Estimating the Time Tag Bias of HY-2A Radar Altimeter and Its Application to Dual-frequency Ionosphere Correction

ABSTRACT

The sea surface height (SSH) derived from radar altimetry is determined by the distance from the satellite to the sea surface and the altitude of the satellite above a reference ellipsoid. The former is measured by the radar altimeter, while the latter is determined by the precision orbit determination (POD). The clock for the POD equipment is independent from that of the radar altimeter onboard the HY-2A satellite. The time tag bias, which is the bias between the time tags provided by the two independent clocks, can greatly affect the SSH measurement accuracy of HY-2A altimeter. This paper estimates the time tag bias of HY-2A radar altimeter using the crossover differences obtained from the sensor geophysical dataset records (SGDR) from February 2014. We obtained a −0.61-ms Ku-band time tag bias and a −5.61-ms C-band time tag bias. After we added the time tag bias corrections to the SSH measurements from Ku and C bands, respectively, the means and standard deviations of the global crossover differences can be significantly reduced. We then applied the SSH measurements with the time tag biases corrected to calculate the HY-2A dual-frequency ionosphere correction, significantly improving the accuracy of the HY-2A dual-frequency ionosphere correction.

KEYWORDS:

• Dual-frequency ionosphere correction
• HY-2A
• radar altimeter
• time tag bias

Acknowledgements

The Jason-2 GDR data and the DAC data were obtained from AVISO, and the data to calculate the pole tide were obtained from IERS. The data we used to calculate the dry and wet troposphere corrections were downloaded from ECMWF. The authors would like to thank AVISO, IERS and ECMWF for providing the data for free. They would also like to thank NSOAS for providing the HY-2A altimeter SGDR data.