Gps, Chart Overview; Principle Of Gps - Furuno GP-1610CF Operator's Manual

GPS, CHART OVERVIEW

4.1 Principle of GPS

What is GPS?
GPS is an acronym meaning Global Position-
ing System. GPS (sometimes referred to as
NAVSTAR) is a highly precise satellite navi-
gation system developed by the U.S. Depart-
ment of Defense.
A constellation of 24 satellites emplaced in
nearly 20,000-kilometer high 12-hour circu-
lar orbits provides highly precise, continuous,
worldwide, all-weather position plus time and
velocity information to GPS receiver-
equipped vehicles, vessels and aircraft.
1 Satellite locations are
calculated.
2 Distances are measured.
3 The GPS fix is the point
of intersection of three
spheres which are drawn
around the three satellites
with diameter d1, d2, and d3.
d1
Figure 4-1 How GPS determines position
How the GPS receiver calculates
position
The GPS receiver's position is continuously
fixed by receiving 3 (or 4) satellites in line-
of-sight of the GPS receiver. The basic steps
in position fixing are as below:
1) GPS satellites continually transmit their
own precise orbital data called ephemeris.
The GPS receiver computes satellites'
position by this data.
2) The GPS receiver measures very accurate
distance to the satellites.
3) Satellite positions and their distances from
the GPS receiver are known. The point of
intersection of the satellites used in fixing
position.
d2
d3
GPS fix
4-1