View Full Version : Time on ADS position reports

10th Jan 2010, 17:42
Hello, I'm a controller doing some work related to a separation standard in a procedural environment utilising ADS(C) postion reports.

The thing I need help with is understanding where the time from the position reports comes from. In other words do all aircraft FMCs have the same time? Does the time given in the ADS report even come from the FMC?! Where and when is the time checked? Do different equipment manfacturers have different sources for updating the time? Do different airlines have different procedures?

Sorry if this is all very vague; what i'm trying to establish is how possible it would be for me as controller to see two position reports from different aircraft indicating five minutes separation between the aircraft when in reality there could be (much) less between them!

Thanks in advance for your help.

10th Jan 2010, 18:03

Those aircraft equipped with GPS will be using the GPS time clock. Other wise they will be using the aircraft's clock which is set manually (normally with the GMT time pips from the radio)

vikrant soni
10th Jan 2010, 18:31
Hello G-OFUK,

The pilot should always set zulu time in the aircraft clock.

If you are using gps it is automatically updated with precise zulu time.

If the FMC is used it also uses gps to update, so it is also very precise.

If the pilot has to use the watch to keep time it should be updated eith precise zulu time by pilot within +/- 30 seconds (ICAO), which is also the accuracy with which the ATC reports time.

And it is always strongly recommended for pilots to always do a time check with ATC prior to flight to set Zulu time within +/- 30 secs. If it is done it will ensure proper and standard time keeping, no matter where you fly.

Hope That helped :)


Spooky 2
10th Jan 2010, 18:58
Highly unlikely that an aircraft equipped for ADS reporting would not be utilizing GPS time but there may be a few running around out there.

10th Jan 2010, 20:12
FANS enabled aircraft utilize GPS actual time for the ADS position report which is UTC.

Spooky 2
10th Jan 2010, 20:19
Actually I believe GPS time lags UTC time by around 15 seconds.:}

10th Jan 2010, 21:09
Wonderful stuff, thanks all for your replies - you've been most helpful.

So, as I understand it from the answers given, all ADS reporting A/C should hold the same time. Is there any way at all the pilot can manually override this? And if so, under what circumstances might this happen?

10th Jan 2010, 22:45

Interesting initial question.

It is possible for a switch on my aircraft's clock to be to placed in the non normal "SET" position rather than the "GPS" position. From the write up (copied below), it would be possible to set a different time to GMT / UTC. I guess this clock time would be transmitted by ADC - but I'm not sure. Somebody else with ADC may be able to confirm.

From FCOM A320
"A fully independent clock is on the right side of the control panel.
It sends time to the centralized fault data interface unit, the flight data interface unit, and the flight management and guidance computer.
The clock performs four functions :
It displays "UTC" (GMT) time in hours, minutes and seconds on the center counter.

It displays elapsed time (ET) (from engine startup) in hours and minutes on the lower counter.

It drives the chronometer (CHR), which measures a time interval (from the pushing of the CHRONO button) in minutes and seconds.

It can replace the UTC with the date.

UTC selector
"GPS" : Time (or date, if selected) is displayed, and this data is synchronized on GPS information.

Note :
If the signal between the GPS and the clock is not detected, dashes are displayed. Only the "INT" and "SET" positions are then available.

If the signal is detected, but GPS data is invalid, the clock automatically runs on its internal time.

The clock will automatically resynchronize on the GPS information, as soon as the GPS data becomes available.

"INT" : Internal time (or date, if selected) is displayed.

Note :
The clock's internal time is always synchronized with the latest valid GPS information.

If there is no valid GPS information at power up, the internal time will be 00:00:00, until the clock is initialized, or until valid GPS information is present.

"SET" : Allows the internal time and date to be initialized. "

10th Jan 2010, 22:56

The same question was asked in http://www.pprune.org/tech-log/398817-what-s-time-please.html . Worth checking.


13th Jan 2010, 01:13
We have some later generation 737ng, you cannot adjust the clock it is automatic via GPS