measurements to correct its navigation solution.  This implementation is more robust in

that it can continue to correct INS errors to some degree even when there are less than

four satellites being tracked.  A major consideration to using tightly coupled integrations

with current systems is that the pseudoranges and deltaranges corrected for SA are

classified.

8.3.3  GPS and Mission Computer/Databus Emulator

The GPS configuration with mission computer/databus emulator (see Figure 8 3) is a

means to provide the equivalent of 1553 bus operation in a vehicle that does not have a

1553 bus. It uses the mission computer (or sometimes a smart CDU) to emulate the ARINC

575 or 429 interface to GPS, as well as to control the interfaces to other navigation

sensors. The main penalty in comparison to a 1553 architecture is the limited data transfer

rate of the ARINC interfaces and the increased complexity of mission computer functions

required to support data formatting and transfer among the different sensors.

CDU OR MISSION

ANTENNA

COMPUTER

INS

ARINC 575

DATABUS

GPS

EMULATOR

RECEIVER

OR ARINC 429

OTHER

SYSTEMS

Figure 8 3.  Configuration with Mission Computer/Databus Emulator

Examples of other systems to be integrated with GPS in this configuration are a doppler

radar (velocity) combined with an AHRS. An AHRS is similar to an INS except that only HV

attitude, attitude rates, heading and acceleration are available (no position). The quality of

its output information is lower than that of a conventional INS. However, an integrated

GPS/AHRS may become a direct replacement for a high quality INS. The estimated cost of

an integrated GPS/AHRS is less than one  half that of a stand alone INS. Additionally,

yearly maintenance costs are expected to be one  third that of an INS.

8.3.4  GPS in a 1553 Databus Configuration

A GPS receiver includes a highly evolved set of interfaces to external systems.  One such

interface is the high speed databus MIL STD 1553 (see Figur e 8 4).  The 1553 databus

may be used by high dynamic, high performance HVs to quickly transfer data between an

external system and the GPS receiver.

Although a GPS receiver usually can only receive and transmit a limited number of well

defined 1553 bus data blocks, the number of options is still large. Nevertheless, an

integration cannot change the data blocks input to or output from a given GPS on the 1553