Spacecraft State Research Articles

Venus gravity west of Beta Regio

Doppler tracking data from the Pioneer Venus Orbiter (PVO) have been used to estimate the anomalous gravity field in the region of Venus west of Beta Regio. The analysis invokes a Kalman filter-smoother to solve the nonlinear spacecraft state estimation problem and a linear Bayesian estimator to perform the geophysical inversion. The topographic map for this region, derived from the PVO radar, has been filtered to have the same distortions and degree of smoothing as the gravity map. The undulations of the gravity are about 0.2 times as large as expected from the topography on the assumption that the latter is uncompensated. A comparison of the gravity and topography by means of the spectral admittance is consistent with Airy compensation at a depth of 50 km if the surface material has a density of 2.6 g/cm 3. However, this is not a unique interpretation.

Application of Model Following Control and Estimation Techniques to Attitude Control of Manoeuvring Spacecraft

The paper presents an approach to sampled data, on-board estimation and control of the attitude motion of manoeuvring spacecraft. Algorithms for a) estimation of gyroscope parameters (drift rate bias, scale factor error) and spacecraft state (attitude, angular velocity, disturbance torque), and b) model following control of attitude manoeuvres, are presented. The algorithms were validated in single-axis software simulations of an attitude control system of the type as used in the Infra Red Astronomical Satellite (IRAS). The considered control system comprises a strapdown rate-integrating gyro and a slit-type star sensor for optical - inertial attitude sensing, a reaction wheel actuator, and a 16-bit on-board computer.The estimation and control algorithms are described, design trade-offs are discussed, and simulation results are presented.

Development and analysis of a twelfth degree and order gravity model for Mars

Satellite geodesy techniques previously applied to artificial earth satellites have been extended to obtain a high‐resolution gravity field for Mars. Two‐way Doppler data collected by 10 Deep Space Network (DSN) stations during Mariner 9 and Viking 1 and 2 missions have been processed to obtain a twelfth degree and order spherical harmonic model for the martian gravitational potential. The quality of this model was evaluated by examining the rms residuals within the fit and the ability of the model to predict the spacecraft state beyond the fit. Both indicators show that more data and higher degree and order harmonics will be required to further refine our knowledge of the martian gravity field. The model presented shows much promise, since it resolves local gravity features which correlate highly with the martian topography. An isostatic analysis based on this model, as well as an error analysis, shows rather complete compensation on a global (long wavelength) scale. Though further model refinements are necessary to be certain, local (short wavelength) features such as the shield volcanos in Tharsis appear to be uncompensated. These are interpreted to place some bounds on the internal structure of Mars.

Orbit Determination for a Jupiter Orbiter Tour of the Galilean Satellites

Current NASA planning envisions an ambitious Jupiter tour mission which will arrive at the planet in the year, 1984. The objectives of this mission are to broaden our knowledge of the outer solar system and in particular to extend our understanding of the physical processes at work in the Jupiter and Galilean satellite environment. To achieve these goals, precise spacecraft navigation is a fundamental requirement inasmuch as very near encounters with the satellites are necessary both for the planned scientific investigations and to reduce fuel requirements necessary for orbital insertion about the planet. This paper, deals with problems of producing precise spacecraft state estimates relative to the Galilean satellites, with the aid of both earth-based radio and on-board video data.

Lunar Radii Determined from Lunar Orbiter 1 V/H Sensor Data

THE Lunar Orbiter series of satellites revealed the potential use of orbiting satellites to gather various types of physical data on a celestial body. Among these data types are surface photography and gravitational field characteristics. In the case of the Lunar Orbiter satellites, another data type was discovered which provided a means of determining the lunar equatorial radius. This data was in the form of an extremely accurate angular velocity reading from an electro-optical device known as the image-motion compensator sensor (V/H sensor) used to stabilize the film during the open period of the shutter of the photographic system. The function of this sensor was to provide a mechanical shaft rotation, proportional to the image motion, to drive the camera platen through a cam and lever arrangement. The ideal result of the platen motion is to completely stabilize the film during the entire time the system shutter is open. The output of the system was divided by the focal length of the camera lens. This ratio was then telemetered back to Earth as a V/H measurement. The V/H measurement together with the spacecraft state vector are used to determine the local lunar radius at the point of measurement.

Sequential estimation of the state and the observation-error convariance matrix

Algorithm for simultaneous estimate of spacecraft state and covariance matrix with observation error vector