Evaluation And Correction Of Path Delay For Tiangong-2 Interferometric Imaging Radar Altimeter

As the first spaceborne interferometric radar altimeter realizing wide-swath ocean topography measurement by adopting small incidence angles with short lines,the Interferometric Imaging Radar Altimeter(In IRA)onboard Chinese Tiangong-2 space laboratory achieves various technological breakthroughs.Meanwhile,it also brings some challenges in data processing.Correcting signal path delays is a premise for In IRA to meet its geocoding and sea surface height(SSG)measurement goals.For one thing,In IRA operates at a single frequency and there is no radiometer onboard,mathematic model based methods need to be developed for path delay calculations.For another,the Tiangong-2 In IRA is different from traditional nadir looking altimeters by taking small incidence angles from 1° to 8°.Therefore,both velocity variation along a line path and the bending effect of radio waves need to be considered.To solve these problems rooted in the path delay correction for the Tiangong-2 In IRA,the thesis focuses mainly on mathematic models and algorithms for correcting the ionospheric and tropospheric path delays.The work includes the following three parts:1.The first part studies ionospheric path delay.According to physical characteristics of ionosphere,a relationship between ionospheric parameters and ionospheric path delay is derived as well as the mathematic formula to calculate ionospheric path delay.The “IGS(International Global Navigation Satellite System Service)-based correction” method is described in detail and three classical models for model based correction method are compared and analyzed.2.The second part studies tropospheric path delay related to dry atmosphere and wet atmosphere.The relationship between atmospheric refractive index and weather parameters is deduced and then the accuracy of refractive index is analyzed under the theory of error.With the formula of calculating tropospheric path delay,we respectively point out the terms as bending of radio waves and velocity variations.Three mathematic models for tropospheric path delay correction are detailed with their advantages and disadvantages summarized.3.The third part is to develop a path delay correction method for the Tiangong-2 In IRA.For ionospheric path delay,“IGS-based correction” was applied.The calculation results of ionospheric path delay exhibit temporal and spatial characteristics,which are consistent with temporal and spatial characteristics of the ionosphere.For tropospheric path delay,an algorithm considering the range of small incidence angles is proposed for correcting the slant tropospheric path delay,which is validated by two calibration campaigns conducted in 2017 and 2018,respectively.Different residual range errors are briefly demonstrated.It is shown that the standard deviation of residual range error is around 6.2cm after slant path delay correction,i.e.centimeter-level accuracy of path delay correction is realized.