A new technique for the satellite remote sensing of greenhouse gases in the atmosphere via the absorption of short-wave infrared laser signals transmitted between counter-rotating satellites in low earth orbit has recently been proposed; this would enable the acquisition of a stable, global set of altitude-resolved concentration measurements. We present the first ground-based experimental demonstration of this new technique, in which the atmospheric absorption of CO₂ near 2.1 μm was measured over a ~144 km path length between two peaks in the Canary Islands (at an altitude of ~2.4 km). The retrieved CO₂ volume mixing ratio of 400.1 ppm (±14.7 ppm) is consistent within experimental uncertainty with simultaneously recorded in situ validation measurements. We conclude that the new method has a sound basis for monitoring CO₂ and other greenhouse gases in the free atmosphere.