Hemisphere GPS Crescent V100 Series, User Manual

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2: Crescent V100 Operation
U
NDERSTANDING THE
C
RESCENT
V100
The purpose of the Crescent V100 is to provide accurate, reliable
heading and position information at high update rates. To
accomplish this task, the Crescent V100 uses a high performance
GPS engine and two multipath-resistant antennas for GPS signal
processing. One antenna is designated as the Primary GPS, while
the other is designated as the Secondary GPS. Positions computed
by the Crescent V100 are referenced to the phase center of the
Primary GPS antenna. Heading data references the vector formed
from the Primary GPS antenna phase center to the Secondary GPS
antenna phase center.
There is a heading arrow located on the bottom of the Crescent
V100 enclosure, which defines system orientation. The arrow
points in the direction that the heading measurement is computed
(when the antenna is installed parallel to the fore-aft line of the
vessel). The secondary antenna is directly above the arrow.
Moving Base Station RTK
The Crescent V100’s internal GPS engines use both the L1 GPS C/A
code and carrier phase data to compute the location of the
Secondary GPS antenna in relation to the Primary GPS antenna
with a very high sub centimeter level of precision. The technique of
computing the location of the Secondary GPS antenna with respect
to the Primary antenna, when the Primary antenna is moving, is
often referred to as moving base station Real-Time Kinematic (or
moving base station RTK).
RTK technology generally is very sophisticated and requires a
significant number of possible solutions to be analyzed where
various combinations of integer numbers of L1 wavelengths to each
satellite intersect within a certain search volume. The integer
number of wavelengths is often referred to as the Ambiguity as
they are initially ambiguous at the start of the RTK solution.