Le Caire - Hier et aujourd'hui - suite Mesure     Like its predecessor ERS-1 (launched in July 1991 by Ariane 4 and successfully put into orbit at an altitude of some 780 km), the ERS-2 satellite launched on 21.04.95 by Ariane 4, monitors the ground day and night under all weather conditions thanks to its powerful sharp-eyed, cloud-piercing radars. ERS-2 also carries an instrument to help monitor the ozone layer.