China's first ocean dynamic satellite HY-2A was launched in August 2011 which carries a dual-frequency (Ku- and C-band) altimeter to measure the global mean sea level. In-orbit calibration is essential for reliably measuring sea-surface height (SSH). This paper concerns a novel approach for HY-2A altimeter in-orbit calibration using a reconstructive transponder, which is different from the existing bent-pipe transponder. The reconstructive transponder captures, tracks, and records the altimeter's pulses, then reconstructs the pulses, and transmits them back to the altimeter. It is carried on a SUV, reduces requirement for a permanent installation site, and is much more flexible and efficient. There is an internal calibration loop to enable the stability of the transponder electronics delay and the system gain. The transponder electronics delay and system gain are obtained through an overall measurement. Another benefit of the signal reconstructive transponder is to accurately estimate the altimeter clock frequency drift. HY-2A altimeter in-orbit calibration campaign started from March 2012, until now more than 70 experiments have been carried out at several sites. On March 31, 2013, HY-2A altimeter side-A failed owing to malfunction of its ultrastable oscillator (USO). And this abnormality is monitored by the transponder. From the successful experiments, HY-2A altimeter side-A and side-B instrument bias are obtained with 1- and 4-cm precisions separately. The USO frequency drifts of the HY-2A altimeter are monitored all through the calibration campaign.
