Given a 24 satellite constellation, GPS will provide 100% four and five satellite

coverage without a PDOP constraint (but with a mask angle of 5 degrees), and six

satellite coverage greater than 99.9% of the time.  However, four satellite coverage with

a PDOP constraint of 6 can drop as low as 99.9%, with a worst case dispersion of the

24 satellites with respect to their nominal orbits.  Even in this event, most users will

experience continuous coverage.  A few isolated locations may experience four 

satellite coverage as low as 96.9%, with a PDOP constraint of 6 and a mask angle of 5

degrees.

Satisfaction of the third design objective requires the ability to remove any individual

satellite from the constellation, and still be able to provide as close to continuous global

coverage as is practical. Satisfaction of this objective requires that at least five

satellites be in view almost continuously.  As shown in Figure 6 1, this is the case with

the 24 satellite constellation design.  Although an explicit requirement is not

established to ensure that multiple combinations of satellites provide adequate solution

geometry at any given time, most of the time at least two and usually more

combinations of four satellites will support a Position Dilution of Precision (PDOP)

constraint of 6 or less.

6.1.3  Service Availability Characteristics

This section defines the GPS availability standards and expected regional and global

service availability characteristics.  The user is provided with information concerning

GPS service availability patterns on a global and regional basis.  Service availability

varies slightly over time, due to routine satellite maintenance requirements.  Note that

the regional service availability values provided below are based upon a global grid

point spacing of approximately 111 x 111 kilometers, with 30 second intervals over 24

hours.

Service availability is described in two basic parts.  The first part concerns the variation

in service availability as a function of temporarily removing a number and specific

combination of satellites from service.  The second part of the assessment applies

service availability variation characteristics to an operational scenario.

6.1.3.1  Service Availability Standards

GPS service will be provided in accordance with the availability standards specified in

Table 6 2.

6.1.3.2  Satellite Outage Effects on Service Availability

Service availability varies predominantly as a function of the number and distribution of

satellite service outages.  With a 24 satellite constellation, the permutations and

combinations of satellite service outages are rather large.  Normally, no more than

three satellites will be removed from service over any 24 hour interval.  This ground

rule bounds the problem to an analysis of the effects of removing each satellite and all

combinations of two and three satellites from service for no more than 24 hours.  The

results of the analysis are summarized in Table 6 3.

6 5