I’d venture that nothing *on the satellite* is aware of UT1. That would be done in the ground segment. The satellite itself probably does not know where it is, it just plays the specified messages generated from the model based on the internal clock. On the ground, they measure the observables, calculate the ephemeris, fit it to a model, and then send the revised model parameters up, which then get sent out as part of the almanac and nav messages.
Sent from my iPad > On Jan 3, 2024, at 6:28 AM, Mike Hapgood - STFC UKRI via LEAPSECS > <leapsecs@leapsecond.com> wrote: > > > 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-boun...@leapsecond.com> on behalf of Tom Van Baak > <t...@leapsecond.com> > Sent: 02 January 2024 14:44 > To: Leap Second Discussion List <leapsecs@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 > > _______________________________________________ > LEAPSECS mailing list > LEAPSECS@leapsecond.com > https://pairlist6.pair.net/mailman/listinfo/leapsecs
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