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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


1.1.4 Geophysical coverage Overview

MIPAS covers the infrared spectral region from 4100 nm to 14600 nm. It is designed to operate in both the day and the night parts of ENVISAT's orbit with an azimuth scan geometry in the anti-flight direction. This geometry ensures a complete global coverage. MIPAS is also capable of pointing perpendicularly to the flight track (i.e. in the range 80 to 110 degrees from flight direction), a scanning geometry that permits diurnal changes to be detected and special events to be observed. Latitudinal resolution is increased by looking backwards.

Figure 1.4 The scanning geometry (rear looking and sideways looking) of MIPAS Basic principles of a Fourier transform spectrometer

The figure below represent a simplified version of a Michelson interferometer on which the design of MIPAS is based. In is essentially made of two flat mirror, perpendicular to one another and of a beamsplitter at 45 degrees. One of the two mirror (mirror #2) can be translated on an axis that is perpendicular to its surface. The system also includes a light detector that returns an electrical signal that is proportional to the intensity of the light that it collects.
On the figure the light from the source enters the interferometer from the left. The light beam strikes the beamsplitter. The beamsplitter separates the beam in two parts: one part is reflected to the mirror #1, the other part is transmitted to the mirror #2. Both part of the beam are reflected by one mirror and return to the beamsplitter. The beamsplitter once again separates the returning beams in two parts: one part is directed toward the detector and one part is directed back to the source. To simplify the explanation, we will suppose that the source is perfect laser, i.e. a source of monochromatic and coherent light.

Figure 1.5

Two components of the initial beam reaches the detector: the beam that was reflected by the mirror #1 and the beam that was reflected by the mirror #2. These two beams are superposed when they reach the detector and their energy will add up.
Light is an electro-magnetic wave and it has a behaviour that is somewhat similar to classical waves. The result of an addition, also called interference,  of two waves depends on the strength (energy) of each wave but also on the relative phase of the waves. If two waves add up so that their maximum are coinciding (figure below on the left), then the resulting wave has a maximum amplitude. This is called a constructive interference. On the other hand, if the two waves add up when a maximum of wave coincide with the minimum of the other wave (figure below, on the right), they cancel out and the result is nothing. This is called a destructive interference. Any other combination will result in a wave that has an amplitude between 0 and the maximum amplitude.

Figure 1.6

Light waves are waves that oscillates in space. Their phase (or the position of the maximum of amplitude) depends on the distance they have travelled. At the moment they are separated by the beamsplitter, the two beams that we were discussing above are in phase (they have the same phase) because they have travelled the same distance. After having been separated by the beamsplitter each beam travels its own path. One beam goes from the beamsplitter to the mirror #1, back to the beamsplitter and then to the detector. The other beam goes from the beamsplitter to the mirror #2, back to the beamsplitter and then to the detector. The difference of phase between the two beams depends on the difference of distance between the mirrors with respect to the position of the beamsplitter.
If this difference of travelled path is such that the two beams are in phase, the will interfere constructively on the detector and the detector will record a maximal signal. If the difference of travelled path is such that the two beams are in opposed phase, the will interfere destructively on the detector and the detector will record no signal. Anywhere in between these two extremes, the detector records a more less strong signal that depends on the difference of phase between the two beams.
The two beams will be in phase on the detector if the difference of distance between the two mirrors is an even multiple of a quarter of the wavelength (image) of the light. At the opposite, the two beams will be completely out of phase if this distance is an odd multiple of quarter of the wavelength of the light. Because the light beams move back and forth between the mirror and the beamsplitter, the total difference of path (optical path) is twice the difference of physical distance.
If the moving mirror is translated back and forth along its axis, the difference of path between the two paths will vary and the interference pattern on the detector will alternate between constructive and destructive interference.  The figure below shows the interference pattern and the corresponding signal recorded by the detector as a function of the position of the moving mirror.


Figure 1.7


The interference pattern recorded by the detector is called an interferogram. The interference pattern depends on the position of the moving mirror and on the wavelength (image) of the light beam. The amplitude of the interferogram also depends on the energy of the light entered the interferometer. The interferogram thus contains information about the wavelength and the energy of the light that entered the interferometer. This information is, however, somewhat hidden in the interferogram but it can be retrieved through the use of a mathematical operation. This mathematical operation is the Fourier transform. Applying a fast Fourier transform (FFT) to the interferogram transforms it into the distribution of energy as a function of the wavelength (a spectrum) of the incoming light. The adjective "fast" is added because it is a simplified Fourier transform function adapted to digital computers.

Figure 1.8

In our example, the interferogram is a cosine and the resulting spectrum is a spike because the incoming light comes from a monochromatic laser. However, the instrument works just as well with polychromatic light. The interferogram (and the resulting spectrum) is just more complicated. You can take a look at the image gallery 1.4. for an example of a typical MIPAS interferogram and its corresponding spectrum.

