[LEAPSECS] UT1 offset

[Richard B Langley]

Hello all:
A couple of days ago, I started to draft a response to the question raised but got waylaid. I think others have already address the issue fairly well but here's my reply for the record (a bit long-winded):

Earth orientation data are provided in the new(ish) GPS CNAV navigation message structure and is documented in IGS-IS-200 (as already point out). They are only needed by those working in the ECI (Earth-Centred Inertial) frame. That leaves out the majority of GPS users. If you are doing orbit integration to improve the orbits of the GPS satellites you might benefit from this data. But I wonder who in the academic community, for example, is actualy using this information in the CNAV messages. I might enquire of some colleagues. By the way, GPS and the other GNSS are used daily (using mostly IGS archived data) to actually determine very accurate Earth orientation data, which is used by the IERS to create data files for Earth rotation/orientation researchers. I used to be one of them. ;-)

30.3.3.5 Message Type 32 Earth Orientation Parameters (EOP)
The earth orientation parameters are provided in Message Type 32. The parameters are defined below, followed by material pertinent to the use of the data.

...

30.3.3.5.1 EOP Content
Message Type 32, Figure 30-5, provides SV clock correction parameters (ref. Section 30.3.3.2) and earth orientation parameters. The EOP message provides users with parameters to construct the ECEF and ECI coordinate transformation (a simple transformation method is defined in Section 20.3.3.4.3.3.2). The number of bits, scale factors (LSBs), the range, and the units of all EOP fields of Message Type 32 are given in Table 30-VII.

30.3.3.5.1.1 User Algorithm for Application of the EOP
The EOP fields in the Message Type 32 contain the EOP data needed to construct the ECEF-to-ECI coordinate transformation. The user computes the ECEF position of the SV antenna phase center using the equations shown in Table 30-II. The full coordinate transformation for translating to the corresponding ECI SV antenna phase center
position may be accomplished in accordance with the computations detailed in Chapter 5 of IERS Technical Note 36: IERS Conventions (2010) and equations for UT1, xp and yp as documented in Table 30-VIII. For UT1, Table 30-VIII documents the relationship between GPS time and UT1 with ΔUTGPS and ΔU̇ TGPS. Users who may need ΔUT1 (UT1-UTC) as detailed in Chapter 5 of IERS Technical Note 36: IERS Conventions (2010) can calculate this parameter from UT1-UTC, or more accurately as (UT1-GPS) + (GPS-UTC), using intermediate quantities (UT1-GPS) and (GPS-UTC) which are produced during calculation of UT1 and UTC. Figure 5.1 on page 73 of that document depicts the computational flow starting from GCRS (Geocentric Celestial Reference System) to ITRS (International Terrestrial Reference System). Ongoing WGS 84 re-adjustment at NGA and incorporating the 2010 IERS Conventions, are expected to bring Earth based coordinate agreement to within 2 cm. In the context of the Conventions, the user may as a matter of convenience choose to implement the transformation computations via either the "Celestial Intermediate Origin (CIO) based approach” or the “Equinox based approach”. The EOPs are used to calculate UT1 (applied in the "Rotation to terrestrial system" process) and the polar motion parameters, xp and yp (applied in the "Rotation for polar motion" process). Details of the calculation are given in Table 30-VIII.

-- Richard Langley

-----------------------------------------------------------------------------
| Richard B. Langley                            E-mail: lang at unb.ca         |
| Geodetic Research Laboratory                  Web: http://gge.unb.ca      |
| Dept. of Geodesy and Geomatics Engineering    Phone:    +1 506 453-5142   |
| University of New Brunswick                                               |
| Fredericton, N.B., Canada  E3B 5A3                                        |
|        Fredericton?  Where's that?  See: http://www.fredericton.ca/       |
-----------------------------------------------------------------------------

________________________________________
From: LEAPSECS <leapsecs-bounces at leapsecond.com> on behalf of Tom Van Baak <tvb at LeapSecond.com>
Sent: January 2, 2024 10:44 AM
To: Leap Second Discussion List
Subject: Re: [LEAPSECS] UT1 offset

✉External message: Use caution.


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