• A Sounding Rocket Attitude Determination Algorithm Suitable For Implementation Using Low Cost Sensors

      Charlton, Mark Christopher; Hawkins, Joseph G. (2003)
      The development of low-cost sensors has generated a corresponding movement to integrate them into many different applications. One such application is determining the rotational attitude of an object. Since many of these low-cost sensors are less accurate than their more expensive counterparts, their noisy measurements must be filtered to obtain optimum results. This work describes the development, testing, and evaluation of four filtering algorithms for the nonlinear sounding rocket attitude determination problem. Sun sensor, magnetometer, and rate sensor measurements are simulated. A quatenion formulation is used to avoid singularity problems associated with Euler angles and other three-parameter approaches. Prior to filtering, Gauss-Newton error minimization is used to reduce the six reference vector components to four quaternion components that minimize a quadratic error function. Two of the algorithms are based on the traditional extended Kalman filter (EKF) and two are based on the recently developed unscented Kalman filter (UKF). One of each incorporates rate measurements, while the others rely on differencing quaternions. All incorporate a simplified process model for state propagation allowing the algorithms to be applied to rockets with different physical characteristics, or even to other platforms. Simulated data are used to develop and test the algorithms, and each successfully estimates the attitude motion of the rocket, to varying degrees of accuracy. The UKF-based filter that incorporates rate sensor measurements demonstrates a clear performance advantage over both EKFs and the UKF without rate measurements. This is due to its superior mean and covariance propagation characteristics and the fact that differencing generates noisier rates than measuring. For one sample case, the "pointing accuracy" of the rocket spin axis is improved by approximately 39 percent over the EKF that uses rate measurements and by 40 percent over the UKF without rates. The performance of this UKF-based algorithm is evaluated under other-than-nominal conditions and proves robust with respect to data dropouts, motion other than predicted and over a wide range of sensor accuracies. This UKF-based algorithm provides a viable low cost alternative to the expensive attitude determination systems currently employed on sounding rockets.
    • Cubesat Attitude Control Utilizing Low-Power Magnetic Torquers & A Magnetometer

      Mentch, Donald B.; Thorsen, Denise (2011)
      The CubeSat Project has lowered development time and costs associated with university satellite missions that conform to their 10 centimeter cube design specification. Providing attitude control to a spacecraft, of such small volume, with a very limited power budget has been a challenge around the world. This work describes the development of an attitude control system based on a very low-power magnetic torquer used in conjunction with a magnetometer. This will be the first flight use of this torquer which is composed of a hard magnetic material wrapped inside of a solenoid. By discharging a capacitor through the solenoid, the magnetic dipole moment of this permanent magnet can be reversed. The completed attitude control system will make the first use of the low-power magnetic torquer to arrest satellite tip-off rates. It will then make the first known use of a dual axis magnetic dipole moment bias algorithm to achieve three-axis attitude alignment. The complete system is standalone for high inclination orbits, and will align the spacecraft to within 5 degrees of ram, nadir, and local vertical, without any requirement for attitude determination. The system arrests tip-off rates of up to 5� per second (in all 3 axes) for a satellite in a 600 kilometer polar orbit expending 0.56 milliwatts of power. Once in the proper alignment, it utilizes 0.028 milliwatts to maintain it. The system will function for low inclination orbits with the addition of a gravity boom. The system utilizes the magnetometer to calculate spacecraft body rates. This is the only known use of a magnetometer to directly measure spacecraft body rates without prior knowledge of spacecraft attitude.
    • Performance Prediction Of A Folding Fin Aircraft Rocket Using Datcom, Sens5D, And 6Dof Gem

      Kralewski, Sara Louise; Goering, Douglas; Lin, Chuen-Sen; Das, Debendra (1998)
      An approach for the performance prediction of a Folding Fin Aircraft Rocket (FFAR) is presented. This prediction was compiled by calculating the gravimetrics, aerodynamics, and trajectory for a FFAR. The trajectory analysis utilized four computer codes: Rogers Aeroscience Rocket Performance Software, NASA Wallops Sens5d Trajectory and Wind-Sensitivity Calculations for Unguided Rockets, the United States Air Force (USAF) Stability and Control DATCOM, and the NASA Langley Research Center LRC-MASS program (GEM). Computations were performed for a rigid body configuration. This analysis was compared to radar data collected during the flight of a FFAR launched in February 1997 at the Poker-Flat Research Range. The comparison shows good agreement between the flight data and the predicted apogee and impact point of the vehicle. In addition, static and dynamic stability analyses were completed for the FFAR. <p>