[time-nuts] UTC and the speed of light?

Magnus Danielson magnus at rubidium.dyndns.org
Tue Aug 30 15:18:19 UTC 2011

```On 30/08/11 09:40, Chris Albertson wrote:
> How is the speed of light accounted for in the definition of UTC?

They haven't.

> In other words, how did they solve the conflict where on one hand we'd all
> expect two "perfect" clocks to "tick" at the same time but wether they do
> depends on the location of the observer?

If we only focus on speed of light, then you will have to compensate the
one-way time transfer with the expected delay between the first clocks
location and the second clocks location. If the locations isn't well
established for the precision needed, or the speed of light changes over
time (such as the ionosphere does) then we need to measure it somehow.
One such method is called two-way time-transfer in which you transfer
time from both clocks and compare with the local clock on reception.
These differences can then be added to form the round-trip delay, and by
reciprocal approximation assume that the delay is the same in both
directions the one-way delay can be estimated by letting it be half the
round-trip. Now, using the estimated one-way delay you can add the
estimated delay to the received time measure, and you will get the time
of the remote clock at the time of reception. Taking the difference
between this adjusted remote clock and the local clock provides you with
the time-error between the clocks and you can use that to lock the local
clock to the remote clock using normal PLL techniques.

Two-way time-transfer thus allow for eliminating the delay effect of
distance and within some bias and unstabilities faithfully reproduce the
remote clock.

GPS is a one-way time transfer technique in which multiple one-way
observation resolve the position and clock bias of the receiver, with
that the pseudo-distance between the receiver and satelites become known
within a certain amount of bias and noise.

It does not however require the UTC to incorperate the speed of light.

However, TAI and UTC does care about gravitational position of the
clocks, as these causes rate differences between clocks. Yet further
effects of relativity plays tricks with us.

Cheers,
Magnus

```