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by: Laurine Willms


Laurine Willms
GPA 3.82


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Class Notes
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This 6 page Class Notes was uploaded by Laurine Willms on Monday September 7, 2015. The Class Notes belongs to ASE 370L at University of Texas at Austin taught by Staff in Fall. Since its upload, it has received 41 views. For similar materials see /class/181854/ase-370l-university-of-texas-at-austin in Aerospace Engineering at University of Texas at Austin.




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Date Created: 09/07/15
CHAMP Scientific Payload and its Contribution to a Sible Attitude Control System Ralf Bock Hermann Liihr Ludwig Grunwaldt GeoForschungsZentrum Potsdam Telegrafenberg D14473 Potsdam Germany httpopgfzpotsdamdechamp AGU Fall Meeting 2000 Poster presentation GG1A 04 AGU Fall Meeting Page 1 7 Presentation G61A 04 Introduction GPS POD Antenna Accelerometer inside the spacecraft at center of gravity Response of the accelerometer to thruster firings 20e 07 0 0e00 20e07 40e07 60e07 Acceleration I39nIS 80e07 10e06 12e06 99 99 99 r 29 33959 99 99 99 Q Q Q Q Q Q 99119 993 er ngz b g 99169 99169 9933 Time UTC Nov 10 2000 Accelerometer measurements are affected by attitude thruster activation and subsequently triggered magnetometry boom oscillations which decay within a few tens of seconds Here the orbit radial component is depicted The maximum amplitude of the spikes about 10396 ms2 exceeds the instrument resolution by nearly a factor of 1000 Such periods of thruster firings boom oscillations have to be excluded from the science data processing thus the number of thruster activations has to be kept at a minimum The frequent thruster pulses in this example are due to a special attitude maneuver AGU Fall Meeting Presentation G61A04 Star Sensors Optical Bench with Fluxgate Magnetometers and Star Sensors Digital lon Driftmeter and Langmuir Probe Overhauser Magnetometer On July 15 2000 the geoscientific satellite CHAMP has been launched into a near polar circular low earth orbit at 460 km altitude Throughout its anticipated 5year mission the altitude will decay to 300 km This mission profile has been chosen to provide full global coverage and to resolve temporal variations The CHAMP scientific payload is dedicated to recover the global gravity field the magnetic field and to perform atmospheric sounding measurements In addition to these scientific objectives the GPS receiver star cameras and vector magnetometers are also used as input sensors for the attitude and orbit control system AOCS In order to separate gravitational from nongravitational orbit perturbations an accelerometer is flown on CHAMP The nongravitational perturbations include air drag solar radiation pressure and attitude control actuator operation Therefore an attitude control system is needed that sparsely performs thruster firing but predominantly uses magnetic torquing instead The reason for this is that the latter produce significantly lower angular accelerations and no linear accelerations thus providing a quiet measurement platform While the Fluxgate Magnetometer delivers undisturbed input for magnetic torquing outages of the GPS receiver and star cameras have occasionally affected the CHAMP AOCS performance Page27 Attitude and Orbit Control System The CHAMP satellite is kept in a threeaxis stabilized earthoriented attitude with the boom pointing in flight direction to ensure minimum air drag The A008 employs a cold gas propulsion system for attitude and orbit change maneuvers A set of three magnetic torquers for pre compensation of environmental disturbances supports the cold gas system The payload instruments Star Cameras GPS Receiver and Fluxgate Magnetorneter are used as attitude sensors along with the Coarse Earth and Sun Sensors for the safe mode Attitude and Orbit Control System AOCS Data Management Unit and Drive Electronics rotational The accelerometer measures and linear accelerations in the range of 104 ms2 with a resolution of 109 msz This led to demanding requirements in the spacecraft and A008 design eg positioning the accelerometer in the center of gravity aligning thrusters to achieve force free torques and limit thruster activation to a few per orbit Thruster firings disturb the accelerometer measurements thus mainly magnetic torquing is used for attitude control Missing or false AOCS data input by the instruments leads to unnecessary thruster pulses AGU Fall Meeting Presentation G61A 04 deg Deviation Angle mA Torqer Current msec 1000 Thruster Pulses 2000 9 90 1 1 39 Example of AOCS control activity 6559 A 9 A gt9 A 39b E 9 39b E 1939 19quot 