Robert H. Tolson

Application of Acclerometer Data to Atmospheric Modeling During Mars Aerobraking Operations

R. H. Tolson∗ North Carolina State University, Hampton, Virginia 23666-6147 G. M. Keating George Washington University, Newport News, Virginia 23602 R. W. Zurek Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109-8099 S. W. Bougher University of Michigan, Ann Arbor, Michigan 48109-2143 C. G. Justus Morgan Research Corporation, Huntsville, Alabama 35805-1948 and D. C. Fritts∗∗ NorthWest Research Associates, Inc., Boulder, Colorado 80301

Atmospheric Modeling Using Accelerometer Data During Mars Reconnaissance Orbiter Aerobraking Operations

Aerobraking as an enabling technology for the Mars Reconnaissance Orbiter mission was used in numerous analyses based on various data types to maintain the aerobraking time line. Among these data types were measurements from spacecraft accelerometers. This paper reports on the use of accelerometer data for determining atmospheric density during Mars Reconnaissance Orbiter aerobraking operations. Acceleration was measured alongthreeorthogonalaxes,althoughonlydatafromthecomponentalongtheaxisnominallyintothe flowwereused during operations. For a 1-s count time, the root-mean-square noise level was 0:004 mm=s 2 , permitting density recovery to 0:008 kg=km 3 , or about 0.023% of the mean density at periapsis, during aerobraking. Accelerometer data were analyzed in near real time to provide estimates of density, density scale height, orbit-to-orbit variability, latitudinal-seasonalvariations,longitudinalwaves,andotherphenomenainthethermosphere.Summariesaregiven of the aerobraking phase of the mission, the accelerometer data analysis methods and operational procedures, some applications to determining thermospheric properties, correlation with the Mars Global Surveyor and Odyssey missions, and some remaining issues on interpretation of the data.

Aerobraking Cost and Risk Decisions

Four missions have successfully employed aerobraking at Venus and Mars to reduce the spacecraft orbit period and achieve the desired orbit geometry. The propellant mass reductions enabled by the aerobraking technique allow the use of smaller launch systems, which translate to significant savings in launch costs for flight projects. However, there is a significant increase in mission risk associated with the use of aerobraking. Flying a spacecraft through a planetary atmosphere hundreds of times during months of around-the-clock operations places the spacecraft in harms way, and is extraordinarily demanding on the flight team. There is a cost/risk trade that must be evaluated when a project is choosing between a mission baseline that includes aerobraking, or selecting a larger launch vehicle to enable purely propulsive orbit insertion. This paper provides a brief history of past and future aerobraking missions, describes the aerobraking technique, summarizes the costs associated with aerobraking, and concludes with a suggested methodology for evaluating the cost/risk trade when considering the aerobraking approach.

Zonal Wind Calculations from Mars Global Surveyor Accelerometer and Rate Data

TheMars Global Surveyor spacecraft was initially placed into a high-eccentricity, nearly polar orbit about Mars with a 45-h period. To accomplish the science objectives of the mission, a 2-h circular orbit was required. Using a method known as aerobraking, numerous passes through the upper atmosphere slowed the spacecraft, thereby reducing the orbital period and eccentricity. To successfully perform aerobraking, the spacecraft was designed to be longitudinally, aerodynamically stable in pitch and yaw. Because the orbit was nearly polar, the yaw orientation of the spacecraft was sensitive to disturbances caused by the zonal components of wind (east to west or west to east) acting on the spacecraft at aerobraking altitudes. Zonal wind velocities were computed by equating the aerodynamic and inertia-related torques acting on the spacecraft. Comparisons of calculated zonal winds with those computed from the Mars thermospheric general-circulation model are discussed.

Improved Method for the Estimation of Spacecraft Free-Molecular Aerodynamic Properties

A numerical procedure for calculating free-molecular aerodynamic properties of spacecraft is revisited and redeveloped using improved discretization and shading techniques. New complex geometric shapes are included, allowing the user to model a high-fidelity spacecraft model divisible into elemental panels of approximately equal area to conserve resolution. The forces on exposed elements are summed across the entire geometry. New analytic methods for determining exposed panels have been developed. Combined with the use of array algebra, the improvements considerably enhance processing speeds. The fidelity of the data is tested by application to the geometry of theMars Reconnaissance Orbiter. The resulting data are comparedwith direct simulationMonte Carlo predictions. Additional case studies serve to inspect implications of geometry design versus output accuracy and processing speed.

