IEEE - Aerospace and Electronic Systems - August 2022 - 6

Maximization of LEO Nanosatellite's Transmission Capacity to Multiple Ground Stations
Figure 1.
Simulated orbit, ground track and telescopes, and GS on the Earth map (coordinates and altitudes are visible in Table 1).
The main contributions of this article are as follows.
The demonstration of the possibility to calibrate
CMB ground telescopes with a low-orbiting microwave
emitting source on CubeSat.
The definition of an approach for orbital parameters
optimization, which takes into account the maximization
of satellite RT with multiple GS.
The definition of the required " calibrator " CubeSat
pointing maneuvers during passage over GS and
their implications on the ADCS design.
The definition of a mission design that allows at
least one daily passage above the ground site, maintaining
contact times that are amply sufficient for
calibration activities. All this can be achieved with
a COTS ADCS targeted to CubeSat and without the
use of additional pointing mechanisms, reducing the
overall mission cost and mass.
The rest of this article is organized as follows. the
" Satellite Visibility by Multiple GS " section reports the sensitivity
study conducted on orbital families with different altitudes
to identify the optimal set ofparameters, achieving both
the maximum GS mean visibility time and the number ofGS
accesses. The " Evaluation of the Required Attitude Control
Strategy " section analyses the effect of the selected orbital
configurations on ADCS requirements. The " Results and
Discussion " section reports the optimal orbit selection
results, and the expected performance in terms of pointing
accuracy and maneuver velocity, conducting a preliminary
tradeoff with commercially available systems that could be
integrated into an orbital demonstrator. Finally, the
" Conclusion " section reports the concluding remarks.
SATELLITE VISIBILITY BY MULTIPLE GS
CONSIDERED GS
The purpose of this study is to explore the feasibility of flying
a nanosatellite as a calibrator for a series ofground-based
telescopes, selecting an optimal combination of orbital
6
parameters and assessing the attitude control strategies
needed to maximize the signal transmission during orbital
evolution. Several telescopes that could benefit fromthis calibration
method have been considered and the provisional
list used in the study is reported in Table 1 along with the
associated geographical coordinates. Figure 1 shows one of
the simulated orbit ground track and telescopes and GS on
the Earth map.
Since the targeted ground telescopes are scattered
around the globe, a polar or near-polar inclination (not
necessarily sun-synchronous) orbit is considered to allow
adequate coverage of the polar regions. In addition, the
orbit shall have some other distinct features:
Guarantee CubeSat-to-telescope visibility with a
minimum time duration of 20 s and maximum
possible frequency of revisit for multipassage
calibration.
Tuned orbital altitude to avoid the high rate of
orbital decay due to atmospheric drag and in parallel
keep an adequate SNR at the receiver.
Both criteria depend on the satellite altitude, which
becomes, therefore, a key parameter that must be discussed
before conducting the study. LEO orbits are highly sensitive to
atmospheric drag, which constantly reduces the satellite's
orbital energy and, therefore, its height and life, so increasing
the orbital semimajor axis would extend the mission duration,
butwould have a negative impact on theRTandwould increase
the electrical power required by the satellite's communication
subsystem. E.g., considering themean atmospheric density conditions,
a drag coefficient of 2.2, a 6U satellite having a 7 kg
mass and a 0.030 m2 drag area, and circular orbits with initial
altitude included between 200 and 550 km, the expected lifetime
is found to vary between few days and 10 years.
SATELLITE POINTING PASSAGE
Considering the mission objective, the microwave active
calibrator (MAC) payload mounted on the CubeSat and
used to calibrate the TUT, must always point toward the
Earth, but a nadir pointing configuration may not
IEEE A&E SYSTEMS MAGAZINE
AUGUST 2022

IEEE - Aerospace and Electronic Systems - August 2022

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