jgg     2015 35 (3):  495-498    ISSN: 0412-1961  CN: 21-1139/TG Determination of the Geopotential and Orthometric Height Difference  Based on the Two-Way Satellite Time and Frequency Transfer  and GNSS Observations 1 School of Geodesy and Geomatics, Wuhan University, 129 Luoyu Road, Wuhan 430079,China Received: 2014-09-15  Revised: null  Online: 2015-06-02 Supporting info