The simplest way to combine GPS and INS is a reset only mechanization in which

GPS is used to periodically reset the INS solution (see Figure 9 3).  In this loosely

coupled, open loop strategy the INS is not recalibrated by GPS data, so the

underlying error sources in the INS still drive its navigation errors as soon as GPS

resets are interrupted.  However, for short GPS interruptions or for high quality INS,

the error growth may be small enough to meet mission requirements.

INS UNCORRECTED

POSITION, VELOCITY

CORRECTED POSITION,

AND ATTITUDE

VELOCITY AND ATTITUDE

INS

INS

KALMAN FILTER

GPS POSITION

AND VELOCITY

GPS

GPS RCVR

KALMAN FILTER

Figure 9 3.  Open loop GPS/INS Aided Architecture

The main advantage of GPS aiding the INS in a closed  loop mechanization is that

the inertial system is continuously calibrated by the Kalman filter, using the GPS

observables (see Figure 9 4). When GPS is lost due to jamming, dynamics or

satellite shadowing, the inertial system can continue to derive its navigation

solution, but now with a greater degree of precision by virtue of its recent

calibration.  However, this loosely coupled, closed loop technique has serious

potential stability problems in cases where the INS feeds back navigation data to

the GPS receiver.

CORRECTED POSITION,

VELOCITY AND ATTITUDE

INS

CORRECTIONS FOR INS

POSITION, VELOCITY

AND ATTITUDE ERRORS

INS

KALMAN FILTER

GPS POSITION

AND VELOCITY

GPS

GPS RCVR

KALMAN FILTER

Figure 9 4.  Closed Loop GPS/INS Aided Architecture

GPS aiding the INS (loosely coupled) in both open and closed  loop modes results

in the simplest implementation: any GPS receiver and any INS with the necessary

data interfaces can be used and it requires a smaller Kalman mechanization of the

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