The High Resolution Fly’s Eye (HiRes, Utah): air-fluorescence detection of UHECR showers, with a near-calorimetric energy measurement independent of the ground-array/MC lateral-distribution technique used by AGASA. Basis for S-60 - Abbasi et al. 2008- HiRes first observation of the GZK suppression (and the later HiRes monocular-flux papers). known_biases: ~10% duty cycle (moonless clear nights only) so exposure-limited statistics at the highest energies; energy scale depends on atmospheric/fluorescence-yield calibration and aperture Monte Carlo. Systematics are largely independent of the AGASA ground-array systematics, which is why HiRes vs AGASA agreement/disagreement is informative rather than circular.