[opendtv] Re: A full explanation of the PSIP time issue.

John posted this in his "Challenge" email earlier today:
"Lesson: the trick is getting the right time value: either UTC, or another time value plus an integer that corrects that time value to its UTC equivalent. UTC requires only UTC; GPS or other time bases require the time and a time offset. Here's another help: At Jan 6, 1980 00:00:00 UTC, GPS displays said Jan 6, 1980 00:00:07."
From all I can find, At January 6th, 1980, 00:00:00 UTC, GPS time was also January 6th, 1980, 00:00:00 GPS. The two did not diverge until 1 July 1981 00:00:00 UTC, when the first leap second since the inception of GPS time was inserted into UTC. Making the equivalent GPS time 1 July 1981, 00:00:01 GPS.
I'm not sure where he got his "GPS displays said Jan 6, 1980 00:00:07", because even in 1980 there had been 9 prior leap seconds inserted into UTC, not 7. Regardless, GPS doesn't care what happened to UTC prior to it's epoch date of 6 January 1980 00:00:00 UTC. That is time zero for GPS and only the leap seconds after that date are calculated when converting between UTC and GPS time. 

John Shutt
----- Original Message ----- From: "Paul Freeman" <paul@xxxxxxxxxxxxx> 



Hi John,
I would like to respond to this, but I have to admit that I am having a hard 
time understanding your post. As I see it, Mark Eyer's example STT
transmitted a system_time field in GPS. At the receiver side I would
subtract the transmitted GPS_UTC_offset to arrive at a time expressed as
UTC. This time would then correspond to exactly 8029 days and 13 hours (and no remaining seconds). I would then add this amount of time to my start time of 00:00:00 Jan 6, 1980 UTC to arrive at the current UTC of 1:00 PM December 
30, 2001.



-Paul





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