摘要：The paper presents a method estimating the HY-2A altimeter ultra stable oscillator (USO) drift with a reconstructive transponder. The frequency of the USO of the in-orbit altimeter changes with age, and a bias between the actual frequency and the nominal one exists. The USO bias contributes a portion of the altimeter range drift. The HY-2A altimeter transmits signals at a fixed time interval during calibration, and the actual interval between adjacent altimeter transitions, which is controlled by the USO, is different from the nominal one due to the USO drift. The reconstructive transponder measures the arrival times of the altimeter transmitted signals accurately with the atomic clock. The drift of the USO on-board the HY-2A altimeter can be estimated accurately by using the ranges from the reconstructive transponder and the HY-2A altimeter. The USO drifts of around 40 cm/year are presented. Furthermore, the multi-mission crosscalibration provides preliminary validation of HY-2A altimeter USO drift.
摘要：Independent clocks provide time tags for the precision orbit determination (POD) equipment and the radar altimeter onboard the HY-2A satellite, and a bias between POD data' time tag and corresponding range observation's time tag from the HY-2A altimeter exists. The time tag bias contributes a bias in the sea surface height observation due to the nonzero time rate of change of the HY-2A altimeter's height. A transponder for in-orbit radar altimeter calibration provides an approach to estimate the time tag bias. The altimeter receives the responding signals from the transponder and generates ranges. Pertinent reference ranges are obtained fromthe POD data and the transponder's coordinate. Using the ranges from the radar altimeter and the reference ranges, the time tag bias between the POD data and the altimeter observations can be estimated. During an in situ HY-2A altimeter calibration campaign using a reconstructive transponder from August 9, 2012, to July 20, 2014, 17 estimations of the altimeter's time tag bias were obtained. The preliminary results are presented in this letter.
摘要：A reconstructive transponder has been utilized for the in-orbit calibration campaign of the HY-2A radar altimeter since March 2012. The precision of final calibration result is influenced by echo signal's quality in the HY-2A altimeter's range window. As an indicator of the signal's quality, echo signal dwell time is analyzed considering its influence on signal quality and its uncertainty. In HY-2A altimeter calibration, the echo signal dwell time is determined by the radial orbit prediction uncertainty and the real-time signal processing mechanism of the reconstructive transponder. The real-time signal processing mechanism of the reconstructive transponder utilizes some incoming signal samples without sending echo signals before transmitting. Comparing with the length of the HY-2A altimeter's range window, the radial orbit prediction uncertainty is large. Large radial orbit prediction uncertainty and signal processing mechanism of the reconstructive transponder are two main factors that limit the echo signal dwell time in HY-2A altimeter calibration. Finally, approaches for increasing echo signal dwell time are briefly proposed.
摘要：This letter presents a method for matching satellite radar altimeter data and transponder data generated during in situ calibration. The transponder generates a measurement error when it measures the arrival time of the altimeter’s transmitted signal and embeds the error in both the transponder’s recorded data and the altimeter’s recorded data. The secondorder finite difference sequence of this error sequence can be extracted from the raw data, thus, the correspondence between two identical but mismatched second-order difference sequences can be uniquely established. The measurement error is utilized,and a data matching method that can uniquely establish the correspondence between the altimeter’s recorded data sequence and the transponder’s recorded data sequence is presented.This post-processing method does not increase the real-time signal processing workload of the transponder. Furthermore,The principles underlying this method can be used for any transponder that can adjust the response signal delay during calibration.