[time-nuts] June 30 2015 leap second
jimlux at earthlink.net
Sat Jan 10 10:49:09 EST 2015
On 1/9/15 4:57 PM, Henry Hallam wrote:
> Such slewing solutions are OK for Google. They wouldn't work well for
> one of the systems I work with, which uses system time to calculate
> the position of a LEO satellite for purpose of pointing a 7.6 meter
> X-band dish. Half a second of error corresponds to a pointing error
> of 0.5 degrees, well outside the main lobe of the antenna beam.
Which is why we use TAI in the space business and don't fool with this
"Greenwich Mean Time" or "Coordinated Universal Time" which is
discontinuous and potentially non-monotonic.
We DO need to compensate for the earth's varying rotational speed,
though, but that's just handled as a separate model for deep space, or
for LEO, where the coordinate system is Earth Centered, absorbed into
the spacecraft orbital elements.. nobody is going to use 1 year old
I do find myself explaining exactly WHY we can't just use PC system
time, etc. and periodic leap seconds are an object lesson in why not.
(or, you can arrange to not being doing any operations at the moment of
leap...that's been done too)
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