Furuno BACK GP-80 - Manual (Page 71)

10 – 1
10. INTRODUCTION
TO GPS
10.1 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.
When full global coverage becomes available,
a constellation of 24 satellites emplaced in
nearly 20,000-kilometer high 12-hour circu-
lar orbits will provide highly precise, continu-
ous, worldwide, all-weather position plus time
and velocity information to GPS receiver-
equipped vehicles, vessels and aircraft.
10.2 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 locations and their distances from
the GPS receiver are known. The GPS
receiver fixes its own position by triangu-
lation.
d 1
d 2 d 3
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 d
1 , d 3 , and d 3 .
GPS fix
Figure 10-1 How the GPS receiver calculates position