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MIPAS Data Formats Products
2 MDSR per MDS 1 forward sweep 1 reverse sweep
2 MDSRs per MDS 1 forward sweep 1 reverse sweep
LOS calibration GADS
Spectral Lines MDS
P T Retrieval MW ADS
VMR Retrieval Parameters GADS
P t Retrieval GADS
Framework Parameters GADS
Processing Parameters GADS
Inverse LOS VCM matrices MDS
General GADS
Occupation matrices for vmr#1 retrieval MDS
MDS2 -- 1 mdsr forward sweep 1 mdsr reverse
Occupation matrices for p T retrieval MDS
General GADS
Priority of p T retrieval occupation matices
P T occupation matrices ADS
Summary Quality ADS
Instrument and Processing Parameters ADS
Microwindows occupation matrices for p T and trace gas retrievals
Scan information MDS
Level 2 product SPH
MDS1 -- 1 mdsr forward sweep 1 mdsr reverse sweep
H2O Target Species MDS
P T and Height Correction Profiles MDS
Continuum Contribution and Radiance Offset MDS
Structure ADS
Summary Quality ADS
Residual Spectra mean values and standard deviation data ADS
PCD Information of Individual Scans ADS
Instrument and Processing Parameters ADS
Microwindows Occupation Matrices ADS
Scan Information MDS
1 MDSR per MDS
Scan Geolocation ADS
Mipas Level 1B SPH
Calibrated Spectra MDS
Structure ADS
Summary Quality ADS
Offset Calibration ADS
Scan Information ADS
Geolocation ADS (LADS)
Gain Calibration ADS #2
Gain Calibration ADS #1
Level 0 SPH
DSD#1 for MDS containing VMR retrieval microwindows data
DSD for MDS containing p T retrieval microwindows data
VMR #1 retrieval microwindows ADS
P T retrieval microwindows ADS
1 MDSR per MDS
VMR profiles MDS (same format as for MIP_IG2_AX)
Temperature profiles MDS (same format as for MIP_IG2_AX)
Pressure profile MDS (same format as for MIP_IG2_AX)
P T continuum profiles MDS (same format as for MIP_IG2_AX)
GADS General (same format as for MIP_IG2_AX)
Level 0 MDSR
Values of unknown parameters MDS
Computed spectra MDS
Jacobian matrices MDS
General data
Data depending on occupation matrix location ADS
Microwindow grouping data ADS
LUTs for p T retrieval microwindows MDS
GADS General
P T retrieval microwindows ADS
ILS Calibration GADS
Auxilliary Products
MIP_MW1_AX: Level 1B Microwindow dictionary
MIP_IG2_AX: Initial Guess Profile data
MIP_FM2_AX: Forward Calculation Results
MIP_CS2_AX: Cross Sections Lookup Table
MIP_CS1_AX: MIPAS ILS and Spectral calibration
MIP_CO1_AX: MIPAS offset validation
MIP_CL1_AX: Line of sight calibration
MIP_CG1_AX: MIPAS Gain calibration
MIP_SP2_AX: Spectroscopic data
MIP_PS2_AX: Level 2 Processing Parameters
MIP_PS1_AX: Level 1B Processing Parameters
MIP_PI2_AX: A Priori Pointing Information
MIP_OM2_AX: Microwindow Occupation Matrix
MIP_MW2_AX: Level 2 Microwindows data
MIP_CA1_AX: Instrument characterization data
Level 0 Products
MIP_RW__0P: MIPAS Raw Data and SPE Self Test Mode
MIP_NL__0P: MIPAS Nominal Level 0
MIP_LS__0P: MIPAS Line of Sight (LOS) Level 0
Level 1 Products
MIP_NL__1P: MIPAS Geolocated and Calibrated Spectra
Level 2 Products
MIP_NLE_2P: MIPAS Extracted Temperature , Pressure and Atmospheric Constituents Profiles
MIP_NL__2P: MIPAS Temperature , Pressure and Atmospheric Constituents Profiles
Glossaries of technical terms
Level 2 processing
Miscellaneous hardware and optical terms
Spectrometry and radiometry
Data Processing
Alphabetical index of technical terms
Frequently Asked Questions
The MIPAS Instrument
Inflight performance verification
Instrument characteristics and performances
Preflight characteristics and expected performances
Subsystem description
Payload description and position on the platform
MIPAS Products and Algorithms
Data handling cookbook
Characterisation and calibration
Latency, throughput and data volume
Auxiliary products
Level 2
Instrument specific topics
Algorithms and products
Level 2 products and algorithms
The retrieval modules
Computation of cross-sections
Level 1b products and algorithms
Calculate ILS Retrieval function
Level 1a intermediary products and algorithms
Product evolution history
Definition and convention
MIPAS Products User Guide
Image gallery
Further reading
How to use MIPAS data?
Summary of applications and products
Peculiarities of MIPAS
Geophysical coverage
Principles of measurement
Scientific background
MIPAS Product Handbook
Site Map
Frequently asked questions
Terms of use
Contact us


