Two micro-spacecraft constellation with solar sails partially compensating the solar gravity allow their placement in the points along the Sun-Earth line behind the standard L1 point, i.e. further out from the Earth and closer to the Sun. The distances of the placement from the Earth and the base distances between such satellites depend on the ratios of their mass to the sail area. It can be selected in the way needed for the warning of the coronal mass ejection arrival several or many hours before the upcoming geomagnetic storm. Measurements of the magnetic field vector are necessary and sufficient for this purpose on each spacecraft. No plasma measurements are needed for this purpose, which makes the weight of spacecraft minimal. The sail can be used also as a solar panel, an antenna oriented on the Earth for the radio transmission and device for obtaining a stable mechanical orientation of the spacecraft axis along the Sun-Earth line. Time delays and known base distances between satellites allow the solar wind velocity measurements with a sufficient accuracy for the warning of the coronal mass ejection arrival to the Earth. Sky maps obtained with micro photo cameras and processed on board can give the accurate spacecraft orientation needed for the determination of the crucial quantity: magnetic field vector orientation. Together with measured magnetic fieldstrength and the solar wind velocity on can obtain information for the robust forecast of geomagnetic storms.