Sea surface salinity（SSS） is a very important parameter of ocean’s circulation and global meteorological and climatic predictions.Salinity data will permit estimates of sea surface density

or buoyancy

which drives the ocean’s three-dimensional circulation.L-band（1400—1427MHz） microwave radiometer is the most effective remote sensing tool of SSS measurement.In L-band

the sensitivity of radiometer brightness temperature variations associated with salinity is 0.5K/psu.Since a salinity change of 0.2psu results in a brightness temperature change of 0.1K

it is necessary to have a microwave radiometer with sensitivity of better than 0.1K.Highsensitivity airborne L-band microwave radiometer uses digital auto-compensation methods for gain-fluctuations and environment temperature variations

and 6-th Chebyshev high effect filter to reduce GPS interference which helps its sensitivity surpass 0.08K.The antenna with dimensions of 1m×1m belongs to microstrip type

antenna 3dB beamwidth is 15°

nadir incidence angle is 35°

and its weight is about 15kg.The antenna gain is 20 dB and side lobe level is lower then-17dB.In September 2005

the aircraft remote sensing measurements were carried out.The L-band horizontl and vertical polarization signals were recived simultaneously.The cross-polarization isolation is ≥30 dB.The microwave radiometer was fixed on CMS3807（Y12 mode） aircraft over flat sea surface conditions at 122°50′E

39°30′N—39°6′N.During the experiment

sea surface salinity were detected on a ship when aircraft was flying in same area of the sea at the same time.The experiments were compared with site test on the sea.The results of these measurements are presented in this paper

with the resolution of better than 0.2psu and the accuracy of 0.42psu.