LIDAR-Aided Inertial Navigation with Extended Kalman Filtering for Pinpoint Landing over Rough Terrain

In support of NASA s Autonomous Landing and Hazard Avoidance Technology (ALHAT) project, an extended Kalman filter routine has been developed for estimating the position, velocity, and attitude of a spacecraft during the landing phase of a planetary mission. The proposed filter combines measurements of acceleration and angular velocity from an inertial measurement unit (IMU) with range and Doppler velocity observations from an onboard light detection and ranging (LIDAR) system. These high-precision LIDAR measurements of distance to the ground and approach velocity will enable both robotic and manned vehicles to land safely and precisely at scientifically interesting sites. The filter has been extensively tested using a lunar landing simulation and shown to improve navigation over flat surfaces or rough terrain. Experimental results from a helicopter flight test performed at NASA Dryden in August 2008 demonstrate that LIDAR can be employed to significantly improve navigation based exclusively on IMU integration.

Anomalistic disturbance torques during the entry phase of the mars exploration rover missions—a telemetry and mars-surface investigation

Shortly after landing on Mars, post-flight analysis of the “Spirit” entry data suggested that the vehicle experienced large, anomalistic oscillations in angle-of-attack starting at about Mach number M = 6. Similar analysis for “Opportunity” found even larger oscillations starting immediately after maximum dynamic pressure at M = 14. Where angles-of-attack of 1–2 degrees were expected from maximum dynamic pressure to drogue deployment, the reconstructions suggested 4 to 9 degrees. The next Mars lander, 2007 Phoenix project, was concerned enough to recommend further exploration of the anomalies. Detailed analysis of Opportunity data found significant anomalies in the hypersonic aerodynamic torques. The analysis showed that these torques were essentially fixed in the spinning vehicle. Nearly a year after landing, the Opportunity rover took pictures of its aeroshell on the surface, which showed that portions of the aeroshell thermal blanket assembly still remained. This blanket assembly was supposed to burn off very early in the entry. An analysis of the aeroshell photographs led to an estimate of the aerodynamic torques that the remnants could have produced. A comparison of two estimates of the aerodynamic torque perturbations (one extracted from telemetry data and the other from Mars surface photographs) showed exceptional agreement. Trajectory simulations using a simple data derived torque perturbation model provided rigid body motions similar to that observed during the Opportunity entry. Therefore, the case of the anomalistic attitude behavior for the Opportunity EDL is now considered closed and a suggestion is put forth that a similar event occurred for the Spirit entry as well.

Mars lander position estimation in the presence of ephemeris biases.

The process of estimating the location of a spacecraft landed on the surface of Mars is investigated through the application of statistical estimation techniques to earth-based radio tracking data. The spacecraft location and the tracking geometry and schedule are consistent with Viking-type mission constraints. With mission control requirements in mind, the investigation is restricted to analysis of a short data arc (approximately 3 days). Statistics of the spacecraft location are obtained through analysis of (direct-link) tracking data for the landed spacecraft and through simultaneous analysis of tracking data for both a landed and an orbiting spacecraft. These estimates include the effects of model uncertainties in the ephemeris of Mars, tracking station locations, the Mars rotational period, the Mars gravity field, and the orientation of Mars axis of rotation. The most significant of these effects is shown to be due to the Mars ephemeris uncertainty. A dual spacecraft tracking technique is presented for substantially reducing these ephemeris effects.

Mars Thermospheric Winds from Mars Global Surveyor and Mars Odyssey Accelerometers

crowley.pdf, Jan. 2003. [6] Crowley, G., Bullock, M. A., Freitas, C. J., Chocron, S., Hackert, C., Boice, D., Young, L. A., Grinspoon, D. H., Gladstone, G. R., Huebner, W.,Wene, G., andWesterhoff, M., “ANew Surface to ExosphereMars Atmosphere Model,” Bulletin of the American Astronomical Society, Vol. 35, Abstract 14.05, 2003, p. 934. [7] Esposito, P., Johnston, M., Graat, E., and Alwar, V., “Navigation and the Mars Global Surveyor Mission,” Proceedings of the 12th International Symposium on Space Flight Dynamics, ESA SP-403, ESA, Paris, Aug. 1997, pp. 371–376. [8] Esposito, P., Alwar, V., Demcak, S., Graat, E., Johnston,M., andMase, R., “Mars Global Surveyor Navigation and Aerobraking at Mars,” American Astronautical Society Paper 98-384, 1998; also available at http://marsprogram.jpl.nasa.gov/mgs/sci/aerobrake/SFD/SFD-Pa-

Program for the Estimation of Spacecraft Aerodynamic Properties

A numerical procedure for calculating free-molecular aerodynamic properties of spacecraft is revisited and improved using a modern computer program, Matlab 2007. This program allows the user to model a spacecraft from geometric shapes which are divided into elemental panels of approximate equal area to conserve resolution. The forces on exposed elements are summed across the entire geometry. The methods for determining exposed panels have been improved with the utilization of array algebra to improve processing time. The fidelity of the data is tested by application to the geometry of the Mars Reconnaissance Orbiter. The resulting data are compared with Direct Simulation Monte Carlo predictions. Additional case studies serve to inspect implications of geometry design versus output accuracy and processing speed.