Local gravity network as a reference station for the IGRF Ilya Oshchepkov Roman Sermyagin Department of Gravimetry and Geodynamics Center of Geodesy, Cartography and Spatial Data Infrustructure (CGCSDI) Moscow, Russia EGU General Assembly 2019 Apr. 8 – 12, 2019 Vienna, Austria
Outline 1 International Gravity Reference Frame (IGRF) 2 Moscow gravity network Structure and collocation Reference, comparison and core station for the IGRF Network maintenance Vertical and horizontal gravity gradients 3 Ensuring traceability of measurements Traceability in time–variable gravity field Loading effects on gravity Measurement frequency requirements Calibration of relative gravimeters 4 Conclusions and future work
International Gravity Reference Frame (IGRF) The work is based on the IAG Resolution 2015 (No.2) within the working group: IAG JWG 2.1.1: Establishment of a global absolute gravity reference system. Basic infrustructure 1 Reference station provide a continuous gravity reference function: 1 very precisely by superconducting (SG) and/or quantum gravimeter (QG) , approximately with the periodical absolute gravity (AG) measurements. 2 Comparison station: reference station + facility to compare ( ≥ 2 ) AG. 3 Core station: reference station + space geodetic technique, link to the ITRF. Frame definition, site requirements and maintenance 1 The reference frame is based on absolute gravity measurements . Uncertainty for measurements: < 10 µ Gal . Levelling network connection + collocated GNSS. Vertical gravity gradients (VGG) are required. Should be documented in AGrav database. 1 From IAG JWG2.1.1 Report to IAG 2015–2019 (draft) by Hartmut Wziontek
Moscow gravity network Structure and collocation TSEL 2 pillars MEND −7 1 pillar+GNSS+SLR − 1 1 9 −6 LEDO TSNI −52 2 pillars+GNSS 3 pillars+GNSS+gPhone 20 27 −7 M O S C O W 6 4 KPRE −19 1 pillar+GNSS ZVEN 2 pillars+GNSS Absolute sites Gravity ties (mGal)
Moscow gravity network Reference, comparison and core station for the IGRF Reference station Very stable : no crustal movements, no tectonics, no volcanos, no GIA, located far from the oceans and seas, etc. No superconducting (SG) or quantum gravimeter (QG) yet. Regular absolute gravity measurements , but not precisely scheduled. Unfortunately, very noisy at some stations (large city). 4 out of 6 stations are connected with the national levelling network. Comparison station Up to three simultaneously working AG at one station. Core station MEND (Mendeleevo) is I GS station (MDVJ) and I LRS station (MDVL) ZWEN (Zvenigorod) is I GS station (ZWE 2 ).
Moscow gravity network Example: Comparison station TSNI (TsNIIGAiK) AGrav database integration
Network maintenance Connection with Key comparisons Indirectly by Russian–Finnish Comparisons in 2013 (RFCAG2013) and FG5–X # 221. Tidal parameters and environmental effects Tides analysis with gPhone (in progress). Collect information about local and global environmental variables. Precise transfer correction Non–linear vertical gravity changes are determined for all stations. Non–linear horizontal gravity changes are determined only at TSNI. Continuous gravity reference function Ensuring traceability in time–varibale gravity fi eld. Regular absolute and relative gravity measurements.
Precise transfer correction Vertical gravity changes
Precise transfer correction Vertical gravity gradients (VGG) TSNI — 109a Z = 27 cm - 16 cm = 11 cm 3320 Z = 71 cm - 27 cm = 44 cm 3255 Z = 126 cm - 71 cm = 54 cm E 3360 3174 3168 3150 3240 3180 3440 30 3400 3186 20 3255 3255 3360 10 Y / cm 0 3280 3300 10 3400 3360 3285 3360 20 3320 3200 3440 3320 3255 3150 30 3162 3280 3280 3240 3240 3225 3270 3156 3156 30 20 10 0 10 20 30 30 20 10 0 10 20 30 30 20 10 0 10 20 30 X / cm X / cm X / cm TSNI — 109b Z = 27 cm - 15 cm = 11 cm Z = 71 cm - 27 cm = 44 cm Z = 126 cm - 71 cm = 54 cm E 3225 3240 3240 3225 3210 3350 3375 3190 30 3400 3200 3425 3450 3255 3440 20 3285 3375 3200 10 Y / cm 3360 3270 0 3180 3375 10 3300 3280 20 3425 3190 3210 3475 3450 3200 3240 3170 3220 30 3500 3225 3230 3350 3255 30 20 10 0 10 20 30 30 20 10 0 10 20 30 30 20 10 0 10 20 30 X / cm X / cm X / cm TSNI — 110a Z = 27 cm - 15 cm = 11 cm Z = 71 cm - 27 cm = 44 cm Z = 126 cm - 71 cm = 55 cm E 3300 3225 3195 3225 3240 3330 3360 3240 30 3255 3390 3180 3440 3420 3190 20 3185 10 3330 3480 3360 Y / cm 3285 3185 0 3300 10 3280 3180 3255 3450 20 3175 3270 3170 30 3200 3225 3165 3225 3360 3240 30 20 10 0 10 20 30 30 20 10 0 10 20 30 30 20 10 0 10 20 30 X / cm X / cm X / cm
