IEEE - Aerospace and Electronic Systems - May 2022 - 5

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several clusters of reconfigurable servers running baseline
ESA ground operations software as well as custom application
stacks.
METHOD
To demonstrate the use of the SDR payload of OPS-SAT
with a real-life scenario, the COSPAS-SARSAT system
was chosen, a global network of ground stations and transponder
instruments on satellites that pick up and relay
distress transmissions originating from terrestrial emergency
beacons. The system allows detection and relay of
emergency transmissions via satellites. Most ships and aircraft
are required to carry such a beacon, also known as an
emergency position indicating radio beacon (EPIRB). The
aviation equivalent is referred to as an emergency location
transmitter (ELT), often mounted in the tail of the aircraft.
Distress beacons can be activated automatically by a variety
of means, including prolonged contact with salt water
for an EPIRB and large G-forces for ELTs. Manual activation
is also always possible. Several protocols exist for the
actual message encoding, but most schemes encode basic
information such as a registration number and approximate
location acquired via an internal GNSS receiver.
Many satellites in low, medium, and geostationary
orbits (LEO, MEO, and GEO) carry repeater instruments
that perform a frequency shift of the received beacon
transmissions in UHF, and downlink them in the L-band
(1.5 GHz) to the local user terminals for on-ground processing
ofthe beacons. Once a beacon activation is verified,
it is forwarded to the nearest rescue coordination center. If
no GPS information is sent in the beacon, time difference
of arrival (TDOA) is used to determine a coarse position.
As opposed to the on-ground signal processing that is
employed in the operational COSPAS-SARSAT system,
this IOD is concerned with the direct in-orbit processing,
and decoding ofbeacon transmissions.
CONCEPT OF OPERATIONS (CONOPS)
Figure 1.
OPS-SAT with deployed solar arrays in the cleanroom (TU Graz).
MAY 2022
The SDR payload and UHF monopole antenna ofOPS-SAT
are used to receive the terrestrial RF transmissions frombeacons
in the 406-MHz band at the experiment preprogrammed
start time t0. The recordings are digitally processed
and files with signal metadata containing the encoded beacon
messages are generated onboard. Upon acquisition of
signal at t1, the files are synchronized with the ground. The
mission operations are performed via the mission timeline,
which is the result of the mission planning system and is a
preprogrammed list of time-tagged commands that shall be
executed onboard and on-ground. This consists ofthe uplink
windows, when the SDR will be recording, and processing
beacons, as well as the timing ofthe downlinks. The timeline
is loaded as XML files into the mission automation system
and consists of timestamps and references to procedures
stored in a database. A procedure consists of one or more
TC to be sent to the satellite, with intermediate TM checks.
TCs are sent and TM is received using the satellite control
and operation system (SCOS), the generic MC software
infrastructure developed and used by ESA. SCOS handles
many TM/TC aspects in space missions and connects using
TCP to the ground station modem via a proxy to exchange
packets on the RF space link. File transfers are performed
using the Consultative Committee for Space Data Systems
IEEE A&E SYSTEMS MAGAZINE
5
IEEE - Aerospace and Electronic Systems - May 2022

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