Various operational aspects of the Earth Radiation Budget Satellite (ERBS) Tracking and Data Relay Satellite System (TDRSS) are described. The ERBS-TDRSS Contract Planning System is an expert system which has been used operationally since June 1987 by the ERBS Flight Operations Team (FOT) at Goddard Space Flight Center to build weekly schedules of requests for service from the TDRSS. The basic operation of the system and significant enhancements and changes are discussed

Beyer, David S.

Littlefield, Ronald G.

Mclean, David R.

English

NASA Technical Reports Server

NS8-16415
Operational Aspects
of a
Spacecraft Planning/Scheduling Expert System
David R. McLean, Ronald G. Llttlefteld, and David S. Bayer
Bendix Field Engineering Corporation
10210 Greenbelt Road
Lanham-Seabrook, MD 20706
ABSTRACT
This paper describes various operatlona] aspects of the
ERBS-TDRSS Contact Planning System. The ERBS-TDRSS Contact
Planning System is an expert system which has been used
operationally since June 1987 by the Earth Radiation Budget
Satellite (ERBS) Flight Operations Team (FOT) at Goddard Space
Flight Center to build weekly schedules of requests for service
from the Tracking and Data Relay Satellite System (TDRSS). The
ERBS-TDRSS Contact Planning System, which ts written enttre]y
tn the C language and runs on an IBM PC-AT, reads ERBS orbit
prediction data from a 9-track tape and builds a 1-week
schedule of requests for TDRSS service in three minutes, with
additional time required to prtnt the schedule that has been
built. By comparison, the ERBS FOT scheduling expert
previously had to spend over six hours per week to do this same
task. Several enhancements have been made to the system since
tt has become operational. In addition, modifications have
been made to the knowledge bases of the system to accomodate
changing operational scenarios and mission constraints imposed
by the TDRSS Network Control Center (NCC). Contributing to the
operational success of thts planning/scheduling expert system
are: (1) the resource window generator which limits search
through the orbtt data, (2) the custom-built strategies
interpreter which applies various strategies to try to schedule
primary and alternative events, (3) the custom-built inference
engine (TIE1) which automatically checks each event proposed by
the strategies interpreter against mission-specific scheduling
constraints, (4) the custom-built user interface to the system,
and (5) the report generator which produces TDRSS schedule
requests In the format required by the NCC.
INTRODUCTION
Thts paper begins by describing the basic operation of the
ERBS-TDRSS Contact Planning System (ERBS-TDRSS CPS). Next,
significant enhancements to the ERBS-TDRSS CPS and changes in
Its operational characteristics are discussed. Finally, some
conclusions based on several months of operational experience
are presented.
303
https://ntrs.nasa.gov/search.jsp?R=19880007033 2020-03-20T07:27:42+00:00Z
A schematic diagram of the ERBS-TDRSS CPS is shown in Figure 1.
GROUND TRACK
ORBIT PREDICTION
TAPE FROM FDF
I
V
I 9-TRACKI
l TAPE I
J_
I
• V
I RESOURCE I
I WINDOW I<--->
CONFIGURATION
FILE < ->
INPUT--
SCHEDULE FILE
I >
V
STRATEGIES KB
A
1 -->
DIRECT USER
INTERACTIONS
IEPS
USER
INTERFACE
STRATEGIES I CONSTRAINT
[ CHECKER
INTERPRETERI (TIE1)
I
OUTPUT
.... > SCHEDULE
FILE
I
V
I REPORT I
I GENERATORI
I
V
SARs
AND OTHER
REPORTS
--->LOG FILE
CONSTRAINT
<..... > KB
Figure i: Schematic Diagram of the ERBS-TDRSS Contact
Planning System.
In its operation, the ERBS-TDRSS CPS first directs a tape
drive to read selected orbital information from a 9-track
Ground Track Orbit Prediction data tape obtained from the GSFC
Flight Dynamics Facility. Orbital information from the tape is
written to the PC that runs the ERBS-TDRSS CPS. This data
includes such items as antenna viewing angles and start and
stop dates and times for each orbit and daylight period.
Next, resource window generator routines on the PC apply
scheduling heuristics to the orbital data to identify resource
windows. This process improves overall system efficiency by
reducing the amount of data which the strategies interpreter
must search through to find TDRSS contact periods.
As the ERBS-TDRSS CPS is run In its automatic scheduling
mode, resource window information is used along with system
configuration information from a configuration file, scheduling
strategies from the strategies KB, and scheduling constraints
from the constraint KB to build a 1-week schedule of TDRSS
requests. As the strategies interpreter applies primary and
alternative strategies to find valid candidates for TDRSS
304
contacts, tentative event Information Is passed to the
constraint checker, the Transportable Inference Engine (TIE1).
TIE1 checks this event information against mission-specific
scheduling constraints In the constraint
whether the candidate event should be sch
prtmary event strategy is rejected by the co
the strategies Interpreter automatically t
schedullng strategies and passes updated event
to the constraint checker. This process is
event until it is scheduled or the str
exhausted.
KB to determine
eduled. If the
nstraint checker,
ries alternative
information back
repeated for each
ategies list is
After a i-week schedule has been produced in the automatic
scheduling mode, the ERBS-TDRSS CPS can be used in the
interactive scheduling mode. In this mode, the Interactive
Experimenter Planning System (IEPS) user interface provides
graphical displays of event timelines and user-friendly editing
capabilities.
During interactive scheduling mode operations, the
constraint checker is automatically invoked to help the ERBS
FOT planner add or delete individual events in accordance with
the scheduling constraints in the constraint KB. When there is
a scheduling conflict, TIE1 produces a diagnostic message to
explain why the event cannot be scheduled. By entering a
password, the ERBS FOT planner can override the constraint
checker and force the system to schedule the event.
