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Y eci (me) Autonomous Mission Management for Spacecraft Rendezvous Using an Agent Hierarchy Figure 5. Low-thrust portion of rendezvous. The low-thrust guidance algorithm works by optimizing the attitude profile during long periods of constant thrust to accomplish rendezvous, minimizing time and fuel consumption for the rendezvous. The low-thrust portion of the rendezvous may take several days, each consisting of dozens of orbits. This means that the mission manager must be capable of planning high-thrust burns to complete the rendezvous from a variety of failure conditions, resulting in a range of phase angles, altitude differentials, and lateral errors. If all goes nominally, the low-thrust guidance delivers the satellite to a point approximately 150 km behind and 30 km below the RSO. Figure 6 shows the final 2500 km of the rendezvous. In the figure, the RSO is at the origin and is moving to the left. The solid green trace is the relative trajectory of the chaser in the orbit plane. As the figure shows, in the nominal case, two final high-thrust burns precisely complete the trajectory to the desired offset point. Altitude Difference (km) Figure 6. Chaser downrange and altitude errors from RSO (+Vbar left). 46 x107 1 0.8 0.6 0.4 0.2 0 -0.2 -0.4 -0.6 -0.8 X eci (me) x106 -1 -8 -6 -4 -2 0 2 4 6 8 10 0 -10 -20 -30 -40 -50 -60 -70 -80 Target offset point Nominal Mission (No Failures) Low-thrust cutoff Downrange (km) Green: Chaser Red: RSO RSO Orbit Chaser lowthrust spiral trajectory High-thrust burns Chaser lowthrust trajectory -90 0 500 1000 1500 2000 2500 When failures occur, the high-thrust system has enough impulse to accomplish a rendezvous from approximately 100 to 2500 km behind the RSO. Should the failure occur outside this region, the mission manager waits until the chaser has “lapped” the RSO and arrived back in the region “behind” the RSO prior to scheduling high-thrust burns. In testing an autonomous rendezvous concept, it is important to include navigation errors, particularly in the knowledge of the target vehicle (RSO) state. Filter convergence and target state updates test the mission manager response at several levels. Smaller corrections cause replanning at lower levels in the hierarchy since they tend to affect only the current or upcoming activities. Large corrections can cause a complete replan of a trajectory or burn sequence. Also, target state estimation updates are very similar, from the mission manager point of view, to target maneuvers. The chaser navigation software includes a dual inertial state extended Kalman filter. This means that the filter tracks and propagates the states of both the chaser and RSO. During far field rendezvous, the chaser inertial state is updated by the onboard GPS sensor, while the RSO state is updated by ground uplinks. Once inside communication range, relative GPS provides continuous relative RSO location information, which is used to update the filter’s inertial RSO state estimate. Navigation state updates and sensor acquisitions can cause the mission manager to execute replanning. The above discussion of the CONOPS has prepared us to list a set of objectives explicitly for the rendezvous portion of the PAMM mission. These will be enacted by the agent hierarchy as described in the sections on temporal and functional decomposition below. These mission objectives are simple and clear and may be stated as: 1. Achieve the target offset point at the specified time using low thrust to 150 km and high-thrust burns thereafter. 2. If #1 cannot be achieved (possibly due to low-thrust system failures outside 150 km, target maneuvers, or large target state estimation errors), achieve the target offset point at the specified time using the high-thrust system from the point of failure. 3. If the target offset point cannot be achieved at the specified time, adjust the arrival time to an achievable value and continue the mission. 4. If the target offset point cannot be achieved with onboard resources, abort the rendezvous and enter a circular orbit. 5. If failures or resource limitations prevent any of the above, enter safe mode.
