Extensive measurements have been made of the upper atmosphere by satellites and the lower atmosphere is measured twice daily by weather balloons. In contrast, the middle atmosphere is a difficult area to measure and therefore has been much less extensively studied. We are currently upgrading an old lidar system to a new system that will be 70 times more sensitive, making this the most sensitive lidar of its kind in the world. The upgrade consists of combining the outputs of 18 and 24 watt Nd:YAG lasers; implementing an optical chain to detect backscattered light using an existing large, four-mirror telescope; four optical fibers; an optical system and mechanical chopper; photomultiplier tubes; a data-acquisition system; and an aircraft detection radar. Moving to this new system will allow us to extend Rayleigh-scatter observations from 90 to 110 km in the mesosphere and lower thermosphere, significantly higher than was possible with the original system. Alternatively, it will enable significantly greater precision or better time resolution for observations in the previous altitude range. After finishing the upgrade, we will use software previously designed for this system to perform the reduction and analysis of the first data to obtain relative density fluctuations and absolute temperatures.