Did you know that NASA's Mars Pathfinder spacecraft mission was planned before and during operation by a software system developed using Allegro CL? The Planner system helped Pathfinder scientists account for new data as it was transmitted from Mars by the Pathfinder spacecraft, and evolve their mission plans in real-time to optimize spacecraft operations.
"This was incredibly important with such a complex, pioneering mission," says David Mittman, a Flight Engineer on the Mars Pathfinder team and key developer for the planning system adaptation used on this mission.
"We were actually able to respond to new information about the status of the spacecraft during operation, and then redesign our operational strategies right away." This real-time capability ended up playing an important role in the overall success of this mission, enabling the Pathfinder spacecraft to operate well beyond their expected lifetimes and sending back more data than ever thought possible.
The Pathfinder mission, managed by the Jet Propulsion Laboratory (Pasadena, CA) for NASA's Office of Space Science, was "primarily a technology demonstration," according to David Mittman.
"Can we land a spacecraft cost-effectively on Mars? That's essentially what this mission was intended to demonstrate." Mittman explains.
In particular, the Pathfinder mission involved landing a solar-powered lander spacecraft (called the Sagan Memorial Station) on the planet, and then deploying a smaller, free-ranging rover spacecraft (called the Sojourner) which toured the planet's surface. Together, these two spacecraft analyzed soil samples, took photographs of the Martian landscape, and gathered weather data, and reported this information back to Earth via a high-gain antenna.
The Pathfinder mission was initially planned, and then re-planned during operation, by a planning system called MPF Plan-IT-II. This system was adapted from Plan-IT-II for use in this mission using Allegro CL.
David Mittman, chief developer of the planning system adaptation for the Pathfinder mission, explains the system's core functionality: "The Planner essentially performs activity and resource modeling for things like solar panel power generation, data storage capacities, environmental conditions and other resources. It places all planned activities on a time-line, and shows us the conditions of the spacecraft at any point in time. For example, it can show us how much power is being used and how much data has been collected."
Mittman continues: "With the Planner, we are able to model highly complex operational scenarios before the launch and plan for various contingencies. For example, what if the temperature ends up being much higher than what we expect? What if there are dust storms? And so on."
Not only was the system used pre-launch to plan out the activities of the spacecraft, but it was also used during mission operations, on each and every day of the Pathfinder mission. "Every day, we would plug in the activities planned for that day. Then, we would take the information we'd gathered on the previous day, input this data into the Planner, and see how the activities were affected. We'd make any necessary modifications, then spit out the new plan," Mittman explains.
Sometimes this capability to change plans in real-time resulted in very significant optimizations that ultimately played an important role in the Pathfinder mission's success.
"Using the Planner," says Mittman, "we discovered early on in the mission design that battery life was going to be important for us. During the daytime, we had to be careful to leave enough excess solar energy unused so that the spacecraft batteries could be recharged. The batteries were used mainly to keep our computer operating overnight."
During the course of the mission, however, new data coming into the Planner enabled the scientists to redesign their energy-usage strategy. "We discovered that we could operate the spacecraft's transmitter for more hours during the day because it wasn't getting as hot as we had predicted. We were getting such great data transmissions from them. We were also expecting 1185 bits per second and we were actually getting closer to 8300 bits per second!"
The unexpected boost in data rate was caused by the successful implementation of a plan to aim the high gain antenna of the spacecraft very precisely (within 1 degree) towards Earth. "It was because we got such a great direct signal that we were able to boost the data rates, and then redesign operations strategy as a result," explains Mittman.
The significance of this redesign was that it enabled the battery life of the spacecraft to be extended far beyond what had initially been planned. The mission was expected to have lasted for 30 days, based on the projected energy requirements of the lander. In actuality, the Pathfinder spacecraft were still sending data back to Earth after 83 days on Mars!
"The mission landed successfully on July 4, and we received our last meaningful data from the spacecraft on September 27," says Mittman. "During this amazing 83-day period, we were able to accomplish more science than we ever thought possible."
The Allegro CL system used to plan the activities of the spacecraft was an adaptation of the Plan-IT-II planning and scheduling program used by NASA for other spacecraft missions.
"The Planner is what we call a 'multi-mission tool'," says David Mittman. "It can be adapted to different spacecraft projects." The adaptation of the system to different projects is facilitated by the Plan-IT-II Adaptation Language, a domain-specific extension to Common Lisp which was built on top of Allegro CL. This language, designed for resource-modeling, makes it easier to efficiently customize the base Plan-IT-II system to particular spacecraft.
In addition, adds Mittman, "because it was Common Lisp we were working with, we were able to easily go beneath the surface of the adaptation language, right to the core of the system, and make changes there as well if this was necessary."
"One of the important things you need to realize about this system," says Mittman, "is that it uses data from many other different systems. These other systems were written in C, Ada, Tcl, Perl, and then there are a couple of commercial packaged systems. The only thing that ties these systems together is that they're all Unix-based."
The Allegro CL system was able to integrate with all of these systems. "Because of the O/S hooks in Allegro CL, it is an open system. We could use the data generated in all of our existing systems as inputs into the Planner. This was very valuable for us."
Other features of Common Lisp important to this particular system included the macro expansion capability used in the development of the Adaptation Language, the incremental compilation and runtime patching features ("a definite win in development as well as in operation" according to Mittman), and dynamic typing, which gives Allegro CL the ability to support very rapid, interactive development.
"This was definitely important for us," comments Mittman. "I didn't have to worry about the changes we were making to the system on an ongoing basis. I knew they weren't likely to mess up the system or cause it to break. And sure, that was important. We were operating in a 'flight environment' here! To bring a complex, evolving piece of code like this into a flight environment is a feat in and of itself!"
"We were using what's known as 'concurrent engineering'," he adds. "We were actually building the Planner at the same time as we were designing the spacecraft and figuring out how it works. We were using the Planner and developing it at the same time! We couldn't have done this if we were not working with Common Lisp."
The Pathfinder Mars mission was a tremendous success in many ways. In all, Pathfinder returned 1.2 gigabits (1.2 billion bits) of data and 9,669 pictures of the Martian landscape. "The data returned by the Sagan Memorial Station and Sojourner has been nothing short of spectacular, and it will help provide a scientific basis for future Mars missions, including a sample return, for years to come," says Dr. Wesley Huntress, NASA associate administrator for space science.
In addition, the mission was also successful in demonstrating that sending useful spacecraft to Mars can be done cost-effectively. "This mission demonstrated a reliable and low-cost system for placing science payloads on the surface of Mars," says Brian Muirhead, Mars Pathfinder project manager at NASA's Jet Propulsion Laboratory.
Landers and rovers of the future will share the heritage of spacecraft design and technologies tested in this "pathfinding" mission, planned and monitored using an Allegro CL-based software system.
For more information on the Pathfinder Mars mission (and to see more cool photographs of the Martian landscape!) please visit http://www.jpl.nasa.gov.
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