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3.3.1.2.1. Constellation Convention
IF demodulation is as per the following counterclockwise convention:
Gives
Where i(t) and q(t) = 0 or 1:
I and Q outputs can be swapped on the IF receiver in case of clockwise convention.
3.3.1.2.2. AQPSK Processing
The AQPSK telemetry chain consists of an IF Receiver feeding two telemetry units (TMU-A and TMU-B) in
parallel. The demodulation algorithm depends on whether TMU-A is used for the highest or lowest bit rate bit
rate.
If TMU-A is used for the highest bit rate, the IF Receiver operates in QPSK mode and it will lock only when both
Bit Synchronizers are locked. This locking process is robust since the performances do not depend on the bit
rates ratio and the data themselves. The drawback is that the acquisition and tracking threshold is based on the
highest bit rate (the higher the bit rate, the higher the threshold). This scenario is recommended when both bit
rates are very close to each other.
Oppositely, if TMU-A is used for the lowest bit rate, the IF Receiver operates in BPSK mode on the lowest rate
channel and it is declared locked as soon as TMU-A is locked and whatever TMU-B status is (locked or
unlocked).
In this mode, the acquisition and tracking threshold will be much better (lower) than in the first scenario. The
drawback is that the highest bit rate signal has to be rejected by the lowest bit rate processing bandwidth to avoid
performance degradation. In other words, this mode should be used only when the bit rate ratio between the two
channels is as high as possible (ideally greater than 4, higher than 2 in any case). It is also desirable that the
lowest bit rate demodulation remains available even if the highest bit rate one is not.
© Safran Data Systems
COMMAND RANGING & TELEMETRY UNIT CORTEX
CRT QUANTUM USER'S MANUAL
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Safran Data Systems
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Ref.
DTU 100042
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5.17
Date:
Dec.
03, 2021Sept. 30, 2021
Page 26
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