What Is the Epoch Time for the WGS84 Coordinate System?

[mjdiaz89]

Hello,

I've been searching all over for the epoch time of the WGS84 coord system. A GPS I'm using says its 06/01/1980, but I don't know if starts from 12:00:00 ET. Is there an epoch by definition or is it arbitrary?

 

[mgb_phys]

There isn't a precise epoch for the coordinate system - the 1984 is just when the data was agreed. There are slight real changes to the values with time, but the real changes come with more accurate measurements.

There is a start epoch to the GPS clock data but that's just a software detail, GPS time doesn't include leap seconds - so GPS and UTC were synced in 1980 but GPS now lags by a number of seconds.

 

[mjdiaz89]

Interesting that there isn't a defined epoch, only a synced time. Yes, I believe the lag is 13 seconds.

Thank you very much :)

 

[mgb_phys]

Yes, astronomical coords need a definite epoch because the star positions are moving.
Although the Earth's shape really changed between say Airy 1837 and WGS84 it's insignificant compared to the change in the measurement accuracy.

Just to confuse things further - most GPS receivers do give UTC, the number of leap seconds is sent to the satellites as part of their update data.