The core task in the design and packaging of the
ALIAS instrument was that of reducing a 1500 pound instrument designed
for a balloon gondola to a 150 pound instrument for aircraft use. Responsibility
for instrument configuration, packaging, mounting and mechanical design
was primarily mine. The weigh limitation was met, and the instrument
first produced in 1993, is still presently in service.
For extensive JPL pages dedicated to this instrument
follow this link to:
Principle of Operation: (Extracted from a paper
entitled Aircraft Laser Infrared Absorption Spectrometer for in-situ
stratospheric measurements of HCl, N2O, CH4, NO2, and HNO3, Applied Optics in
1994, by Webster, May, Trimble, Chave and Kendall)
The ALIAS instrument is a very high resolution scanning tunable diode
laser spectrometer which makes direct, simultaneous measurements of NO2,
HCl, CH4, and N2O (including vertical profiles of CH4 and N2O) at sub-part-per-billion
level sensitivities over a 3-30 second integration time. Unique features
include a sample inlet/throttle system designed to achieve near-isokinetic
sampling in PSC events, an in-flight wavelength reference cell rack, a
mechanical fringe-spoiler, a four-laser/four-detector dewar with 24-hour
hold-time operating at a fixed temperature without electrical regulation,
and in-flight fast correlation routines for spectral drift compensation
prior to spectral addition. Laser light in the 3.4 to 8 micron wavelength
range from four lasers is injected into a one-meter 80-pass Herriott absorption
cell fitted with mechanical fringe-spoiler in a flowing configuration
with a cell flush time of 1 to 2 seconds. ALIAS is run using a 486-based
computer and 200 MByte sealed hard-disk.
Accuracy: Typically 5-10%,
depending upon IR spectral parameters and SNR Precision: Depends
upon observed atmospheric concentrations and SNR Detection Limits:
NO2 (0.05 ppbv); HCl (0.05 ppbv); CH4 (0.2 ppbv); N2O (0.1 ppbv) Response Time: 3
second data collection: 30 second full integration time for above sensitivities Location on ER-2:
Spear pod on right wing.
About this photo: The image above shows the ALIAS
instrument package installed in a standard ER2
rectangular instrument rack, ready for mounting in the
under-wing-pod of the ER2 aircraft. The white probe is
G-10 fiberglass, 15 cm in diameter and contains heaters
which help ensure that the air sample obtained is purely
in gaseous form, rather than a gas and crystal mix.
The cell behind the white probe is the Herriott absorption
cell in which the multi-pass spectroscopy is done.