IEEE - Aerospace and Electronic Systems - August 2022 - 5

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(around 100 depending on configuration). On the other hand,
Dai et al. [17] designed a constellation with a multiobjective
genetic algorithm for a regional terrestrial satellite network.
In their article, they reduce the total number of satellites in
the constellation by considering the ratio of actual coverage
ofa single-orbit constellation and the area oftargets.
Given the orbit definition, another factor to be considered is
the ground target scheduling in the case ofsimultaneous calibration
satellite visibility frommultiple GS; as an example, Li et al.
[18] investigated the mission scheduling problem for an Earth
observation satellite with specific and temporal requirements.
Considering the mission scenario the following main
requirements may be highlighted:
1) an orbit with global coverage is needed to serve
seven telescopes spread around the globe;
2) a relatively short time (20 s) is required to transmit
the calibration signal;
3) the strength of the received signal shall produce a
signal-to-noise ratio (SNR) greater than 30 dB;
4) the pointing accuracy of the polarized calibrator
toward the receiving telescope shall be sufficient so
that the signal attenuation is not greater than 50%
compared to the ideal pointing conditions.
A near-polar LEO orbital configuration can, therefore, be
considered, allowing high bandwidth and low communication
latency. Regarding pointing accuracy, most used nanosatellite
ADCS architectures foreseen a suite ofsensors, such as gyros,
magnetometers, horizon sensors, star trackers, and sun sensors,
to estimate the attitude and rotational velocity of the
spacecraft (S/C), while pointing maneuvers are carried out by
attitude actuators, such as reaction wheels (RWs) and magnetic
torquers. In the last few years, it was possible to reach
CubeSat high pointing accuracy even using commercial offthe-shelfADCS.
As an example, Mason et al. [19] found that
the pointing accuracy ofthe XACT three-axis ADCS [20] was
AUGUST 2022
between 15 and 42 arcsec for an orbit altitude ranging from
190 to 400 km. To obtain higher pointing accuracy, some
authors present a two-step pointing architecture. For the
ASTERIA mission, Pong [21] developed a control architecture
based on an XACT ADCS and a piezo stage to tilt the
imager focal plane, pointing accuracies of0.5 arcseconds over
20 minutes were achieved. Subarcsecond accuracies were
obtained with a similar control strategy by Beierle et al. [22].
However, the mission scenarios considered in [19], [21], and
[22] foresee a pointing manoeuvre with null or minimal slew
rate. In the presented case, to ensure that the strength of the
signal received by the telescope under test (TUT) remains
constant during the passage of the satellite, it is necessary to
provide a certain slew rate during pointing, thus a lower accuracy
is expected. For the identified scenario, ADCS actuators
consist of at least three RWs, since they provide sufficient
authority for high precision maneuvers and three magnetorquers
that will be used to compensate the magnetic disturbances.
Inamori et al. [23], [24] presented a technique to improve
nanosat pointing performance with residual magnetic moment
compensation with magnetic torquers. Cervettini et al. [25]
presented a testbed based on spherical air bearing and Helmholtz
cage dedicated to CubeSatmagnetic attitude control system
(ACS). The work by Johnson et al. [26] reported the
feasibility study of a CubeSat-based system for CMB telescope
calibration, however, the analysis considers only one
possible orbit and no implications on the control torques
required by the ADCS are presented.
In general, the selection ofthe operative orbit for a satellite
is a complex andmultidisciplinary process, driven by different
design variables ranging across all aspects of the
mission and strongly related with the requirements ofsatellite
subsystems and onboard payloads; for nanosatellites requiring
calibrated transmission to the ground and with no or the
very limited possibility of orbital mobility, the design of the
ADCS in charge ofS/C attitude control and payload pointing
becomes crucial for orbit selection.
IEEE A&E SYSTEMS MAGAZINE
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IEEE - Aerospace and Electronic Systems - August 2022

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