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8
GPS Overview
Calculating positions
The GPS satellites transmit signals relative to their position
and the timing signal. By multiplying the time the timing
signal takes to reach the receiver by the velocity of radio
wave (3 x 108 m/s), the GPS receiver calculates the distance
between the satellite and the receiver itself. The receiver's
position is defined by the cross point of spheres whose
radius correspond with the distance from each satellite.
Therefore, the receiver can calculate its position: latitude,
longitude, altitude' when it receives signals from four
satellites.
«The unit calculates the altitude based on the World Geodetic
System (WGS84) coordinates. This altitude ts not relative to the
height above sea level.
Introduction
Methods of position
The unit can provide you with either a two- or three-
dimensional position, depending on satellite availability.
* Two-dimensional position: The unit uses three satellites to
determine a two-dimensional position: latitude and
longitude.
The unit employs the altitude that has been
calculated during the latest positioning.
¢ Three-dimensional position: The unit uses four satellites to
determine a three-dimensional position: latitude, longitude,
and altitude.
GPS system specifications
Satellite altitude:
About 20,000 km (10,900 miles)
Satellite orbit:
About a 12-hour-cycle elliptical orbit
Signal format:
P code — used for military applications:
C/A code — used for civilian applications.
The IPS-760 uses the C/A (Coarse
Acquisition) code signals.
C/A code:
Frequency
1575.42 MHz
Clock
1.023 Mbps
Orbit data speed
50 bps
Signal strength
—160 dBw
Modulation:
Direct Sequence
Accuracy:
30 m (98 feet), DOP" < 6, with S/A'' OFF
DOP (Dilution of Precision) is an indication of the accuracy of
the position based on satellite geometry (the position of the
satellites relative to the earth). When the GPS satellite
constellation is complete, there will be a minimum of five
satellites in view with a DOP of six or less to any user
worldwide.
xxx S/A (Selective Availability) is a method for the government to
control the accuracy of the GPS signals. With S/A on, the
accuracy may be degraded to 100 meters.
Warning:
GPS system signals are maintained and controlled by the
United States Department of Defense (DoD). Without notice,
the DoD can change the characteristics of this signal, which
will degrade the accuracy of this unit. Use the IPS-760 at
your own risk.
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