  According to the Consultative Committee for Space Data Systems (CCSDS)
recommendation for TeleCommand (TC) synchronization and coding, the
Communications Link Transmission Unit (CLTU) consists of a start sequence,
followed by coded data, and a tail sequence, which might be optional depending
on the employed coding scheme. With regard to the latter, these transmissions
traditionally use a modified Bose-Chaudhuri-Hocquenghem (BCH) code, to which
two state-of-the-art Low-Density Parity-Check (LDPC) codes were later added. As
a lightweight technique to detect the presence of the tail sequence, an
approach based on decoding failure has traditionally been used, choosing a
non-correctable string as the tail sequence. This works very well with the BCH
code, for which bounded-distance decoders are employed. When the same approach
is employed with LDPC codes, it is necessary to design the tail sequence as a
non-correctable string for the case of iterative decoders based on belief
propagation. Moreover, the tail sequence might be corrupted by noise,
potentially converting it into a correctable pattern. It is therefore important
that the tail sequence is chosen to be as much distant as possible, according
to some metric, from any legitimate codeword. In this paper we study such
problem, and analyze the TC rejection probability both theoretically and
through simulations. Such a performance figure, being the rate at which the
CLTU is discarded, should clearly be minimized. Our analysis is performed
considering many different choices of the system parameters (e.g., length of
the CLTU, decoding algorithm, maximum number of decoding iterations).