Autonomous Mission Management for Spacecraft Rendezvous Using an Agent Hierarchy PAMM Mission Temporal Decomposition The temporal decomposition process starts with the CONOPS and the mission objectives. Once these are understood, a nominal mission timeline can be developed with the goal of identifying mission activities and their interface states. As a rule of thumb, activity boundaries are set whenever a new subsystem or guidance mode must be invoked to continue the mission. From the CONOPS then, the PAMM temporal decomposition will include a low-thrust activity and a high-thrust activity (Figure 7). During the low-thrust rendezvous, the mission manager invokes the low-thrust agent and assigns it a target state relative to the RSO. The lowthrust agent continuously uses the optimal guidance scheme described above to assess the vehicle’s capability to reach the low-thrust target state given current resources. The agent also monitors the low-thrust propulsion system for failure indications and resource usage. The low-thrust agent can correct for small errors that may accrue in the trajectory due to navigation errors or thrust control errors. If for some reason the low-thrust agent determines that the target state cannot be achieved from the current vehicle state, given the current resources (propellant) and health status, then it reports this failure to the mission activity agent. The mission activity agent may change the low-thrust target state to a state that is both reachable by the low-thrust system and from which a high-thrust rendezvous may be completed to the target point. The low-thrust target then, is the interface state between the two agents as described previously in the Hierarchical Planning section. The high-thrust rendezvous nominally consists of two burn events with coasting flight between the burns as described in the CONOPS section. The first burn raises the orbital apogee to the altitude of the desired RSO offset Low-Thrust Rendezvous High-Thrust Rendezvous Mission Figure 7. Temporal hierarchy for PAMM mission. point, while the second burn places the chaser in an orbit that is co-elliptic and co-altitude with the RSO. The second burn is timed to cause the chaser to arrive at the offset point at the desired time. As we shall see later, additional burns may be required to achieve the offset point if the high-thrust agent is invoked for any reason at a starting state other than close to the nominal low-thrust termination state. The high-thrust maneuvers include periods of thrusting and periods of nonthrusting attitude hold or “pointing.” In contrast, the segments of low-thrust maneuvering involve continuous thrusting while pointing of the space vehicle (and thrust vector) to effect the solution to the minimum-time rendezvous problem according to an adaption of the Kechechian solutions. Because the lowthrust trajectory employs a constant thrust approach, the minimum time solution is also the minimum fuel solution. To employ this low-thrust algorithm effectively, the mission manager must account for these periods of constrained attitude requirements in order to ensure the satisfaction of other mission constraints such as power generation, target sensing, and navigation. During the high-thrust rendezvous, the burns will be short and long periods will be spent in the pointing activity. The mission planner may schedule pointing to optimize solar power generation, improve thermal conditions, acquire the RSO with sensors, etc. Pointing activities could be planned onboard, scheduled on the ground, and tasked as mission objectives, or ground-planned activities could be treated as constraints to the mission manager. Since the focus for this activity was on the rendezvous problem, pointing and attitude hold activities were not implemented, and the mission manager was implemented in a 3-degree-of-freedom (DOF) simulation. The implemented activity agents are shaded in Figure 7. Reference [3] Burn Burn Burn Thrust Point Height Point CoCo- Point Safe Raising ellipticelliptic Abort Proximity Operations Docking 47
Page 1 and 2: THE DRAPER TECHNOLOGY DIGEST 2006 V
Page 3 and 4: Introduction by Vice President, Eng
Page 5 and 6: niversary N O L O G Y D I G E S T e
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Page 23 and 24: FalconView graphical map interface
Page 25 and 26: Table 1. Initial GN&C-Related Drago
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Page 33 and 34: REFERENCES [1] X-72 Data Sheet, htt
Page 35 and 36: Detection of Biological and Chemica
Page 39 and 40: Intensity (a.u.) Detection of Biolo
Page 41 and 42: shown here, we could move toward a
Page 45 and 46: Autonomous Mission Management for S
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Page 71 and 72: Fluid Effects in Vibrating Micromac
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Page 87 and 88: The Howard Musoff Student Mentoring
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