[time-nuts] TymServe 2100 1995 Issue - A Kludgy Fix

[Tom Van Baak]

Hi Chuck,

It's not that simple. First, it's not 20 years, but 1024 weeks (19.6 years). And not UTC weeks (which may have leap seconds) but GPS weeks (which do not, and are always 604800 seconds long).

So you have to convert the incorrect UTC date and time back to GPS date and time, then apply the 1024 GPS week correction, and then convert back to UTC. This requires knowledge of all leap seconds during the past 1024 week cycle and this information is not present in the GPS signal or in the binary or NMEA messages that come out of a GPS receiver. Don't forget to account for all the leap years during that period too: 1024 weeks is 19.638 normal years but 19.585 leap years. And when you power-on the GPS receiver it may have the wrong leap second count as well, wrong for both 1995 and wrong for 2015. You have to wait up to 12.5 minutes for that information to come down at 50 baud.

Not saying it isn't possible, but it's not easy. And then you need to test it against last week and this week, and the week before and after June 30 of this year when the next leap second occurs. I realize the TymServe 2100 issue is unrelated to leap seconds. But leap seconds severely complicate any "simple" conversion between time scales, especially if you are interested in second or sub-second accuracy.

/tvb

----- Original Message ----- 
From: "Chuck Harris" <cfharris at erols.com>
To: "Discussion of precise time and frequency measurement" <time-nuts at febo.com>
Sent: Tuesday, May 05, 2015 6:33 AM
Subject: Re: [time-nuts] TymServe 2100 1995 Issue - A Kludgy Fix


> Seems to me that the obvious simple answer works this way:
> 
> Since the GPS must use an RS232 connection to communicate
> its information to the other devices in the telescope, all
> that need be done is to write a fairly trivial program to
> run on a PIC, or Arduino, that when presented with the date,
> adds 20 to the year, and then sends it on to the rest of the
> system.  Everything that is not a date gets passed through
> unmolested.
> 
> -Chuck Harris