APL's robust systems engineering philosophy guides the development of systems. A systems engineering approach defines system needs and functionalities early in the development cycle, documents those requirements, and then monitors the requirements throughout the design, build, and system validation. Trades studies, system performance, and system optimization are considered at the beginning of each mission and weighted against the operational requirements. A life-cycle view ensures that requirements affecting other aspects of development, production, and operation are considered and evaluated before implementation. APL's systems engineering approach requires interdisciplinary skills as well as teamwork.
Spacecraft Autonomy Work and Research at APL
Autonomy is the ability of a spacecraft to act independently from ground control. APL has developed and steadily, over three generations, increased the capability of a flexible and expressive autonomy system spanning seven spacecraft programs. Today, APL is embarking on the development of a new set of autonomy systems that will meet the critical challenges of our national security space and NASA customers into the future.
New Horizons Autonomy System
The New Horizons mission to Pluto and to the Kuiper Belt offers several unique autonomy challenges based on the great distance this spacecraft has to travel.
Autonomous First Aid
Even though New Horizons left Earth in 2006 at the fastest launch speed ever recorded for a man-made object, it took 12 years to reach Pluto. During cruise, the spacecraft spends up to a year without ground commands, so any problems it encounters during the mission must be fixed autonomously.
As the New Horizons spacecraft makes its final approach to Pluto, signals traveling from the spacecraft take 4 hours to reach Earth, which means that ground operators who might need to respond to anything happening onboard during the final approach would be unable to reach the spacecraft in time. So New Horizons has to operate autonomously for the final approach to Pluto. The pressure is on. After a 12-year voyage, the spacecraft could not make any mistakes, or it could miss a 1 in 230 year opportunity. Luckily, APL autonomy is on the job ensuring that New Horizons keeps going.
MESSENGER Autonomy System
The MESSENGER autonomy system was the first in the third generation of APL autonomy systems, and it is followed by other third-generation systems in the NASA STEREO and New Horizons missions. The MESSENGER autonomy system offered amounts of flexibility and expressiveness, allowing large amounts of telemetry to be manipulated and combined in order to identify and detect multiple onboard faults. The harsh environment around Mercury required a robust autonomy system. The MESSENGER spacecraft was tucked behind a heat shield to stay cool, but mere minutes outside the shield overheats the spacecraft. The MESSENGER autonomy system's job is to make sure that didn't happen...ever.
Spacecraft Autonomy Visualization
Spacecraft are intensely complicated devices producing huge amounts of data. At approximately 5,000 unique data values per second (18 million per hour), the ability to quickly review the data in order to quickly understand the current state of the system is daunting. This is important for autonomy systems during testing in order to ensure that autonomy judgments and responses are correct. This is also important during flight, when operators have to understand what the spacecraft has done autonomously in between scheduled contact times. Engineers at APL have developed Spacecraft Autonomy Visualization (SAV), a visualization technique that combines a matrix-like data display with TiVo-like time controls. The result is a single view of all 5,000 data points with the ability to examine onboard autonomy response over time and across the entire data set. SAV is used on the NASA STEREO mission and was used on the NASA MESSENGER mission.
ExecSpec, short for Executable Specification, is the fourth generation in APL autonomy systems. It defines autonomous operation through diagrams. In ExecSpec, a user, without any programming capability, can draw diagrams describing how he or she wants the spacecraft to act. The diagrams are then loaded directly into the spacecraft, which interprets the diagrams in order to control the spacecraft. This capability involves more engineers in the design process and makes the system easier to review and understand. ExecSpec will make its first appearance on the Solar Probe Plus.
Onboard autonomy systems usually function in a reactive (condition leads to response) or time-based (at this time do this) mode. Hierarchical Activity Planning (HAP) is a development and execution system for time-based sequences. Similar to ExecSpec, HAP focuses on diagrams to explain how the system should behave. However, the main feature of HAP is the ability to change sequences on the fly without endangering the spacecraft or current ongoing sequence. National security space customers may desire the ability to add observations to an on-orbit spacecraft and accomplish that observation in minutes to hours. The current ability to add observations takes days to weeks and may result in a specific target eluding detection.