IEEE - Aerospace and Electronic Systems - August 2022 - 9

Avram et al.
and disposition of the solar panels onboard the satellite.
The S/C pointing sequence, the input attitude quaternion
relative to the ECI frame fqECIg and the input
body frame's angular velocity fvg¼ fvx; vy; vzg are
calculated as follows:
i) As a first step, the site access sequence is scheduled.
Figure 3.
Topocentric Horizon Coordinate system (ENZ frame) on the surface
of the oblate Earth in cyan.
telescopes latitude f and longitude u, frS/Cg and
frGSg as follows:
vector f^rg¼ frg
2
6
6
S/C azimuth a and elevation E are related to the unit
jjfrgjj of the line-of-sight direction in the
ENZ frame by the following equation:
sin u
frg¼ 4
cos u
cos f sin u
f^rg¼
<
:
8
cosEsin a
cosEcos a
sinE
9
=
;
:
(4)
By using (4), the elevation E is given by E ¼ arcsin ^rz
and the azimuth is given by a ¼ arctan ^rx
r^y. The satellite is
considered visible by the TUT if E Emin. If the elevation
of the satellite seen from the ground telescope is
below the minimum value, no visibility is deduced, otherwise,
the beginning and end time of contact is registered
by the algorithm.
EVALUATION OF THE REQUIRED ATTITUDE CONTROL
STRATEGY
ATTITUDE CONTROL STRATEGY CALCULATION
To facilitate communication with the S/C and optimize
the control strategy, the satellite is in the nadir pointing
configuration for the longest part of its operative
life, except when it enters the access area where it
points toward the TUT. Moreover, this strategy, in
combination with the selection of sun-synchronous
orbits, simplifies the electrical power subsystem design
AUGUST 2022
sin f cos usin f sin u cos f
cos f cos u
sin f
frS=CgfrGSg :
3
7
7
Elevation profiles EiðtÞ are calculated for each the ith
TUT by means of (4), if E Emin the S/C is considered
in the access zone. Due to the location of the ground
telescopes and to the minimum elevation considered, the
satellite may be visible from two to three sites simultaneously,
and considering the presence of only one antenna
on board, it is necessary to choose one ground telescope
among the others, that will be tracked by the satellite during
the passage. Figure 4 shows the case of the simultaneous
visibility of the satellite by three sites close to the
South Pole.
When more than one telescope is in sight, afirst-comefirst-served
basis rule is applied, thus the S/C will point to
the first station seen. The pointing sequence is stored in a
database containing for each the ith site, the jth passage
number, the passage starting time tij;start, and the passage
end time tij;end. Based on this rule, Figure 5 shows
an example of the site access schedule.
ii) When the S/C reaches the access area, it starts to
5
(3)
8
f^zS/Cg¼
<
:
frGSi gfrS=Cg
jjfrGSi gfrS=Cgjj
frS=Cg
jjfrS=Cgjj
t 2½tij;start;tij;end
t 2tij;end;tiþ1j;start½
(5)
point its z-axis toward the ith ground telescope. The primary
alignment vector f^zS/Cg of the S/C at a time instant
t, expressed in the ECI coordinate system, is given by
Figure 4.
Representation of the simultaneous visibility of the satellite by
three sites close to the South Pole.
IEEE A&E SYSTEMS MAGAZINE
9
IEEE - Aerospace and Electronic Systems - August 2022

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