IEEE - Aerospace and Electronic Systems - August 2021 - 39

Cividanes et al.
Table 2.
Onboard HML Library Into Flight Software
HML's Item
Onboard HML library into flight software
Element An Element has functions (actions) and static
attributes (resources and timelines)
Onboard HTN Planning System
An Element does not act directly with
the planning algorithm, but it is a
crucial component of the model for
interfacing with other HML
components
Action A primitive action is a static function with
preconditions, effects, and mapping to the ECSS
PUS command
Resource A Resource is a static attribute that belongs to an
Element. Each Resource provides access to
calculate consumption, and mechanisms for
reading and setting resources
Timelines
(state
variables)
A Timeline is a static attribute belonging to an
Element. Each Timeline has functions for reading
and setting state variables, as well as the
interface with the OBSW by real-time telemetries
Domain Value constraints on state variables
It is part of one or more HTN and used
as a way to find a valid plan. A
controller can also transform primitive
actions into ECSS PUS telecommand,
monitoring the plan execution
Check the finite resources of the
satellite, being also used by the state
inference mechanism. It is typically
explored in the deliberative phase
Defines the initial problem with the
current state of the space system and
controls the status of the satellite
throughout the whole planning
process
Using the domain, the planner
indirectly verifies the preconditions
and effects of the planning actions
and methods, while the controller
checks expected effects after the
action is performed by the OBSW's
executor
Event
Global function with preconditions and effects
It is used during the state inference
mechanism by the planner; a
controller could check temporal
endogenous events, to produce a final
plan with defined time points
HTN Goals Translates a high-level task sent by the ground
station or satellite into a hierarchical task network
HTN Task " Task table " that represents the onboard
hierarchical model: task identification, type
(abstract or primitive), methods, subtasks, and
whether it is a state inference point
HTN
Methods
Static functions with preconditions for
decomposing HTN methods related to an
abstract task
resource can have one or more consumers, based on the
producer-consumer paradigm (see Listing 2). By extending
this approach-generally used in temporal planning-
to the hierarchical domain, a new representation of the
resource consumption is created for HTN planners, that
unify planning and scheduling, enabling the process of
state inference.
AUGUST 2021
HTN goal can be customized with
parameters and other goal time
constraints not known during the
domain description
It is used throughout the
decomposition process by the HTN
planning algorithm
Check constraints for reduction into
low-level HTN tasks, or use alternative
methods to achieve task reduction
HML MODEL AS A LIBRARY TO SATELLITE FLIGHT
SOFTWARE
A parser translates the HML model into compiled C code-
as a static library-that are linked to the flight software
together with the real-time operating system. Thereby, the
onboard model becomes executable by the space processor.
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