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d. The customer platform receiving system will search for and acquire the
command channel PN code (if applicable) and carrier. Normally, a customer
MOC will not be transmitting forward service data to the NISN data transport
system until the forward service signal has been acquired by the customer
platform and the acquisition verified by the customer MOC from customer
platform return service telemetry. If the NASA fourth generation standard
transponder is used, its design implementation requires that there be no data
transitions during the signal acquisition process, while others may merely result
in longer acquisition times. Data transmission is inhibited by the SN ground
terminal during PM carrier frequency sweep.
e. For QPSK modulation, the customer platform receiving system will search for
and acquire the range channel PN code upon acquisition of the command
channel PN code and carrier.
f. Depending upon the customer platform receiving system design, upon
completion of forward link acquisition and subsequent customer platform
transition to signal tracking, the customer platform transmitting system may
either switch to a coherent mode or remain in a noncoherent mode until
commanded by the customer MOC to switch.
g. The SN ground terminal will continue Doppler compensation of the TDRSS SSA
forward service signal unless requested by the customer MOC to inhibit the
Doppler compensation.
h. Tacq in the customer platform receiver is a function of the customer platform
receiver design and signal-to-noise density ratio. For the purpose of an
example, Table 6-6 provides the acquisition characteristics for the fourth
generation transponder when receiving an SSA QPSK signal. The Tacq values
listed in Table 6-6 are contingent on the customer MOC defining the customer
platform receiver center frequency, fo, to an accuracy of +700 Hz in each
service support schedule add request (SAR). The customer platform forward
service acquisition time must be considered in determining the overall return
service acquisition time for customer platform with a coherent mode of
operation.
i. Appendix A provides example link calculations for the TDRSS SSA forward
service.
6.3 SSA Return Services
6.3.1 General
The RF signals provided by the customer platform to the TDRS and the characteristics
of data provided at the SN ground terminal interface are defined in paragraphs 6.3.2
through 6.3.6. This discussion assumes that an appropriate return service has been
scheduled and a data signal is present at the TDRS interface. The SSA return service
supports data services using the SN ground terminal receivers and an automated IF
service.
Revision 10 6-17 450-SNUG