NASA Logo, National Aeronautics and Space Administration

Remote Agent

                During its mission, DS1 performs
                <a href="">Autonomous Optical
                Navigation (OpNav)</a> to determine its position in space.
                One task in this activity involves taking pictures of
                several asteroids using DS1's Miniature Integrated Camera and Spectrometer (MICAS). PS generates the plan for this
                task, which is much more complex than it may at first
                seem. Let's see how PS does it.
              <h2>Obtaining the Mission Goals</h2>
                In order to begin, the PS obtains the main goals for a
                particular part of the mission. These goals can be
                programmed in before lift-off or can be uploaded
                during the mission by ground controllers.   For the DS1
                mission, these goals are divided into four separate
                categories which are each put on a separate timeline. We
                need to be concerned about two of these for our example:
                waypoints and OpNav. The waypoints timeline tells PS
                the start time and end time for a plan. This period of
                time is called a "schedule horizon." Most
                schedule horizons are about one week long. The OpNav
                timeline tells the PS when OpNav activities should take
                place. During DS1 OpNav will take place once every week.
                The diagram below is a picture of what the waypoints and
                OpNav timelines might look like.
              <img src="/m/project/remote-agent/images/new2.png" alt="Way Point Diagram" height="119" width="490">
                The waypoints timeline tells us that this plan will
                span 66 hours, beginning at 6 hours and ending at 72
                hours. The OpNav timline tells us there will be one OpNav
                activity during this 66 hour period. The dotted line
                represents that the OpNav activity could take place at
                any point along the timeline. Many goals have large
                windows of time like this in which the goal could be
                accomplished, rather than a specific time at which it
                must happen. 
              <h2>Planning Experts</h2>
                When you create your plan to go to the store, there are
                certain parts of your plan that you may leave up to other
                "experts." For example, if there is no price tag on
                the milk, you may ask the cashier how much it costs so
                that you will know how much money to take out.
                PS also relies on experts to figure out some details
                of the plan called "planning experts." In our
                example, PS would gather information from the Autonomous
                Navigation software program (AutoNav) and the Attitude
                Control System (ACS). AutoNav would provide information
                on where the asteroids are that will be photographed and
                when the photographs should take place. The ACS will tell
                PS how much and in what direction to turn in order to
                face MICAS toward the target asteroids. As you can see,
                many parts of DS1 are constantly communicating with one
                another. Each one has its own area of expertise.
               It is now the job of PS to break down the goals into
              smaller tasks, or tokens,that must take place in order for the
              goals to be accomplished. This process is called
              subgoaling. For example, the OpNav activity goal can be
              subgoaled into a series of turns and photographs. Turns
              and photographs can then be further subgoaled. The diagram below shows the subgoaling of
              MICAS taking a picture.  
              <img src="/m/project/remote-agent/images/diagram3.gif" alt="Sub-Goaling Diagram" height="180" width="441">
                To generate a plan, PS must also take
                into account rules, called constraints, which control
                when the tokens can or cannot take place with regards to
                other tokens. The tokens are laid out on a series of
                separate timelines which follow the contraints. 
                The diagram below represents the last 20 hours of a
                possible plan that PS would generate for the schedule
                horizon shown in the waypoints timeline above to
                accomplish the OpNav activity.   The attitude
                timeline describes when the spacecraft should turn. The
                Solar-Electric Propulsion timeline describes when the
                spacecraft should thrust. The planning timeline includes
                directions for PS to begin generating the next plan so it
                will be ready as soon as this plan has been executed.
                Check the constraints below to make sure they are
                being followed in the plan.
              <img src="/m/project/remote-agent/images/diagram4.png" alt="Last 20 Hours of the Planning Horizon" height="409" width="490">
                  The Solar-Electric Propulsion system and MICAS
                  cannot be on at the same time due to power
                  availability and the possibility of blurring the
                  The Attitude Control System must not be turning
                  the spacecraft while MICAS is taking a picture,
                  or it will blur the picture.
                  The Attitude Control System must not be turning
                  the spacecraft while the Solar-Electric
                  Propulsion system is thrusting or the trajectory
                  will be altered.
                  The Attitude Control System must turn the
                  spacecraft toward the asteroid before MICAS takes
                  a picture.
                  Only 2600 watts of power is available at any
                  given time. This limits how many devices on the
                  spacecraft can work at once. The Solar Electric
                  Propulsion system uses 2300 watts of power when
                  it is thrusting.
                  A maximum of 237,000 seconds of Solar Electric Propulsion
                  thrusting can take place. (That is the total length of the
                  schedule horizon, so thrusting can take place as
                  often as possible.)
                Once the plan is ready, PS sends it to Smart
                Executive for execution.
First Gov logo
NASA Logo -