1.1.3 Principles of measurement

In this section we describe how MIPAS measures the interferograms that will later, during the level 1b and level 2 processing phases, be processed into spectra of atmospheric spectral radiance and in vertical profile of concentration of atmospheric molecules. Overview

MIPAS 5.1. is a rapid scanning Fourier transform spectroradiometer. Compared to other types of spectrometers, Fourier transform spectrometers usually have a higher spectral resolution in the infrared, are easier to radiometrically calibrate and since they do not require a light-limiting slit, they tends to be more sensitive and can have a larger field of view. Like most Fourier transform spectrometers (FTS), MIPAS is based on a variant of the traditional Michelson interferometer. The figure below shows the MIPAS interferometer.

Figure 1.3

Infrared radiation (in blue on the figure) coming from the atmosphere is collected by the input telescope of the instrument. The radiation  is then collimated by the input collimator and directed to one of the input port of the interferometer. In the interferometer, the radiation beam is divided in two equal fractions by the beamsplitter. About 50% of the radiation is reflected by the beamsplitter and the remaining 50% is transmitted by the beamsplitter. Both the transmitted beam  and the reflected beam (in purple on the figure) are then directed to two cube corner retro-reflectors. The retro-reflectors reflect the radiation beams back to the beamsplitter where they are recombined. One more time, the beamsplitter separate the recombined beam in two separate beams (in green on the figure) and each beam is directed to an output port where they are collected by infrared detectors. The recombined beams generate an interference pattern. The intensity reaching the detector will depend on the interference pattern: the intensity is minimal if the radiation reaching the detector is in destructive interference; the signal is maximal if the interference is constructive.

If the corner-reflectors are moved with respect to one another, the difference in optical path between the reflected and transmitted beams will vary. If the optical path between the two beams varies, the interference pattern will also varies. The detector will record a succession of varying intensity as the interference changes from destructive to constructive and destructive again. This series of varying intensity is called an interferogram.

The interference pattern depends not only on the optical path difference between the two arms of the interferometer but also on the wavelength and on the spectral intensity of the incoming radiation. The interferogram thus contains information on the spectral distribution of energy of the incoming radiation. This information can be retrieved by "decoding" the interferogram. By performing a numerical Fourier transform of the recorded interferogram, the interferogram is transformed into the spectrum of the incoming radiation.

Because the corner-reflectors shear the beams, the radiation beam going to the corner-reflector do not overlap with the radiation beam reflected by the corner-reflectors. Because of that, the interferometer has two distinct output ports. Detectors are placed at each output ports. By symmetry, the system also has two input ports but the second input port is blocked by a cold target.

The spectral resolution of a FTS is mostly determined by the maximum optical path difference (MPD) between the two retro-reflectors. MIPAS has a MPD of +/-20 cm. This is achieved by independently moving the two cube corners by 5 cm backward and forward. The spectral resolution is about 0.6 / MPD. For MIPAS it is 0.035 cm-1. This corresponds to about 0.06 nm at a wavelength of 4150 nm. It would be difficult, if not impossible, to achieve such a fine spectral resolution with a type of spectrometer other than a Fourier transform spectrometer.

This is very complicated, please explain more how a Fourier transform spectrometer works .

The spectrum of the incoming radiation from the atmosphere contains information about the chemical constituents of the atmosphere and their concentration. A given molecule or atom of the atmosphere absorbs and emits radiation at specific wavelengths. It is thus possible to identify the presence of a specific molecule by checking if the studied spectrum contains the spectral line of that given molecule. The intensity of the line is proportional to the concentration of the molecules. Pressure and temperature also affect the pattern of spectral lines emitted by a given molecule. Since pressure and temperature are function of the altitude and because MIPAS takes measurement at several elevation angles, it is thus possible to determine vertical profiles of the concentration of given molecules from the spectra acquired with MIPAS. Detector and spectral ranges

A set of four detectors by output port (for a total of eight detectors) records the interferogram. The data recorded by each output port are coadded to increase the signal to noise ratio (SNR). This set-up also provides a certain redundancy: if a detector in one output port fails, the corresponding detector in the second output port still provides useful data.
The four detectors are Hg:Cd:Te detectors but each has been optimized for a particular spectral range. Together, the four detectors cover the spectral range from 685 cm-1 to 2410 cm-1 in wavenumbers. In wavelengths, the corresponding spectral range is about 4150 nm to 14600 nm.
The four detectors are named A, B, C and D. Detectors A, C and D of port 1 and 2 are identical. Detectors B are different in port 1 and port 2. These 8 detectors can be combined in five spectral bands.
Table 1.2
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 Sampling the interferograms

As the corner-reflectors are moved, the interference pattern moves over the detectors. To record a useful interferogram, the modulated output has to be sampled at very regular optical path difference intervals (the required sampling accuracy for MIPAS is about 30 nm). This is done with help of a laser beam transmitted in the same optical set up, which is used to trigger the sampling electronics behind the detector at very precise values.
The interferogram of the monochromatic laser is pure sine wave. The interferogram is detected by a dedicated metrology detector and a  fringe counter determines the OPD by the phase of the sine wave. The fringe counter  forms a “clock signal that is sent to the ADC in the on-board signal processor electronics (SPE). The fringes trigger the sampling of the interferogram.

