GPS System Segments - Understanding GPS: Principles and Applications - page 84

new capabilities needed from GPS, primarily to add new military and civil ranging
signals. The GMSDT was formed as a government/Federally Funded Research and
Development Center (FFRDC)/industry team to evaluate the deficiencies of the
existing signal structure and recommend a new signal structure that would address
the key areas of modulation and signal acquisition, security, data message structure,
and system implementation. Today's M code signal structure is the result of those
studies. (M code is discussed in Section 4.5.3.) The complete list of ranging signals
provided by the IIF SV is shown in Table 3.4. It should be noted that the new ranging
signals also carry improved versions of the clock and ephemeris data in their respec-
tive navigation messages. This eliminates some of the resolution limitations the orig-
inal navigation message had imposed as the URE has continued to improve.
The original flexibility and expandability features of the IIF SV in both the
spacecraft and navigation payload designs allowed the addition of these new signals
without major revisions to the IIF design. An exploded view of the Block IIF SV is
depicted in Figure 3.12. The figure shows all of the components of the spacecraft
subsystems. These include the attitude determination and control subsystem, which
keeps the antennas pointing at the Earth and the solar panels at the Sun; the electri-
cal power subsystem that generates, regulates, stores, and shunts the DC power for
the satellite; and the TT&C subsystem, which allows the MCS operators to commu-
nicate with and control the satellite on-orbit. To support the increase in DC power
requirements due to the increased transmit power, the solar arrays were switched
from silicon technology to higher efficiency triple-junction gallium arsenide. Addi-
tionally, the thermal design had to be revised to accommodate the additional trans-
mitter thermal loads. Other than some realignment to maintain weight and thermal
balance, no other modifications were required for the spacecraft.
The navigation payload on the Block IIF SV includes two RAFSs and one cesium
AFS per the contract requirement for dual technology. These AFSs provide the tight
frequency stability necessary to generate high-accuracy ranging signals. The NDU
generates all of the baseband forms of the ranging signals. The original NDU design
included a spare slot that allowed the addition of M code and the L5 signal within
the same envelope. The original NDU computer was designed with 300% expansion
memory margin and 300% computational reserve (throughput margin), so that
there was sufficient reserve to support the generation of the new navigation mes-
sages for M code and L5 plus other modernization requirements. The computer pro-
gram is reprogrammable on-orbit and is loaded from onboard electrically erasable
programmable read-only memory (EEPROM) when power is applied, avoiding the
need for large blocks of contact time with the ground antennas. The L-band subsys-
tem generates about 350W of radio frequency (RF) power for transmitting the three
sets of signals in Table 3.4.

Table 3.4 Block IIF Ranging Signal Set
Link (Frequency) L1 (1,575.42 MHz) L2 (1,227.6 MHz) L5 (1,176.45 MHz)

Civil (open) signals C/A code L2C L5

Military (restricted)
signals

P(Y) code
M code

P(Y) code
M code

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