Global Land Surface Backscatter at Ku-Band Using Merged Jason1, Envisat, and Jason2 Data Sets

A unique method for investigating continental surfaces uses backscatter data measured by a radar altimeter at the nadir point of a satellite, in contrast to other microwave sensors designed to work at oblique angles, such as scatterometers and synthetic aperture radar. To improve the altimetry resolution over land, we generated 0.5° × 0.5° merged altimetry backscatter maps covering 66° N to 66° S over the global land surface every six days for the period from January 2002 to June 2009 by combining three altimeter data sets (Jason1, Envisat, Jason2) in the Ku-band. The four backscatter products of Envisat RA2 from different retracking algorithms were evaluated prior to merging with the Jason1 and Jason2 data. The global pattern and the seasonal variation of the merged altimetry backscatter were examined, which show the merged results have better spatial sampling for the regional to global geophysical process due to the combination of three altimeters. To understand how the altimetry backscatter is related to land surface parameters, the advanced integral equation model for bare soil and water cloud model for vegetation are used to simulate the Ku-band backscatter response to soil and vegetation parameters at an incidence of 0°. Furthermore, we compared the time series of merged altimetry backscatter with the leaf area index (LAI) determined by an optical sensor and the backscatter coefficients obtained from a scatterometer (QuikSCAT) over seven selected vegetated areas over six years. The results confirm the sensitivity of ocean altimetry to vegetation. Further studies relating altimetry backscatter to geophysical parameter are needed.