[ntp:questions] garmin 18x and linux

unruh unruh at wormhole.physics.ubc.ca
Fri Sep 9 16:37:09 UTC 2011

On 2011-09-07, Chris Albertson <albertson.chris at gmail.com> wrote:
> On Wed, Sep 7, 2011 at 9:32 AM, Greg Hennessy <greg.hennessy at cox.net> wrote:
>> I'm not even sure the USNO provided time sync to GPS is at the 2nS
>> level.
> This is not an accuracy issue. it is an intentional offset to account
> for the earth's slowing rotation rate.  here are various time scales
> and at this level they only more or less ageree as each is defined
> differently

The second has been divorced from the earth's roatation rate for a long
time, whether that is a gps second or a utc second. The link is made via
leapseconds which is an entirel ydifferent issue than how accurately gps
is steared.

> What we are talking about when we say "accuracy" is the amount of uncertainty.

No we mean how closely the gps absolute time agrees with the utc
absolute time (modulo leap seconds). 

> Here is another paper to read:
> http://gpstime.com/files/tow-time2011.pdf
> Slide #21 shows an older VT+ Receiver's PPS output compared to a hydrogen maser.
> Slide #24 is pretty good. It shows the RMS error from a newer MT+.

And the new MT+ have an offset built in. Also, if you only have a view
of part of the sky, the troposphere and ionosphere delays will not
average out, and give a time uncertainty. 


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