[LEAPSECS] UT1 offset

[Tom Van Baak]

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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