John,Here is the nub of the differences between you and me. In my world you were not off by 2 seconds, you were off by 7.
First of all, an "epoch" date for GPS means that on that date both clocks were synchronized to the same time. "Time zero" if you wish. So on 6 January 1980, 00:00:00 UTC, the GPS time was also 6 January 1980, 00:00:00 GPS. If you don't believe that then all I can do is point you yet again to the definition of GPS time on the National Institute of Standards and Technology website that you will again refuse to read because you object to it as a "secondary source" and therefore not reliable.
If, as you assert, that GPS time is offset from UTC time by all leap seconds that have been observed prior to 6 January 1980, 00:00:00 UTC in addition to all of the ones observed after, then ask yourself this question: Why is the current GPS to UTC offset 14 seconds? We can both agree that the current GPS to UTC offset is 14 seconds?
By using your assumption that GPS time was 00:00:09 when UTC time was 00:00:00 on 6 January 1980, the current GPS to UTC offset should be 23, because there have been 23 leap seconds observed in UTC since the first one was inserted on 23:59:59 30 June 1972.
Is the GPS to UTC offset 23 seconds? No, it is 14 seconds. Many places on the web confirm that fact. Why is it 14 seconds and not 23, John? Because on 6 January 1980, GPS time and UTC time were the same, and since then there have been 14 additional leap seconds observed in UTC, and those are the only ones that concern GPS time.
In addition to the definition of GPS time from the NIST, I also gave you a reference (at the bottom of my response to your Challenge) to a website with a JavaScript clock that showed the current time in UTC, GPS, TAI (Atomic time), and Loran C time, based on your PC's clock. From that website, you can see that the author of that program also thought that GPS time is currently 14 seconds ahead of UTC time. Working backwards from now to 6 January 1980, 14 leap seconds have been observed in UTC, so of we subtract those 14 seconds of offset, we come to find that GPS time and UTC time were identical on 6 January 1980, not 9 seconds offset.
However, as I said previously to you, please continue to labor under your understanding of GPS time because in the end it doesn't matter. The <system_time> variable in the STT is expressed as the number of GPS seconds that have elapsed between GPS epoch and the current moment. The EIT variable <start_time> is also conveyed as the number of GPS seconds that have elapsed between GPS epoch and the program start time.
You are of course absolutely correct that the integer time value, in GPS or UTC, is never expressed in the form MM.DD.YYYY HH.MM.SS, anywhere in STT or EIT, and because of that it doesn't matter what you think GPS time should read at any given moment, nor what I think is should read. It doesn't matter if you call the values contained in <system_time> and <start_time> UTC times and I call them GPS times. All that matters is that your PSIP generator need to spit out the correct values for <system_time>, <start_time> and the hundreds and hundreds of other values required for PSIP, and your software does it extremely well. The fruits of your labor speaks volumes.
I must say, John, that after reading A/65C, I am truly in awe of any one person who could write code that can keep track of all the crap that has to go into PSIP, and do so flawlessly every few milliseconds. The MGT, STT, RTT, TVCT, rotating EITs and ETTs, etc. It boggles my puny mind that any one person could "grok" all of that and end up not only with a working product, but with his sanity intact. And you have done it not once, but several times as you had to recreate your work from scratch through a nasty hard drive crash, as I recall. It is a monumental task that I know I could never even begin to duplicate. I respect you for your accomplishments, John, and I'm sorry to have angered you.
Regards, John----- Original Message ----- From: "johnwillkie" <johnwillkie@xxxxxxxxxxxxx>
To: <opendtv@xxxxxxxxxxxxx> Sent: Thursday, May 24, 2007 10:23 PM Subject: [opendtv] Re: A full explanation of the PSIP time issue.
So, I was off by 2 seconds on a 27 year old time value. Franky, I couldn'tremember if it was 7, 8 or 9, and I didn't care to look it up, because GPS representations of time are irrelevant to STT time, although the count of GPS seconds is important. Less worse than being off 14 seconds now. So, starting on July 1, 1991, you couldn't code STTs using GPS time displays, and you can't now. This is before the grand alliance was even formed, so you are tending now to support my view? John Willkie
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