This paper describes research to evaluate the role of mutual coupling between antennas in the performance of synthetic aperture radiometers. Measurements, taken from the aircraft prototype instrument ESTAR (electronically scanned thinned array radiometer), are presented to demonstrate the importance of mutual coupling. Theory for the ESTAR configuration (an array of dipoles) is presented and checked against the measurements. This theory is then used to assess the effects of small displacements in the elementary antennas in the synthesis array of a hypothetical sensor in space. The measurement errors that result from small displacements (as small as a few hundredths of a wavelength) can be important for applications such as the measurement of soil moisture. Therefore the design approach for future systems should include the control of these and should also include the capability to perform onboard calibration to monitor and remedy detrimental effects. A synthetic aperture radiometer is particularly suited to the measurement of mutual coupling because the basic hardware (a correlation radiometer) measures the product for each antenna pair in the array. Hence, in conclusion, this paper proposes a concept for monitoring changes in mutual coupling to correct for such effects in formation of images. ¿ 1998 American Geophysical Union