Precise transfer correction Horizontal gravity changes TSNI — 109a Level: 16 cm Level: 27 cm Level: 71 cm Level: 126 cm Gal -2 - 2 - 3 1 -4 2 2.5 30 -3 -1 - 3 20 0.0 -2 0 10 Y / cm 0 0 2.5 -1 -3 10 5.0 -1 0 20 -4 -5 - 4 -6 -4 -4 30 -6 2 - -5 -7 - 5 7.5 - 7 30 20 10 0 10 20 30 30 20 10 0 10 20 30 30 20 10 0 10 20 30 30 20 10 0 10 20 30 X / cm X / cm X / cm X / cm TSNI — 109b Level: 15 cm Level: 27 cm Level: 71 cm Level: 126 cm Gal 2 3 30 1 -1 0 20 1 -2 10 0 0 Y / cm - 3 4 -1 0 -1 - 1 10 - 2 -5 - -2 6 3 - 3 - 2 -2 20 -5 - -5 3 - 3 - 4 6 -7 6 -3 -4 5 - - - 30 -7 -8 -7 9 -9 -5 -6 30 20 10 0 10 20 30 30 20 10 0 10 20 30 30 20 10 0 10 20 30 30 20 10 0 10 20 30 X / cm X / cm X / cm X / cm TSNI — 110a Level: 15 cm Level: 27 cm Level: 71 cm Level: 126 cm Gal -9 -4 2 - 5 - 4 - -4 4 - -1 - 6 2 -2 0.0 -5 - -8 - - -1 7 -1 7 -4 -2 -3 -2 -6 -3 20 3 -1 - - 2.5 4 - Y / cm 1 0 3 5.0 - -5 20 7.5 -3 - 3 10.0 30 20 10 0 10 20 30 30 20 10 0 10 20 30 30 20 10 0 10 20 30 30 20 10 0 10 20 30 X / cm X / cm X / cm X / cm
Ensuring traceability in time–variable gravity fi eld National gravity network and other users/applications Through absolute and relative gravimeters. 1 2 Through measurements at the reference station/network. Absolute gravimeters Directly through comparisons at several points. 1 2 Indirectly through the reference station/network. Relative gravimeters Through calibration between the reference stations. But gravity is changing. How to ensure traceability? Current requirement: mesure gravity with AG every two months.
Loading effects on gravity Atmospheric, hydrological and ocean non–tidal loading Estimations are taken from EOST Loading Service. TSNI is included in EOST computations (code: MOSC). Ocean non–tidal loading is negligible. Atmospheric loading was averaged over months. Strong correlation between FG5 and hydrological data.
Measurement frequency requirements There are several data gaps. Continuous gravity reference function is just linear. RMS before substructing: 3 . 4 µ Gal , RMS after substructing: 1 . 3 µ Gal . Residuals are from − 2 . 7 µ Gal to 2 . 5 µ Gal ≈ ± 3 µ Gal . Measurements with AG performed every two months will reduce residuals to ± 2 µ Gal .
Calibration of relative gravimeters Traceability to relative gravimeters Needed in precise gravity measurements in the national gravity network. Should be based on absolute gravity measurements. Precision requirements: d Y 1 = d(∆ g ) − d(∆ z ) Y 1 ∆ g ∆ z For d Y 1 = 10 − 5 . . . 10 − 4 , ∆ z ≈ ∆ g = 52 mGal , d(∆ z ) = 5 µ Gal Y 1 we get for the reference difference d(∆ g ) : d(∆ g ) = 5 . 5 . . . 10 . 2 µ Gal this leads to the uncertainty ≈ 3 . 9 . . . 7 . 2 µ Gal of the individual absolute measurement. It is very di ffi cult in a time–variable gravity fi eld. Very high standards of measurements, including relative gravimeters. Ties between all nearest stations: 10 ties between 6 stations. Ideally, twice a year. Absolute and relative measurements are carried out simultaneously for no more than one month (10 days for relative measurements only). Measurements performed in 2015 — 2018. Every year is adjusted with absolute measurements.
Conclusions and future work A local gravity netowrk can be accepted as a reference statioin for the IGRF. The network is better than one station, because relative gravimeters can be calibrated on it. But it is also a more time–consuming task to maintain it. Maintenance should include: connection with key comparisons, vertical and horizontal gravity changes determintaion, regular absolute and relative gravity measurements, tides analysis with continuously operating gravimeter, analysis of the environmental non–tidal effects. The Moscow gravity network is maintained and it is a possible cadidate for the International Gravity Reference Frame (IGRF). It is ( fi naly!) included in AGrav database. Future work Finish tidal analysis for gPhone data. Continue regular absolute gravity measurements, increase measurement frequency. Use local data for hydrology, snow, etc.
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