Elongated plasma density irregularities in the magnetosphere could guide extraordinary mode waves from a satellite and produce detectable ducted echoes. The ducted waves are undamped and immune to spatial attenuation. For instance, a 10% depletion 100 km wide is sufficient for echoes 10-30 dB above nonthermal continuum, regardless of the distance. The remote measurement of plasma density and magnetic field structure in the magnetosphere would be practical using such echoes.