Airborne Laser Infrared Absorption Spectrometer (ALIAS-II)

Measurement Description: The Airborne Laser Infrared Absorption Spectrometer (ALIAS-II) is a very high resolution scanning tunable diode laser spectrometer which makes direct, simultaneous measurements of selectable combinations of HCl, NO₂, CO, CO₂, CH₄, and N₂O at sub-part-per-billion levels over a 3-30 second integration time. The measurement technique is based upon using tunable lead-salt and/or quantum cascade lasers operating from 3.4 to 8 microns wavelength scanning over absorption lines at 10 Hz recording second harmonic spectra. The instrument features an open-cradle multipass Herriott absorption cell with 15.24-cm diameter spherical zerodur mirrors coated with gold on chrome. The separation between the mirrors is adjustable allowing for a relatively small cell (0.75-m to 1.5-m) to contain an optical path length up to 120-m, depending on the spacing of the mirrors. Lasers and detectors are contained in a lightweight aluminum liquid nitrogen Dewar which can achieve a 28-hour hold time with only a 2 liter charge of liquid nitrogen. The instrument features custom laser current drives, signal chains, InSb detectors and preamps, 16-bit signal averager, analog signal conditioner, and digital I/O which are controlled by an onboard Pentium processor. Data is written to a ruggedized 2-Gb hard disk every 30 seconds and simultaneously transmitted via telemetry to ground station computers which provide backup storage of the data. The instrument weighs 36 kg. and requires <56 watts for operation. Additional power up to 250 watts is available for structural heaters and current draw varies with atmospheric conditions. For launch on the Jet Propulsion Laboratorys Observations from the Middle Stratosphere gondola the optical assembly is deployed outside the gondola to sample free stream air and the electronics remain inside the gondola. Before the flight is terminated the optical assembly is retracted and stowed to withstand up to 10-Gs. ALIAS-II has flown successfully on seven missions out of Ft. Sumner, New Mexico, Fairbanks, Alaska, and Juazeiro do Norte, Brazil.