Planning and scheduling is a complicated part of spacecraft control. Planning, for our purposes, can be thought of as determining all the small tasks that must be carried out in order to accomplish a goal. Let's say your goal is to buy a gallon of milk. It may sound like a simple task, but if you break it down, there are many small tasks involved: obtain keys, obtain wallet, start car, drive to store, find and obtain milk, purchase milk, etc. Planning also takes into account rules, called constraints, which control when certain tasks can or cannot happen. Two of the many constraints in this example are, you must obtain your keys and wallet before driving to the store and you must obtain the milk before purchasing it.
Here is what a simple plan for buying milk at the store might look like:
Scheduling can be thought of as determining whether adequate resources are available to carry out the plan. Two resources that scheduling would have to take into account for our example above are fuel and time. If it takes two gallons of gas to get to the store and back and your car only has one gallon, you must develop a plan which includes a stop at the gas station. If it takes 15 minutes to drive to the store, the store closes at 10:00, and it is currently 9:30, you must also take that time constraint into account when scheduling your tasks.
On an autonomously controlled spacecraft like Deep Space One (DS1), creating plans and schedules is the work of the Planner/Scheduler (PS) component of Remote Agent. Mission Control engineers program Remote Agent with the general goals of the mission, and PS designs the plans to achieve those goals. A goal-based, on-board planner has many benefits. PS can create plans that are more flexible and better able to take advantage of unexpected opportunities than plans created by ground controllers. Leaving planning and scheduling to Remote Agent's PS also helps decrease costs; planning and scheduling normally is performed through communication with the Deep Space Network (DSN) on the ground, and this communication time is very costly. Less expensive missions mean more missions and more scientific knowledge gained.
PS creates detailed plans of what must happen during a given time period, called a "schedule horizon." The time periods are usually about one week long. The following is a summary of the work of PS: