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The University of Torontos Balloon-Borne
Fourier Transform Spectrometer Debra Wunch,
Jeffrey R. Taylor, James R. Drummond Clive
Midwinter, Kimberly Strong Department of Physics,
University of Toronto Hans Fast Environment Canada
The University of Torontos Fourier transform
spectrometer (U of T FTS), derived from an
ABB-Analytical DA5 Michelson-type interferometer,
was rebuilt and flown on the Middle Atmosphere
Nitrogen TRend Assessment (MANTRA) high-altitude
balloon platform in September, 2004.  The U of T
FTS has a resolution of 0.02 cm-1, a spectral
range covering 1200-5000 cm-1, and InSb and MCT
detectors that measure simultaneously. The
spectrometer was originally built in the 1980s
and purchased by the Meteorological Service of
Canada.  To prepare the instrument for flight,
new software was necessary and was replaced by
LabVIEW control software, creating a robust and
easily-controlled instrument, adaptable to either
remote control or lab-based work.  The software
contained accessible housekeeping information,
downlink capability and an embedded scheduler. 
In addition to replacing the software, nearly all
the original electronics were replaced using
off-the-shelf components.  The dynamic alignment
system is the only original electronic system
remaining.  A small suntracker was incorporated
which facilitated alignment of the spectrometer
with the balloon payload. Despite telemetry
and gondola pointing system failures during the
MANTRA 2004 campaign, two spectra were recorded
on each detector during sunset from a float
height of 35 km.  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 data that were acquired will be
shown.  The spectrometer has since participated
in a three-month-long ground-based FTS
inter-comparison campaign. The results from the
intercomparison show that the U of T FTS
retrieves, on average, ozone columns that are
within 3 of a co-located Brewer
Spectrophotometer.
The Instrument and its Scientific Roles The
University of Torontos balloon-borne Fourier
Transform Spectrometer (U of T FTS) is a
Michelson-type interferometer with a 0.02 cm-1
resolution and a spectral range covering
1200-5000 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 U of T FTS
has participated in two Middle Atmosphere
Nitrogen TRend Assessment (MANTRA) high-altitude
balloon campaigns in 2002 and 2004. The role of
the FTS aboard MANTRA is to advance MANTRAs
science goal of understanding mid-latitude ozone
depletion, and, specifically, to determine the
role of nitrogen compounds in the depletion of
ozone. The U of T FTS contributes to this project
because it can simultaneously measure many trace
gas species related to nitrogen and ozone, with
high vertical and spectral resolution. The FTS
Refurbishment The U of T FTS was initially
purchased by the Meteorological Service of Canada
in the early 1980s from what is now known as
ABB-Analytical, a company that produced a
research-grade FTS with an excellent optical
design. The main asset of this design was the
dynamic alignment system that used the stationary
mirror to compensate for the inevitable angular
changes of the scanning mirror (see Figure 1).
The instrument control software provided by
ABB-Analytical allows the user to manually
control the main functions of the FTS in a simple
and repeatable way. This, however, was not
appropriate for our balloon application, which
requires that interferograms be recorded during
solar occultation (sunrise and/or sunset) with
limited telemetry uplink and downlink. For this
application, it is necessary to control the
instrument remotely and have a robust automatic
mode. The original software is embedded in the
original control computer for the instrument, so,
in order to update the software, the control
computer and nearly all of the original
electronics had to be replaced, while ensuring
that the dynamic alignment system was kept
intact. 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 (voltages, temperatures
and instrument status information), downlink
capability and an embedded scheduler. Delta-Trac
ker 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. 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).
Figure 1 shows a schematic of the instrument and
delta-tracker. Results from the MANTRA 2004
Flight Due to a telemetry and gondola pointing
system failure during the MANTRA 2004 flight, the
payload rotated during sunset. Since the
delta-tracker has only 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 MCT spectrum, seen to the
right in Figure 2, was recorded while looking
through the atmosphere from a float altitude of
approximately 35 km at a 89 solar zenith angle.
Signatures of O3, N2O, CH4 and CO2 are clearly
visible in the spectra, and a selection of
microwindows for these gases are displayed in the
lower panels. Long-Term FTS Intercomparison
Campaign The U of T FTS participated in a
long-term ground-based intercomparison campaign
with the Toronto Atmospheric Observatory FTS
(TAO) that lasted from May 2005 through early
September 2005. The TAO FTS, is a linear,
ABB-Analytical DA8 FTS, and is housed at a
complementary NDSC site in a rooftop laboratory
at the University of Toronto It is solely a
ground-based FTS, and records solar absorption
measurements during every clear-sky opportunity
throughout the year. TAO has been in commission
since 2001. Also on-site during the campaign was
a Brewer Spectrophotometer, for ozone total
column comparisons. The main scientific goal
of the intercomparison is to answer the question
What is the effect of instrument resolution on
the retrieved column amounts of trace gas
species? To be able to answer this question, we
eliminated possible differences due to retrieval
methods and a priori trace gas profiles, by using
the retrieval algorithm SFIT2 and the same a
priori information. We eliminated possible
differences in the atmospheric conditions
themselves by measuring simultaneously with both
instruments, and using a pick-off mirror to
supply the U of T FTS with sunlight from the TAO
sun tracker. This isolates the problem to one of
instrumentation. Results from the Long-Term
Intercomparison Campaign Results for ozone, N2O,
CH4, HCl and CO have been retrieved for this
campaign. Only the ozone results are shown here.
Figures 3a, b, and c below show the agreement
between the U of T FTS, the TAO FTS and the
Brewer Spectrophotometer. Figure 3a shows the
total columns and Figure 3b and c show the
percent difference between the TAO FTS and the
Brewer and the U of T FTS and the Brewer,
respectively. The U of T FTS is, on average, 3
lower than the Brewer, with no discernable
structure in the residuals of Figure 3c. The TAO
FTS reads lower than the Brewer by 3.5 on
average.
Figure 1. A schematic of the University of
Torontos Balloon-Borne Fourier Transform
Spectrometer.
Figure 2. Top panel an MCT spectrum recorded
from the MANTRA 2004 balloon during sunset
bottom panels a closer look at some molecular
signatures.
a
c
b
Figure 3. Results from the Long-Term
Intercomparison. In a, the total columns in b,
the percent difference between the TAO FTS and
the Brewer in c, the percent difference between
the U of T FTS and the Brewer.
Summary and Future Work The University of
Torontos Fourier transform spectrometer was
successfully modernized and recorded good data
under difficult conditions. New software and a
delta-tracker were key components in enabling
its success on board the MANTRA 2004 payload.
The ground-based long-term intercomparison with
the TAO FTS has revealed some interesting
results. Future work will concern investigating
the reasons for the apparent bias between both
FTS instruments and the Brewer.
The authors wish to thank C. Tom McElroy, Pierre
F. Fogal, John Olson and the MANTRA Science team
for their invaluable advice and assistance.
MANTRA is supported by the Canadian Space Agency
(CSA), Environment Canada, CFCAS and NSERC. The
figure in the background is from an ozonesonde
launch during the MANTRA 2004 campaign. To
contact Debra Wunch, email debra_at_atmosp.physics.ut
oronto.ca