Fundamentals of Satellite Navigation - Understanding GPS: Principles and Applications - page 26

R

(a)

Plane of intersection

(b)

Figure 2.6 (a) User located on surface of sphere. (b) User located on perimeter of shaded circle.
( Source: [2]. Reprinted with permission.) (c) Plane of intersection. (d) User located at one of two
points on shaded circle. ( Source: [2]. Reprinted with permission.) (e) User located at one of two
points on circle perimeter.

a user on the Earth's surface, it is apparent that the lower point will be the true posi-
tion. However, users that are above the Earth's surface may employ measurements
from satellites at negative elevation angles. This complicates the determination of an
unambiguous solution. Airborne/spaceborne receiver solutions may be above or
below the plane containing the satellites, and it may not be clear which point to
select unless the user has ancillary information.

2.2 Reference Coordinate Systems

To formulate the mathematics of the satellite navigation problem, it is necessary to
choose a reference coordinate system in which the states of both the satellite and the