CORRECTED POSITION,

VELOCITY AND ATTITUDE

INS

CORRECTIONS FOR INS

POSITION, VELOCITY

AND ATTITUDE ERRORS

GPS/INS

KALMAN FILTER

GPS PR AND dPR

MEASUREMENTS

GPS RCVR

Figure 9 6.  Closed loop Integrated GPS/INS Architecture

9.4.2.2  The INS Navigation Process

The INS Kalman filter will model and estimate some of the INS errors.  An INS

consists of, as a minimum, an Inertial Measurement Unit (IMU) and a computer to

perform processing. The inertial sensors are accelerometers and gyroscopes which

measure, loosely speaking,  inertial accelerations and rotations.

Most IMU s fall into one of two classes. The first, and oldest type, is a gimballed

platform. In a gimballed IMU, the accelerometer triad is mounted on a platform that

is maintained stable in inertial space by the gyros. A locally level navigation frame

is usually mechanized so that the gyros are "torqued" for transport and earth rates

to force the platform to remain perpendicular to the local gravity vector. Thus the

accelerometers measure accelerations directly in the local level plane.  Many

gimballed platforms also include a third accelerometer orthogonal to the two in the

platform plane that measures vertical specific force. The attitude of the IMU case

with respect to the platform is determined through electrical pickoffs of the gimbal

structure.

The second type of IMU is the strapdown system. In a strapdown IMU, the gyros are

rate integrating and are essentially hard mounted to the host vehicle. The gyros

measure incremental angular change. The computer must keep track of the angular

changes so that the attitude of the host vehicle can be determined. The three

accelerometers measure linear accelerations as in the case of a gimballed IMU,

although the accelerations are in the vehicle  body frame. The transformation from

the platform to navigation frames is used to convert the accelerations to navigation

coordinates where they are integrated to produce velocity and position.

9 14