GPS System Segments - Understanding GPS: Principles and Applications - page 71

form of space vehicle number (SVN); for example, 60 refers to NAVSTAR satellite
60. The third notation represents the configuration of the PRN code generators
onboard the satellite. These PRN code generators are configured uniquely on each
satellite, thereby producing unique versions of both C/A code and P(Y) code. Thus,
a satellite can be identified by the PRN codes that it generates. Occasionally, the
PRN assignment for a given SVN can change during the satellite's mission duration.
3.2.2 Constellation Design Guidelines
As discussed in Section 2.3.2, several tradeoffs are involved in the design of the GPS
constellation. One primary concern is the geometric contribution to navigation
accuracy; in other words, is the satellite geometry sufficiently diverse to provide
good observability to users throughout the world. This geometry is measured by a
parameter called dilution of precision (DOP) and is described in more detail in Sec-
tion 7.3. Studies continue concerning tradeoffs on different possible satellite config-
urations. Some studies have investigated the use of 30 satellites in three orbital
planes as well as the utility of geostationary satellites. Most of this work is done
with a nominal constellation assuming all satellites are healthy and operational, but
a new dimension for study is introduced when satellite failures are considered. Sin-
gle or multiple satellite failures provide a new dimension around which to optimize
performance from a geometry consideration. Another design consideration is line-
of-sight observability of the satellites by the ground stations to maintain the
ephemeris of the satellites and the uploading of this data.
3.2.3 Space Segment Phased Development
The continuing development of the control and space segments has been phased in
over many years, starting in the mid-1970s. This development started with a con-
cept validation phase and has progressed to several production phases. The satel-
lites associated with each phase of development are called a block of satellites.
Characteristics of each phase and block are presented in the following sections.
3.2.3.1 Satellite Block Development
Five satellite blocks have been developed to date. The initial concept validation sat-
ellites were called Block I. The last remaining prototype Block I satellite was dis-
posed of in late 1995. Block II satellites are the initial production satellites, while
Block IIA refers to upgraded production satellites. All Block I, II, and IIA satellites
have been launched. Block IIR satellites, denoted as the replenishment satellites, are
being deployed. At the time of this writing, modified Block IIR versions denoted as
Block IIR-M were scheduled for launch in 2005. Block IIF satellites, referred to as
the follow-on or sustainment satellites, are being built. GPS III satellites are in the
planning stage for a post-2010 deployment. Since satellites are launched only as
replacements for a satellite failure, their scheduling is difficult to predict, especially
when most satellites have far outlived their design lifetime. At the time of this writ-
ing, the constellation consisted of 27 operational satellites. Table 3.2 describes the
configuration of the current satellite constellation. Thus, the current optimized con-