GPS System Segments - Understanding GPS: Principles and Applications - page 78

Coarse
sun
sensor

Low
band
antenna

Antenna
farm

Solar
array

Earth
sensor
assembly

S-band
antenna

Solar
array

High
band
antenna

Antenna

Coarse
sun
sensor

Figure 3.8 Block IIR satellite components. ( Source: Lockheed Martin Corp. Reprinted with

permission.)

The RAFS has a MMD of 7.5 years. It is coupled with a redundant voltage con-
trolled crystal oscillator (VCXO) and software functionality into what is called the
time keeping system (TKS). The TKS loop provides a timing tuning capability to sta-
bilize and control satellite clock performance.

IIR Accuracy
An accurate onboard AFS provides the key to good GPS PVT accuracy [5]. Figure
3.9 shows the 1-day Hadamard deviation for mid-2004. Hadamard deviation mea-
sures frequency stability—the lower the number, the more stable the AFS.
Hadamard deviation (as opposed to Allan deviation) currently provides the best
way to measure frequency stability in AFS with nonzero frequency drift. (Appendix
B provides descriptions of both the Hadamard and Allan deviations.)
The IIR specification requires that the total IIR URE (defined in Section 3.2.3.5)
value should be less than 2.2m when operating a RAFS. The URE performance for
GPS IIR has averaged approximately 0.8m or better for several years [6]. Thus, the
required specification is easily surpassed.
There is also a significantly improved solar pressure model (by an order of mag-
nitude compared to the II/IIA model) used in the MCS when computing the orbit of
the IIR [7, 8]. This increases the accuracy of the ephemeris modeling on the ground.

Enhanced Autonomy
The advanced capabilities of the Block IIR SV include a redundancy management
system called REDMAN, which monitors bus subcomponent functionality and pro-
vides for warning and component switching to maintain SV health.