On-Ground Retracking to Correct Distorted Waveform in Spaceborne Global Navigation Satellite System-Reflectometry  
[PERSON]  
1 School of Electronic and Information Engineering, Beihang University, Beijing 100191, China; [EMAIL_ADDRESS] (F.W. ); [EMAIL_ADDRESS] (W.Y.) [PERSON]  
1 School of Electronic and Information Engineering, Beihang University, Beijing 100191, China; [EMAIL_ADDRESS] (F.W. ); [EMAIL_ADDRESS] (W.Y.) [PERSON]  
2 Earth Observation Research Group, Instituto d'Estudis Espacials de Catalunya (ICE-CSIC/IEEC), 08191 Barcelona, Spain; [EMAIL_ADDRESS] (Correspondence: [EMAIL_ADDRESS]; Tel. : +86-010-8231-7229 201  
who used UK-DMC data to validate the feasibility of retrieving wind speed using spaceborne GNSS-R. The new spaceborne mission, UK-TechDemoSat-1 of the Surrey Satellite Technology Ltd (SSTL) in Guildford of England [4], has been providing more spaceborne GNSS-R data, and the first retrieval of wind speed were presented in [5]. CYGNSS mission of NASA launched in December of 2016 aims to resolve the genesis and rapid intensification phase of the tropical cyclone lift cycle with sufficient frequency [6]. At present, with the increasing of the available spaceborne GNSS-R data, current studies mainly have focused on developing retrieval approaches or models of wind speed [5; 7; 8; 9; 10], sea surface height [11], soil moisture [12; 13], sea ice [14; 15; 16]. To improve the spatial resolution of the observation, some approaches, such as Delay-Doppler Map (DDM) inversion [17; 18] and stare processing [19] have been proposed. Delay waveform as the basic observable of GNSS-R, which is obtained by correlating the reflected GNSS signals and local replicas at different delays, could be used to retrieve sea surface height and wind speed. In general, incoherent averaging among a number of single snapshot should be used to reduce the speckle noise. However, the delay difference between direct and reflected GNSS signals changes rapidly with the motion of the GNSS and LEO satellites [20]. This change should be compensated on each snapshot to avoid the distortion of measured waveform and improve the retrieval performance. The procedure of the compensation is called as tracking. Both GNSS and LEO satellite parameters including height, moving direction and elevation angle are first considered to estimate the delay difference changes (DDC) [21]. The impact of Doppler compensation was accounted for and demonstrated using UK-DMC [22]. A delay tracking method was proposed by [PERSON] [23] for altimetry by assigning the specular point index of each snapshot on each coherent integration time period. However, a high tracking refresh rate may require too many instrument resources or too much data to be uploaded from the ground station [23]. To reduce the refresh rate above, one way is to decrease the requirement of tracking error and retrieval performance; and the other is to incoherently average each waveform samples on-ground, but needs all waveform samples to be downloaded to the ground station. This paper focuses on an on-ground retracking method to recovery the waveform from the distorted one which is caused because of the low tracking refresh rate on board. Proposed method aims to use post-processing to replace the compensation on each coherent integration time period on-board to reduce the requirement of too many instrument and much data to be uploaded from the ground station. The simulation scenarios and models used in this paper are presented in Section 2. Section 3 briefly reviews the speckle noise statistics and the incoherent averaging which could significantly mitigate the influence of the speckle noise on the waveform. Section 4 describes the waveform distortion caused by the time-varying observation geometry of GNSS-R. In Section 5, the sensitivity of Arrival Time of Leading Edge (AToLE) to sea surface height and Peak Waveform(PW), Leading Edge Slope (LES), Trailing Edge Slope (TES) to wind speed are analyzed. The detail of the methodology is described in Section 6.