IEEE - Aerospace and Electronic Systems - July 2022 - 25

Collins et al.
Table 2.
Comparison of Gimbal Systems
MRO HGA
gimbal
assembly
Coverage AZ -105 to 42
EL -180 to 160
Mass
Tracking
(slew) rate
Resolution
Disturbance
45 kg[19]
0.534/s
0.119[20]
-
MOOG
EPGA[17]
350
180
6.1 kg
0.6/s
0.002
-
Sierra Nevada
SNC EH-25[18]
350
150
5.8 kg
0.08/s
0.0025
-
Requirements
82.4
-14.5 to 345.5
-
Min -0.392/s, max 0.398/s
See the " Pointing Accuracy " section
No amplification of spacecraft jitter
over the range 0.01-500 Hz
in angles of [-82.4, +82.4] in elevation and [-14.5,
+345.5] in azimuth, which serves as a fundamental
requirement to be levied on the pointing system.
SLEWRATE
During the portion of the orbit when the spacecraft is
occluded by the planet, the pointing system needs to reset
the alignment of the telecommunications system to the
opposite extents of the FOR to prepare for the next link as
the craft approaches the limb. In order to complete this
maneuver within the time of the occultation window, the
telecommunications system needs to slew at a rate of
0.534/s. This value sets the second primary requirement
to be levied on the pointing system.
POINTINGACCURACY
The guidance, navigation, and control capability on a deep
spacecraft is driven by the needs ofthe primary science instrument(s).
In the MRO case, the HiRISE imager imposes a
pointing accuracy of 100 mradians ( 0:0057)orbetter,
which establishes a baseline requirement for the pointing system
to independently actuate the telecommunications system
[16]. A pointing mechanism ofsufficient precision could
be used here to point the imager independently of the spacecraft.
It is important to optimize the performance ofdata links
between spacecraft and Earth by precisely pointing the high
gain aperture (RF or optical) to minimize boresight losses,
which will render a maximal potential data rate. Equally
important is to try to maximize the throughput during the mission
by allowing independent alignment between the communications
and science instrumentation. Decoupling these two
spacecraft functions will increase the duty cycle spent performing
each.
The pointing resolution requirement for the Canfield
joint is fluid, because the task of pointing the payload
JULY 2022
relative to the spacecraft is shared among a set of subsystems
comprising concentric control loops, including
spacecraft attitude control, vibration suppression, and
fast-steering mirrors (FSM). The ultimate requirement
of this pointing budget is a resolution of4.5 m rad,
in order to ensure the incident power at the ground terminal
is within 3 dB of the peak irradiance; i.e., 50%
coverage of a ground station by an optical beam subtending
an angle of9 m rad.
DEVELOPMENT OF THE CANFIELD JOINT
The pointing requirements listed in the " Concept of Operations "
section are not met by several COTS gimbals, as
summarized in Table 2. Crucially, none can articulate a
mass greater than its own, which limits the possibilities
for payloads to be pointed independently of the spacecraft
bus. However, these specifications may be realized
through a carpal-wrist joint architecture, adapted from the
" Canfield Joint " mechanism [21], which allows for continFigure
5.
Illustration of the gimbal apparatus used on the MRO, using
Moog components.
IEEE A&E SYSTEMS MAGAZINE
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