To summarize, an INS consists of three major subsystems:

1. Attitude subsystem where the orientation of the IMU with respect to the

navigation frame is maintained

2. Specific force sensing subsystem, usually three single axis accelerometers,

that measure accelerations

3. Computation subsystem, in which INS navigation equations of motion are

solved.

Depending upon the instrument quality, INS position error rates can range from 0.1

nm/hr to 10.0 nm/hr.  Although the INS errors can grow unbounded, their temporal

behavior has a very well defined frequency behavior.  Horizontal errors will oscillate at

the Schuler frequency (with an 84 minute period) modulated by earth rate (24 hour

period) and the second order Foucault frequency (period depends on vehicle velocity,

direction, and latitude).

The pure inertial vertical channel of an INS is unstable with a time constant of 9.5

minutes.  For this reason many inertial navigation systems incorporate baro  altimeter

stabilization of its vertical channel.  Note that for GPS/INS integrated filters, the baro 

altimeter error can be estimated through GPS position measurements.  Since direct

GPS measurements of position yield reference ellipsoid altitude, the baro altimeter

error estimated includes the offset of local mean sea level from the ellipsoid.

An important error source in an INS is an error in knowledge of the orientation or

alignment of the INS sensor package with respect to its navigation frame.  These

misalignment errors are usually expressed as three small rotation angles and are

referred to as platform tilts.  In a strapdown INS, the three angles are related to pitch,

roll and heading error, while in a gimballed system the angles are identified with

rotations of the platform about the level and vertical axes.

The three accelerometers in an INS are usually mounted in a mutually orthogonal triad,

each one measuring a component of specific force along its sensitive axis.  Typical

accelerometer errors include bias, scale factor, and misalignments among others.  A

bias error means that the instrument reading is always off by a fixed amount of

acceleration.  A scale factor error refers to an error in the accelerometer output by a

constant multiplicative factor.  An accelerometer misalignment will cause the

accelerometer to sense components of  accelerations that occur along an axis

supposedly orthogonal to its input axis.

An INS usually has gyros mounted with their sensitive axes in a mutually orthogonal

triad.  The gyros may be single or dual axis (2 degree of freedom) in nature.  Two 2

degree of freedom gyros provide outputs along 3 orthogonal axes and along 1

redundant axis.  The gyro axes have a known, fixed orientation with respect to the

accelerometers.  A gyro drift rate or bias is a constant angular rate of change of the

platform tilts.  Gyros errors also include scale factor errors, misalignments, and in the

case of spinning mass gyros, g sensitive drift rates.  The gyro bias errors are the

primary cause of increasing horizontal position errors  and consequently are the errors

most necessary to minimize for longer missions.

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