Precise attitude determination requires that attitude sensor and gyro parameters be calibrated accurately. It is often the case that attitude sensor and gyro calibration are not carefully considered in the design and operation of a system, or the calibration algorithm and its software implementation are limited in their capability or performance. A well designed calibration algorithm and software tools are necessary to conduct early trade studies and to predict performance, to quickly design and implement a system, and to achieve optimum system performance during operation.
A large number of spacecraft carry a Redundant IMU (RIMU). A RIMU is an Inertial Measurement Unit (IMU) that comprises more than three rotational sense axes. (In some contexts an IMU includes accelerometers.) In this context we are talking about active redundancy rather than passive functional redundancy. Examples of a RIMU include
- the Northrop Grumman SIRU (comprising four HRG gyros),
- four or more interferometric fiber-optic gyros (IFOG),
- a set of two or more two-axis gyros such as dry-tuned rotor gyros (DTG),
- two 3-axis IMUs.
I can show you how to achieve maximum performance, maximum active and passive redundancy, and improved fault detection from multiple sense axes. This improves reliability, as well as performance, at little to no extra cost.
Calibration filters in common use treat the RIMU as a 3-axis IMU either by using data from 3 gyro sense axes or by combining data from more than 3 gyro sense axes to obtain a 3-axis measurement. Furthermore, they generally estimate a minimal set of calibration parameters or only gyro bias. They often do not use the correct process noise matrix. As a result, they do not fully exploit the capability of a RIMU, and so performance and real-time redundancy are sacrificed.
The Redundant IMU (RIMU) Attitude Determination/Calibration (RADICAL TM) Filter
The RADICAL filter (pronounced rad•i•cal) is an Attitude Determination and Calibration Kalman filter is implemented in C. The RADICAL filter takes full advantage of the redundancy in a RIMU to optimally estimate spacecraft attitude and a full set of calibration parameters for n ≥ 3 gyro sense axes and relative misalignments (interlocks) for m attitude sensors. The IMU calibration parameters include, for each gyro, bias, symmetric and asymmetric scale factors, and a pair of sense axis misalignments. The RADICAL filter is based on an advanced calibration model and numerically stable algorithms for reliability and accuracy.
The RADICAL filter is ideal for
- Desktop analysis and design,
- Automated or semi-automated ground-based processing of attitude sensor telemetry,
- Real-time on-orbit attitude determination and sensor calibration.
The RADICAL software incorporates not only the best algorithms, but also relevant experience. RADICAL has advantages in
- Cost effectiveness
- Dedicated expertise
- Continued improvement
- Fully detailed documentation
The RADICAL solution makes sense in a world of tighter requirements, shorter procurement schedules, smaller design teams, and limited research and development funds. RADICAL is a cost-effective alternative to in-house development.
RADICAL has been used to support several missions and has processed thousands of hours of telemetry. Its performance has been verified through extensive testing via simulation and processing of in-flight telemetry, and demonstrated by accurate geolocation results based on attitude estimates produced by the RADICAL filter.
RADICAL is offered as licensed COTS software at reasonable cost. I can also provide technical assistance, system design, analysis, and data processing services through a support contract.
Contact ACS to discuss how I can address your technical concerns and requirements.
Copyright © 2021 by Mark E. Pittelkau and Aerospace Control Systems, LLC