HY-2A Altimeter Time Tag Bias Estimation Using Reconstructive Transponder

Abstract

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 from the 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.

Global Assessments of the HY-2B Measurements and Cross-Calibrations with Jason-3

“…It was also proven that the accuracy of the GDR products of the HY-2B satellite's radar altimeter had displayed little differences in time and space. There were no time scale bias problems as observed in the HY-2A satellite [3][4][5]. Figure 19b shows the mean distributions of the sea level anomalies differences at the crossovers of ascending and descending ground tracks.…”

“…The main design parameters of HY-2B radar altimeter are given in Table 1. In order to accurately measure the distances from the satellite to the sea surfaces while avoiding the effects of drifts caused by the ageing of the device [2][3][4][5], the HY-2B satellite carries a rubidium atomic clock and a Global Positioning System (GPS) second pulse design to ensure that the timing remains accurate between and within one second. At the same time, in order to improve the accuracy of the distance measurements, the HY-2B's radar altimeter has been modified as a vertical surface direction (nadir-looking) device.…”

Significant Wave Height Estimation Using Multi-Satellite Observations from GNSS-R

“…Sea surface wind fields, significant wave height (SWH), and sea targets are important physical parameters that reflect the sea-state and navigation environment. Traditionally, maritime information is acquired by marine stations, synthetic aperture radar (SAR), radar altimeters, unmanned aerial vehicles (UAVs)-borne measurements and other active observational approaches [2][3][4][5][6][7][8]. Global Navigation Satellite System Reflectometry (GNSS-R) is a new type of remote sensing technology with all-weather operability, multi-source signals, small weight and volume, low power consumption, and low cost.…”