IEEE - Aerospace and Electronic Systems - May 2022 - 9

Mladenov et al.
Figure 8.
I and Q samples of a detected burst, showing the initial carrier
lock at sample 2700 and start of bit sync (BS) at sample 3100 and
frame sync (FS) at sample 3250.
of the preamble begins, the power swings back and
forth between the positive and negative imaginary
components following the two Manchester phase levels
according to the biphase-L encoding scheme. Only the
samples from the PLL Q-branch are passed down further
as these data contain the phase and, therefore, the
encoded message.
SYMBOL TIMING RECOVERY AND DECODING
Symbol recovery is done by feeding phase symbols to
a clock_recovery_mm block, which performs symbol
timing recovery according to the Mueller and M€uller
timing synchronization algorithm [7]. This GNU Radio
block is provisioned with the estimated symbol rate of
800 symbols/second. The latter is an estimated rate since
the sampling rate of the SDR is not exact. After symbol
recovery, the output is also phase symbols in the f32 format,
but resampled to 1 sample/symbol, hereafter called
" soft symbols. " The soft symbols are converted to hard
symbols by means of a binary slicer. This block converts
any value equal or less than 0 to a hard 0, and any
positive value to a 1. The conversion from soft symbols to
hard symbols in the preamble sequence is illustrated
in Figure 9.
The obtained logic levels of the hard symbols are still
representing the Manchester symbols and not the actual
bits. A differential decoder is used to map the Manchester
symbol pairs to bits. The bit synchronization pattern consisting
of 15 " 1 " s (30 Manchester symbols) can be seen in
Figure 9. The decoding is performed in the C++ code and
is implemented as a tag correlator where the hard symbols
are shifted continuously through a synchronization register.
This register is checked at every bit shift to see how
well it correlates with the expected preamble sequences.
As the phase information is relative to the carrier that the
MAY 2022
Figure 9.
Q-branch sample plot showing the resampled phase symbols at 1
sample/symbol and conversion from soft symbols (red) to hard
symbols (blue).
PLL locks to, the absolute phase of the transmission is not
known. Therefore, it is also necessary to correlate against
the inverted preamble, resulting in the following four
possibilities:
(15x 1)+ 000101111 (normal polarity and normal
mode);
(15x 0)+ 111010000 (inverted polarity and normal
mode);
(15x 1)+ 011010000 (normal polarity and self-test
mode);
(15x 0)+ 100101111 (inverted polarity and self-test
mode).
If there is a match between the contents of the synchronization
register and one of the four possible preambles,
then a transmission is detected, after which the
subsequent symbols are shifted into a separate message
register. Once the message register is filled, it is differentially
decoded and the output logged.
DEPLOYMENT
Prior to code uplink to the satellite on-orbit, a validation is
performed on the engineering model (EM). This consists
of uplinking the application code with the same file transfer
pipeline that will be used during a pass with the satellite.
Time-tagged commands are sent to the EM, and the
SEPP resource usage is monitored live using the MCS.
This process is shown in Figure 10.
GNU Radio version v3.7.13.4 in a headless configuration
is cross-compiled for the arm-linux-gnueabihf
architecture, packaged into compressed archives, and is
uploaded to the satellite together with the compiled GNU
Radio flow graph. The archives containing the shared
GNU Radio libraries are available to other future users as
IEEE A&E SYSTEMS MAGAZINE
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