Time tag bias correction of HY-2 satellite altimeter by in-orbit calibration using reconstructive transponder

The sea surface height (SSH) measured by the satellite radar altimeter is determined by the distance from the satellite to the sea surface and the altitude of the satellite above the reference ellipsoid. The former is measured by altimeter and the latter is determined by precision orbit determination. If the two systems use different clocks, there may be a time tag bias between their time series. This time tag bias reduces the accuracy of SSH measurements by satellite radar altimeter. To solve the problem of time tag bias of the HY-2 altimeter, we conducted nearly 20 in-orbit calibration experiments from November 2021 to September 2022 using a reconstructive transponder, and statistically analysed the effect of time tag bias on the altimeter height measurement. The results of time tag bias of HY-2B is 0.03 ms, that of HY-2C is 0.07 ms, and that of HY-2D is 0.1 ms. As an independent calibration method, the reconstructive transponder can be deployed on land, which makes it unaffected by error sources introduced by sea surface dynamics, resulting in calibration with more accuracy. The altimeter system delay after correcting the time tag bias is also given, which provides a guarantee for the quality of the subsequent data products.