This section defines the GPS coverage standards, GPS constellation design objectives,

and the characteristics of GPS coverage which are expected with a 24 satellite

operational constellation. The user is provided with general information concerning how

coverage will vary over time on a global basis, and a worst case projection of coverage

on a regional basis.  The data provided in the discussion is based upon a global

assessment of grid points spaced equally, approximately 111 kilometers apart, every

30 seconds over a 24 hour period.

6.1.2.1  Service Coverage Standards

GPS Service will be provided in accordance with the coverage standards presented in

Table 6 1.

Table 6 1.  Service Coverage Standards

Coverage Standard

Conditions and Constraints

 99.9% global average

  Probability of 4 or more satellites in view over any 24 hour interval,

averaged over the globe

  4 satellites must provide PDOP of 6 or less

  5  mask angle with no obscure

  Standard is predicated on 24 operational satellites, as the

constellation is defined in the almanac

 96.9% at worst case

  Probability of 4 or more satellites in view over any 24 hour interval,

point

for the worst case point on the globe

  4 satellites must provide PDOP of 6 or less

  5  mask angle with no obscure

  Standard is predicated on 24 operational satellites, as the

constellation is defined in the almanac

6.1.2.2  The GPS 24 Satellite Constellation

The 24 satellite constellation is designed to optimize global coverage over a wide

range of operational conditions.  Specific constellation design objectives are listed

below:

  Provide continuous global coverage with specified geometry and mask angle

constraints.

  Minimize coverage sensitivity to expected satellite orbital drift characteristics.

  Mitigate the effects on service availability of removing any one satellite from

service.

Several factors affect GPS coverage.  These factors must be taken into consideration

in the constellation design.  The factors are:

6 3