Airbus GPS / Clock

[PGA]

I know it's pure trivia, but it has made me wonder anyway.

While selecting GPS monitor in the FMGC the GPS time is displayed for both GPS 1 and 2. According to the FCOM the onboard clock takes it time from said GPS, however, it is always around 7/8 seconds fast. This goes for both the 320 / 330 / 345, I have flown and seen this in all 3 types.

On the 343 it's different since the onboard clock doesn't use the GPS time, and somehow the minutes tick over when the second hand reaches 30'

Does anybody know why the onboard clock is running ahead of the GPS clock, even though it supposedly takes it's time from the GPS clock?

Thanks for sharing your thoughts :)

[Check Airman]

On the 343 it's different since the onboard clock doesn't use the GPS time, and somehow the minutes tick over when the second hand reaches 30'

My Math teacher would call that premature rounding off:=

[A Squared]

Does anybody know why the onboard clock is running ahead of the GPS clock, even though it supposedly takes it's time from the GPS clock?

I'm not claiming to know, but one possibility is that the onboard clock is not running fast. The time displayed on the FMS in GPS monitor mode may be slow because of the latency of the display. The internal time of the GPS receiver is accurate to a micro-mini second, but the actual display may lag behind that some amount because of delays in the system.

[aveng]

Are you looking at the time on the Status display? Cos this is derived from the CMC, not directly from the GPS clock.

[ATCast]

I am not familiar with the Airbus systems and what time is displayed on the FMS. As mentioned by A Squared latency in the display may very well be the cause.

Another factor could be that GPS time is 15 seconds ahead of UTC. It's not that GPS is fast, UTC is slow. UTC has had 15 leap seconds since the start of GPS time on January 6th, 1980 00:00.000000000 UTC.
LORAN, running since January 1958, is now 24 seconds ahead of UTC.
In fact next month UTC will be 16 seconds behind GPS as another leap second is introduced on the 30th of June this year.

The following NANU was published recently:
NOTICE ADVISORY TO NAVSTAR USERS (NANU) 2012034

SUBJ: LEAP SECOND
1. CONDITION: THE INTERNATIONAL EARTH ROTATION AND REFERENCE SYSTEMS
SERVICE (IERS) HAS ANNOUNCED THE INTRODUCTION OF A LEAP SECOND TO
OCCUR AT THE END OF JUN 2012

2. COORDINATED UNIVERSAL TIME (UTC) WILL SEQUENCE AS FOLLOWS:
30 JUN 2012 23 HOURS 59 MINUTES 59 SECONDS
30 JUN 2012 23 HOURS 59 MINUTES 60 SECONDS
01 JUL 2012 00 HOURS 00 MINUTES 00 SECONDS

3. FOR GPS, AS WITH PREVIOUS LEAP SECOND UPDATES, THE UTC DATA IN
THE NAVIGATION MESSAGE WILL CHANGE IN ACCORDANCE WITH IS-GPS-200.

BEFORE THE LEAP SECOND
GPS-UTC IS 15 (GPS IS AHEAD OF UTC BY 15 SECONDS)

AFTER THE LEAP SECOND
GPS-UTC WILL BE 16(GPS WILL BE AHEAD OF UTC BY 16 SECONDS)

4. POC: CIVILIAN - NAVCEN AT 703-313-5900, HTTP://WWW.NAVCEN.USCG.GOV
MILITARY - GPS OPERATIONS CENTER AT HTTPS://GPS.AFSPC.AF.MIL/GPSOC, DSN 560-2541,
COMM 719-567-2541, [email protected], HTTPS://GPS.AFSPC.AF.MIL
MILITARY ALTERNATE - JOINT SPACE OPERATIONS CENTER, DSN 276-3514,
COMM 805-606-3514, [email protected]
GPS receivers can output their time in UTC; the offset of UTC from GPS time is included in the navigation messages downloaded from the satellites.
If the GPS outputs GPS time instead of UTC on the databus, this would cause the displayed time to be 15 seconds ahead. It does not really match with the 7/8 seconds you have seen.

Of course it could also be that someone hard coded a 7 seconds difference between UTC and GPS time when they designed the box in the early 90's. I have seen bigger mistakes.

[FlightPathOBN]

Latency sounds about right, you didnt mention what FMS you are using, latency on the Universal is 0.74 seconds, which is close to what you are reading...

[MD83FO]

I also understand GPS satellite clocks are off a few seconds to account for special relativity.