Re: [LEAPSECS] UT1 offset
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: l...@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 on behalf of Tom Van Baak 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
Re: [LEAPSECS] UT1 offset
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 > 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 on behalf of Tom Van Baak > > Sent: 02 January 2024 14:44 > To: Leap Second Discussion List > 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 ___ LEAPSECS mailing list LEAPSECS@leapsecond.com https://pairlist6.pair.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] UT1 offset
Most spacecraft don’t know any sort of time in an absolute time scale. They have a free running counter at some rate, and everything is done in terms of SCLK. (Spacecraft Clock). Someone on the ground does a process called “time correlation” to relate local clock on spacecraft to some other system (TAI, UTC are popular). You send commands up with a “do it at this time” in terms of the local clock ticks, and telemetry comes down time stamped with the local clock. That may get turned into TAI or UTC on the ground, or even if you’re working in TAI on board, there’s some stuff required to deal with the jump from “no fix” to “have fix”. Once you have a fix, if the GPS solution goes away, you just flywheel on forward, and either have a transient when GPS comes back or try to smoothly pick it up. Recently, (last 20 years) with the advent of inexpensive GPS receivers, a lot of cubesats use them in some way, but a lot don’t actually use GPS time on board - after all, you need to have some time scale to use before you’ve got a GPS fix. That GPS time (and position) would probably be reported in telemetry. It might also be fed to your attitude control and determination subsystem, so it can be used with the star tracker to determine attitude and position, which then gets propagated forward. But that’s a fairly low precision need (1 second), and there have been plenty of satellites where the GPS receiver failed, and they just uplinked the “current GPS time” to replace it. For satellites that are making precision measurements (e.g. radio occultation with GPS) the GPS onboard data is just sent to the ground as raw observables, often without ever trying to get a solution on board. The manipulation to some Earth based time scale is all done on the ground in post processing. It’s pretty unusual for there to be a need for time knowledge on board in a standard time scale to “better than a second” kinds of precision. My own SunRISE mission needs to know time to a microsecond onboard, but it doesn’t have to be any particular time scale, just the same across all six spacecraft. It happens to be GPS time. We do determine time in post processing to a bit better than a nanosecond. This is actually becoming more important - if you want spacecraft autonomy, AND you want multiple spacecraft working together, having them all on a common time scale is important. But it’s still sort of done in an ad hoc way - that is, while there’s plenty of CCSDS standards on how to report and keep time, the implementations vary. Sent from my iPad > On Jan 3, 2024, at 6:28 AM, Seaman, Robert Lewis - (rseaman) > wrote: > > > 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: > > External Email > > 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) i
Re: [LEAPSECS] UT1 offset
Yo Tom! On Tue, 2 Jan 2024 06:44:01 -0800 Tom Van Baak wrote: > 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? From the IS-200M: 30.3.3.5.1.1 User Algorithm for Application of the EOP. "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. r RGDS GARY --- Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703 g...@rellim.com Tel:+1 541 382 8588 Veritas liberabit vos. -- Quid est veritas? "If you can't measure it, you can't improve it." - Lord Kelvin pgpza1wa4c5t8.pgp Description: OpenPGP digital signature ___ LEAPSECS mailing list LEAPSECS@leapsecond.com https://pairlist6.pair.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] UT1 offset
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: External Email 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
Re: [LEAPSECS] UT1 offset
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 on behalf of Tom Van Baak Sent: 02 January 2024 14:44 To: Leap Second Discussion List 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
