The UHSAS, originally manufactured by Particle Metrics, Inc. and now by Droplet Measurement Technologies, measures the concentration and size distribution of aerosol particles having diameters from 0.060 --1.0 µm, in 100 bins that are approximately logarithmically spaced. The instrument flies in an under-wing canister and senses particles in the airstream via light scattering from individual particles. The maximum counting rate for which coincidence error may be neglected is about 3000/s, so the maximum concentration (at a sample rate of about 0.83 volume cm³/s) is about 3600 cm^-3.  Flow control is via a volumetric flow controller, and concentrations are reported in the data files at ambient (not instrument) conditions. Data are transferred via RS-422 to the aircraft data system and recorded there, and are optinally recorded on the laptop as well.

The specifications for the instrument limit its operation to > -40°C and < 12 km altitude, and there have been some operational problems at higher altitude and lower temperature, notably noise in the first channels or anomalously low measurements of concentration. These have been improved, but not eliminated, through modifications to the instrument.  Small-diameter noise in particular continues to be a problem.  The probe covers the full size range through the use of two different detectors, each having two gain circuits, and there is some overlap among the sensitive regions of these four channels so the instrument must choose the best signal for each size pulse. The ranges change somewhat during flight, so at times gaps or discontinuities develop in the size distributions. This also has been addressed in recent changes and is expected to be of less concern in projects beyond 2012.

The UHSAS must receive an initialization record, which can come from a host laptop PC with a LabView application, or from a static record sent by a Data System Module (DSM). The instrument fits in a standard 6" instrument wing pod.