Fundamentals of Satellite Navigation - Understanding GPS: Principles and Applications - page 52

ECEF Cartesian coordinate system. Vector s represents the position of the satellite
relative to the coordinate origin. Vector s is computed using ephemeris data broad-
cast by the satellite. The satellite-to-user vector r is

rsu =− (2.15)

The magnitude of vector r is

r =− s u (2.16)

Let r represent the magnitude of r

r =− su (2.17)

The distance r is computed by measuring the propagation time required for a
satellite-generated ranging code to transit from the satellite to the user receiver
antenna. The propagation time measurement process is illustrated in Figure 2.16. As
an example, a specific code phase generated by the satellite at t 1 arrives at the

Satellite-
generated code

t 1

Code phase generated by
satellite at Code arriving

from satellite

t arrives t seconds later ∆

1

t 2

RCVR

Receiver-generated
replica code

RCVR

Receiver-generated
replica code shifted
seconds

∆ t

RCVR

∆ t

t

t 1 t 2

Figure 2.16 Use of replica code to determine satellite code transmission time.