Variable phase delays in the detection electronics would also result in sampling jitters, thus the sampling frequency has to stay within a narrow range, which in turn leads to a requirement to have a very constant optical path difference speed (i.e. drive speed of the retro reflectors). Calibration measurements

To radiometrically calibrate the spectra that will be generated during the Level 1b processing, it is necessary to have two calibration measurements: a "cold" and a "warm" measurement. These two measurements are necessary to determine the effective gain and offset of the instrument. These parameters are essential to perform the radiometric calibration of the measurements made by MIPAS. Offset measurement

Looking at the deep space provides a "cold": scene, i.e. a scene with negligible infrared radiance. The interferogram acquired while looking at the deep space is mostly due to the instrument self-emission. Deep space measurements are made frequently, one every four elevation scans, in order to account for changing instrument self-emission due to temperature variations along the orbit. These measurements are performed at a reduced spectral resolution. The spectral resolution is coarser than the highest resolution of MIPAS by a factor 10. A single measurement will last 0.4 second.
The Offset Calibration is performed every four scans, and uses six sweeps at low resolution (three forward and three reverse), which must be combined to reduce the noise level to acceptable level.  The baseline scenario uses 300 sweeps at low resolution in both forward and reverse directions. The total duration of the offset measurement is 16.15 seconds (including transition times), and measurements are then made every 300.5 seconds.
For an orbit of 100 minutes, assuming all measurements are performed with the same scan scenario, there are about 20 offset measurements per orbit, which is well above the necessary minimum determined from an estimation of the expected temperature variations. Gain measurement

The "warm" measurement, i.e. a measurement with a relatively high radiance, is performed while the instrument is looking at an internal source. This source is a well characterized calibration blackbody with a controlled temperature.
The baseline is to look at the internal calibration blackbody once a week. These measurements are also performed at a reduced spectral resolution. The spectral resolution is coarser than the highest resolution of MIPAS by a factor 10. A single measurement will last 0.4 second.
In order to reduce the noise in the calibration measurements, many measurements are recorded and then coadded on the ground. The baseline scenario uses 300 sweeps at low resolution in both forward and reverse directions.

Calibration measurements are performed after the instrument slides have been stopped in order to re-establish a phase reference. The calibration sequence is therefore commanded as the first operation in any nominal measurement sequence. Radiometric Deep Space Calibration measurements precede those made looking towards the calibration blackbody to cover the worst case condition: i.e. the instrument entering measurement mode directly after the boost heater phase of the calibration blackbody . Onboard processing

The sampled interferograms are converted into digital number by the onboard analogue-to-digital converter (ADC). Before being sent to ENVISAT, the digital data is numerically filtered, it is decimated, the size of the digital word is reduced over a certain fraction of the interferogram and the data is compressed. See the level 0 processing for details. The goal of all this processing is to reduce the data rate of the instrument to less than 550 kbits/s. The digital filtering and decimation can be disabled by telecommand from the ground but, in that case, the data rate increases to 8 Mbits/sec.
ENVISAT is in charge of telemetring the data to the ground station were it will be further processed. Ground processing

The ground processing of MIPAS data is divided in two phases called level 1b and level 2.
The goal of the level 1 b processing is convert the raw interferogram  measured by MIPAS and sent by ENVISAT into validated, corrected, radiometrically calibrated, spectrally calibrated and geolocated spectra of atmospheric radiance. The radiometric calibration uses the gain and offset measurements described above . The spectral calibration is based on a comparison of the measured spectra with the spectral position of well known atmospheric lines used as reference. See the level 1b processing for further details.

The goal of the level 2 processing is to retrieve vertical profile of atmospheric molecules from the calibrated spectra. It relies on state-of-the-art inversion models that derive vertical profile of temperature, pressure and volume mixing ratios of selected molecules from the fine spectral information provided by MIPAS. The five priority molecules that will be routinely retrieved on regular basis are water vapour (H2O), ozone (O3), methane (CH4), nitrogen dioxide (NO2), nitreous oxyde (N2O) and nitric acid (HNO3). Other molecules can also be retrieved from the data. See the level 2 processing for details. Spatial coverage

MIPAS can collect data from various altitudes and various positions by using two scanning mirrors to point at different angles to the side and to the rear of ENVISAT. This scanning capability combined with the orbit of ENVISAT allows MIPAS to achieve a complete coverage of the Earth. See the section on geophysical coverage 1.1.4. for more details.

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