By giving a good overview of polar regions (-82.5°S / 82.5°N), Envisat mission is well adapted. Envisat altimeter (RA-2) data are delivered on DVD-Roms or by FTP and can be obtained from EOHelp.
This dataset has another advantage. The Envisat GDR brings the opportunity to use algorithms other than the classical ocean-oriented ones, that are better suited for non-ocean surfaces. One of them is optimized for continental surfaces, the so-called ICE-2 retracker.
The RA-2 altimeter on Envisat platform is a dual-frequency radar operating at Ku-band (13.575 GHz) and at S-band (3.2 GHz).
By using the Broadview Radar Altimetry Toolbox (BRAT), we map the radar altimeter backscattering coefficient in S-band in order to classify major global surface properties and to study their temporal evolution: date of the first snows, thickness of snow or vegetal cover in relation to the season, inundated areas, etc.
This example is made on a global scale without any geographical limits.
In the “Datasets” tab, we have selected one cycle per dataset, carefully named (from Datasets_cy30 to Datasets_cy40) from the DVD GDR Envisat. Finally, we have 11 datasets, each with ~980 files (up to 1002).
A first operation is created, selecting longitude for “X”, latitude for “Y” and for the “Data expression”, we use the S-band ICE-2 backscatter coefficient at 18 Hz (hz18_s_ice2_bscat field).
The data editing limits the backscatter coefficient boundaries to eliminate the most erroneous data. An ocean mask is also applied to keep data only on continental areas: (altim_landocean_flag > 0) && (is_bounded(0, hz18_s_ice2_bscat, 45)).
Next operations are created from the first one using the “Duplicate” button but be careful to link up each operation to the good cycle dataset.
In the “Views” tab, we add each operation-cycle in the same plot. Click on “With Animation” in the “General Plot Properties” and “Execute”. In the Display screen, you can change the speed for each frame to be displayed or you can stop on each frame and save it independently.
Results and comments
At a global scale, the backscatter coefficient values mainly ranges from 0 to 20 dB in S-band. The backscatter coefficient is high (~20 dB and more) on very flat surfaces: deserts, river basins or wetlands due to the specularity of the return echo. But it is low over mountainous regions. Strong variations can be seen, especially for regions which are covered by snow in winter or with a contrasted rainy season.
The backscattering coefficient results from a surface scattering echo and a volume scattering echo inside a medium. Over land surfaces, three different cases can be described:
- the return signal is only due to the surface echo: water surfaces, inundated soils, melting snow,
- the return signal is due to reflecting surfaces located under volumetric scatterers: sparse forests, inundated forests, dry snow,
- the return signal is due to reflecting surfaces located above volumetric scatterers: dry or arid areas, wet snow
|fig 1. Backscatter coefficient Sigma0-S from ICE-2 retracker for Envisat cycle 034 (top) and 040 (bottom): several river basins such as Orinoco, Mississipi, Amazon, Parana, Lena, etc, and areas with melting snow during boreal summer are clearly visible.|