The Fourier transform spectrometer (FTS) has three main advantages over other types of spectrometers:

  1. It does not require a slit like most other spectrometers so that it can accept a very large beam of light. This is useful to detect sources that are very faint.
  2. It requires only one detector and not one detector per spectral interval like other types of spectrometers.
  3. The spectral resolution is not limited by the size of the detector, it depends mainly on the maximum difference of path between the two mirrors. The longer the path is, the finer the spectral resolution is. It is relatively easy to achieve very fine spectral resolutions with a FTS. Pointing

The overall MIPAS observational objectives can be described as being able to observe atmospheric parameters in the altitude range between 5 km and 160 km of altitude, globally, with step sizes between 1 and 10 km. To attain these objectives, MIPAS has two pointing mirrors: the elevation mirror and azimuth mirror.
The elevation mirror selects the limb altitude and corrects for variation in orbital altitude and the Earth's geoid geometry. The orientation of the azimuth mirror and the position of the satellite determine the latitude and the longitude of the observed part of the atmosphere. The azimuth mirror provides access to any limb target rearward within a 35 degree wide range around the anti-flight direction and sideways within a 30 degree wide range in the anti-Sun direction.
The baseline strategy is to keep the azimuth mirror at a fixed angle, while data from various altitudes are acquired by changing the orientation of the elevation mirror. The azimuth mirror is then moved to another orientation before acquiring new data. Varying the azimuth angle between scans allows MIPAS to measure from pole to pole. Since the azimuth mirror remains stationary during an elevation scan, the migration of the field of view due to the Earth's rotation (for backward looking) and to satellite movement (for sideways looking), has to be taken into account. For cross-track observation, simultaneous changes of both pointing mirror is possible in order to improve the horizontal resolution of a single atmospheric layer.
The time required to acquire one complete spectrum at a fixed elevation is 4.5 seconds at full spectral resolution. An elevation scan consists of 15 spectra (also called sweeps). Considering the time lost between sweeps and the time lost to reposition the pointing mirrors,  the time required to complete an elevation scan is about 72 seconds. Typically there is a spacing of 3 km between successive measurements in the altitude range from the upper troposphere to the upper stratosphere and larger spacing above.

Typically, elevation scans performed in the rear and sideways looking modes while the satellite is moving, project a staircase pattern on which the single elevation scan will be positioned. The sampling intervals change their shape according to the sampling frequency during one elevation scan and the spacing between successive samples at the various altitudes. However since measurements in the mesosphere and thermosphere are not planned on a regular basis, an equal sampling distance has been assumed between the upper troposphere and the upper stratosphere. The altitude of the lower starting point of the elevation scan may be adjusted to the climatological height of the tropopause along the orbit. During sideways looking, the effective horizontal width of the FOV in the flight direction will be in the order of 60 km, due to the motion of the IFOV during interferogram recording. Spatial coverage

In its nominal mode, MIPAS will have global coverage. Specialized observation modes will  be used occasionally to study specific region of the globe.

At the tangent point, the IFOV of MIPAS is about 3 km in elevation by 30 km in azimuth.

Depending on the observation mode, the direction of looking and the angle of the elevation mirror, the horizontal spacing of a single measurement is between 100 and 800 km. Depending on the observation mode, the direction of looking and the angle of the elevation mirror, the vertical spacing between two consecutive measurements is between 1.5 and 10 km. See the section on observation modes for details. Temporal coverage

A typical elevation scan comprises fifteen high-resolution atmospheric scene measurements, each at one elevation. At full resolution, one measurement (i.e. one sweep) takes about 4.5 seconds. Considering the time lost between sweeps and the time lost to reposition the pointing mirrors,  the time required to complete an elevation scan is about 72 seconds. Alternatively, an elevation scan can include up to 75 scenes measurements but with a spectral resolution reduced by a factor 10.
MIPAS will be in operation continuously over the full orbit of ENVISAT-1. Observation modes

To cover the various scientific objectives of MIPAS, several observation modes are planned. Each mode is associated with a particular scientific objective and has different altitude coverage, altitude resolution and horizontal resolution.
Table 1.3
Observation Mode Scientific Objective Pointing direction  Coverage Altitude range (km) Vertical Spacing(km) Horizontal spacing (km)
Nominal Stratospheric chemistry and dynamics rear Global  6 - 68 3 - 8 530
Polar Winter Chemistry Polar chemistry and dynamics rear Regional or occasional 8 - 55 2 - 10 450
Tropospheric- Stratospheric Exchange Exchange between stratosphere and troposphere, troposphere chemistry rear Regional or occasional  5 - 40 1.5 - 10 400
Upper Atmosphere Upper atmosphere rear  Regional or occasional 20 -160 3 - 8 800
Dynamics Small-scale structures in the middle atmosphere rear  Regional or occasional 8 - 50 3 - 8 500
 Diurnal Changes Diurnal changes near the terminator  side   Regional or occasional  15 - 60  3  100
 Impact of Aircraft  Study of major air traffic corridor side   Regional or occasional  6 - 40  1.5 - 10  500


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