Compensating the PTR and LPF Features of the HY-2A Satellite Altimeter Utilizing Look-Up Tables

The point target response (PTR) and low-pass filter (LPF) features of the altimeter (ALT) onboard China´s HY-2A satellite are presented and analyzed. Via Monte-Carlo simulation, the look-up tables (LUTs) of the range (epoch), significant wave height (SWH), and backscatter (Sigma0) biases with respect to SWH and attitude angle (ATT) are calculated accounting for the instrument PTR and LPF features using two distinct retracking algorithms (MLE3 and MLE4). The performance of both retracking algorithms are also evaluated in terms of the size of the biases (corrections) produced, and MLE4 is found better. The actual PTR and the Bessel-expression flat surface impulse response (FSIR) are integrated in the simulation procedure to achieve more accurate results. The simulated biases are tabulated by very fine grids of SWH and ATT so that they can be used as corrections in the ground preprocessing procedure (processing prior to Level 2 product) to get better operational ocean products. As verified by the crossover analysis with the Jason-2 ALT, the corrections pertaining to the instrument features significantly improve the accuracy of HY-2A ALT sea level products.