This communication addresses the problem of the automatic mosaicking of raw images acquired by airborne pushbroom imagers. Using appropriate ancillary data issued from GPS and inertial measurements, we show how the mutual information criterion can be used to improve the co-registration and direct georeferencing of overlapping flight lines by estimating unknown or inaccurate elevation data. The proposed approach does not require any control point to work, and requires only few iterations to improve the initial pose. The mosaicking task itself is performed in a very simple manner. We describe the proposed system, and assess the robustness of our method with an example of application to the mosaicking of multi-track real multispectral image data.