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c. Asynchronous Data Modulation. For data rates < 300 kbps, the forward service
data received at the SN ground terminal from the NISN data transport system is
directly modulo-2 added by the SN ground terminal to the command channel
PN code sequence. The forward service data will be asynchronous with the
carrier and the PN code.
NOTE:
When the command channel does not contain any actual
forward service data, the forward service command channel
signal is the command channel PN code sequence with or
without an idle pattern.
d. Functional Configurations: A further description of the functional configurations,
the I-Q channel ambiguity, and data polarity ambiguity is found in Appendix B,
paragraph B.2.2.
e. Doppler Compensation. The TDRSS MA forward service carrier frequency (F)
and the PN chip rate transmitted by a TDRS can optionally be compensated by
the SN ground terminal for Doppler. When compensated, the carrier, F , R
arrives at the customer platform receiving system within a predictable tolerance
(E) of fo as defined in Table 5-1. This feature minimizes the Doppler resolution
requirements of the customer platform receiver and is available continuously to
facilitate reacquisition by the customer platform in the event of loss of lock of
the TDRSS MA forward service signal. Doppler compensation may be inhibited
and the TDRSS will transmit a fixed frequency MA forward service carrier and
PN code chip rate.
5.2.2.2 BPSK Signal Parameters
a. BPSK Modulation (PN Modulation Enabled). The I channel is used to transmit
the customer command data and is referred to as the command channel.
TDRSS MA forward services with data rates equal to and below 300 kbps
should incorporate spread spectrum modulation techniques to satisfy flux
density restrictions imposed on the TDRSS forward services by the NTIA. The
command channel includes a rapidly acquirable PN code and contains the
forward service data. The PN code chip rate is coherently related to the TDRS
transmit frequency in all cases. This feature permits the customer platform
receiver to use the receiver PN code clock to predict the received carrier
frequency, thereby minimizing receiver complexity and reducing acquisition
time. 451-PN CODE-SNIP defines all the salient characteristics for the forward
command channel PN code libraries. The Agency Spectrum Manager
responsible for PN code assignments will allocate a customer platform-unique
PN code assignment from these libraries. The GSFC Spectrum Manager is
responsible for NASA PN code assignments.
Revision 10 5-6 450-SNUG