Position Determination Principle - Kenwood TM-255A Product Manual

4 GPS
4.1 What is GPS?
GPS, standing for Global Positioning System, is becoming common nowadays. Following is a
brief introduction.
The American Defense Department developed GPS originally for military operations. The system
is available for use by the general public. For example, anybody can use GPS in association with
modern vehicle navigation systems. Position precision for public use is approximately 15 m.
Moreover, precision can be improved when used for vehicle navigation and aboard ships through
use of map matching technology and differential GPS techniques.
A total of 24 GPS or more satellites are at a high-level altitude of approximately 20,000 km on six
circular tracks with orbital radii of approximately 26,000 km. Therefore, there are four or more
satellites located in each orbital track. For civilian use, the RF carrier frequency of the satellites is
1575.42 MHz. Spread spectrum (SS) technology is used over a bandwidth of 2.046 MHz to
prevent interference among all the satellite signals on a common carrier frequency. Due care
must be taken when installing amateur radio equipment and antennas for use on the 430 MHz or
1200 MHz bands to prevent from interfering with GPS signals.
Figure 4-1 GPS IIR Satellite in Orbit

4.1.1 Position Determination Principle

The GPS receiver receives the radio signals transmitted from the GPS satellites. The receiver
measures the time duration between when the signal leaves the satellite and when the signal
arrives at the GPS receiver. Knowing this time allows calculation of the distance that the signal
traveled from that particular satellite. By receiving multiple signals from different satellites and
doing this distance calculation multiple times, the intersection of the spherical surfaces that
represent the respective radius distances to the various satellites will determine a single point.
The precision of the determination of the intersecting point relies on the satellites and the data
collected.
TM-D710A/E 13
CONTENTS