Modal identities for multibody elastic spacecraft: An aid to selecting modes for simulation

The question: Which set of modes furnishes a higher fidelity math model of dynamics of a multibody, deformable spacecraft (hinges-free or hinges-locked vehicle modes) is answered. Two sets of general, discretized, linear equations of motion of a spacecraft with an arbitrary number of deformable appendages, each articulated directly to the core body, are obtained using the above two families of modes. By a comparison of these equations, ten sets of modal identities are constructed which involve modal momenta coefficients and frequencies associated with both classes of modes. The sums of infinite series that appear in the identities are obtained in terms of mass, and first and second moments of inertia of the appendages, core body, and vehicle by using certain basic identities concerning appendage modes. Applying the above identities to a four-body spacecraft, the hinges-locked vehicle modes are found to yield a higher fidelity model than hinges-free modes, because the latter modes have nonconverging modal coefficients; a characteristic proved and illustrated

Momentum accumulation due to solar radiation torque, and reaction wheel sizing, with configuration optimization

This paper has a two-fold objective: determination of yearly momentum accumulation due to solar radiation pressure, and optimum reaction wheel sizing. The first objective is confronted while determining propellant consumption by the attitude control system over a spacecraft's lifetime. This, however, cannot be obtained from the daily momentum accumulation and treating that constant throughout the year, because the orientation of the solar arrays relative to the spacecraft changes over a wide range in a year, particularly if the spacecraft has two arrays, one normal and the other off-normal to different extent at different times to the sun rays. The paper first develops commands for the arrays for tracking the sun, the arrays articulated to earth-pointing spacecraft with two rotational degrees of freedom, and spacecraft in an arbitrary circular orbit. After developing expressions for solar radiation torque due to one or both arrays, arranged symmetrically or asymmetrically relative to the spacecraft bus, momentum accumulation over an orbit and then over a year are determined. The remainder of the paper is concerned with designing reaction wheel configurations. Four-, six-, and three-wheel configurations are considered, and for given torque and momentum requirements, their cant angles with the roll/yaw plane are optimized for minimum power consumption. Finally, their momentum and torque capacities are determined for one-wheel failure scenario, and six configurations are compared and contrasted

Applied research and teaching contributions in space flight vehicles guidance, navigation and control

by H. B. Hablan

Fusion of redundant aided-inertial sensors with decentralised Kalman filter for autonomous underwater vehicle navigation

Most submarines carry more than one set of inertial navigation system (INS) for redundancy and reliability. Apart from INS systems, the submarine carries other sensors that provide different navigation information. A major challenge is to combine these sensors and INS estimates in an optimal and robust manner for navigation. This issue has been addressed by Farrell1. The same approach is used in this paper to combine different sensor measurements along with INS system. However, since more than one INS system is available onboard, it would be better to use multiple INS systems at the same time to obtain a better estimate of states and to provide autonomy in the event of failure of one INS system. This would require us to combine the estimates obtained from local filters (one set of INS system integrated with external sensors), in some optimal way to provide a global estimate. Individual sensor and IMU measurements cannot be accessed in this scenario. Also, autonomous operation requires no sharing of information among local filters. Hence a decentralised Kalman filter approach is considered for combining the estimates of local filters to give a global estimate. This estimate would not be optimal, however. A better optimal estimate can be obtained by accessing individual measurements and augmenting the state vector in Kalman filter, but in that case, corruption of one INS system will lead to failure of the whole filter. Hence to ensure satisfactory performance of the filter even in the event of failure of some INS system, a decentralised Kalman filtering approach is considered.by Vaibhav Awale And Hari B. Hablan

Air-to-air tracking of a maneuvering target with gimbaled radar

This paper is concerned with the steering of an antenna beam for an air-to-air tracking system where an airborne radar antenna senses an airborne maneuvering target. A mechanical gimbal-mounted airborne radar is used to steer the antenna beam. The difficulty with a gimbaled radar is that the radar measurement becomes inaccurate if the target is outside the antenna beam width, which is usually less than 2 deg. An extended Kalman filter is developed to estimate the relative position, relative velocity, and absolute acceleration of the target in Cartesian coordinates, which are further used to drive the antenna beam. Realistic noise models of sensors are incorporated in the measurements. A high-fidelity model is developed to accommodate the sensors operating at different frequencies. The mathematical formulation of initializing the state vector and process noise matrix using measurements, the measurement sensitivity matrix, and the process noise covariance matrix are presented. The flight dynamics of two fighter aircraft, both executing 3g, maneuvers is simulated to validate the proposed model. The accuracy achieved in the line-of-sight rate and line-of-sight angle estimation are deemed to be adequate for the tracking scenario. Furthermore, a Monte Carlo analysis of 50 runs is provided to validate the extended Kalman filter. Read More: http://arc.aiaa.org/doi/10.2514/1.G001184by Vinod Saini and Hari B. Hablan

Relative navigation of GEO satellites in formation using double-difference carrier phase measurements with integer ambiguity resolution

by Amolika Soni and Hari B. Hablan