天宫二号三维成像微波高度计大气斜距时延校正

Correction of the tropospheric slant path delay of Tiangong-2 Interferometric Imaging Radar Altimeter

2016-09-15随天宫二号空间实验室发射升空的三维成像微波高度计(简称天宫二号成像高度计)是国际上第一个采用小入射角、短干涉基线实现宽刈幅海面高度测量的高度计。由于天宫二号没有为成像高度计配备用于大气校正的微波辐射计,因此需要采用模型方法对大气时延进行精确估计。传统高度计采用星下点观测,通常只考虑大气折射率的改变引起传播速度变化的时延。天宫二号成像高度计采用偏离星下点1°-8°的小角度观测,因此在进行大气传输时延校正时不仅要考虑雷达信号传播速度的改变,还应考虑由于传播方向改变带来的路径弯曲效应。本文针对天宫二号成像高度计的观测几何特性,提出基于数值天气模型校正大气斜距时延的算法:采用欧洲中期天气预报中心的天气数据和大气分层模型,通过气象参数计算大气折射率;根据高度计参数和各层大气折射率,依赖折射定律和信号传播的几何关系在路径上逐层积分,计算大气斜距时延的估值。通过对天宫二号成像高度计的陆地角反射器定标实测数据进行处理,经大气斜距时延校正后,角反射器的剩余距离误差的标准差约为6.2 cm,达到厘米量级的斜距测量精度,验证了在不同入射角情况下,所提出的大气斜距时延校正算法的有效性和可靠性。

Launched on September 15,2019,the Interferometric Imaging Radar Altimeter(InIRA)onboard the Chinese Tiangong-2 space laboratory is the first spaceborne interferometric radar altimeter that can obtain wide-swath ocean topography measurements by adopting small incidence angles from 1°to 8°with a short baseline.InIRA achieves various technological breakthroughs,meanwhile,it also brings some challenges in data processing because no radiometer is onboard the Tiangong-2 space laboratory.Considering signal path delays is a premise for InIRA to meet its geocoding and Sea Surface Height(SSH)measurement goals,a mathematical model-based method for tropospheric path delay correction should be developed.Unlike traditional nadir-looking altimeters,which only require the propagation delay related to the velocity variation along a line path,the Tiangong-2 InIRA must consider the additional bending of radio waves for its small incidence angles.In this study,a tropospheric slant path delay correction algorithm is developed using the ray-tracing technique based on Fermat’s principle and on the numeric weather model from the European Center for Medium-range Weather Forecasts.Two calibration campaigns are conducted in March 2017 and September 2018,which recorded 12 measurement data from 9 corner reflectors.Results show that the standard deviation of the residual error range after the tropospheric slant path delay correction is approximately 6.2 cm,which indicates that a centimeter-level range accuracy is realized.Therefore,the effectiveness and reliability of the proposed algorithm in different small incidence angles are validated.