The traditional difficulty in designing such an instrument is that cosmic ray production seems to be a random process. It wouldn't be very fruitful to point a cosmic ray detector in one direction because it may have to wait a very long time to detect many cosmic rays. Due to cost and engineering difficulties, any instrument based on this detection method would be far to small to gather much data - even over the course of many years. The latest ideas are based on sending a fluorescence detecting (FD) telescope into space to look at the Earth's atmosphere to try to detect the faint Cherenkov light that we currently are looking for with our FD's at the ground level. This idea does have merit, but it would cost a great deal more money to put the FD into space than to place it on the ground (as we currently do).