4.2.2  ARINC 429 Digital Information Transfer System

The ARINC 429 data link is commonly used in commercial as well as military aircraft.  It

is a single point to multi point asynchronous half duplex data link.  That is, an

equipment can transmit data to several other pieces of equipment.  Each link is

programmed to output specific data formats at specific data rates.  The ARINC 429

specification defines standard data formats and rates for data transfer between a wide

variety of commercial avionics equipment.  However, the GPS data formats were

designed for a commercial ARINC 743A GPS/ GLONASS receiver.

4.2.3  Uses of the MIL STD 1553 and ARINC 429 Interfaces

The following paragraphs give several examples of ancillary equipment that might

communicate with a GPS receiver over the MIL STD 1553 or ARINC 429 data links.

4.2.3.1  Control and Display Unit

A CDU is often required when a GPS receiver in an equipment compartment must be

controlled remotely from a crew compartment.  The CDU allows the operator to enter

initialization data and control parameters, display status and position data, and can

provide access to related functions, such as, waypoint navigation functions.  Examples

of two types of CDUs procured by the U.S. DoD are discussed below to clarify typical

CDU capabilities.

The U.S. DoD has procured dedicated CDUs as well as multifunction CDUs.  A

dedicated CDU (or "dumb" CDU) is essentially a remote control and display panel that

possesses no processing capability, relying on the GPS receiver for all computation

functions.  A multifunction CDU (or "smart" CDU) is designed to control a GPS receiver,

perform other navigation or control functions, and may interface with additional

navigation equipment as well.  The multifunction CDU includes onboard processing

capability for functions, such as, calculating a composite positioning solution using

GPS and other navigation sensors, or performing the waypoint navigation function.

4.2.3.1.1  Dedicated CDU

A view of the front panel of a dedicated CDU with a sample display is shown in Figure

4 1. The CDU has a four line, 13 character display controlled by two rotary switches,

four line select keys, a display freeze key (Mark), a waypoint mode key, a page slew

key, and an alphanumeric keypad.  The MODE switch selects the receiver operating

mode, the DATA switch selects which parameters are to be displayed, and the

keyboard is used to make parameter entries.

In addition to the basic position, velocity, and time displays, the CDU also provides

status information on various display pages.  Some of this information is the external

interface configuration, satellite tracking status, estimated position error, age of satellite

almanac, PTTI 1 pulse per second time difference, and the Built In Test (BIT) fault log

data.  Control functions

4 2