The difference between the planned orbit and the orbit actually achieved during

the launch and orbit insertion process,

  Orbit variation dynamics, and

  Frequency and efficiency of satellite station keeping maneuvers.

6.1.2.3  Expected Service Coverage Characteristics

Proper support of the first design objective (from above) requires that at least four

satellites are continuously in view with an acceptable geometry and mask angle

anywhere in the world.  An impli cation of this requirement is that most of the time

significantly more than four satellites will be visible.  As shown in Figure 6 1, eight

satellites will be visible on average for any location in the world, over 24 hours.  Very

seldom will a user see only four satellites when all 24 satellites are providing usable

ranging signals.  If the 24 satellites in the GPS constellation were all launched with no

deviations into their planned orbits, and no drift were allowed, the constella tion would

provide virtually 100% (0.99999714) four satellite coverage with a PDOP constraint of

six.

Figure 6 1.  Satellite Global Visibility Profile

Unfortunately, variations in final orbits based upon launch uncertainties and routine

drift do occur. The second design objective is supported by evaluating how changes in

each satellite s orbital elements affect nominal coverage characteristics.  Bounds are

applied to orbital element deviation from the nominal orbit to ensure that constellation

coverage does not degrade beyond allowed limits. Degraded coverage areas drift and

change slightly in shape over time, but their average number and duration will remain

approximately constant for a given constellation.  Changes in the number of satellites

or significant shifts in satellite orbits,  however, can dramatically change the attributes of

degraded coverage areas.

6 4