Finally, report generation routines in the ERBS-TDRSS CPS
are used to produce reports including: selected data tape
records, resource window listings, TDRSS contact request
worksheets, and specially formatted Schedule Add Requests
(SARs). The SARs that are produced are sent (via a Mission
Planning Terminal) to the NCC.
Three papers [1], [2], and [3] that provide additional
information about the ERBS-TDRSS Contact Planning System are
listed in the reference section.
OPERATIONAL EXPERIENCES
When the ERBS-TDRSS CPS began operating, Ground Track Orbit
Prediction data from a 9-track tape was read into an IBM
mainframe in the Command Management Facility and then
transferred (via a modem) to the PC in the ERBS FOT area. It
took about 20 to 30 minutes to transmit the data required for a
1-week schedule. To enhance the ERBS-TDRSS CPS, a tape drive
with flexible software was added. This tape drive allowed
orbital data for one week to be read from a 9-track tape in
less than a minute. In addition, selected orbital data tape
records and resource window listings could be quickly verified
by the ERBS FOT planner.
305
After the ERBS-TDRSS CPS operations began, it became
evident that the ERBS FOT planner was not comfortable with the
multitude of DOS operating system commands required to run the
ERBS-TDRSS CPS. To make the control of the ERBS-TDRSS CPS more
user friendly, a commercial product was used for adding a high
level menu-driven user interface to the system. This interface
includes options for: loading the data tape, selecting the
scheduling mode (automatic or interactive), entering start
dates, selecting antenna service, running the scheduling
process, and printing reports. In addition, since the
orientation of the spacecraft relative to its direction of
travel affects its ability to view TDRSS (and the spacecraft is
occasionally "flipped" end-for-end), a menu prompts the ERBS
FOT planner at the start of each scheduling session to specify
the orientation of the spacecraft and when the spacecraft is
scheduled to be flipped.
In response to changing scheduling requirements imposed on
the ERBS Flight Operations Team, significant alterations were
made to the knowledge bases of the ERBS-TDRSS CPS. For
example, several new types of Tape Recorder Dump (TRU Dump) and
Stratospheric Aerosol and Gas Experiment (SAGE) events (along
with their scheduling strategies and constraints) were added to
the knowledge bases. Also, the durations of some tape recorder
dump events were changed and separate knowledge bases for
scheduling SAGE events during periods of "full-sun" were
added. These knowledge base changes provided new scheduling
capabilities for the ERBS-TDRSS CPS.
CONCLUSIONS
Although the ERBS-TDRSS CPS has only been operational for
several months, significant changes have been made to the
system. Some of these changes, such as the addition of the
tape drive and the menu system, were made to increase the
performance of the system and to make the system more user
friendly.
Other changes were made to support new scheduling
requirements imposed on the ERBS FOT and resulted in new and
different operating capabilities for the system. It is
expected that maintenance expertise to support changing
scheduling requirements will be required for the life of the
mission. For example, when TDRS-West becomes operational, the
ERBS-TDRSS CPS will probably have to be modified.
An important aspect of maintaining the ERBS-TDRSS CPS has
been the ability to add new scheduling capabilities by editing
its knowledge bases. For example, rules and frames in the
strategies and constraint knowledge bases were quickly and
easily modified to provide capabilities for scheduling several
new types of events, each with its own antenna configurations,
306
Modifications such as these would likely have been much more
difficult and time consuming to make in a system with a more
conventional software architecture,
Another important factor in maintaining the ERBS-TDRSS CPS was
having access to the source code. For example, changes to the
source code were required in order to enhance the resource
window generator. Most commercial expert system shells do not
include access to the source code.
Finally, having a portable, C-based system that runs on
conventional hardware, such as IBM PC-ATs under DOS or MC/68020
workstations under UNIX, has been very helpful. Maintenance
and development work has been done mainly on the 68020-based
workstation utilizing its powerful development environment.
The updated source code was then ported to the ERBS-TDRSS CPS
PC and recompiled.
ACKNOWLEDGEMENTS
The authors would like to thank Patricia Lightfoot, William
Macoughtry, and Carolyn Dent of The Spacecraft Control Programs
Branch at NASA-GSFC and Ellen Stolarik of Bendix Field
Engineering Corporation for their many contributions to the
Interactive Experimenter Planning System task. In addition,
the authors would like to thank Doris Tallman of the ERBS
Flight Operations Team at NASA-GSFC for her expert help. This
work was supported by NASA contract NAS5-27772.
REFERENCES
[1] McLean, David R., "The Design and Application of a
Transportable Inference Engine (TIE1)," PROCEEDINGS OF THE
1987 GODDARD CONFERENCE ON SPACE APPLICATIONS OF ARTIFICIAL
INTELLIGENCE (AI) AND ROBOTICS, Goddard Space Flight
Center, Greenbelt, MD., May 15t 1986.
[2] McLean, David R., Llttlefield, Ronald G. and Beyer, David
S., "An Expert System for Scheduling Requests for
Communications Links between TDRSS and ERBS," PROCEEDINGS
OF THE 1987 GODDARD CONFERENCE ON SPACE APPLICATIONS OF
ARTIFICIAL INTELLIGENCE (AI) AND ROBOTICS, Goddard Space
Flight Center, Greenbelt, MD, May 13-14, 1987.
[33 McLean, David R., Littlefleld, Ronald G., and Macoughtry,
William 0., "Defining and Representing Events in a
Satellite Scheduling System: the IEPS (Interactive
Experimenter Planning System) Approach," 1987 INTERNATIONAL
TELEMETERING CONFERENCE, San Diago, California, October
26-29, 1987.
307


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