9 1 1quot 7 Time UTC Aug 7 2000 A 9 63 5 Pg 366 379 569 These plots over 25 orbits each scale mark represents a quarter orbit starting at a northbound equator pass at 2000 UTC show that the attitude is predominantly controlled by magnetic torquing and that regional patterns of attitude deviation angles and torquer activities exist For example the roll axis can not be controlled by torquers over the equator at 2000 2046 2133 UTC etc If the deviation angles exceed 2 or the estimated rates are above 0004 degsec the thrusters are activated Unnecessary thruster firing occur due to false sensor input that lead to wrong rate estimates eg 2020 UTC Page37 GPS Receiver Effects Apart from the scientific objectives like precise orbit determination and occultation measurements the GPS TRSRll space receiver forwards its navigation solution to the A008 These position and velocity data are required to provide the A008 with the attitude reference direction aligned with velocity vector and are needed to calculate the actual attitude from the star sensor reading During the first 3 months in orbit the CHAMP GPS receiver has experienced 4 software resets per day but when navigation solutions are unavailable the A008 propagates the orbit based on latest inputs from GPS More harmful were false navigation solutions that led to wrong attitude deviations and caused thruster firings The first shortcoming has been significantly reduced while the latter has been eliminated by a GPS software update on November 8 2000 GPS outages 141 lt 6 min 469 69 min Available Not Available 22 978 390 gt 9 min The unavailability of GPS navigation solutions due to resets is 22 statistics over 622 hours This graph shows the distribution of those data gaps with respect to duration The updated GPS software causes less resets but it requires the acquisition of 5 satellites in lock before the navigation solution will be forwarded to the A008 While the number of resets per day has decreased the overall time of data gaps is the same but the distribution has shifted to longer periods Reaction of the AOCS to a failed GPS position fix 7000 6000 5000 4000 3000 iAttltude Thrusters iDev Roll iDev Pitch Yaw Thrusttime ms 2000 Attitude Deviation Angles deg 1000 o 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 N369 459 N199 N799 N799 N799 W799 599 W751 N799 W756 W732 99 Mgr9 WW5 WES Time UTC After a SNV reset the GPS receiver performs an open sky search for the navigation signals of the GPS constellation The old SNV version started to output navigation solutions as soon as 4 satellites were in lock but the quality check was insufficient In case of a false solution the A008 onboard orbit propagator assumes a sudden change in the orbital position of the spacecraft and tries to steer the attitude according to this improper data This causes excessive thruster firings and deviations from the nominal attitude The GPS receiver clears the erroneous situation by a selfinduced reset after a few minutes and produces good navigation data afterwards The A008 corrects for the improper attitude and brings CHAMP back into nominal orientation within short time No more GPStriggered attitude deviations have occurred since the software update AGU Fall Meeting Presentation G61A 04 Page47 Star Camera Effects h budyrmuumed and mm n5 mmammn Wm M AOCS m mam N 2 accexmmem sensuv and m F uxgate magnemmehy assemb y my me buum These nunrmagneuc cameva head um s ave uwn m 012 my me m duanead Snlzr prnlnns can a lake slzls an the star camera Ingvs s aunched mum mm mm 15n72umwnv197 gym mvdes and ummuus nunr s eHav mans 12d mvdes Pmmns 355mg thvuugh me cameva chm age 143 ummuus amen WEYE detected u m h 71 a a s Denveasmg me sensmvny and mstaumg a hmspm 1212mm has awed m pyumem mu FEM123mg Presematxan Gm Am Wes7 Tracks of the stars cameras on the sky 90 60 Line of Sight Star Camera Head 1 Declination deg 30 60 Line ofSight Star Camera Head 2 60 120 180 240 300 Right Ascension deg 360 Attitude data are not available when the sun or the moon are in the field of view The line of sight in the celestial reference frame of two camera heads over several orbits is depicted The A008 uses only one of the 4 camera heads which is chosen according to its availability in a fixed order The switch from one camera head to another especially from body to boom mounted heads may cause thruster activation in case the transfor mation angles into the common reference system are not properly adjusted Especially gracing incident sun light causes the frequent switching between camera heads Software updates and angle adjustments have significantly reduced this effect AGU Fall Meeting Presentation G61A 04 Page67


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