DGPS is not yet routinely available, but the rapidly advanc-

ing EPS market may soon make its use commonplace. One of the more se-

vere limitations of kinematic DGPS is the need to have the fixed shore

station within 12 to 20 km of the surveying platform. However, industry

advances will likely extend this distance.

An additional factor that should be considered in barge positioning is

the placement of receiving/transmitting equipment on the barge or vessel.

For instance, when a barge is being towed to the disposal site by a tug,

there may be significant offsets between actual material disposal location

and positioning antennae. If the positioning antennae is located on the

tug, then the recorded placement location may differ by as much as 200 m

from the actual placement due to offsets from the positioning antenna on

the tug to the center of the barge. In addition, there may also be lateral

offsets from the vessel track line that are on the order of a barge width.

Therefore, for most capping projects where placement location is critical

and will be recorded, it is recommended that the antennae be located on

the barge. To be most effective, the EPS requires a visual display in the

vessel's pilot house to accurately navigate and position the vessel.

Chapter 5 Equipment and Placement Techniques