[LEAPSECS] UT1 offset

[Mike Hapgood - STFC UKRI]

Hi Tom,

The key issue is simply that the Earth is a spinning body - and UT1 is a measure of the phase of that spin.

For GNSS to work the satellites must know where they are relative to the spinning Earth. So the ephemeris data uploaded to each satellite are presented in way that delivers a coordinate system spinning in phase with the Earth. Typically these data are pseudo-Keplerian orbit elements, and the algorithms for their use are described in a range of user documents, textbooks and presentations.

But the satellite ephemeris data are not the actual Keplerian elements, as would be applied in an inertial coordinate system. However, inertial coordinates are the natural baseline for recording the evolution of satellite orbits, e.g. by assimilation of tracking data including bearings, ranging and Doppler - also for exchange with other operators (e.g. for collision avoidance). So GNSS ground systems have to transform their satellite positions from inertial to spinning Earth-centred coordinates. That requires UT1.

So UT1 is important for GNSS. End users do not need to know the details, but it would be good to raise awareness that those details are handled well on their behalf. Similarly, it would be good if users were aware that GNSS is an engineering application of general relativity.

Best wishes,
Mike
________________________________
From: LEAPSECS <leapsecs-bounces at leapsecond.com> on behalf of Tom Van Baak <tvb at LeapSecond.com>
Sent: 02 January 2024 14:44
To: Leap Second Discussion List <leapsecs at leapsecond.com>
Subject: Re: [LEAPSECS] UT1 offset


Hi Mike,

> the system needs an estimate of current UT1

Can you give some references to your observation? I don't recall seeing UT1 mentioned in the first couple of decades of GPS documentation. The system runs on GPS time, the WGS84 coordinate system, broadcast ephemeris including SV clock corrections. Where does UT1 appear in those?

> That estimate is applied internally so the end user does not need to know the details

Right, the user is shielded from many details. But I didn't think even GPS receivers had knowledge of UT1, nor the satellites themselves. So where in "the system" does UT1 apply?

Thanks,
/tvb

On 12/28/2023 1:23 AM, Mike Hapgood - STFC UKRI via LEAPSECS wrote:
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

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