RocketSat VII Construction of an Attitude Determination System for a Sounding Rocket COSGC Symposium April 9, 2011

Colorado Space Grant Consortium RocketSat VII Mission Overview RocketSat VII shall design an attitude determination system for future RocketSat-C missions Colorado Space Grant Consortium RocketSat VII 1

Background and Research RocketSat VI Mission required angle of attack of rocket Attitude determination system was not successful Algorithm chosen required attitude sensors with more accuracy than what was used “Prototype Development of a Low-Cost Sounding Rocket Attitude Determination System…” by Jan Kenneth Bekkeng Purpose: development and analysis of a low-cost attitude determination system for a spin-stabilized sounding rocket, with an attitude accuracy of 1-2 degrees used an Inertial Reference Unit (gyroscopes), a high speed CMOS image sun sensor and a commercial three-axis magnetometer Colorado Space Grant Consortium RocketSat VII 2

Attitude Determination Method TRIAD Algorithm Requires 2 absolute body measurements Takes unit vectors of each measurement to create body frame An inertial frame is created from outside measurement sources Utilize both frames to create the T matrix Code will be able to convert attitude solution of the rocket in terms of ECEF or an ECI frame Colorado Space Grant Consortium RocketSat VII 3

Colorado Space Grant Consortium RocketSat VII Attitude Sensors Two absolute sensors Need two to fully determine the attitude solution Will be using sun sensor and three axis magnetometer One relative sensor Three axis gyroscope Used to find attitude at any time by integration of values and propagation of error Colorado Space Grant Consortium RocketSat VII 4

Attitude Algorithm: Code Flow Chart Organize all data by timestamp Identify where first Sun vector and magnetometer vector are determined Perform Transport Theorem to obtain absolute vectors at the same time Find next absolute update from magnetometer or sun sensor and update the TRIAD solution Take data from gyroscopes and use rotational matrix to propagate rockets attitude over time Use TRIAD function to obtain first attitude update and rotational matrix Check the validity of the T matrix, run T matrix through T better code until back within tolerance Colorado Space Grant Consortium RocketSat VII 5

Colorado Space Grant Consortium RocketSat VII Sun Sensor Algorithm Procedure: Import image Convert to grayscale and analyze intensity Identify centroid RocketSat’s high angular rate/ blurred/glared images Matlab’s Image processing toolbox Colorado Space Grant Consortium RocketSat VII 6

Magnetometer Algorithm Procedure Convert data from “counts” to milligauss Store components of magnetic field strength into vector form -Magnetic field vector in body frame Obtain magnetic field vector in inertial frame -Use known position of rocket to determine vector (from World Magnetic Model) Colorado Space Grant Consortium RocketSat VII 7

Colorado Space Grant Consortium RocketSat VII Error Analysis Calibration testing Determine correct offset and sensitivity for accuracy of gyroscope and magnetometers Dynamic Testing Determine drift on gyroscope over time Colorado Space Grant Consortium RocketSat VII 8

Colorado Space Grant Consortium RocketSat VII Design Overview One aluminum plate Alignment Flexing Machining accuracy L-Brackets For vertical optical sensor placement Sit in pocket in plate Main Board Holds gyroscope and magnetometer Hangs underneath plate Colorado Space Grant Consortium RocketSat VII 9

Colorado Space Grant Consortium RocketSat VII Error Stackup Budget Purpose: quantify structure error Maximum two sensors are misaligned Error steps Pocket Standoffs Faces Between CMOS 0.734 degrees in plane 0.919 degrees normal CMOS to Board 0.453 degrees in plane 0.63 degrees normal 1.95 Theta .975 1.55 3.10 .003   Colorado Space Grant Consortium RocketSat VII 10

QUESTIONS? 11