A data system generates information on the position and orientation of a pointing instrument (e.g., a telescope or a laser) that could be mounted on a moving platform (e.g., an aircraft) and generate platform-steering commands for pointing the instrument at a known target (e.g., a target on the ground, a celestial body, or an artificial satellite). The subsystems of this system include an inertial navigation system (INS) and a Global Positioning System (GPS) antenna and receiver.
Unlike other instrument-pointing systems, the INS mounts directly on the pointing instrument instead of on the platform. As a result, the INS decouples from the platform in the sense that it determines the position and orientation of the instrument independently of the platform. In further contradistinction to other instrument-pointing systems, the pointing instrument is not slaved to the platform; instead, as explained below, the platform is slaved to the pointing instrument and the INS.
The position and orientation of the INS relative to the pointing instrument are known and remain fixed, unlike in other systems, in which INSs are located elsewhere and it is necessary to measure suspension angles and to utilize multiple platform and suspension coordinate transformations subject to buildup of platform-bending and misalignment errors. The direct-mounting scheme thus eliminates several sources of instrument-pointing error and simplifies design and operation (thereby also reducing cost).
The INS contains triads of gyroscopes and accelerometers and associated analog and digital electronic circuits. The GPS antenna and receiver are mounted on the pointing instrument along with the INS. The GPS signals are used to correct drift errors in the INS position and orientation outputs. The position and orientation data (including GPS corrections) from the INS are sent to a computer, along with GPS time data. The computer selects the relevant data and processes these data through a series of transformation routines to generate command angles or instructions, in a preferred navigation coordinate frame, for pointing the instrument at a known stationary or moving target.
The computer calculates the required instrument-pointing angles in the navigation coordinate frame. The computer then compares the required instrument-pointing angles with the actual instrument-pointing angles determined by the INS. Next, using control logic, the computer processes the results of this comparison into corrected pointing angle commands. It is desirable to keep the corrected pointing angle commands as close to zero as possible. These commands are used for fine pointing of the instrument as the platform moves.
In some applications, steering instructions from this system are used for navigating the platform. In such an application, an angular-position encoder provides data on the orientation of the platform relative to the instrument. The encoder may, if necessary, be far less accurate than the encoders used in other instrument-pointing and navigation system; its accuracy need not exceed that of the platform autopilot or steering control subsystem.
In normal operation with respect to a celestial target viewed from an aircraft, an operator first gives the computer the celestial coordinates (e.g., right ascension and declination) of the target. The computer then generates a command to steer the aircraft onto a heading that will enable the instrument to acquire the target. The heading command can be routed to the autopilot. Once the aircraft is on the desired heading, the computer commands the pointing instrument to turn to the required elevation, azimuth, and line-of-sight angles for tracking the target.
This work was done by Robert Yee and Fred Robbins of Ames Research Center.
This invention has been patented by NASA (U.S. Patent No. 5,809,457). Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to
the Patent Counsel
Ames Research Center
Refer to ARC-14060.