The in-flight absolute calibration of spaceborne instruments working in the solar reflective region (0.4-2.5 mu m) has often been performed with large uniform ground areas, such as White Sands, U.S.A., or La Crau, France. The method presented here describes an alternative for high spatial resolution sensors employing artificial small ground targets. The required target size is seven times the size of the sensor's spatial resolution, the ground projected instantaneous field-of-view (GIFOV), if the Gaussian full width 2 sigma of the image point spread function (PSF), atmospheric blurring removed, is less or equal to 1.5 * GIFOV. For near-future multispectral instruments with a spatial resolution of 1 to 10 m the required ground reference areas with very homogeneous reflectance properties will be sized 7 m * 7 m to 70 m * 70 m, respectively. The proposed method can also be employed for panchromatic sensors.