TIDAS-SPU: Development and testing of a system for infrared FTS imaging of the atmosphereNeil Humpage1, John Remedios1, Alex Wishart2, Thomas McCoy2, Hugh Mortimer3, Kevin James4, Clive Arthurs4

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 2AS3. 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 H2O, O3, and other gases of a more anthropogenic nature such as

CH4 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 7th Earth Explorer mission.

Figure 1: 1st 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 μm. 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): Mean moderate spectral resolution (blue) and low spectral resolution (orange) N2O 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 N2O absorption band for background, N2O

and transmittance (N2O / 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 N2O 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

Transmittance

High spatial resolution mode

Background Transmittanc

e

Low spatial resolution mode