Aerospace Control Systems

Spacecraft GNC Systems Engineering

RADICAL RIMU Attitude Determination/Calibration Filter - Advanced Capabilities PDF Print E-mail
Article Index
RADICAL RIMU Attitude Determination/Calibration Filter
Software Components
Advanced Capabilities
Filter States
Further Reading
All Pages

Advanced Capabilities

In addition to estimating a full set of calibration parameters, the RADICAL filter has several features that give flexibility when processing actual telemetry. These include a warm and cold start, check points, a covariance bump, fault detection and performance monitoring, and logically defined parameter tables. Telemetry data are automatically synchronized, and small time tag differences are compensated. These are just some of the many features incorporated into RADICAL, and are described in more detail below.

Warm and Cold Start

The filter is first initialized by using a “cold start”, the calibration parameter estimates are set to a given default value with a given covariance that may be large. Convergence of the parameters can take several minutes or longer, depending on attitude motions such as a calibration maneuver. One feature of the RADICAL filter is that it can process disjoint or interrupted telemetry streams and telemetry in multiple files. The attitude estimate, attitude covariance, and attitude cross-covariance are reset when there is a break in the gyro data. The parameter covariance remains intact (in UD factorized form). This is called a “warm-start” of the calibration filter.

The importance of being able to process disjoint telemetry streams and applying the covariance bump is that the filter does not have to be reinitialized, and the filter is nearly converged when the prior converged estimates and their covariance are used to warm-start the filter. This can be of benefit in autonomous on-board calibration. Convergence problems are avoided when a prior estimate and a small prior covariance are used to warm-start the filter, and a shorter calibration maneuver may then be sufficient for re-convergence of the calibration parameters and their covariance. This can be of benefit during mission operations to reduce risk, to reduce interruption of payload operations, and to reduce the volume of telemetry dedicated to calibration. The warm-start feature permits a calibration maneuver to be segmented under certain operational constraints.

Check Points

A check point can be saved at anytime during processing so that processing can be resumed in case there is a problem with the telemetry some time after the check point is saved. After loading the check point, the filter is resumed with a warm start. The filter can be restarted at any time in the telemetry after the check point. (It could be started prior to the check point, but doing so would result in error correlation and suboptimal performance; a cold start would avoid that problem.) Of course, this feature is not used in real time, but only with stored telemetry data.

Covariance Bump

A covariance bump can be applied at any time during processing in RADICAL. In particular, the covariance “bump” can be applied upon a warm start to model uncertainty due to a change in parameters since the epoch of the previously processed telemetry stream. A bump can also be applied to the attitude covariance. The covariance bump is simply a specified increase in the covariance of any estimated parameter or attitude, and is applied upon a warm-start or at any time upon command. The bump is applied directly to the UD factors of the covariance matrix for computational efficiency and to ensure numerical accuracy and stability.

Fau lt Detection and Performance Monitoring

The fault detection function monitors a large number of variables and compares them to threshold limits and tests for numerical faults. The fault detection function provides a small set of fault detection signals that can be monitored by a centralized C&DH fault detection system. This reduction to a small set of signals greatly simplifies integration and test, and it protects the inner workings of the calibration filter from its software environment.

Several variables that are indicators of performance are provided in the output packets. These include attitude covariance information, residuals and residual statistics, and a pair of scalars that measure the overall accuracy of the filter.

Parameter Tables

Parameters are logically arranged into a small number of tables to support simple table upload and maintenance. The required data is simple in nature and requires very little ground support to produce a table. For example, instead of storing the (very large) process noise matrix in a table, only the gyro noise variances and parameter process noise variances are stored in the process noise table. When using the command interpreter, the parameter tables can be read from files and written to files in either binary or ascii.

Data Synchronization

Telemetry data read from files are automatically synchronized, which simplifies processing. For both telemetry read from files and for real-time data, there can be small time tag differences between the star tracker and gyro data. Small time tag differences are compensated to avoid attitude estimation error that would appear with non-zero angular rate.