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RE: GJD to HJD
Even if the computer clock is accurate to .001 seconds the accuracy
of the exposure time is not. The exposure time is subject to several
1) The shutters are a mechanical system controlled through the stamp.
2) The shutters do not open and close simultaneously but in sequence
so the I exposure is actually 2 seconds ahead or behind the V.
3) The exposures are an averaged image of 1 to 2 minutes long.
For these reasons an error budget of 2 seconds is not out of line.
From: Chris Albertson [mailto:firstname.lastname@example.org]
Sent: Friday, June 14, 2002 12:10 PM
To: Michael Koppelman; TASS
Subject: Re: GJD to HJD
"Perfect" What a word! One centimeter of error and you are not
perfect. You would have to know the location
of the observer on the Earth's surface and not assume he was
at the center of the Earth. I think also you would need to take
into effect the pull of the moon on the path of the Earth's
orbit. The way I've seen these things done is first you pick
an allowable error. Let's say in our case, you picked
0.1 seconds. How far does light travel in 0.1 sec? OK that is
your allowed error budget for error in the observers position
relative to the sun. Do you need to use a four body (Earth, moon,
Sun, Jupiter) model or can the simple two body (Earth Sun) model
work? No way to know without doing it both ways.
The folks across the way from here run the GPS system. They worry
about things like the different speeds of light in air in
pressure gradient then in vacuum and special relitivy's
effect on a clock in orbit vs. on the ground and still they
don't get "perfect".
It would appear that
the JD -> HJD conversion is the largest source of timing uncertainty.
Our computer clocks are very easy to keep to <<1 second.
I'd say that unless we can get the error introduced by the conversion
to HJD two orders of magnitude below other sources of timing error
we keep UTC for the archive. I know we can live with a few seconds
of error but it is just plain embarrassing that our data reduction
process should be the largest source of error in a measurement.
--- Michael Koppelman <email@example.com> wrote:
> I'd appreciate if someone could double-check my calculation. As I
> it is based on the same code as the SLA library, so it should be spot
> but I did have to do a few date manipulation things and I'd
> appreciate a
> double-check. Here are two examples:
> digital [10:47pm] % ./jd2hjd 2452425.6907291 179.470833 6.375277
> correction is 0.001791
> digital [10:41pm] % ./jd2hjd 2452431.6260590 179.470833 6.375277
> correction is 0.001234
> The arguments above are jd, ra and dec.
> This matches within 4 seconds of Lew Cook's spread sheet. He claims
> accuracy near 3 seconds. Can anyone check this with something known
> to be
> Michael Koppelman
Home: 310-376-1029 firstname.lastname@example.org
Office: 310-336-5189 Christopher.J.Albertson@aero.org
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