f.  Modes of operation   Will the GPS receiver be required to operate as a

primary or sole means of navigation and/or as a sensor to an INS, DRNS, or

mission computer? The intended use of GPS greatly affects the integra tion

strategy under signal loss or failure conditions.

8.3  INTEGRATION ARCHITECTURES

When requirements are well understood, the integration design has to address: (1) the

selection of GPS and other equipment, (2) the selection of data transfer and interfaces and

(3) the selection of a software concept.

There are essentially five architectures for integrating GPS in a system navigation solution.

The resulting system architecture for GPS integration will be one of these basic structures

as discussed in the following. There is a clear distinction in the potential performance

available from the stand alone receiver (see 8.3.1) and the aided/integrated configuration

(see 8.3.2 to 8.3.4). However, the differences between the performance realized in the

aided/integrated configurations may be small unless the mission computer makes

aggressive use of GPS inputs.

8.3.1  GPS Stand Alone/Baro/Clock Aided

The GPS stand alone configuration (see Figure 8 1) shows a GPS receiver with inputs  from

an antenna set and options for barometric altimeter and clock aiding. The receiver outputs

can drive any CDU or (analog) instrument that may be required.

This configuration would be used primarily for operation in low  dynamics.  The user must

be willing to accept reacquisition times of up to two minutes or more if the receiver loses

lock because of jamming or dynamics.

ANTENNA

ARINC 461

ALTIMETER

CDU

GPS RECEIVER

ARINC 429

CLOCK

INSTRUMENTS

Figure 8 1.  GPS Stand alone Configuration

A barometric altimeter or an external clock are aiding options to coast the receiver

through short periods when visible satellites are less than four.  Especially for a

low dynamic environment, a barometric altimeter, can be very cost effective. The GPS

receiver clock or a typical external clock can have drift rates that vary widely, from 10

 10

s/s to 10

 7

 s/s. Thus, position error could grow to 100 m (95%) in a few seconds.