Global Positioning System Reference
In-Depth Information
Table 3.3
IR-M Modification Summary
Component
Magnitude of Change
Description of Change
Antenna panel
Moderate redesign
Replace L-band elements with broadband
proprietary elements
L-band subsystem
Technology upgrade
Replace five separate components with three
multifunction assemblies (L1 and L2 high-
power amplifiers and a WGMIC)
L-band structural panel
New design, similar to
those flown on
communication SVs
Higher-power dissipation requires integral
heat pipes in the panel honeycomb structure
Payload control
electronics
Minor modification
Add power switching and fusing to
accommodate additional power
Power regulation unit
Minor modification
Spare pins connected to additional power
and return lines for higher power
3.2.3.7 Block IIF—Follow-On Sustainment Satellites
In 1995, the Air Force (GPS JPO) released a request for proposal (RFP) for a set of
satellites to sustain the GPS constellation, designated as Block II follow-on, or IIF.
The RFP also requested the provider to include the modifications to the GPS CS nec-
essary to operate the IIF SV. While necessary for service sustainment, the IIF SV pro-
curement afforded the Air Force the opportunity to start adding new signals and
additional flexibility to the system beyond the capabilities and improvements of the
IIR SV. A new military acquisition code on L2 was required, as well as an option for
a new civil L5 signal at a frequency within 102.3 MHz of the existing L2 frequency
of 1,227.6 MHz. The L5 frequency that was eventually settled upon was 1,176.45
MHz, placing it in a frequency band that is protected for ARNS. (The L5 signal is
described in Section 4.5.2.)
The RFP also allowed the provider to offer additional “best value” features
that could be considered during the proposal evaluation. Boeing (then Rockwell)
included several best value features in its proposal and was awarded the IIF contract
in April 1996. Several of these features were to improve service performance,
including a URE 3m or less in AutoNav mode, an age of data for the URE of less
than 3 hours using the UHF crosslink to update the navigation message, and design
goals for AFS Allan variance performance better than specification. (Appendix B
contains details on the Allan variance.) Other features supported the addition of
auxiliary payloads on the IIF SV and reduction of operational complexity for the
operators via greater use of the UHF crosslink communication system.
The original planned launch date for the first IIF SV was April 2001. However,
due to the longevity of the Block II and IIA SVs and projected service life of the IIR
SVs, the need date for a IIF launch was extended sufficiently to allow the Air Force
to direct modifications to the IIF SV that resulted in the present design. The first
modification was enabled when the Delta II launch vehicle (LV) was deselected for
IIF, leaving the larger evolved expendable launch vehicle (EELV) as the primary LV.
The larger fairing of the EELV enabled the “Big Bird” modification to the IIF SV,
which expanded the spacecraft volume, nadir surface area, power generation, and
thermal dissipation capability. Around the same time, extensive studies were per-
formed by the GPS Modernization Signal Development Team (GMSDT) to evaluate
Search WWH ::




Custom Search