On 16 Feb 2016 at 18:36, Richard Aiken wrote:

> The Imperium ignores individual planetary time, clocking everything
> on Sylean (e.g. Imperial) time. This drives the locals crazy, of
> course. Particularly because it's merely a legal fiction (since there
> is something in quantum mechanics - ask somebody with the math
> exactly what - which makes keeping accurate time on an interstellar
> scale impossible).

Actually, it's not that hard. a bit expensive, and definitely time
consuming, but quite doable. And it'd be needed for long range
surveying of distant stars anyway.

Below is a rough draft of an article that'l eventually get posted on
my website.

Jump calcs need very high precision. That means that you either need
to know where you are jumping to (relative to your starting point)
with great precision or you have to aim for an exit point that is
well away from anything. (fortunately, the very nature of jump means
you can't jump in too close to anything).

Because of this, the ISS has set up a grid of observatories with very
long baselines to survey the Imperium and beyond.

The parsec derives from the use of such a baseline by astronomers on
Earth to survey the nearer stars. For this, the baseline was the
diameter of earth's orbit (2 AU). A parsec is the distance at which a
star showed a shift of one second of arc in position between
observations made at opposite points on the earth's orbit.

That makes a parsec arctan(1 sec of arc) AU. Which is a bit over
200,000 AU.

The ISS needed something much better. So they (or rather the
astronomers and stellar cartographers who preceded them) set up a
baseline between two stars.

Each was far out in its star system. They used powerful lasers
modulated to carry a precise time signal. It took years for the beam
to cross the gap between the systems, And longer still for the return
of the time signal. But once the round trip had been made, they knew
the precise distance between the stations.

Relative motion (radial motion) is determined from the frequency
shift in the beams.

The data sent is essentially "at our time X, we received your signal
sent at time Y". That easily allows calculation of the separation at
the moment the reply was sent. Add in the relative velocities, and
you get a good estimate of the current distance.

You can also use this data to "synchronize" the clocks between the
two stations to high precision.

Once this is done, you take high precision measurements of stellar
positions at each station, just as was originally down at opposite
sides of a planet's orbit. Only this time your baseline isn't 2 AU,
it's a parsec or more.

At even 100 parsecs distance, the position shift of the stars
observed is measured in degrees not mere seconds of arc. This means
you know to very high accuracy where all the stars for several
sectors around your baseline are.
[add some calcs and table]

Being far out in their respective star system also allows for the use
of huge telescopes. Even if the mirrors are actually synched,
multiple mirrors, you still have a huge aperture.

This means that planets and even asteroids in other systems can be
tracked and often imaged.

So when Scouts are sent out to explore "new" systems, they already
know where the planets and many of the smaller bodies are, what their
orbits are like, and much of the physical data about the planet.

They'll have the size digit of the UPP nailed. They'll have a good
idea of the atmosphere (thickness, yes, taint, maybe, maybe not)[add
table]. And a rough idea of the hydro digit (say +/- 3)

You need baselines in several directions as your accuracy goes down
the farther the stars observed are from being at a right angle to the
baseline.

It still takes years to set up (figure 7-8 years per parsec between
nodes, after the stations go online).

For the most part, you'll want reasonable pop and tech levels for
systems that are one end of a baseline. Makes it easier to maintain
them.  But you also want to cut down on the time it takes to
establish the link. Which argues for closer in.

It could take a century or two for a link over 30 parsecs (a sector).
But to the Imperium, it might be worth it. Though I suspect that
you'd have severe signal strength problems.

The time signals allowing synchronization lead to a secondary use.
Due to the vagaries of jump, you can't just stick an atomic clock
onto a ship and jump to another system to synchronize time between
them.

But sufficiently powerful radio transmission (or laser links) can
cross between systems and be used to synch clocks even if the system
isn't part of a baseline link. This is how systems not on a baseline
keep their local timebase close to Imperial Standard Time.

Even though laser links are more efficient, radio broadcasts (of
extreme power) are used. This allows ships that have misjumped to
locate themselves if they can rig an antenna and pick up three
separate time signals.

Again, not all systems will have time signals being sent
interstellar. But most average pop systems with at least TL ??? will.

--
Leonard Erickson (aka shadow)
shadow at shadowgard dot com