Pushing the limits of SPOT HRV resolution with steered viewing modes

SPOT 1 to 4 satellites have been designed to provide pictures with a swath of 60 km for nadir viewing and operate in two modes: a panchromatic mode with a sampling interval of 10 meters, and a multispectral mode with a sampling interval of 20 meters. Since the modulation transfer function (MFT) is significant at Nyquist 2D-frequencies, C.N.E.S. has been undertaking new studies to find a way to increase the sampling density and therefore improve the resolution. Without any change in the spacecraft electronics or optics, a method which consists in accurately steering the satellite attitude around its 3 axis was proposed and validated on SPOT1 during in-flight campaigns in December 1998 and June 1999. The acquired pictures actually demonstrated the resolution improvement. After a presentation of the SPOT1 image chain, this paper describes the different steered modes, focuses on ground processing methods and compares the results on the pictures in order to evaluate their actual resolution.