Precision Instrument Pointing Control for the Earth Observing System

The High Resolution Imaging Spectrometer (HIRIS) is a remote sensing instrument which will be placed on board the first NASA Polar Orbiting Platform (NPOP-1), as part of the Earth Observing System (EOS). HIRIS is an F/5.4 aperture spectrometer with a focal length of about 1222 mm, and uses a precision two-axis gimballed pointing mirror system to image and track targets. The instrument is in early development and is scheduled for launch in 1998. Gimbal position and rate sensors provide the feedback signals to maintain the pointing accuracy at 585 arcsec (peak-to-peak), and the pointing stability at less than 2.65 arcsec in 33 msec. A state-space controller implemented with a digital computer is used to provide a position loop bandwidth of 1 Hz and a rate loop bandwidth of 7 Hz. This paper will address the design concept developed for the HIRIS pointing system, including an overview of the system design, flight hardware and software. In addition, a simulation assessment of the pointing system that takes into consideration issues such as non-linear effects, sensor noise, and non - collocated sensors and actuators in a flexible structure, will be discussed.