Airbus GPS / Clock
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Airbus GPS / Clock
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
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
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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'
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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?
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Leap seconds?
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:
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.
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:
Code:
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]
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.
Last edited by ATCast; 11th Jun 2012 at 12:13.
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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...
Last edited by FlightPathOBN; 11th Jun 2012 at 16:05.