[LEAPSECS] FW: comments on DRR TF.460-6

[Matsakis, Demetrios]

I asked an unquestioned expert on celestial navigation about its UT
requirements.  This was his response:


For observed stars that are near the celestial equator or at low
declinations, the error can be up to 0.25 nm per uncorrected second.
This
is a systematic error, i.e., all lines of position for a fix are shifted
in
the same direction, either east or west (although not necessarily by the
same amount).  Given that  marine sextant sights by an experienced
observer
under good conditions typically have errors of around an arcminute, or 1
nm,
anything more than  about a four-second uncorrected UT1 time error would
dominate the error of a fix.

Stars near the celestial poles, like Polaris, work well for azimuth or
latitude determinations because those observations are quite insensitive
to
time errors, but they don't provide a longitude component.  If you want
accurate longitude, you can't get around the need to know UT1 at no
worse
than the several-second level.

********************************

> 

> From: ashtongj

> Sent: Tue 9/21/2010 11:17 AM

> To: Leap Second Discussion List

> Subject: Re: [LEAPSECS] comments on DRR TF.460-6

> 

> Tony asked "Are there any requirements for mean solar time other than

> astronomy and celectial navigation?"

> 

> 

> Land surveyors still use sun and star sights to find azimuth. While

GPS

> can also be used for this function, the total station (theodolite and

> laser distance meter combined) is an indispensable piece of capital

> equipment. Smaller firms and single-person firms can avoid the expense

> of GPS equipment by doing sun and star shots and not bidding on

projects

> where GPS is essential. Land surveying GPS requires a site relatively

> free of trees. Trees of course can also interfere with astronomical

> observations, but there could be gaps that would be sufficient for

> astronomical observations but inadequate for GPS.

> 

> The tree cover problem can be overcome through the time and expense of

> setting up a GPS line of known direction and traversing to the wooded

area.

> 

> The problem of not being able to obtain DUT1 directly from audible

> shortwave time broadcasts could be overcome by obtaining delta UT1

from

> the internet. I don't know if any of the software surveyors use to

> reduce their results limit delta UT1 to < 0.9 s or not.

> 

> Observe that the Astronomical Almanac still publishes Polaris tables,

> suggesting that someone out there is still obtaining directions from

> Polaris. I understand, although I haven't done the math myself, that

> depending on the position of Polaris, the time accuracy required to

> obtain an azimuth accuracy of 1 arcsecond ranges from 4 seconds to a

few

> minutes. One could easily argue that by the time the error becomes

great

> enough to create real problems, star sights will be totally abandoned.

> But the time accuracy requirement for other bodies is stricter, and it

> may not be possible to observe Polaris because of local obstructions.

> 

> Gerry

> 

> On 2010-09-21 8:26 AM, Tony Finch wrote:

>> On Mon, 20 Sep 2010, Robert Seaman wrote:

>>> 

>>> To say that "leap seconds were devised to keep the UTC time scale in

>>> close alignment with earth time making UTC useful for celestial

>>> navigation" is to suggest two unsupported assertions.  First that no

>>> other requirements for "earth time" exist, and second that UTC is

>>> responsive only to the evolving needs of those who used to have a

>>> requirement for celestial navigation.

>> 

>> Are there any requirements for mean solar time other than astronomy

and

>> celectial navigation?

>> 

>> Tony.

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