I. Introduction
The ABSOLUTE accuracy of microwave remote sensing radiometers is an important consideration for many applications. One method of achieving absolute radiometer calibration is through the use of blackbody calibration targets to provide reference brightness temperatures for the radiometer to view. These targets are widely used with spaceborne, airborne, and ground-based microwave instruments. While studies have been published that assess overall instrument calibration performance [1], [2] or calibration methods [3], [4], or identify error sources relative to specific instruments or groups of instruments [5], [6], few studies exist which investigate the calibration target-related sources in detail.