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November 2003 KC-135 SPHERES flight test results Mark O. Hilstad, Simon Nolet, Dustin Berkovitz, Alvar Saenz-Otero, Dr. Edmund Kong, and Prof. David W. Miller MIT Space Systems Laboratory 2003-11-24

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Nov. 2003 KC-135 results2 Overview Attitude control and beacon tracking –Tracking of a hand-held beacon by a sphere. –Tracking of a sphere by another sphere. Simple search pattern –Open-loop three-axis rotation Coordinated search pattern –Elements of the “lost in space” maneuver Docking –Initial approach stage only Identification of inertia and center of mass Lessons learned A series of four KC-135 flights during the week of 3 Nov 2003 was sponsored by the Jet Propulsion Laboratory’s Terrestrial Planet Finder program. These flights were used to test algorithms designed by several members of the SPHERES team, and by NASA Ames. The results of these tests are presented herein. This presentation references the following video files: KC135_Nov03_flight2_para23.mpg KC135_Nov03_flight4_para02.mpg KC135_Nov03_flight4_para03.mpg KC135_Nov03_flight4_para10.mpg KC135_Nov03_flight4_para13.mpg KC135_Nov03_flight4_para18.mpg KC135_Nov03_flight4_para19.mpg KC135_Nov03_flight4_para29.mpg

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Nov. 2003 KC-135 results3 Beacon tracking experiments Objective –Demonstrate tracking of a beacon and optimal rotation along the shortest path –Validate the 3-D control law by demonstrating off-axis 3-D attitude control Experiment description –Tracking of a beacon randomly located in the test volume –Tracking of a free floating sphere’s on-board beacon –Two spheres simultaneously tracking each others’ on-board beacons Flight 2, parabola 23 Flight 4, parabola 18

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Nov. 2003 KC-135 results4 Beacon tracking results Flight 4, parabola 18 –Pointing error is reduced in each test (q 1, q 2 and q 3 tend toward zero, while q 4 tends toward one) –Body rates show angular acceleration and deceleration, as expected

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Nov. 2003 KC-135 results5 Simple search Flight 4, parabola 2 Flight 4, parabola 3 Initial conditions –A single sphere stationary with respect to the KC frame Open-loop spin –Try to point the onboard beacon in as many directions as possible Attempt to map 4 steradians Alternating thrusters –Two thrusters on at any given time Very limited time, so maximize actuation –Used to change the plane of rotation –Thruster state changes once per second. A propellant-efficient algorithm will be used for SPHERES-TPF.

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Nov. 2003 KC-135 results6 Simple search results Day 4, parabola 2 –Alternating z, y, x torque commanded –Some coverage due to thrusters, some due to disturbances such as bumping walls –Sparse coverage in the -x direction, but within the half-cone angle of the beacon –Quaternion integration issues add uncertainty to validity of results Quaternion behavior appears erratic, but rapid change is expected Rate gyroscope saturation leads to inaccurate quaternion integration Quaternion normalization errors also led to inaccurate integration –Video shows multi-axis rotation, as intended. Coverage maps

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Nov. 2003 KC-135 results7 Coordinated search Experiment description –Two spheres begin with the beacon faces offset from their common line of sight by 135°. –Both spheres initiate open-loop z- axis spins. –Acquisition occurs when one sphere hears the other’s beacon. –When a sphere hears a beacon, it sends a stop message to the other sphere. –When a sphere receives a stop message, it initiates rate damping. –Both spheres use the same algorithm. Flight 4, parabola 10 Flight 4, parabola 13

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Nov. 2003 KC-135 results8 Coordinated search results Flight 4, parabola 10 –Local 0° are offset from common line by -135° –Perform +z spin at ~30°/s until acquisition/communication, followed by rate damping Beacon half angle is ~30-45°(equivalent to 1-1.5 seconds of spin time) –Telemetry from both spheres shows expected behavior Sphere 1 shows rate damping at z-quat≈0.6 → z-angle≈106° Sphere 2 shows rate damping at z-quat≈0.45 → z-angle≈127° –Primary maneuver (spin search, then decelerate upon acquisition) is circled. Sphere 1 Rate damping begins q 3 at start of rate damping Sphere 2 Rate damping begins q 3 at start of rate damping

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Nov. 2003 KC-135 results9 Glide slope docking experiments Flight 4, parabola 19 Flight 4, parabola 29 Objective –Demonstrate the first phase of the docking approach computed by a glide slope algorithm Experiment description –The two spheres maintain their orientation toward each other –The chasing sphere initiates a translation along its x-body axis to move toward the target –The algorithm is set such that docking should occur in about 8 seconds!

