The University of Toronto’s Balloon-Borne Fourier Transform Spectrometer Debra Wunch, James R. Drummond, Clive Midwinter Department of Physics, University of Toronto Hans Fast Meteorological Service of Canada A Fourier Transform Spectrometer was flown on the Middle Atmosphere Nitrogen TRend Assessment (MANTRA) high-altitude balloon platform in September, The spectrometer was derived from a Bomem DA5 Michelson-type interferometer built in the 1980s and purchased by the Meteorological Service of Canada. The software and nearly all the original electronics were replaced in preparation for flight. A small “delta- suntracker” was incorporated which facilitated alignment of the spectrometer with the balloon payload. Despite balloon launch and gondola pointing system failures during the MANTRA 2004 campaign, two spectra were recorded on each detector during sunset. The data indicate that the instrument performed well throughout the flight, and, had the payload pointing been under control, would have retrieved a full set of occultation data. The spectrometer will be flown on future MANTRA campaigns. The FTS Refurbishment Control Software The original software for instrument control was not written with balloon-borne measurements in mind: it required user interaction. This software was embedded in the old electronics of the FTS (see Figure 1), so, in order to update the software and create something suitable for remote measurements, nearly all of the original electronics had to be replaced. This was done using off-the- shelf components, which resulted in a significantly smaller instrument that was reduced in mass from around 90kg to 55kg, with reduced power consumption from around 140W to 65W. The original software was replaced by LabVIEW control software, creating a robust and easily- controlled instrument, adaptable to either remote control or lab-based work, containing accessible housekeeping information, downlink capability and an embedded scheduler. Figure 2 shows a photo of the instrument post-renovation. Dynamic Alignment The dynamic alignment system is the signature component of a Bomem DA-series instrument. It ensures that the system maintains its alignment as the moving mirror of the spectrometer scans. This system was the only electronic system that was left untouched during the refurbishment of the FTS. It is now controlled by the LabVIEW software. “Delta-Suntracker” While gondola pointing systems can be good to a few degrees in zenith and azimuth, FTS instruments require more precise solar pointing abilities. A small, light-weight, 2-axis suntracker with ±10° zenith and azimuth field of view was used to partly decouple the FTS from the main gondola pointing system (see Figure 2). This allowed for easy instrument integration on the payload and a reasonably simple transition to ground-based measurements (two extra mirrors were required to direct the sun into the tracker on the ground). The Instrument and its Scientific Roles The University of Toronto’s balloon-borne Fourier Transform Spectrometer (FTS) is a Michelson-type interferometer with a 0.02 cm -1 resolution and a spectral range covering cm -1 across two detectors. The two detectors, an InSb and an MCT detector, measure simultaneously. The instrument is configured to measure atmospheric trace-gas species in occultation mode: that is, measuring solar absorption through sunrise and sunset. It is designed primarily for high-altitude balloon-borne measurements. The FTS has participated in two Middle Atmosphere Nitrogen TRend Assessment (MANTRA) high-altitude balloon campaigns: in 2002 and The role of the University of Toronto’s FTS aboard MANTRA is to complement MANTRA’s science goals of measuring ozone depletion and the molecules that contribute to the ozone budget by measuring HCl, O 3, N 2 O, CH 4, CO 2 and CO. The FTS will also be used to validate satellite data and foster Canadian expertise in Fourier transform spectroscopy. Results from the MANTRA 2004 Flight Due to a pointing system failure during the MANTRA 2004 flight, the payload was rotating during sunset. Since the delta-suntracker only has a ±10° zenith and azimuth field of view, only two spectra were recorded on each detector. There is every indication that, had the payload been properly oriented, the instrument would have recorded a full occultation of spectra. The spectra, seen below in Figures 3 and 4, were recorded while looking through the atmosphere from a float altitude of approximately 35 km at a 91° solar zenith angle. Signatures of H 2 O, CO 2, N 2 O, CH 4 and O 3 are clearly visible in the spectra, and a selection of microwindows are displayed in the lower panels. Summary The University of Toronto’s Fourier transform spectrometer was successfully modernized and recorded good data under difficult conditions. New software and a “delta-suntracker” were key components in ensuring its success. The current state of the instrument is a vast improvement over the original and there are plans for the instrument to fly again on the MANTRA 2006 payload. The authors wish to thank C. Tom McElroy, Pierre F. Fogal and John Olson for their invaluable advice and assistance. MANTRA is supported by the Canadian Space Agency (CSA), the Meteorological Service of Canada (MSC) and NSERC. Launch support is provided by Scientific Instrumentation Limited (SIL) of Saskatoon. The figure in the background is from an ozonesonde launch during the MANTRA 2004 campaign. To contact Debra Wunch, Figure 1. Top right: the instrument prior to refurbishment; bottom left: the electronics box that contained the software. Figure 2. Centre: the instrument after refurbishment; bottom left: the LabVIEW control software; top right: plan view of the delta-suntracker. Figure 3. Top panel: an MCT spectrum recorded from the MANTRA 2004 balloon during sunset; bottom panels: a closer look at some molecular signatures. Figure 4. Top panel: an InSb spectrum recorded from the MANTRA 2004 balloon during sunset; bottom panels: a closer look at some molecular signatures. Environment Canada Meteorological Service of Canada Environnement Canada Service météorologique du Canada