



5.3.2 Spectrometry and radiometry
Bands:
MIPAS detectors are combined in five spectral
bands. The table below gives the detectors
configuration and the spectral range of these
bands.
Band

Detectors

Spectral range (cm^{1})

A

A1 and A2

685  970

AB

B1

1020 
1170

B

B2

1215 
1500

C

C1 and C2

1570 
1750

D

D1 and D2

1820 
2410

Beamsplitter:
In MIPAS, the beamsplitter is a
plate of transparent material with optical
coatings designed to reflect 50% of the incoming
infrared radiation and let the remaining
fraction goes through. In other words, it is a
semitransparent mirror.
Blackbody:
An ideal body that completely absorbs all
radiant energy striking it and, therefore,
appears perfectly black at all wavelengths. The
radiation emitted by such a body when heated is
referred to as blackbody radiation and is given
by the Planck's equation. Black body
radiation is only function of the temperature of
the black body emitting it. Almost ideal
blackbodies are often used as reference sources
in infrared radiometry.
Calibration:
Radiometric calibration is the
process by which physical units are attributed
to the raw spectrum derived from the
measured interferograms. The
radiometric calibration is applied as follow:
FFT{I  I_{o}
} = G x (S  S_{o}
)
where L is the calibrated radiance,
G is the radiometric
gain, S is the raw
spectrum to calibrate and I is its
correspondin interferogram, I_{o}
is the interferogram acquired
while looking at the deep space (offset
measurement 1.1.3.5. ) and S_{o}
is its corresponding raw
spectrum. Click here for details.
Spectral calibration is the
process by which the wavenumber of every point
of the spectrum are validated and corrected
if necessary. Spectral calibration is done
by verifying the spectral position of
reference atmospheric lines. Click here for details.
Cube corner retroreflectors:
A cube corner retroreflector is
made of three perpendicular flat mirrors. Any
ray of light that strikes the inside of the cube
corner is reflected in the same direction
that it came.
Decimation:
The act of systematically
rejecting points at given intervals in the
interferogram. Decimation is used to reduce
the data rate. The decimation factor is the
integer ratio of the initial sampling
frequency to the new one. Decimation factors
are programable. The following table
list the nominal decimation factors for the
bands of MIPAS:
Band

Detectors

Decimation factor

A

A1
and A2

21

AB

B1

38

B

B2

25

C

C1
and C2

31

D

D1
and D2

11

Doppler shift:
A spectral shift of the observed radiation as a
result of the Doppler effect. The Doppler effect
is due to a relative motion between the
source and the observer. The radiation emitted
from a source that moves away from an observer
appears to be of lower frequency than the
radiation emitted from a stationary source. The
radiation emitted from a source moving toward
the observer appears to be of a higher
frequency than that from a stationary source.
For MIPAS, the Doppler shift is caused by the
relative motion between ENVISAT and the
atmosphere. The random relative
motion of the atmospheric molecules causes a
broadening of the atmospheric spectral lines,
called Doppler broadening.
Since the relative motion of the atmospheric
molecules depends on pressure and temperature,
the importance of the Doppler broadening
contains information about the altitude and the
temperature of the air.
Gain
In MIPAS 5.1. the radiometric gain,
G, is computed as follow:
FFT{I_{bb}
 I_{o}
}) = L_{bb}
/ (S_{bb}
 S_{o}
)
where L_{bb}
is the theoretical radiance of the calibration
blackbody. I_{bb}
is the interfeogram acquired while
looking at the calibration blackbody (gain measurement), I_{o}
is the correspomding raw spectrum,
Io is the interferogram acquired while
looking at the deep space (offset
measurement 1.1.3.5. ) and I_{o}
is the corresponding raw spectrum. See
radiometric calibration.
Emissivity:
The ratio of an object's radiance to that emitted by a
blackbody radiator at the same
temperature and at the same wavelength. A
perfect blackbody has an emissivity of 1.
Exitance:
Exitance is the amount of radiant flux emitted by a source per
unit source area. It is the surface
density of power on a source. It depends only on
the source. For example a spherical source with
a radius of 1 cm that emits a radiant flux
of 10 W has an exitance of 10 W divided by the
surface (4 pi times 1 cm^{2}) or 25000 W
m^{2}.
Flux:
Radiant flux (also called radiant
power) is the amount of energy emitted by a
source or received by a detector per unit time.
For example if a source emits 100 Joules
of radiant energy in 10 seconds, its radiant
flux is 10 Watt.
Instrumental line shape:
The instrumental line shape
(or ILS) is the unapodised instrumental response
to a stimulus of negligible spectral width. The
ILS varies as a function of the wavenumber. The ILS also
determines the spectral resolution of the
instrument.

