Towards a consistent set of parameters for the definition of a new GRS

Marti, Urs

doi:10.5281/zenodo.17404081

Towards a consistent set of parameters for the definition of a new GRS

Marti, Urs

Publisher: Zenodo
DOI: 10.5281/zenodo.17404081
Repository: zenodo.org
Language: en

Abstract

In the last few years, a consistent set of parameters for a consistent GRS was developed. The goal is to replace GRS80 as the official conventional system. This change becomes more and more necessary because since the introduction of GRS80, the knowledge in geodesy has improved significantly (e.g. GNSS, gravity space missions), the IERS conventions have changed several times and the use of the parameters became inaccurate and inconsistent over time. The acceptance of the IAG Resolution No. 1 in 2015, which defines the potential at sea level (W0), even increases the inconsistency in the geodetic parameters of the conventional GRS. The new set of parameters is based on the four fundamental parameters: W0 (potential at reference level), J2 (dynamic form factor), GM (geocentric gravitational constant) and ω (angular velocity of the Earth). All these quantities are well observed and monitored by various geodetic space techniques. Most of the defining parameters change with time. This includes seasonal variations and long-term trends. The main problem is the discussion if the replacement of GRS80 as the conventional system is really necessary. Many users do not see the advantage to replace it, as it is seen just as a conventional model for the conversion of geocentric coordinates or for the calculation of gravity anomalies. However, the working group is in favor of working towards an IAG resolution that defines a new GRS. The main goal is not to increase accuracy, but to bring back consistency into geodetic applications.

Full text

Federal Department of Defence, Civil Protection and Sport DDPS Federal Office of Topography swisstopo Geodesy and Federal Directorate of Cadastral Surveying Towards a consistent set of parameters for the definition of a new GRS Urs Marti IAG Scientific Assembly, GGOS Days / September 2025 •GGOS WG under the BPS: •Title: “Consolidation of a best estimate GRS based on the adopted W0of the IHRF” •Main goals: •To bring consistency into the use of geodetic parameters •Replace GRS80 by a more accurate reference model of the Earth • Focus is on “consistency” (not on “best estimate”) •The goal is not to determine better defining parameters that are not documented •Bases are published values •latest IERS conventions (GM, ω, J2) •SLR time series (J2) •IAG resolutions (W0) 2 New GRS • IAG 2025 Goals of the WG •Geocentric gravitational constant GM (including atmosphere) •Dynamic form factor J2 •Gravity potential on the level ellipsoid U0= W0(IHRF) • Mean angular velocity of Earth rotation ω •Physical meaning •Observable (including temporal variations) •semi-major axis a is a derived quantity 3 New GRS • IAG 2025 Four defining parameters •C20 estimations from SLR •Cheng et al. 2013 (NASA, GSFC) •Loomis et al. 2025 (NASA,GSFC) •Koenig et al. 2025 (GFZ) converted to Zero Tide (original is in Tide Free) •I see no reason to use a different value than the one from the IERS2010 conventions: J2= 1.0826359 × 10−3 ± 4.5 × 10−11 •The GRS80 value of J2is clearly off of the time series 4 New GRS • IAG 2025 Temporal variations of J2(Zero Tide) angular velocity ω from 1962 to 2025 relative to the nominal value ωN= 72921151.467064 picorad/sec value used in IERS2010/GRS80: ω = 72921150.0 picorad/sec (TCG) no significant difference between TT and TCG (Δa ≈ 0.015 mm) source: IERS EOP Combined Series 20 C04 No significant difference between ωNand ωIERS2010: IERS 2010 value can be kept semi-major axis: aωN –aIERS2010 = 4.9 mm semi-minor axis: bωN –bIERS2010 = 4.4 mm gravity at equator: γEωN –γEIERS2010 = -0.97 μGal 5 New GRS • IAG 2025 Earth Angular Velocity ω •No real time series available •Only different solutions by various authors •GM is dependent on time system •IERS conventions 2010: GM = (398 600 441.8 ± 0.8) ×106m3s-2 (TCG) •or converted to TT GMTT = GMTCG∙(1-LG) = (398 600 441.5 ± 0.8) ×106m3s-2 Difference between using GMTCG and GMTT : semi-major axis: aTT –aTCG = -4.5 mm semi-minor axis: bTT –bTCG = -4.4 mm gravity at equator: γETT –γETCG = 0.70 μGal •(GMGRS80 = 398 600 500 ×106m3s-2 ) 6 New GRS • IAG 2025 Geocentric gravitational constant GM •GRS80 represents the knowledge of the 1970ies •before GNSS •before gravity space missions •before satellite altimetry •the value of GM is inaccurate •people tend to use “better” values •this leads to inconsistencies in geodetic products •the tidal system is not properly defined •J2“excluding the permanent tidal deformation” -> TF ? •semi-major axis a: Tidal system is not defined •This again, leads to inconsistent products •The adopted value of W0(2015) for the IHRF does not fit to GRS80 and cannot really be used except for the realisation of the IHRS 8 New GRS • IAG 2025 Why not just continue to use GRS80 •The new GRS is a static system •easier to handle •more robust •time dependent GRS is not suitable for monitoring changes (gravity, sea level, ...) •Tidal system: •The new GRS is calculated in the Zero Tide System •Conversion to other tidal systems are provided •Keep it simple (neglect some second order terms) •Time system: •Everything is calculated in TCG (differences to TT are small) •We do not want to redefine TT (keep LGas it is) •LGis a defining parameter for IAU •LGis not based on the adopted value of W0 9 New GRS • IAG 2025 Some basic principles of the new GRS •W0= U0: •Adopted value from IAG resolution 2015 [Sánchez et al. 2016] •U0= 62 636 853.4 ± 0.02 m2s-2 •GM : •from IERS conventions 2010 [Ries, 2007] in TCG •GM = (398 600 441.8 ± 0.4) ×106m3s-2 •J2: •from IERS conventions 2010 (SLR time series) •J2= 1.0826359 × 10−3 ± 4.5 × 10−11 • ω : •conventional value from IERS conventions 2010 (identical to GRS80) (TCG, no significant difference to TT) •ω = 7 292 115 ×10−11 ±10−12) rad s−1 10 New GRS • IAG 2025 Current Choice of the defining parameters