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--> Reference stations should be those where gravity is monitored with an uncertainty of 10^-8 (10 muGal) over time.
---> Each GGOS core-stations should have gravity reference at some level in the future. TO BE ADDRESSED through the GGOS core-sites requierements!
2) Categories of reference stations:
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The Frame is realized through absolute gravity measurements traceable at a reference stations or through comparisons. If that is the case, all measurements of that instrument would be in realize the ITGRF. That would allow to include field absolute stations and would be realizing the ITGRF for a a certain epoch and with a given uncertainty.
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