Figure 5.1 
Intensity:
Intensity is the amount of
radiant flux radiating in a given
direction per unit solid angle. For
example an isotropic source that emits a radiant
flux of 10 Watt has an intensity of 10 W
divided by the solid angle of sphere (4 pi
steradians) or 0.8 W sr^{1} in any
direction and at any distance from the source.
Interference:
The additive process whereby the
amplitudes of two or more overlapping radiation
beams are systematically attenuated and
reinforced.
Interferogram:
Photographic or electronic
recording of an interference pattern.

Figure 5.2 
A typical interferogram
Click here to see a simulated
inteferogram of MIPAS.
Irradiance:
Irradiance is the amount of
radiant flux received by a surface
perpendicular to the incident radiation per unit
area of the surface.
Maximum path difference:
Maximum path difference (MPD) is
the maximum distance between a mirror and ZPD. It is half the
maximum effective distance between the two
mirrors of the interferometer. For MIPAS, the MPD is 20 cm.
Noise Equivalent Spectral Radiance:
The noise equivalent spectral
radiance (NESR) is the rms noise of a given
measurement expressed in unit of radiance.
Radiance:
Radiance is the amount of radiant flux
propagating in a given direction per unit area
and unit solid angle. It is the most general radiometric quantity. It
is used to describe both emitted and received
radiation.
Radiometric quantity:
The various quantities used in
radiometry to quantify radiation. The usual
radiometric quantities, the symbols used in this
document to describe them and their SI
units are given in the following table:
Spectral radiometric quantity are
given per unit wavelength or per unit wavenumber.
Radiometry:
The science interested by the
detection, measurement and quantification of
electromagnetic radiation in terms of energy.
Signal to noise ratio:
Signal to noise ratio is the
signal divided by the noise, both quantities
being given in the same units. It gives the
relative importance of the measured signal
compared to the noise in the measurement.
Spectral binwidth:
The spectral binwidth or spectral
interval is the difference along the spectral
axis between two consecutive points in a
spectra. This is to be distinguished from
the spectral resolution. The
spectral interval in wavenumber units of spectra
generated by a given Fourier transform
spectrometers is a constant and is given by 0.5
/ MPD.
Spectral resolution:
Spectral resolution determines
the ability of the instrument to distinguish
closely spaced spectral features. An instrument
cannot distinguish two spectral features
that are closer than its spectral resolution.
Spectral resolution is usually determined by the
full width at half maximum of the instrumental line
shape.Spectral resolution is often
confused with spectral bindwidth.
Spectroradiometer:
An instrument that is
calibrated so as to measure radiation (energy)
amplitude as a function of wavelength.
Spectrum: A
curve that shows a spectral radiometric quantity
as a function of wavelength (or wavenumber).

Figure 5.3 
A typical atmospheric spectrum
Click here to see a simulated MIPAS
spectrum.
Sweep:
Recording for a single interferogram. A sweep
can be forward or reverse. The
adjective forward or reverse are
attributed to the sweep depending on the
direction of motion of one of the cube corner of the
interferometer.
Wavenumber:
In spectroscopy, the wavenumber
is the inverse of the wavelength. In infrared
spectroscopy it is customary to express the
wavenumber in cm^{1}. Wavenumbers
are preferred to wavelengths in Fourier
transform spectroscopy because the spectra
measured by such instruments are of
constant step size (along the spectral axis)
when expressed in wavenumbers.
Zero Path Difference:
The expression "Zero
path difference" or ZPD is used to describe
the location of the moving mirrors such that the
two arms of the interferometer are of
equal optical path length. At ZPD, the interference is constructive
for every wavelengths and the interferogram is at its
absolute maximum in one of the output port (and
thus at its absolute minimum in the other
output port).



 
Keywords: ESA European
Space Agency  Agence spatiale europeenne,
observation de la terre, earth observation,
satellite remote sensing,
teledetection, geophysique, altimetrie, radar,
chimique atmospherique, geophysics, altimetry, radar,
atmospheric chemistry
