# Inertial/GPS Pointing and Positioning System 

### This system operates independently of the moving platform that carries the instrument. 

#### *Ames Research Center, Moffett Field, California*

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.

 

![](media/image1.jpeg){width="2.6034722222222224in" height="2.84375in"}\
The **INS Is Mounted Directly on the Pointing Instrument** instead of on
the platform as in other systems. This direct-mounting scheme affords
the advantages of less complexity, greater accuracy, and lower cost,
relative to other instrument-pointing systems.

 

*This work was done by Robert Yee and Fred Robbins of* **Ames Research
Center.** *For further information, access the Technical Support Package
(TSP)* **free on-line at [www.nasatech.com](http://www.nasatech.com/)**
*under the [**Electronic Components and Systems
category**](http://www.nasatech.com/TSP/ecs.html).*

*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, (650) 604-5104. Refer to ARC-14060.*

*Tech Briefs article published February 2001.*