Edward C. Wong

Guidance and control design for hazard avoidance and safe landing on mars

To ensure successful future Mars landing missions, the lander must be capable of detecting hazards in the nominal landing zone and maneuvering to a new and safe site. Trajectory guidance and attitude commanding are formulated for the terminal descent phase when the lander is off the parachute. The autonomous six-degree-of-freedom controls are accomplished using engines and thrusters and guided by onboard hazard-avoidance sensors. The algorithms determine the available landing zone, survey them for hazards, select the best or alternate landing site based on state estimates and available propellant, and then maneuver the lander to land safely at the selected site. Computer simulations have demonstrated the satisfactory performance of the algorithms for safe landing on Mars with assumed atmospheric environments.

Guidance and Control Design for Powered Descent and Landing on Mars

Future Mars landing missions must be capable of autonomously delivering highly capable and mobile rovers safely and gently in an upright orientation. The airbag landing system used to deliver earlier rovers (Mars Pathfinder and the two Mars Exploration vehicles) is incapable of landing the Mars Science Laboratory (MSL)-class rover. The design of a novel sky-crane landing concept to land the proposed Mars Science Laboratory rover is presented here. The descent is guided and actively controlled in six degrees of freedom. Terminal guidance is robust to terrain variations-induced altimeter noise. A terminal descent sensor provides surface relative velocity and altitude measurements, the inertial measurement unit measurements help propagate the vehicle attitude and positions. Guidance and control system commands eight throttle-able Mars lander engines to actively control the vehicle attitude and translations. Computer simulations demonstrate the viability of this concept in the presence of various environmental, configuration, and hardware imperfections.

A Constraint Monitor Algorithm for the Cassini Spacecraft

The Cassini spacecraft is commanded to turn from one point in space to another by commanding attitude, rate, and acceleration profiles, which the Attitude Controller is required to execute faithfully. Thc Constraint Monitor algorithm performs appropriate checks to ensure the legality and the realizability of the instantaneous attitude, rate, and acceleration commands. When the command is found to be unrealizable because of hardware limitations or itlegal because it may enter a constraint space, Constraint Monitor modifies it appropriately such that the legality of the command is maintained. In doing so it ensures that the rate and acceleration are not outside the capabilities of the attitude control hardware and that certain sensitive spacecraft boresights are protected from exposure to bright objects. The Cassini spacecraft, scheduled for launch in October 1997, will arrive at Saturn in 2004. On its way to Saturn, it will fly by Venus, Earth, and Jupiter to pick up the needed gravity assists. The spacecraft will be carrying a probe intended for delivery into the Titan atmosphere. The probe entry into the Titan atmosphere will occur about four months after Saturn arrival. Tltc spacecraft will conduct a tour of the Saturnian system for approximately four years. Several close flybys of Titan and Saturns icy satellites are planned. The nominal mission will conclude in the year 2008. Cassini science objectives include investigation of Saturns atmospheric composition, winds and temperature, configuration and dynamics of the magnetosphere, structure and composition of the rings, characterization of the icy satellites, and Titans atmospheric constituent abundance. The rack mapper will perform surface imaging and altimetry during each Titan flyby. Cassini was originally onc of the two spacecraft of Mariner Mark 11 series intended for multi-mission purpose: the CRAF (Comet Rendezvous and Asteroid Flyby) and Cassini. The CRAF mission was to follow a comet ,artd conduct scientific investigations for 120 days and Iatcr flyby an asteroid. NASA budget constraints ncccssitzzted the cancellation of CRAF and dcscoping of Cassini spacecraft. Both the high and low precision scan platforms and their structural booms were deleted, m was the turn table which carried a fields and particles experiment. The spacecraft basebody zmurncd the role of the observation platform and the entire spacecraft now had to turn in order to do earth pointing, star tracking, and remote science pointing. This makes the detection and avoidance of spacecraft attitude constraints ever more difficult since movement of one boresight requires turning …

Attitude controller for the atmospheric entry of the Mars Science Laboratory

This paper describes the attitude controller for the atmospheric entry of the Mars Science Laboratory (MSL). The controller will command 8 RCS thrusters to control the 3- axis attitude of the entry capsule. The Entry Controller is formulated as three independent channels in the control frame, which is nominally aligned with the stability frame. Each channel has a feedfoward and a feedback path. The feedforward path enables fast response to large bank commands. The feedback path stabilizes the vehicle angle of attack and sideslip around its trim position, and tracks bank commands. The feedback path has a PD/D control structure with deadbands that minimizes fuel usage. The performance of this design is demonstrated via computer simulations.

Parameter Identification of Linear Discrete Stochastic Systems with Time Delays

An identification algorithm that uses the maximum likelihood technique to identify the unknown time delays, plant parameters, and noise covariances of linear discrete stochastic systems is presented. Cases of additive white noise and colored measurement noises are considered. The likelihood function is evaluated using either a minimum-variance (Kalman) filter or a minimal-order observer. The Kalman filter is used in the identification algorithm to provide minimum-variance estimates. The minimal-order observer is a lower-dimensional and computationally simpler filter, and is advantageous especially for systems with long delays. It provides a less optimal solution to the minimum-mean-square state estimation problem. The colored-noise observer algorithm has the disadvantage of having to compute an extra error covariance matrix of lower order.

An identification algorithm for linear stochastic systems with time delays

Linear discrete stochastic control systems containing unknown multiple time delays, plant parameters and noise variances are considered. An algorithm is established which uses the maximum-likelihood technique to identify the unknown parameters. An estimated likelihood function is evaluated based on the previous parameter estimates, which in turn generates a new descent direction vector to update the unknown parameters. The delays and plant parameters are identified in their respective parameter spaces. An example of a second-order stochastic system has been implemented by digital simulation to demonstrate the applicability of the algorithm.

Relative Motion Estimator for the Shuttle Radar Topography Mission

This paper presents the design, implementation, and post-mission tuning of the Outboard Estimator for the Shuttle Radar Topography Mission attitude and orbit determination avionics ground data processing The Outboard Estimator estimates the interferometric baseline, relative position and orientation of the outboard antenna with respect to the inboard antenna, using measurements from the ASTROS Target Tracker and an Electronic Distance Meter. It is implemented as a Kalman filter in which the measurements get blended with a nonlinear model of the mast flexible dynamics excited by the Shuttle vernier jet attitude control firings.

Minimal-order observer for linear stochastic time-delay systems

A minimal-order observer for obtaining state estimates in linear stochastic time-delay systems is presented. The system model considered contains white noise disturbance in both the process and the output measurements. An observer gain matrix is derived which minimizes the estimation error in the mean-square sense. The error variance of the observer has been compared to that of an optimal Kalman filter using a second-order time-delay system as an example