Since the launch of ENVISAT satellite in 2002

a considerable amount of programs has been set up in application of the new generation of Advanced Synthetic Aperture Radar（ASAR） instrument for monitoring agricultural crops

which extends the mission of SAR instruments onto ERS-1/2 and provides the continuity on agricultural monitoring.Under the framework of Dragon Project

which is a three-year（2004—2007） cooperation program between NRSCC and ESA focused on science and applications development in China mainly using data from the ERS and ENVISAT missions

a project theme on rice monitoring was set and endeavored to validate the efficiency of ASAR data for rice monitoring in China.Under these circumstances

this paper intends to propose a practical method for rice mapping

taking advantage of ASAR Alternative Polarization Mode（APMode） product which allows the acquisition of radar images with two simultaneous polarizations selected from the four polarizations HH

HV

VH and VV.Therefore

during the early rice season of Gaoan district of Jiangxi Province in 2006

two scenes of ASAR APMode products were acquired on the date of May 8 and June 12

in a specific radar configuration of HH and VV polarization and at approximately 40° incidence angle.Meanwhile

six differential GPS samples containing different crops and other land surface objects

with size of 1 km×1 km for each

were collected during the ground campaign.Several dominant crop calendars were also recorded as ancillary information for image interpretation.The preprocessing of ASAR images includes the calibration

speckle noise filtering

co-registration

and geo-reference.Backscattering coefficients of different crops and other land surface objects were obtained from ASAR images overlaid with the GPS samples and all were averaged for further analysis.Before the rice mapping

the backscattering difference of land surface objects was analyzed by the maps of multi-polarization backscattering difference for each date and multi-temporal backscattering difference of the same polarization which were calculated from the received ASAR data.Accordingly

paddy rice showed significant disparity of backscattering coefficient between VV polarization and HH polarization

while the multi-polarization backscattering difference was not obvious for other land surface objects.Moreover

high sensitivity of HH polarization radar wave to the temporal change of paddy rice was observed

which indicated multi-temporal backscattering difference of HH polarization contained more paddy rice information.However

it should be that the ASAR images employed in the above analysis were acquired at different rice growth stage one of which is at rice transplanting stage and the other at booting stage.During the rice transplanting

the dominant scattering mechanism is the surface-volume scattering

while single-volume scattering becomes the dominant scattering mechanism during the booting stage since paddy canopy has become much denser.Therefore

an optimal combination of backscattering difference images used for paddy rice mapping was obtained.Then

two classification methods are compared: threshold classification and supervised classification were applied separately to retrieve paddy rice from the combined image.As a result

a high rice identification accuracy of 84.92% was achieved using the supervised classification.The relative inferior performance of threshold classification could be caused by the remained speckle noise

because the threshold classification was performed at pixel scale.Finally

based on the multi-remporal and multi-polarization backscattering difference

the practical approach presented in this study not only simplifies traditional rice mapping methods

but also keeps a high mapping accuracy of rice crop.However

limited by the available ASAR data

further study should be extended for investigation of ASAR data on rice mapping of different rice calendar and for rice yield estimation using multi-temporal and multi-polarization radar images.