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Nov. 2003 KC-135 results10 Glide slope docking results Flight 4, parabola 19 –The first 3.5 seconds of the docking maneuver were successfully achieved –The spheres maintain relative pointing, as shown by the constant quaternions –Similar data were acquired by both spheres using their own sensors

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Nov. 2003 KC-135 results11 Inertial property identification Online gyro-based mass property identification –Use gyro data and calibrated thruster information to identify the error from nominal values of center of mass offset and moment of inertia –Algorithms by SPHERES team and NASA Ames Dry inertia (kg m 2 ) I xx 0.022901 I yy 0.020546 I zz 0.018165 I xy 0.000196 I xz -0.000066 I yz -0.000212 CM offset (mm) Full tank CM x -0.054495248 CM y -0.810686405 CM z 0.244119942 Empty tank CM x -0.015665375 CM y -0.821387572 CM z 3.081773236 Pre-flight estimated inertial properties KC-135 flight goal: perform one long test over many parabolas –Allows more time for estimates to converge –Provides the most amount of raw data for download Problem: disturbances corrupt identification –Update stops by itself when gyro saturates, no thrusters are firing, or SNR is too low –Pause/resume command used when sphere is handled

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Nov. 2003 KC-135 results12 Inertia ID sample data Estimated angular acceleration (using rate sensors) –Data are taken from parabolas 32-40, flight #4 –Green lines indicate when test was paused –Delays in pausing and unpredictable motion caused issues Parabola 35: attached proof mass (a spare battery pack) on the -x face of the sphere Parabola 37: replaced the full tank with one containing only 28g of gaseous CO 2 (no liquid) to remove effects of propellant slosh Parabola 38: removed the proof mass Parabola 35 Parabola 37 Parabola 38 a y and a z drop When proof mass Is attached

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Nov. 2003 KC-135 results13 Inertia ID results Deviations from the previous best-estimate inertia –These are “dry” values; propellant is subtracted out The inertia in the y (green) and z (red) axes jumps when the proof mass is added –The change is about 80% of what was expected; this is under investigation Propellant slosh is not an issue ~ 3.2e-3 change (expected 3.73e-3)

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Nov. 2003 KC-135 results14 Center of mass ID results Deviations from the nominal CM offset. –These are “dry” values; propellant is subtracted out Since it was impossible to filter out all handling of the sphere using pause and resume commands in real-time, the online estimate was corrupted Saved high-frequency IMU data from the flight were used successfully in an identical off-line algorithm (accurate to floating-point precision) to produce promising results for both inertial and CM estimation. Parabola 35 Parabola 37 Parabola 38 ~ 5 mm change (expected 6.3)

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Nov. 2003 KC-135 results15 Lessons learned Care must be taken when integrating separate tests into one program –Tests being integrated in one program must have compatible initialization settings –Special attention is required when the tests are written by different individuals Unexpected infrared noise causes confusing behavior –We have identified a series of fixes for use in any future KC flights IR-opaque, visible-transparent sheeting on lights (will also improve video quality) Form-fitted, pre-cut curtains to block IR from other experiments Point laptop screen away from test area We expect these fixes to fully alleviate all infrared problems –Recent test results on the ISS suggest that infrared noise will not be a problem; however, experiences like this continue to help us better understand noise sources and to identify new ways to address the issue if it arises. SPHERES core code changes –Static variables must be explicitly initialized at the start of each test, not just at the start of the program –Order of process initialization problems manifested on flights as corrupt IMU data Identified problem and fixed by changing initialization procedure –Will add data to the telemetry stream to notify us of suspected IR noise.

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Nov. 2003 KC-135 results16 Conclusion Objectives accomplished –Showed that the attitude control algorithm works as expected in 3D –Validated critical maneuvers and demonstrated key components of the “lost in space” sequence Open-loop search Beacon acquisition Intercommunication Stop and hold response Beacon tracking Saved precious experiment time on ISS –Gained confidence in current algorithms, and identified areas for improvement prior to flight. –Improved the inertia and center of mass estimates of the spheres.

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