DME (Direct Measurement Explorer), also known as Explorer 31, was a satellite launched together with the Canadian Alouette 2. The double-launch project, known as ISIS-X, was the first in a new cooperative NASA-Canadian Defense Research Board program for International Satellites for Ionospheric Studies. DME was placed in an orbit with an apogee (highest point) slightly higher than Alouette's and a perigee (lowest point) slightly lower. Eight ionospheric experiments sampled the environment both ahead and behind the satellite.