Furuno GP-1610CF manual GPS, Chart Overview, Principle of GPS, What is GPS?

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.

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.

1Satellite locations are calculated.

2Distances are measured.

3The GPS fix is the point of intersection of three spheres which are drawn around the three satellites with diameter d1, d2, and d3.

d2 d3

d1

GPS fix

Figure 4-1 How GPS determines position

4-1