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

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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

12. QUESTIONS? 11