402
5 X-ray Pulsar-Based Navigation: Theories and Experiments
Fig. 5.45 Structure of navigation satellite system
which is called generational gap, which is considered as a sign that the “crossingthrough-generation” occurs in the development of navigation satellite system. The
so-called “crossing-through-generation” refers to the essential change of the mechanism of navigation satellite system, rather than some improvements of functions and
performances of the system’s components. In that way, is it necessary for the navigation satellite system to cross through generation and how is it to cross through
generation? It is a major strategic issue facing the development of current navigation
satellite system, and also the problem determining the development orientation of
the Third-Generation Navigation Satellite System (3G-NAV).
From the prospective of the development of GPS system, there are seven kinds of
GPS satellite series launched into the orbits since 1978, including Block I, II, IIA, IIR,
IIR-M, IIF and III-A, as shown in Table 5.9, from which it can be seen that each GPS
satellite series is compatible with the basic functions of the former series; the platform
and payload technologies of the GPS satellites have been evolving constantly and
their performance improving; the number of the launched GPS satellites has reached
75 as of June 2020. Although the performance of the space constellation, ground
control and user terminal segments of GPS system has been improving for the past
5 X-ray Pulsar-Based Navigation: Theories and Experiments
Fig. 5.45 Structure of navigation satellite system
which is called generational gap, which is considered as a sign that the “crossingthrough-generation” occurs in the development of navigation satellite system. The
so-called “crossing-through-generation” refers to the essential change of the mechanism of navigation satellite system, rather than some improvements of functions and
performances of the system’s components. In that way, is it necessary for the navigation satellite system to cross through generation and how is it to cross through
generation? It is a major strategic issue facing the development of current navigation
satellite system, and also the problem determining the development orientation of
the Third-Generation Navigation Satellite System (3G-NAV).
From the prospective of the development of GPS system, there are seven kinds of
GPS satellite series launched into the orbits since 1978, including Block I, II, IIA, IIR,
IIR-M, IIF and III-A, as shown in Table 5.9, from which it can be seen that each GPS
satellite series is compatible with the basic functions of the former series; the platform
and payload technologies of the GPS satellites have been evolving constantly and
their performance improving; the number of the launched GPS satellites has reached
75 as of June 2020. Although the performance of the space constellation, ground
control and user terminal segments of GPS system has been improving for the past