Engineering topics

Underwood, S.

Publications and source records attributed to Underwood, S..

4 records

NASA NTRS1994

In-flight estimation of gyro noise on the Upper Atmosphere Research Satellite (UARS) and Extreme Ultraviolet Explorer (EUVE) missions

This paper characterizes the low-frequency noise response of the Teledyne dry rotor inertial reference unit (DRIRU) gyroscopes on the Upper Atmosphere Research Satellite (UARS) and the Extreme Ultraviolet Explorer (EUVE). The accuracy of spacecraft attitude estimation algorithms that use gyro data for propagating the spacecraft attitude is sensitive to gyro noise. EUVE gyro data were processed to validate a single-axis gyro noise model, which is used onboard various spacecraft. The paper addresses the potential impact of temperature effects on the gyro noise model and the overall impact on attitude determination accuracy. The power spectral density (PSD) of the gyro noise is estimated from UARS in-flight data by Fast Fourier Transform (FFT). The role of actuator dynamics on the PSD function is also discussed.

Lee, M.↗

NASA NTRS1994

Operational support for Upper Atmosphere Research Satellite (UARS) attitude sensors

The Upper Atmosphere Research Satellite (UARS) has several sensors that can provide observations for attitude determination: star trackers, Sun sensors (gimbaled as well as fixed), magnetometers, Earth sensors, and gyroscopes. The accuracy of these observations is important for mission success. Analysts on the Flight Dynamics Facility (FDF) UARS Attitude task monitor these data to evaluate the performance of the sensors taking corrective action when appropriate. Monitoring activities range from examining the data during real-time passes to constructing long-term trend plots. Increasing residuals (differences) between the observed and expected quantities is a prime indicator of sensor problems. Residual increases may be due to alignment shifts and/or degradation in sensor output. Residuals from star tracker data revealed and anomalous behavior that contributes to attitude errors. Compensating for this behavior has significantly reduced the attitude errors. This paper discusses the methods used by the FDF UARS attitude task for maintenance of the attitude sensors, including short- and long-term monitoring, trend analysis, and calibration methods, and presents the results obtained through corrective action.

Lee, M.↗

NASA NTRS1993

Upper Atmosphere Research Satellite (UARS) attitude accuracy using coarse attitude sensors

The Upper Atmosphere Research Satellite (UARS) uses fixed-head star trackers (FHST's) and inertial reference units (IRU's) to determine and control its attitude. This combination of fine sensors results in attitude knowledge accuracies to better than 10 arc-seconds (1 sigma). UARS also has a variety of coarse attitude sensors on board: the three-axis magnetometer (TAM), the coarse Sun sensor (CSS), the fine Sun sensor (FSS), and the Earth sensor assembly (ESA). By comparing attitude solutions using coarse sensors with FHST-determined attitude solutions, estimates can be made of the accuracy of the coarse sensors. This paper presents the results of an analysis that compares attitude solutions using various combinations of UARS coarse attitude sensor data with FHST attitude solutions.

Krack, K.↗

NASA NTRS1993

Upper Atmosphere Research Satellite attitude disturbances during shadow entry and exit

The Upper Atmosphere Research Satellite (UARS), as with the Landsat-4 and Landsat-5 spacecraft, experiences large attitude disturbances when entering and exiting the Earth's shadow. Previous investigations have provided some evidence linking these disturbances to rapid bending of the solar array but have also raised questions. For example, the magnitudes of the roll attitude disturbances have shown an unmolested asymmetry, and the timing of the disturbances at sunrise appears to disagree with the modeled timing. A better understanding of this phenomenon is important in assessing the implications for UARS science gathering and for future mission design analysis. To this end, UARS attitude, sensor, and actuator data are used to evaluate the disturbances as they vary with solar beta angle and solar array drive angle. The attitude data are examined during specific periods of interest, such as the month in which the solar array was parked in its high-noon position, and are also tracked from the beginning of the mission to determine any trends that may result from changing mass properties due to cryogen boiloff and propellant usage. Attitude rate and torque profiles are derived from inertial reference until data and related to the disturbances seen in the attitude data. The timing of the disturbances with respect to spacecraft sunset and sunrise is characterized to allow event predictions. Stability during the disturbances is discussed in terms of science instrument requirements. Finally, the results are compared with the behavior predicted by models that are based on solar array bending.

Lambertson, M.↗