GPS System Segments - Understanding GPS: Principles and Applications - page 76

2-hour period and is valid over a 4-hour interval. For the remainder of the first 14
days, the navigation message is broadcast for a 4-hour period with a validity period
of 6 hours (2 additional hours). Following this initial 14-day period, the navigation
message data broadcast periods gradually extend from 6 hours to 144 hours. With
this additional onboard storage retention capability, the satellites can function con-
tinuously for a period of 6 months without ground contact. However, the accuracy
of the CS ephemeris and clock predictions and thus the accuracy of the navigation
message data gracefully degrade over time such that the user range error (URE) will
be bounded by 10,000m after 180 days. (The URE is the contribution of the
pseudorange error from the CS and space segment.) Typically, the URE is 1.4m (1 ).

(Pseudorange errors are extensively discussed in Section 7.2.) With no general
onboard processing capability, no updates to stored reference ephemeris data are
possible. So, as a result, full system accuracy is only available when the CS is func-
tioning properly and navigation messages are uploaded on a daily basis. Block IIA
electronics are radiation-hardened. Nineteen Block IIA satellites were built by
Rockwell International, with the first launched in November 1990 from Cape
Canaveral Air Force Station in Florida and the last launched in November 1997.
The life expectancy of the Block IIA is the same as that of the Block II. At the time of
this writing, 16 Block IIA satellites remained in the constellation, with a projected
MMD of over 10.3 years. A Block IIA satellite is shown in Figure 3.6.

σ

3.2.3.6 Block IIR—Replenishment Satellites
The GPS Block IIR (replenishment) satellites (Figure 3.7) represent an ever-growing
presence in the GPS constellation. Over half of the original 21 IIR SVs have been

Figure 3.6 Block IIA satellite.