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    24-Jul-2014
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MIPAS Data Formats Products
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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
MIP_NLE_2P SPH
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
Glossary
References
Glossaries of technical terms
Level 2 processing
Pointing
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
Calibration
Latency, throughput and data volume
Auxiliary products
Level 2
Instrument specific topics
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Level 2 products and algorithms
Products
The retrieval modules
Computation of cross-sections
Level 1b products and algorithms
Products
Calculate ILS Retrieval function
Level 1a intermediary products and algorithms
Product evolution history
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MIPAS Products User Guide
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2.8.2 Calibration

2.8.2.1 Level 1B


The following calibrations are considered necessary for MIPAS level 1b processing (see index)
 

2.8.2.1.1 Radiometric offset calibration

Offset calibration is performed by observation of cold space to determine the internal emission of MIPAS (which will be the major source for offset in the spectra). It has to be performed frequently to determine all variations of the instrument self-emission due to temperature variations. It is envisaged to perform an offset calibration after four elevation scan sequences (or every five minutes). This calibration measurement 1.1.3.4. takes about 20 s. It comprises several low-resolution interferometer sweeps that are co-added by the ground segment to reduce noise.
The offset measurements are corrected for detector non-linearity and for fringe count errors.
The algorithm that computes the radiometric offset is described here 2.4.3.1.1. .

2.8.2.1.2 Radiometric gain calibration

Gain calibration is performed by observation of the internal calibration blackbody 3.1.3.1.1.3. source to calibrate the instrument response throughout the spectral bands. Gain calibration also provides the information about phase distortions used for the phase correction of the interferograms during ground processing. It is planned to be performed much less frequently (once per day or less). It comprises a number (about 1,000) of blackbody-cold space measurements 1.1.3.4. performed at low resolution, which are coadded on ground to reduce the random noise. The temperature of the calibration blackbody is also downlinked to provide the basis for the conversion into an absolute radiance units.
The gain measurements are corrected for detector non-linearity and for fringe count errors.
The algorithm  that computes the radiometric gain is described here 2.4.3.1.2. .

2.8.2.1.3 Line of sight

LOS calibration is required for the in-flight determination of the actual line-of-sight pointing biases and harmonic variations. This LOS-calibration is based on the observation of stars moving through the IFOV with the short wavelength channels. The actual time of star observation is correlated with the expected time as computed by the pointing information from the attitude and orbit control system (AOCS) of Envisat-1. Thus all biases and slow pointing variations between the star tracker package of Envisat-1 (providing the pointing reference for the AOCS) and the LOS of MIPAS are derived and used for pointing corrections.
The algorithm that performs the LOS calibration is described here 2.4.3.1.6. .

2.8.2.1.4 Spectral calibration

Spectral calibration means that the spectral axes of the radiometrically calibrated limb spectra will be recalibrated typically once per elevation scan. The calibration parameters will be retrieved from subsets of the atmospheric limb measurements, so that the routine scene data acquisition is not interrupted.
The goal of the spectral calibration is to correct the measured spectra for the Doppler shift caused by the relative motion of the satellite and the atmosphere. It also corrects shifts of the diode laser used to sample the interferograms 1.1.3.3. .

Spectral calibration is performed by comparing measured atmospheric spectra in selected spectral windows with theoretical spectra. The spectral lines and windows used are listed in the table below 2.13 . The spectral position of pre-selected lines are compared in both the observed spectra and theoretical spectra. The measured spectra are shifted in groups so that the spectral position of the compared lines matche the reference lines.

The algorithm that performs the spectral calibration is described here 2.4.3.1.3. .

2.8.2.1.5 Instrument line shape

The ILS is the instrumental response to a stimulus, in other words it is a function that describes how the instrument "sees" a spectral line of negligible width. The main contributor to the ILS is the maximum path difference between the two mirrors of the interferometers, the longer the MPD is, the thinner the ILS is. Other contributor to the ILS are the finite optical resolution of the instrument (blur) and misalignment of the optical components.

image
Figure 2.30

The ILS of the instrument is retrieved from the measurements by fitting the measured spectra to a theoretical spectrum built from the convolution a parameterised ILS model and known atmospheric spectral lines. The floating parameters of the model are determined by this fit.
The fitting method is an iterative simplex that stops when the squared difference between the observed and fitted line shape falls below a predefined threshold. The ILS retrieval is performed on recentered spectra and is thus independent of the spectral calibration. The spectral lines and windows used are listed in the table below:

Table 2.13 Reference lines and spectral windows used for spectral calibration and ILS retrieval
Band Target gas Peak position
(cm-1)
Spectral interval
(cm-1)
Tangent altitude
(km)
No. of coadditions Used for
Spectral calibration
Used for ILS
Retrieval
A O3 802.5074 802.40 - 802.62 30 1 yes yes
AB O3 1125.2085 1125.10 - 1125.30 30 1 yes no
B H2O 1409.9686 1409.85-1410.08 50 1 yes yes
C H2O 1672.4750 1672.40 - 1672.55 50 1 yes yes
D H2O 1966.2615 1966.00 - 1966.50 50 1 yes yes

The ILS is retrieved from subsets of the routine scene data (as for spectral calibration), at a repetition rate of approximately once per week. It is included in the Level 1b product. It useful to track the evolution of the instrument (alignment, quality of the optical components, etc.). It can also be used during higher level processing to correct the measured spectra so as to remove the effect of the ILS in the measurements.

The ILS retrieval algorithm is described here 2.4.3.1.5. .

2.8.2.1.6 Radiometric calibration

The radiometric calibration is the series of operations that applies the offset and gain measurements to the scene measurements in order to obtain atmospheric spectra in units of spectral radiance.
In a first step, the offset interferogram is subtracted from the scene interferogram. The subtraction is performed only for a reduced number of data points centred around the ZPD position. This reduced range corresponds to a tenth of the spectral resolution used in acquiring deep space measurements. Due to the absence of fine spectral features in the deep space measurements, the offset interferogram is nearly zero outside that range.
In a second step the length of the corrected interferogram (scene minus offset) is extended to next higher power of two (2N) by adding zeroes symmetrically on both ends. This operation is called "zero filling".
Next, the zero-filled offset-corrected scene interferogram is fast fourier transformed into a raw spectrum with arbitrary units. The prior zero-filling operation allows the use of a time efficient FFT algorithm. The gain interferogram are also zero-filled and FFT'ed. Next the spectral calibration 2.8.2.1.4. is applied to the resulting raw offset-corrected scene spectrum. This raw spectrum is vector of complex values: it has both a real and an imaginary component.

The gain interferogram undergoes the same processing: offset subtraction, zero filling, FFT, spectral calibration.

The radiometric gain is obtained by dividing the theoretical spectral radiance of the calibration blackbody by the corrected gain raw spectrum. Since the gain spectrum is complex, the radiometric gain is also complex.

The last step of the radiometric calibration is the multiplication of the raw scene spectrum by the gain. The real part of this multiplication is the level 1b product: the calibrated atmospheric spectral radiance. The imaginary component, which should only contain noise, is used for quality verification and for computing the NESR of the measurement.

The application of the radiometric calibration is part of an algorithm described here 2.4.3.1.4. .


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