Arecibo Incoherent Scatter Radar is made up of the actual transmitter-receiver system as well as the secondary and tertiary reflectors. The primary reflector is, of course, the 305-m main reflector. Thus the reflectors are numbered in the order in which the received signal hits them and not, as one might think, in the order the transmitted signal hits them. This might have to do with the antenna being mostly (in terms of operating time) used for radio astronomy in a passive mode without transmitter.
Photos: Thomas Ulich.