In 1986, a second Fly's Eye detector site was completed with 36 mirrors. The new detector was referred to as Fly's Eye 2 (FE2), and was located 3.4 km from the original site, which was henceforth denoted as FE1. The new mirrors are identical in design and construction to the original FE1 mirrors.
The addition of the FE2 detector allowed the trajectory of the air shower to be reconstructed by a process of triangulation. This "stereoscopic" reconstruction technique was much more accurate than the timing fit described previously, and hence gave a much more reliable energy measurement than was afforded by monocular measurement.
Both FE1 and FE2 sites were operated until 1993. The total monocular exposure of the original FE1 detector reached ~930 km2-sr-yrs for cosmic rays with energy 5x1019 eV (the aperture of the detector, measured in km2-sr, is energy-dependent). The final stereo exposure accumulated was ~150 km2-sr-yrs.