The GPS (GlobalPositioning System) is based
on a GPS receiver’s ability to accurately meas-
ure the propagation time of signals transmitted
from orbiting satellites.
These satellites transmit accurately timed sig-
nals along with a navigation message containing
the satellite’s position, precise time correction
signals, as well as almanac data for all of the
satellites in the constellation.
The
RAYSTAR
100
GPS sensor measures the time-of-arrival ofeach satellite
signal and calculates the range to each tracked satellite. If the range to the
satellites is known, the position of your vessel can be determined by
triangulation of the range data of the satellites in view, and presented in
Latitude and Longitude.
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The satellites continuously broadcast their
I sat
t1
1
:%t
n2
navigation messages at a
frequency
of
1575.42
/’
Mhz (for civilian use). Superimposed on the
Ranbe
1
/
I
navigation message
isa
high rate C/A (Coarse
I
l&c
2
Acquisition) code used for precise position-
,
\
/’
ing
measurements and positive satellite iden-
‘\
,
,./
tification. The C/A ID code permits the user
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__ _
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to determine and select the ‘best satellites”to
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use in position calculations.
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If it were possible to measure true satellite ranges directly, it would only
be necessary to track data from any two satellites to obtain a vessel’s
latitude/longitude. In actual practice, for marine navigation the receiver
must trackaminimum
ofthree
satellites.This
is necessaryto resolve timing errors including
the receiver’s own internal clock timing bias
error which must be factored into the various
range calculations.
Normally, the
RAYSTAR
100
tracks up to five
satellites
(if
visible)
and uses the best four of
the five for calculating position fixes. By us-
ing four satellites, the processor can deter-
mine the amount of clock error in each range
INTRODUCTION 6
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