Engineering topics

Dougherty, H.

Publications and source records attributed to Dougherty, H..

12 records

NASA NTRS1986

Space Telescope Antenna Pointing System analysis and test

The Space Telescope (ST), carrying five astronomy instruments, is a long-life spacecraft designed to achieve and maintain ultrafine stabilization jitter performance. The rms jitter specification for pointing performance is 7 marcsec (1sigma) during periods of scientific observation. Spacecraft attitude control is achieved by onboard computer processing of attitude and rate sensor data to generate reaction-wheel torque commands. The ST Antenna Pointing System is used to position a high-gain antenna for relaying science and engineering data from the spacecraft to the ground station.

Dodder, R.↗

NASA NTRS1986

Performance characterization of the Hubble Space Telescope Rate Gyro Assembly

An account is given of the design features and performance characteristics of the Rate Gyro Assembly employed by the Hubble Space Telescope's Pointing Control System to effect accurate spacecraft maneuvers and support fine pointing telescope operation, in conjunction with the fine guidance sensors. The rate sensor attitude quantization is 0.00025 arcsec. Fine-pointing stability is the driver for gyro noise performance, while spacecraft-pointing accuracy requirements at the end of maneuvers has driven the gyro scale factor and mechanical alignment stability performance.

Dougherty, H.↗

NASA NTRS1984

Space Telescope pointing control

The Space Telescope, a long life, high performance spacecraft deployed by the Space Shuttle, will carry five scientific instruments on its first mission. Its pointing control system will permit target-to-target maneuvering and precision pointing on a target star to support scientific objectives. Spacecraft attitude control is achieved by onboard computer processing of attitude and rate sensor data to generate reaction wheel torque commands. A momentum management control system is provided to desaturate the reaction wheels. This paper discusses the pointing control system and the control hardware investigations and improvements leading to system design.

Dougherty, H.↗

NASA NTRS1983

Space Telescope pointing control

The Space Telescope Pointing Control System is used to slew the optical axis of the telescope from one target star region of the celestial sphere to the next, and to maintain precision pointing on the target star. A digital computer is employed in the processing of attitude and rate sensor data, in order to generate torque commands for the reaction wheels. The System comprises four major elements: a command generator, the electronic control system, attitude update processing, and momentum management. Emphasis is given to control-related flexibility effects of the Space Telescope's solar cell array and vehicle, and the acquisition methodology and development testing employed, in addition to Pointing Control System design features.

Dougherty, H.↗

NASA NTRS1983

Noise characterization and minimization of a precision gyroscopic rate sensor

A program has been conducted to evaluate the noise signature of Space Telescope gyros. The Space Telescope gyros provide a three-axis attitude reference using six single-degree-of-freedom gyroscopes operated with pulse rebalance electronics; the rate sensor attitude quantization is 0.00025 arcsec. The results of the noise evaluation program, including error source characterization and identification of candidate design modifications, are summarized together with the results of prototype hardware testing. It is shown that the proposed design modifications can significantly improve the rate sensor noise performance.

Dougherty, H.↗

NASA NTRS1982

Space telescope pointing control system

The Space Telescope is a free-flying spacecraft designed for Space Shuttle launch. The Space Telescope's pointing control system slews the optical axis from one target star region of the celestial sphere to the next, and maintains precision pointing for the target star for up to 24 hours. The spacecraft digital computer processes the precision attitude and rate sensor data to generate torque commands for the reaction wheels. The pointing control system has four major elements: the command generator, the control system, the attitude reference processing, and momentum management. The emphasis is on relating design requirements to the hardware and software implementation.

Dougherty, H.↗

NASA NTRS1982

Space telescope - Meeting the pointing control challenge with today's technology

The pointing control system of the Space Telescope, which provides target-to-target maneuvering capability and precision pointing on the target star (with 0.007-arcsec stability and 0.01-arcsec accuracy), is described. Spacecraft attitude control is undertaken by onboard computer processing of attitude and rate sensor data that generates reaction wheel torque commands. The Space Telescope Operations Control Center communicates with the Space Telescope via the synchronous altitude tracking and data relay satellite system, and determines vehicle attitude more precisely by means of sun sensors, magnetometers and fixed-head star trackers. Such disturbance torques as those of gravity gradients and aerodynamics act on the Space Telescope, causing the speeds of the four reaction wheels to increase. In order to prevent the wheels from reaching a speed-saturated condition, a momentum control system is provided for the management of reaction wheel speed buildup. Attention is given to development testing and control hardware investigations and improvements.

Dougherty, H.↗

NASA NTRS1982

Space Telescope Pointing Control System software

The Space Telescope Pointing Control System software is in the advanced development stage, having been tested on both the airbearing and the static simulator. The overall structure of the software is discussed, along with timing and sizing evaluations. The interaction between the controls analysts and software designer is described.

Dougherty, H.↗

NASA NTRS1982

Space telescope - The next generation

The command handling approach as applied to fine guidance sensor guide star acquisition is described in order to illustrate the flexibility it provides to users of the Space Telescope. User control of pointing operations is detailed, and the pointing control system/fine guidance system interface is depicted along with the guide star acquisition sequence. Reaction wheel interaction with vehicle structural modes and the rate gyro assembly noise level are discussed and some test data are shown.

Dougherty, H.↗

NASA NTRS1981

Magnetic control systems for large spacecraft with applications to space telescope

Magnetic control systems for large space vehicles offer the advantage of a simple, reliable, low cost augmentation to the primary control system. When used for momentum management, a magnetic torque source offers a long life and noncontaminant environment when compared to a mass expulsion torque source. These qualities make such systems suitable for employment with the Space Telescope, which is a long life, high performance vehicle with optics and scientific instruments which would be degraded by contamination due to mass expulsion products. The various applications of magnetic systems on the Space Telescope are considered. The future trend in magnetic control of large space vehicles lies in providing a known three axis reference for backup operations, such as recovery of the primary control mode.

Dougherty, H.↗

NASA NTRS1981

A method of testing attitude control systems during the development phase

A technique, utilized on the Space Telescope Program, and used for testing satellite attitude pointing and control systems during the engineering and development phases is presented. The technique verifies the hardware models used in design phase computer simulations, verifies the interface between the flight hardware and flight software, and uncovers hardware/software switching or mode logic problems. The testing is accomplished in two phases: a dynamic hardware simulator phase using hardware electronic simulators and an electronic vehicle motion simulator; and a second real hardware phase utilizing engineering model gyros and reaction wheels on an airbearing table. Both phases use an engineering model of the flight computer, flight algorithms and software, and a breadboard data management and computer hardware interface for timing simulations. The purpose of each test and the test phases are described, and examples of closed loop test results for both attitude hold and maneuvering models are given.

Besonis, A.↗

NASA NTRS1981

Control technology as applied to Space Telescope

The pointing control system (PCS) of the Space Telescope is addressed. The sensors employed in the primary PCS mode are the rate gyro assembly and the fine guidance sensors, while four reaction wheel assemblies provide control torques. Stability considerations based on structural modes are discussed, noting the stability margin criteria used in the design. Acquisition methodology is considered with regard to the PCS component hardware simulators and vehicle dynamics simulation.

Dougherty, H.↗