[time-nuts] Using the HP 58503a to correct your PC clock

Chris Albertson albertson.chris at gmail.com
Wed Aug 3 02:28:06 EDT 2016

The WRONG way to adjust a PC clock is to set the TIME periodically from
some standard.  When you do this then the time on the PC is not running at
a constant rate.  The correct way to do this is to adjust the PC's clocks
RATE.  You make it runs slightly faster if you notice it is getting behind
and slightly slower if it is running fast.

Think about what you would do to a real physical clock.  You would not set
it every few minutes, you'd adjust the rate and wait a little while to see
if the adjustment needs refinement or not.

Most computers can run "NTP" what does this.  In fact, using NTP you don't
need a GPS receiver to keep the clock to within a few milliseconds of UTC
as NTP will synchronize with other NTPs running on servers on the
Internet.  Yes you can pull millisecond level time over the Internet.   But
using a GPS receiver NTP can keep your PC's internal clock accurate at the
hundreds of microseconds level.

NTP is an interesting piece of software.  With the very long delays over
the Internet you'd think you'd never be able to accurately transfer time
but you can get millisecond level accuracy even over a communications path
that has a 100 millisecond delay.  The trick is to think about what you
would do if you lived in the 1700's (before the telegraph) and owned a
grandfather clock that was to big to carry and where given the task of
setting it to match a clock that was a ten minute walk away in a different
house.   You best plan would be to first walk the round trip between the
two houses many times and measure the time it takes then add /2 the round
trip delay.    Later you find that other neighbor own clocks so you start
making round trips to their houses too and keeping good notes on the trip
times between all the clocks.  You could set everyone's clock based on the
consensus of other nearby clocks.   Next you'd notice that some clocks are
just poorly made and you'd ignore these when building the consensus.   This
is exactly what NTP does

Most operating systems in use today run NTP to keep their clocks in order.
Well most OSes except for Windows.  Microsoft uses a vey much simplified
version of this that does the wrong thing and periodically sets the PC's
clock.   You could enable this and likely, maybe reach your +/- 100ms goal.
  Not the "real" NTP is a free program and not hard to set up so you can
have 1ms level accuracy without much effort and better with some work.

On Tue, Aug 2, 2016 at 8:13 PM, Ron Ott <ronott at sbcglobal.net> wrote:

> This has probably been covered in the past, but is there a way correct or
> control a PC (Windows 7) clock with the HP 58503A GPS receiver? I just
> bought one (on the way now) and have a copy of satstats50 on hand. I've
> been using Dimension 4 and I'm surprised at the size of correction every
> couple minutes to my PC clock.  I'd be happy if my PC clock were accurate
> to plus/minus 100ms.
> Ron
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Chris Albertson
Redondo Beach, California

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