[LEAPSECS] UT1 offset

[Seaman, Robert Lewis - (rseaman)]

Hi Tom and Mike and all,

I suppose we weren’t talking about DUT1 time signals?

See http://futureofutc.org/2011/program/presentations/AAS_11-675_Malys.pptx.pdf for details about the flipside question of operating a GNSS constellation (current as of a dozen years ago).

One shouldn’t find it surprising (at least, I don’t find it surprising) if navigating and calibrating constellations of Earth-orbiting satellites requires knowledge of Earth orientation. At some point during the 2011 Exton workshop, there was a discussion of GPS being able to detect motions due to plate tectonics. Earth orientation doesn’t necessarily need to be provided in terms of UT1, and the temporal geophysicists presumably need higher-order moments as well. One doubts the majority of satellites need such precision.

Rob

On 1/2/24, 7:45 AM, "LEAPSECS" wrote:


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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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