IEEE - Aerospace and Electronic Systems - August 2021 - 35

Cividanes et al.
The proposed declarative language (named as
" Hierarchical Modeling Language " -HML) aims to
describe physical and behavioral aspects of the spacecraft
so that it can respond to internal and external events in the
space environment.
The language is based on an extended HTN represenFigure
1.
Different task abstractions passing through the layers of a Hierarchical
Architecture for Real-time mission Planning in intelligent
space Applications (HARPIA). The exemplified tasks are aimed
at an Earth observation mission.
INTEGRATION OF THE ONBOARD MODEL INTO THE
FLIGHT SOFTWARE
Concerning the architectural solution, satellite flight software
typically has a hierarchical approach [27]. Such a
structure maintains direct compatibility with the HTN
domain representation, as well as layered architecture for
autonomous control. Usually, it is structured in three
layers, whose main advantages are simplicity and modularity
[12]. Figure 1 illustrates our proposed example of a
mission planning system integrated with OBSW (Onboard
Software), emphasizing the different actions abstraction
crossing the layers, from high-level goals (e.g., Playback
Image) to lower level instructions (e.g., commands related
to the equipment bus protocol) executed by the space
processor.
This idea preserves the nature of a hierarchical architecture,
in which a flight plan unfolds at a more detailed
level of abstraction as it passes through its lower layers.
Moreover, we advocate using the same HTN planning in
all layers among the components. This approach also
allows AI specialists to focus on the upper layers that
involve planning and scheduling activities, leaving the
details of onboard supervision tasks and hardware interface
to the OBSW team.
HIERARCHICAL DEFINITION LANGUAGE FOR
SATELLITE ONBOARD PLANNING AND ACTING
Automated planners require an accurate description of
planning tasks. Roughly, a planning language is a tool
used to describe the model actions that can be performed
in the controlled system. It provides the environment
state specifications and the intended goals to
be achieved.
AUGUST 2021
tation that allows describing the planning, scheduling, and
execution of tasks with various levels of abstraction for
the operational activities. Unlike similar proposals,
onboard planners and controllers can use a single HML
model to synthesize the sequence of actions that fulfill the
goal. This could avoid the need for an intermediate language
for executing the plan, as proposed by Rajan et al.
[28] and Chien et al. [29]. Due to the HTN-based paradigm,
HML is classified as a configurable domain. This
performs better than classical languages (e.g., PDDL)
according to Nau [22], and usually offers a good tradeoff
between efficiency and generality, which is crucial for
space applications like satellites.
The purpose of our language is to produce intelligible
domain description and keep it as simple as possible. This
is particularly useful for complex systems, such as satellites,
that require multidisciplinary engineering knowledge
(e.g., thermal control, onboard computing, data handling,
attitude, and orbit control, power supply, science payload,
etc.) to correctly model their behavior. The HML domain
is set down by two complementary descriptions: Static
and Behavioral, as shown in Figure 2.
STATIC DESCRIPTION
The Static Description is comprised of two representations:
Hierarchical and Structural, which are described in
XML-based formatting. We use XML to promote data
exchange with the possibility of developing graphical
tools for visualizing and building hierarchical plans by
end users.
The hierarchical description keeps the flight plans and
their operational tasks. Its main purpose is to set down the
highest level tasks unfolded into operational activities. It
encodes the HTN models written by the domain experts
Figure 2.
Onboard domain of extended HTN-based language.
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