Figure 4 5.  Flight Instruments and GPS

4.3  PRECISE TIME AND TIME INTERVAL INTERFACE

4.3.1  Introduction

GPS is becoming recognized as the primary time dissemination system for military and

commercial applications.  An example of a system which may use time transfer from

GPS is the calibration of atomic clocks.

4.3.2  Precise Time Inputs

A time input is used to reduce the uncertainty of the receivers initial time estimate and

thus reduce TTFF, or it may be used instead of a satellite in the navigation solution. 

The precise time input to a GPS receiver is accomplished by using a 1 pulse per

second rate representing UTC one second rollover and a Binary Code Decimal (BCD)

time code describing the pulse per second time from an atomic clock.  The pulse input

indicates the moment of the time to UTC, and the BCD time code identifies what time it

was at the UTC one second rollover.

The MIL STD 1553 PTTI Input Message time transfer mechanism uses the same time

rollover pulse input.  However, instead of labe ling the time with a BCD time input, the

HV supplies a PTTI input message via the MIL STD 1553 MUX bus to label the time

epoch.

4.3.3  Precise Time Outputs

The primary function of these outputs is to calibrate an atomic clock, or to support

other systems that require precise time.  The outputs are 1 pulse per second or 1

pulse per minute to indicate the one second or one minute rollover of UTC, and a

BCD time code that indicates  the time at the rollover epoch (Hours, Minutes,

Seconds, Day of Year, Time Figure  of Merit (TFOM)).

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