Interferometric monitor for greenhouse gases (IMG)

Introduction

Growing awareness on climate change has created an increasing interest to observe concentrations of gases contributing to the warming of the climate. A specially effective way of doing this is to observe this from space, as this enables to an excellent spatial coverage of earth on the flight path of the satellite. Necessary technologies had been unlocked and thus a joint co-operative was launched to create a sensor suitable for the task.

Interferometric monitor for greenhouse gases (IMG) is a satellite born sensor based on FTIR developed in co-operation by JAROS, NASDA and NASA. IMG measured upwelling infrared radiance at fine spectral resolutions. The instrument was designed to have a very high spectral resolution (0.05 cm).The sensor will obtain very accurate spectra of of the thermal infrared radiation from the atmosphere and this data can be used deduce concentrations of greenhouse gases and water vapour.

http://suzaku.eorc.jaxa.jp/GLI2/adeos/Project/images/img-fig2.gif

Working principle

Interferometry
Fourier-transform infrared spectroscopy

Interferometry:

Interferometry is a study of wavelenghts, light and interference (Hence the name). The usual component to study is electromagnetic radiation. Waves are superimposed, meaning they are manipulated in a way to give some meaningful analytical results. Usually light is used as the source of the electromagnetic waves.

The basic principle of a Michelson - interferometer: A light is introduced into the system and it is manipulated to travel two different paths. The paths are then guided to meet at a point (The detector). The difference in the path travelled creates an interference pattern and this pattern can be studied and interpreted. This yields e.g. a refractive index which can be purposefully understood.

A schematic of a michelson interferometer

2. http://cnx.org/content/m22326/latest/graphics2.jpg

Fourier Transform Infrared Spectroscopy FT-IR

FT-IR is the prevalent method for infrared spectroscopy. It has replaced the dispersive infrared instruments.

Background

Infrared spectroscopy is used for identifying one material from another. It is based on the fact that every material has a unique combination of atoms; therefore, it is impossible to get the exact same infrared spectrum from two different compounds.

The infrared spectrum of a material consists of absorption peaks corresponding to vibration frequencies present between the bonds of its atoms. The size of a peak indicates the amount of material present.

The need for FT-IR was felt because of the limitations of dispersive-type instruments. The major problem with the older technology was that the scanning process was slow. There was a need for a process that could simultaneously measure all infrared frequencies rather than measuring one frequency at a time.

Interferometer Output Signal

The signal produced by an interferometer is unique in the sense that it contains all infrared frequencies. Measurement of the signal is quick, typically, requiring a second or so. Therefore, the time required for processing a signal is reduced to a few seconds instead of many minutes.

Fourier Transformation

In order to make identifications, the analyst will need a frequency spectrum (plotting of intensity value for each frequency value). This information is not provided by the measured interferogram signal. In order to get the individual frequencies, the measured signal needs to be “decoded.“ The decoding process is where Fourier transformation is utilized. A computer performs the transformation and presents the required spectral information for analysis.

figure 3: Conversion of measured signal into frquency spectrum by application of FFT (fast Fourier transformation)

Source: http://chemwiki.ucdavis.edu/@api/deki/files/9220/data_process.png

Relative Scale

To establish a relative scale against which all measurements can be compared, measurements are taken without any sample in the beam. The comparison with relative scale gives the “percent transmittance.” This technique produces a spectrum which eliminates all instrumental characteristics from the data. Thus, the spectrum seen is produced solely by the sample.

Sensor details

The detectors are housed in the IR-detector below the cooling unit. There are three detectors, each corresponding to three different spectral channels.

Two InSb (Indium Antimony, narrow-gap semiconductor from 3-4 group) detectors for the shorter wavelengths (3.3 - 4.3 nanometers)
One HgCdTe (Mercury Cadmium Telluride) a semiconductor with a bandgap between 0 and 1.5 eV. This is for the longer wavelengths 4 - 15 nanometer) (4.)
An onboard blackbody for calibration
The mirror can also be moved to point toward space for calibration purposes

The sensors are cooled with the cooler to a temperature of below 80 K.

The incoming light beams are splitted into different wavelengths and reflected into the IR-sensor and then the different semiconductors absorb them in their rispectable operating wavelengths. These portions of the reflected beam power electrons to move onto the the next conduction band, thus causing that particular semiconductor to produce a current. This current is then registered and analyzed.

!!ADD THE SENSOR DETAILS!! CAN BE FOUND FROM LINK OF SOURCE 4!!

Detection and monitoring of the vertical and spatial distribution of:

CO2
CH4
O3
Nitrogen based molecules (N2O, NO2)
CFC

Applications

IMG has been deployed on atleast two occasions.

ADEOS 1 (Advanced Earth Observing Satellite): launched in 1996 and flew in the polar orbit.
ADEOS 2: launched in 2002

Sources

http://www.sciencedirect.com/science/article/pii/S0273117799009266 (Accessed 15.4.2014)
http://www.webpages.uidaho.edu/~vonw/pubs/WaldenEtAl_2010.pdf (Accessed 15.4.2014)
http://suzaku.eorc.jaxa.jp/GLI2/adeos/Project/Img.html (Accessed 15.4.2014)
http://www.sciencedirect.com/science/article/pii/S0273117799009266/pdf?md5=d94773c8e7a93436573522036217bdd1&pid=1-s2.0-S0273117799009266-main.pdf (Accessed 22.2014)

Child pages