ERDC/CHL CHETN-IX-14
March 2004
avoid obstructions to satellite signals. Each GPS antenna was mounted on a 2.54-cm (1-in.) diam
mast that was easily secured to the vessel with elastic cords. Each antenna position was carefully
measured relative to the ship. The bow antenna was measured as the distances aft of the bow and
from the center line. The bridge antennas were measured relative to the face of the bridge and the
edge of the ship. The antenna height was measured above the deck of the ship. The X-Y-Z locations
of the antennas on the 12 ships are listed in Table 2.
Table 2
GPS Antenna Locations
Bow, m
Port, m
Starboard, m
Vessel
X
Y
Z
X
Y
Z
X
Y
Z
0.00
-0.49
1.30
-0.94
0.13
2.36
-0.94
0.13
2.29
Alligator Liberty
1
5.89
0.00
2.12
-0.41
3.49
2.34
-0.41
3.49
2.29
MV Aniara
-1.04
0.84
2.15
-0.46
0.12
2.30
-0.36
0.12
2.19
Chetumal
0.00
0.00
2.20
-0.40
0.09
2.67
-0.40
0.09
2.93
Dorothea Rickmers
0.00
0.00
2.16
-8.49
0.24
1.84
-8.54
0.10
2.53
Ever Diamond
0.08
0.59
1.83
0.55
0.56
2.29
0.51
0.55
2.19
Golden Farmer
0.00
0.78
2.16
-0.35
2.17
1.91
0.37
0.21
2.58
Gulf Bridge
-1.19
1.87
1.99
-1.45
0.15
2.29
-1.50
0.15
2.34
London Maersk
0.00
0.00
0.57
-0.23
0.24
2.56
0.15
0.06
1.61
Madision Maersk
0.00
0.70
2.15
0.37
0.32
1.65
-0.36
0.20
2.53
Munkebo Maersk
0.00
0.00
2.16
-2.32
0.14
2.35
0.94
1.13
2.52
Newark Bay
-0.45
1.46
2.37
0.99
0.24
2.27
-0.93
0.24
2.29
MSC Sabrina
1
MV Aniara bow GPS antenna is actually with respect to stern.
The GPS receiver consisted of a logging unit placed in a self-contained backpack with batteries. The
data-logging rate was one sample per second (i.e., 1 Hz) from the Highway 17 bridge to the end of
the entrance channel. Due to memory limitations with the loggers, however, it was set at once every
3 sec (i.e., 0.33 Hz) inshore of the Highway 17 bridge.
A problem that arises during ship tracks is that there are times during each day when the satellite
coverage is not sufficient to conduct even a static survey, much less a survey where some of the
satellites could be periodically blocked by the ship's superstructure. Since it was not possible to
request when the ship would transit the channel, there were some data sets that did not obtain
centimeter-level accuracy. Another problem occurred near the Highway 17 bridge when some of the
GPS signals reflected off the bridge.
Vertical Control. Vertical control was important in the survey. GPS operates on an earth model
known as the WGS84 ellipsoid. The Charleston District uses NGVD29 (National Geodetic Vertical
Datum, for year 1929 datum) for vertical control in the Charleston area. Water levels and channel
depths are referenced to mean lower low water (mllw). The NGS benchmarks were in NAVD88
(North American Vertical Datum, for year 1988 datum). NOAA tidal benchmarks in the area have
relationships between mllw and NGVD29. NGVD29 and the new vertical datum NAVD88 are based
on gravitational leveling techniques. High accuracy survey work generally discourages trans-
formations between NGVD29 and NAVD88, but there was no choice in this situation. A NGS
program called Geoid96 was used to transform elevations between the WGS84 vertical and the
NAVD88 vertical. Another program called Vertcon was used to convert elevations from NAVD88 to
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