TIDAS-SPU: Development and testing of a system for infrared FTS imaging of the

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24 Νοε 2013 (πριν από 3 χρόνια και 6 μήνες)

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TIDAS
-
SPU: Development and testing of a system for infrared FTS imaging of the
atmosphere

Neil Humpage
1
, John Remedios
1
, Alex Wishart
2
, Thomas McCoy
2
, Hugh Mortimer
3
, Kevin James
4
, Clive Arthurs
4


1.
Earth Observation Science, Department of Physics and Astronomy, University of Leicester,

UK LE1 7RH (email: nh58@le.ac.uk)

2.
Astrium

Ltd,
Stevenage
, UK SG1 2AS

3.
RAL Space, Rutherford Appleton Laboratory,
Didcot
, UK OX11 0QX

4.
Selex

Galileo, Southampton, UK SO15 0EG

1. Science and Technology Context



Infrared FTS systems such as IASI and MIPAS are excellent for measuring height
resolved profiles of H
2
O, O
3
, and other gases of a more anthropogenic nature such as
CH
4

and CFC related species. In addition, the sensitivity of the thermal infrared FTS
technique enables the observation of a wide range of less abundant species, including
organic compounds such as hydrocarbons, PAN and acetone (see Figure 1).



The application of 2D detector array technology to infrared FTS techniques would
significantly enhance the science achievable through IR earth observation, owing to
greatly improved spatial resolution and global coverage.
This technology development
could also significantly increase our ability to probe gas mixtures either in the laboratory
or in remote sensing field instruments.

TIDAS
-
SPU Project Outline

The objective of the CEOI TIDAS
-
SPU (Thermal Infrared Detector Array System


Signal Processing Unit) project has been to determine the design and operational
issues involved in the use of detector arrays for infrared Fourier transform spectroscopy
(FTS).
Potential applications for this technology include the development of new
lab experiments and field instruments, in addition to the enhancement of
techniques currently used for remote sensing of the Earth from space.





Integrating a 2D detector array into an infrared FTS instrument offers a significant
multiplexing advantage over existing earth observation methods, since spatial
information is acquired simultaneously with spectral information. An imaging infrared
FTS concept is currently being considered for ESA’s 7
th

Earth Explorer mission.

Figure 1: 1
st

August 2003 MIPAS retrievals of acetone (courtesy of David Moore, University of Leicester), demonstrating the
global coverage currently achievable using a single detector instrument

2. TIDAS
-
SPU Demonstration Setup



Figure 2 shows most of the elements involved in the TIDAS
-
SPU demonstration setup.
The spectrometer used is a
Bruker

IFS
-
66, provided by the Molecular Spectroscopy
Facility (MSF) at STFC
-
RAL. The IFS
-
66 includes an internal MIR (mid
-
infrared)
globar

source, and a compartment for holding the sample gas cell. An internal
DLaTGS

detector
is used in parallel with the TIDAS
-
SPU detector to monitor the spectrometer alignment,
and to provide reference spectral measurements. The spectrometer has been retro
-
fitted
with a step
-
scanning mechanism, enabling complete control over the mirror scan speed.



The detector used is a
Selex

Galileo LW Hawk 640x512 pixel Mercury Cadmium
Telluride (MCT) array. The pixels are sensitive to IR radiation with wavelengths between 5
and 9.4
μ
m.
An 80x96

pixel window of data is recorded at a frame rate of 400Hz and
digitised using a digital interface module (DIM), also supplied by
Selex
, which performs 14
bit ADC.



The signal processing unit, supplied by
Astrium
, has been designed using COTS
hardware together with customised interfaces, software and firmware developed
specifically for this study. It takes the digitised pixel array data output by the
Selex

DIM and
processes it, following standard FTS algorithms.

Figure 2: TIDAS
-
SPU setup at the MSF, STFC
-
RAL


Figure 3: Some preliminary results from the TIDAS
-
SPU test phase. (a):

M
ean moderate spectral resolution (blue) and low
spectral resolution (orange)
N
2
O
spectra, along with mean background spectra (black). (b): From top to bottom: individual

moderate spectral resolution spectra for each high spatial resolution mode
superpixel
;

individual low spectral resolution
spectra for each
high spatial resolution mode
superpixel
; and maps of integrated intensity in the N
2
O absorption band for
background, N
2
O and transmittance (N
2
O / background). (c): Same as (b), except the output is in low spatial resolution mode

3. Preliminary Results



Figure 3 shows an example of preliminary results taken during February and March
2011 at the MSF. A number of
interferogram

datacubes

were recorded, with N
2
O chosen
as a target sample since it has strong, well
characterised

absorption features in the MIR.



TIDAS
-
SPU may be operated in two modes, which determine the spatial resolution: a
high spatial resolution mode where pixels are co
-
added into
superpixels

of 5x1; and a low
spatial resolution mode where pixels are co
-
added into
superpixels

of 20x4.



Interferograms

are interpolated onto a regular optical path difference grid using the
Brault

method (
Brault
, J.W. 1996. New approach to high precision Fourier transform
spectrometer design.
Applied Optics,
35
(16), 2891

2896).

Outlook and Future Work

These initial TIDAS
-
SPU results demonstrate the potential for imaging infrared FTS
as a technique for atmospheric sounding, though further work is needed to achieve
desired signal
-
to
-
noise levels and on
-
board data processing rates. In addition to
further laboratory testing involving
characterisation

of the spatial response of the
array, the TIDAS
-
SPU team hope to test the system further by making ground
-
based
atmospheric measurements using a solar tracker.



(a)

(b)

(c)

Background

N
2
O

Transmittance

High spatial resolution mode

Background

N
2
O

Transmittance

Low

spatial resolution mode