COMMAND RANGING & TELEMETRY UNIT CORTEX
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Page 95
3.3.8.
Frame Synchronization
Frame synchronization is performed only if parameter
Frame Synchronizer
is set to
on
.
3.3.8.1.
Frame Synchronization Strategy
Whatever the telemetry operating mode may be (Normal, Test mode 1 or Test mode 2), the technique used to
identify the telemetry frames is the same: it consists of finding the synchronization word which marks the
beginning of the frame in the NRZL signal output by the bit synchronizer and in reacting correctly to a loss of
lock. This process is illustrated by next figure:
SEARCH phase (S):
In this mode, the frame synchronizer compares the signal bit by bit, until it matches the
expected synchronization word to within n bits where « n » is a programmable error factor,
referred to as the synchronization threshold (SYN). The synchronization threshold can vary
from 0 to 7 (inclusive).
A programmable mask can also be applied to the synchronization word to ignore corrupted
bits in the synchronization word resulting from a spacecraft failure.
CHECK phase (C):
Before moving onto the LOCK mode, the frame synchronizer will check « i » consecutive
occurrences of the synchronization word (to within n errors). The contents of the telemetry
block between two synchronization words is not checked. During this phase, any occurrence
of an incorrect synchronization word returns the frame synchronizer to the SEARCH mode. «
i » is programmable between 0 and 7 (inclusive) and referred as the CTL (Check-To-Lock)
threshold. To go from SEARCH to LOCK, the frame synchronizer must receive i+1 correct
synchronization words except when i = 0 (in that case, the frame synchronizer will transmit
frames starting from the frame containing the first occurrence of a correct synchronization
word).
LOCK phase (L):
In the LOCK mode, the frame synchronizer merely checks the occurrence of a new
synchronization word (to within n errors) every frame. The window in which the
synchronization word should be found is enlarged by N bits (N = bit slip parameter = 0, 1
or 2 bits) to cope with potential bit slip in the telemetry demodulation process.
FLYWHEEL phase (F):
Transmission errors (signal heavily affected by noise) or loss of PSK demodulator lock may
occur: the frame synchronizer will then only return to SEARCH mode after « j » consecutive
occurrences of incorrect synchronization words (number of errors greater than the
synchronization threshold). « j » is programmable between 0 and 7 (inclusive) referred to as
the LTS (Lock-To-Search) threshold.
