Manual Tracking for a Space-to-Space Photographic Mission

The extension of terrestrial photography to space-to-space operation is a logical expectation. However, this extension is complicated by the relative motion between space vehicles and the need for accurate tracking during photography to reduce resolution losses. This paper is a first attempt to define a manual tracking system for coupled-camera photography of one space vehicle from another when a wieldable viewfinder is used. The proposed system incorporates simultaneous manual control of the two viewfinder gimbals and automatic control of the three vehicle attitudes. The major tool of this study was a physical simulation of the visual scene in conjunction with an analog computer simulation of the equations of relative motion. Data are presented on such typical performance factors as relative velocity, look angles and rates, and line-of-sight accelerations. These data were obtained by substituting automatic control loops with appropriate compensation for the pilot. For certain performance factors, a strong dependence on the specific trajectory flown was noted. An interesting fundamental of the manual control problem was our use of a two-axis control handle when there are five variables to control: the two viewfinder gimbal angles and the three vehicle attitude angles. Two variables can be controlled directly from the two-axis handle, but additional principles must be hypothesized for control of the other three variables. Four such control systems for maneuvering the vehicle to reduce the relative line-of-sight travel are presented and evaluated. We also show how integral compensation could be used to reduce tracking errors.