Showing papers on "Inertial reference unit published in 1978"

Precision attitude determination for multimission spacecraft

Abstract: Attitude determination algorithms for a multimission spacecraft are derived and their performance analyzed. The attitude determination system is composed of a strapdown Inertial Reference Unit (IRU), two fixed head star trackers, and an onboard computer. IRU data is processed to maintain real-time knowledge of spacecraft attitude relative to an inertial reference frame. Star tracker data is processed using Kalman filtering techniques to estimate and correct the attitude determination errors and the gyro drift compensation errors. The results of a star availability analysis for stellar, solar and earth pointing missions are presented. Linear covariance analysis techniques are used to evaluate nominal attitude determination performance, the effects of sensor measurement accuracy variations, the effects of errors in knowledge of sensor measurement accuracy, and the effects of star tracker misalignment errors. Results of a nonlinear simulation analysis of attitude determination performance are also presented. These analyses show that precision attitude determination for stellar, solar and earth pointing missions is achieved.

The Psi-Angle Error Equation in Strapdown Inertial Navigation Systems

Abstract: A detailed development is presented of the psi-angle vector differential equation as applied to the error analysis of strapdown inertial navigation systems. The coordinate systems involved and the psi misalignment vector are clearly defined. It is proven that apart from a sign change the psi-angle differential equation in the error analysis of strapdown inertial navigation systems is identical to the one used in conventional gimbaled inertial navigation systems.

Processor for an inertial measurement unit

Abstract: A processor is provided for an inertial measurement unit which computes its angular velocity and translational acceleration in terms of dynamic variables chosen because they uniquely define the motion of the inertial measurement unit in terms of linear combinations of the outputs of the transducers used in the inertial measurement unit. In one embodiment, the processor includes a demodulator for obtaining d.c., in-phase and quadrature sets of signals from the outputs of the transducers, a combining circuit for deriving some of the dynamic variables from the in-phase and quadrature sets of signals, and a microprocessor for transforming the dynamic variables into angular velocity and translational acceleration vectors. The processor is used with a small rugged, precise inertial measurement unit in which vector components are measured by the use of accelerometers fixed in a spinning rotor, at least one of which is off-set from the rotor axis and has a sensitive axis parallel to that of the rotor. This off-set/parallel accelerometer permits obtaining the signs of the angular velocities in addition to their magnitudes such that complete vector components are obtained. In an alternative embodiment, two orthogonally oriented rotor systems are utilized which permits all a.c. signal processing, thereby eliminating the necessity of d.c. measurements. Error isolation and correction are easily accomplished in a specialized combining circuit which simplifies initial alignment of the instrument. Another alternative embodiment employs three rotors and provides complete redundancy.

Acceleration sensitive gyroscope stabilized platform

Abstract: An inertial measuring unit which precesses about the direction of apparent acceleration as a consequence of the introduction of a mass unbalance to each of the gyroscopes in the inertial measuring unit. The mass unbalances and the rotational moments of inertia of the gyroscopes are carefully controlled so that all gyroscopes within the inertial measuring unit precess at the same rate about any apparent acceleration. The precession of gyroscopes causes certain of the gyroscope errors to be averaged about zero thus reducing the error in the inertial measuring unit. The effect of precession upon the spatial orientation of the inertial measuring unit is compensated for by sensing the apparent acceleration and computationally compensating for the precession due to such acceleration.

Failure Modes and Redundancy Analysis for the Multifunction Inertial Reference Assembly (MIRA).

Abstract: : This report analyzes potential failure modes for various gyroscope and accelerometer designs and their associated electronics and considers the most appropriate mechanizations to accomplish a fail-operational/fail-operational Inertial Reference Assembly. (Author)

Redundant Integrated Flight-Control/Navigation Inertial Sensor Complex

Abstract: The preliminary design of a redundant strapdown navigation system for integrated flight-control/navigation use is presented. Based on tuned-gimbal gyros, a compact configuration (13 in. x 13 in. x 14 in.) has been achieved for fail-operational/fail-operational redundancy. Hardware and software design of a four-channel system configuration is given. The system consists of four gyros, eight accelerometers, and four processors. Compact packaging into four identical chassis is based on the symmetry properties of an octahedron. The design matrix for least-squares combinations of n two-degree-of-freedom gyros is derived, and general parity equations are shown for extracting errors from measurements. A limitation on the amount of sensed information for failure isolation of some second-gyro failures is described, and system failure probability is calculated from hardware failure rate, channel interconnection, and signal-to-noise ratio. Test data are presented for skewed, dual-redundant strapdown inertial measurement units, including typical parity equation responses during physical motion, simulated gyro and accelerometer failures, and a 180-deg turn with a 20-deg roll angle. Fault detection and isolation time for various failure amplitudes and likelihood of false alarms for the assumed error detection amplitudes are given.

Application of inertial navigation systems to geodetic postion and gravity vector survey

Abstract: Inertial navigation systems are mechanized so as to measure the change in geodetic latitude and longitude and geometric height relative to a specified earth's reference spheroid or datum. Since this is the basic task of geodetic survey, conventional inertial navigation systems, which have been used for decades for missile and aircraft navigation, are in essence low-quality survey systems. Navigation errors result, in inertial systems, from the angular orientation error of the accelerometer-sensing axes relative to the reference surface, the scale factor error, and noise that exists in the measurements themselves. One of the principal noise sources contaminating the specific force measurements made by the accelerometers is the anomalous behavior of the gravity vector relative to the reference spheroid assumed that is not accounted for by a gravity model contained in the system computer, Errors induced in the system-computed velocity and position by the gravity disturbance, observed by the use of appropriate reference sensors, serve as one means for determining the anomalous behavior of the gravity vector along a survey vehicle's travel route. Additionally, if the inertial system is brought to rest, the deviation of the gravity vector from the computer gravity model can be directly determined from the accelerometer measurements. Stopping of a survey vehicle carrying an inertial navigation system also allows the error in the system-computed velocity to be observable to high precision. Stopping of the vehicle at points whose geodetic and astronomic positions and gravity value have been determined by independent means provides observations of the error in system-computed geodetic position and the system-estimated anomalous gravity vector. This tutorial paper discusses the manner in which the observations of the error in the systemcomputed velocity and position and the gravity vector are employed in realizing a practical inertial surveying system of high accuracy. System performance, assuming the application of optimal filtering and smoothing techniques is also indicated.