GPS System Segments - Understanding GPS: Principles and Applications - page 81

older Block II/IIA SVs continue to surprise with their longevity. IIR-M brings the
new military and civilian services to users at least 3 years earlier than if modernized
capabilities were to wait for just Block IIF and Block III.
At the time of this writing, the IIR-M SVs were undergoing the design modifica-
tions and were available for launch in 2005. Early testing of L2C and M code (new
civil and military signals, respectively) will occur for some time following the first
IIR-M launch while more modernized SVs are added to the constellation.
Modernized Signals
New L-band signals and increased L-band power will significantly improve naviga-
tion performance for users worldwide. Three new signals will be provided: two new
military codes on L1 and on L2, and a new civilian code on L2. The new L2 civil sig-
nal denoted as L2C will be an improved signal sequence over L1 C/A, enabling ion-
ospheric error correction to be done by civilian users. It will be ground-selectable,
allowing selection of either L2 C/A or a proposed new L2C code or L2C off. The
new signal structure will be totally backward-compatible with existing L1 C/A and
P(Y), and L2 P(Y). (Refer to Section 4.5.1 for further details.)
The M code on L1 and L2 for the military user will also be ground-selectable
and will include a pseudo-M code to be used during testing activities. The new M
code will provide the authorized user with more signal security.
Modernized Hardware
The new navigation panel boxes consist of a redesigned L1 transmitter, a redesigned
L2 transmitter, and the new waveform generator/modulator/intermediate power
amplifier/converter (WGMIC) (Figure 3.11). The WGMIC is a new box developed
by ITT coupling the brand-new waveform generator with the functionality of the L1
signal modulator/intermediate power amplifier (IPA), the L2 signal modulator/IPA,
and the dc-to-dc converter. The waveform generator provides much of the new
modernized signal structure and controls the power settings on the new transmit-
ters. To manage the thermal environment of these higher-power boxes, heat pipes
were incorporated into the fabrication of the structural panel. Lockheed Martin has
used similar heat pipes on other satellites it has built.
The improved IIR antenna panel discussed earlier in this section will also be
installed on all IIR-M SVs. This will provide greater signal power to the user. The
antenna redesign effort was begun prior to the modernization decision but will sig-
nificantly enhance the new IIR-M features. L-band power will be increased on both
L1 and L2 frequencies. L1 will be increased by at least double the power, and L2
will be increased by at least quadruple power at low elevation angles.
The UHF performance has also been improved. This does not directly affect the
user, but it enhances intersatellite communication: data transfer, commanding, and
crosslink ranging.
Other total navigation payload (TNP) modifications include new MDU soft-
ware that is also useable in the classic IIR SVs. Table 3.3 highlights some of the
other SV modifications, including the new antenna array (which includes the
downlink signal L-band array), new power switching capability, as well as other
component and harness changes.