Earth\'s ecosystems and human activities are threatened by a broad spectrum of hazards of major importance for the safety of ground infrastructures,space systems and space flight:solar activity,earthquakes,atmospheric and climatic dis-turbances,changes in the geomagnetic field,fluctuations of the global electric circuit.Monitoring and understanding these major hazards to better predict and mitigate their effects is one of the greatest scientific and operational challenges of the 21 st century.Though diverse,these hazards share one feature in common:they all leave their characteristic imprints on a critical layer of the Earth\'s environment:its ionosphere,middle and upper atmosphere (IMUA).The objective of the International Meridian Circle Program (IMCP),a major international program led by the Chines Academy of Sciences (CAS),is to deploy,integrate and operate a global network of research and monitoring instruments to use the IMUA as a screen on which to detect these imprints.In this article,we first show that the geometry required for the IMCP global observation system leads to a deployment of instruments in priority along the 120°E-60°W great meridian circle,which will cover in an optimal way both the dominant geographic and geomagnetic latitude variations,possibly complemented by a second Great Circle along the 30°E-150°W meridians to capture longitude variations.Then,starting from the Chinese Meridian Project (CMP) network and using it as a template,we give a preliminary and promising description of the instruments to be integrated and deployed along the 120°E-60°W great circle running across China,Australia and the Americas.