IEEE - Aerospace and Electronic Systems - May 2022 - 6

Implementation of a GNU Radio-Based Search and Rescue Receiver on ESA's OPS-SAT Space Lab
Figure 2.
Overview of the ConOps involving the various ground and space components.
(CCSDS) File Delivery Protocol (CFDP). The resulting files
from completed CFDP transfers during a contact window
with the satellite are forwarded via secure file transfer protocol
to another server and contain the encoded messages that
originated from the EPIRB/ELTs. The experiment ConOps
is illustrated in Figure 2.
ONBOARD PROCESSING
The onboard process is detailed in Figure 3. After setting
the carrier frequency and sample rate of the SDR, in-phase
and quadrature (IQ) samples representing the real and
imaginary components of the digitized RF signal are
acquired via a 12-bit parallel bus and stored in the fieldprogrammable
gate array random access memory (RAM).
The API reads the samples using direct memory access
and transfers them to the Linux userspace and high-level
scripts in order to store the samples on a temporary filesystem.
The samples are then read by an executable binary
(burst decoder), which is dynamically linked with GNU
Radio libraries in order to perform the signal processing.
Two types of files are written to the onboard embedded
multimedia card permanent storage, JSON and IQ-files.
The JSON products contain decoded metadata from the
transmissions, such as encoded message, burst frequency,
and errors, while the decimated IQ-files contain the raw
transmission for offline analysis. The raw RF input files
stored in RAM that were originally captured are automatically
deleted since they are no longer required. Finally,
upon a command from the ground, the files are downloaded
using CFDP through a controller area network bus,
which links the main interface of the SEPP to the ground
via the S-band TMTC encoder/decoder.
The next sections will elaborate further on the technical
details of the beacon transmitter standards and how
they were used to test and design a GNU Radio signal processor
to run on the satellite.
BEACON TRANSMISSION CHARACTERISTICS
Figure 3.
Onboard data flow overview.
6
In order to implement a suitable GNU Radio signal processor
for the beacon transmissions, the beacon protocol was
first investigated using the published standards. Modern
EPIRB/ELT transmissions occur in the protected 406-MHz
band and are phase-modulated at 400 bits/second with a
phase offset of 1.1 radians. Manchester encoding is used to
encode the beacon information, resulting in a modulation
rate of 800 baud. This encoding is used to ensure the presence
of periodic transitions in the case of many identical
bits. The periodic transitions in turn allow for a better symbol
clock recovery. The latter is illustrated in Figure 4. Due
to the biphase nature of the linecode, a residual carrier will
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MAY 2022
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