[LEAPSECS] UT1 offset

[Mike Hapgood - STFC UKRI]

Jim outlines a calculation I've done many times. But there's a similar calculation for GNSS systems (GPS, Galileo, Beidou, etc). If you want to use GNSS to determine positions on Earth's surface to accuracy of a few metres, the system needs an estimate of current UT1 accurate at least to a few milliseconds. That estimate is applied internally so the end user does not need to know the details, just as that user does not need to know about the relativistic clock corrections or corrections for ionospheric signal delay that also underpin safe use of GPS. But the bottom line is that knowledge of UT1 (i.e. the spin phase of the Earth) is essential for GNSS - and many other space systems.

Mike

________________________________
From: LEAPSECS <leapsecs-bounces at leapsecond.com> on behalf of Jim Lux <jimlux at earthlink.net>
Sent: 26 December 2023 15:40
To: Leap Second Discussion List <leapsecs at leapsecond.com>
Subject: Re: [LEAPSECS] UT1 offset

Let’s back of the envelope the impact of a 1 second error in a longitude sight.
The Sun moves 360 degrees in 86400 seconds. A one second error is then about 0.004 degree.  But in equatorial km, let’s assume 40,000 km circumference, so 40,000 km in 86,400 seconds (yeah, it’s actually less, sidereal day and all that). Or an error of about 1/2 km.

For the vast majority of boat driving purposes, this is sufficiently accurate - you need to get to within visual range of the coast, so you can get into the desired port.

For flying a plane by celestial navigation, you might care more, because the plane is flying faster than a boat moves.


Sent from my iPad

> On Dec 26, 2023, at 7:10 AM, Gary E. Miller <gem at rellim.com> wrote:
>
> Yo Hal!
>
>> On Mon, 25 Dec 2023 00:25:07 -0800
>> Hal Murray <halmurray+leapsecond at sonic.net> wrote:
>>
>> Who uses DUT1 via radio?  Who will be using it 50 years from now?
>>
>> Is it needed for anything other than navigation and astronomy?
>
> I just asked my brother that did a lot of transoceanic navigation
> by sextant.  He did not even know what UT1 is.  He certainly would
> not know what to do with DUTs.
>
> RGDS
> GARY
> ---------------------------------------------------------------------------
> Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
>    gem at rellim.com  Tel:+1 541 382 8588
>
>        Veritas liberabit vos. -- Quid est veritas?
>    "If you can't measure it, you can't improve it." - Lord Kelvin
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