CHAPTER 8:  POSSIBLE INTEGRATIONS OF GPS

8.1  INTRODUCTION

There are two ways to achieve integration with GPS: aiding which results in a hybrid or

blended solution and aiding which results in a GPS solution.  Implementations can range

from a GPS receiver coupled with an external sensors such as an altimeter or a time aiding

source, to integration with a navigation system.  Examples of navigation systems include an

INS, an AHRS, or a Doppler Radar Navigation Systems (DRNS).

GPS integrated with a navigation system provides the flexibility to have the navigation

solution determined by the GPS receiver, by the host navigation system, or by a

combination of the two.  The GPS receiver can be aided by the host navigation system and

provide the blended solution, or the navigation system may process GPS data to form the

combined navigation solution. Another alternative is to have a central processor separate

from the receiver or host navigation system, receiving data from both the GPS and host

system and producing the blended  solution. This last case essentially treats both the GPS

receiver and host navigation system as sensors. The GPS receiver may reside on a card

that is embedded into the host navigation system box so that the receiver physically

becomes part of the host navigation system.

The benefits of integrating GPS with other navigation systems are significant and diverse.

Basically, each system may have important shortcomings when used in a stand alone

mode, although together they can be integrated to mitigate most of these liabilities.  An INS,

AHRS, or DRNS, for example, is subject to an ever growing drift in position accuracy

caused by various instrument error sources that cannot be eliminated in  manufacturing,

calibration or alignment. Other INS short comings include somewhat lengthy static

alignment period or, alternatively, continuous operation in a powered alert status. 

Furthermore, a high quality INS can be a complex electro  mechanical device with

significant risk of component failure.  Additional shortcomings of Doppler systems include

the reliance on an external heading input to form a navigation solution. Mission or flight

critical implementations of these navigation systems require substantial redundancy in

installation to detect, isolate, and recover from such failures for fail  safe operational

performance.

A stand alone GPS has its shortcomings as well.  GPS is somewhat vulnerable to loss of

signals due to RF interference, antenna shadowing, aircraft attitude maneuvers, or other

causes.  Receiver solution update rates may be fairly slow; i.e., in the 1 Hz region due to

the complex processing of the radio frequency signals into a position or velocity solution.  A

1 Hz update rate may be sufficient for most navigation applications, however, some

avionics systems and weapons require much higher rates. 

GPS used in conjunction with an aiding source can solve some or all of these limitations,

depending on the aiding source used.  An aiding source such as an altimeter or a precise

time source can be substituted for a temporarily masked satellite.  An INS, for example, can