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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
Algorithms and products
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
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
Services
Site Map
Frequently asked questions
Glossary
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2.4.3.1.5 Calculate ILS Retrieval function

Level: 1b

Main objectives:

The Calculate ILS Retrieval function performs the instrument line shape (ILS) retrieval from radiometrically and spectrally calibrated spectra. The result of this operation is made available to the output data products.
Specific objectives:
Specific objectives of the function are:
  • Select specific microwindows containing precisely one reference peak of well-known wavenumbers.
  • Obtain or generate the reference theoretical spectral line corresponding to this microwindow.
  • Fit an ILS to the incoming spectrum by minimizing residuals between the reference line and the parametric ILS.
  • Store the iterated ILS parameter set and the specific wavenumbers as a Level 1b product.

  •  
Organigram:
 
image
Figure 2.10


Input:


Output:


Detailed description:

ILS retrieval has been studied extensively in the technical note (see Ref. [1.8 ] ). A deconvolution approach has shown to be inadequate, but a second approach has shown to give good enough results. The chosen ILS retrieval method is called the “ Parametric ILS Fitting Method (PIFM). This method proceeds with a theoretical ILS, obtained by a modelization with a limited number of parameters, convolved with the theoretical line and iteratively fits the results onto the experimental data.
Appropriate peaks for spectral calibration that represent known features of standard scene measurements have been identified and studied in the document (see Ref. [1.8 ] ). The precision of the peak identification algorithm is proportional to the number of equivalent scenes that are coadded, as the noise affecting the signal decreases when multiple readings are superposed. This number will probably vary between 2 and 10, and will be defined in auxiliary data.
The operation of ILS retrieval is more computer intensive than others tasks presented up until now, but this operation will be requested only from time to time, not on a regular basis as the computation of spectral calibration for example. Topics of the exact frequency at which the ILS retrieval shall be done is addressed here.
It has been chosen to extract the ILSin each detector band of the instrument on an appropriate spectral line located anywhere inside the band. The list of reference spectral lines will be stored in a table kept as auxiliary data.
The auxiliary data file containing retrieved ILS parameter data and spectral calibration data shall be produced by the Level 1B processor according to the processing parameter file. An initial ILS and spectral calibration auxiliary file will be given as an input to the processor at all processing stations and shall be used until the next file will be made available. ILS and spectral calibration data will be written to the auxiliary file simultaneously (i.e., only ca. once per week). Otherwise the file shall not be modified by the processor.

2.4.3.1.6 Calculate Pointing function

Level: 1b

Main objectives:

The Calculate Pointing function performs the line of sight (LOS) pointing calibration in order to generate corrected LOS pointing angles. This includes:
  • Compute correction of elevation pointing angle,
  • Compute corrected pointing angles of actual scene (sweep).

Specific objectives:

Specific objectives of the function are:
  • Compute the actual pointing error at time of ZPD crossing
  • Compute actual azimuth pointing angle
  • Compute correction of elevation angle
  • Compute actual elevation pointing angle

Organigram:

image
Figure 2.11

Input:

  • Measured LOS angles
  • LOS calibration data

Output:

  • Corrected LOS angles

Detailed description:

The Calculate Pointing function is based on the following assumptions. It is assumed that commanded elevation angles are only partially corrected with respect to known pointing errors according to the best knowledge based on-ground characterisation and LOS calibration measurements. The remaining elevation error, obtained from LOS calibration measurements, shall be computed in the ground segment (PDS) and be used to correct in measurement mode the measured elevation angles. The corrected elevation angles and the measured azimuth angles are used to compute the geolocation (height/longitude/latitude) of the actual scene (target).

2.4.3.1.7 Calculate Geolocation function

Level: 1b

Main objectives:

The main objectives of the Calculate Geolocation function are:
Specific objectives:
Specific objectives of the function are:
  • Compute orbital position of spacecraft at ZPD time
  • Compute tangent height, longitude and latitude
  • Estimate error on computed tangent height
Organigram:
image
Figure 2.12
Input: Output: Detailed description:
The Calculate Geolocation function calculates the tangent point geolocation and related information. The function has as input the orbit state vector and corrected pointing angles.

2.4.3.1.8 Detection and correction of spikes

Level: 1b

Main objectives:

Detect and correct spurious spikes in an interferogram
 
Specific objectives:
Specific objectives of the function are:
  • Inspect the interferogram around the ZPD to detect the presence of spikes
  • Reject the interferogram if it is in a calibration measurement, otherwise replace detected spikes by the mean of the neighbor points.

  •  
Organigram:
 
image
Figure 2.13


Input:

  • Interferogram to inspect and correct


Output:

  • Corrected interferogram
  • Spikes position


Detailed description:

This function has the purpose of detecting spurious spikes in an interferogram. The presence of spikes in an interferogram can be caused by cosmic radiation or transmission errors. The affected points in a scene interferogram are corrected by taking the mean between immediate non-affected points. This scene will be flagged of having corrected for one or more spikes. If a spike is detected in a gain or in an offset measurement, this measurement will be discarded in order to avoid corrupting all of the subsequent calibrated spectra.
The algorithm performing spike detection scans groups of points in the interferogram (odd number with central block corresponding to ZPD block) in search of spikes. In each block, except for the central ZPD region of the middle block, the standard deviation of the interferogram values is computed, and a spike is identified if a given point amplitude exceeds a predefined threshold for values in the real or the imaginary parts. To improve the accuracy of the algorithm, A second pass is done excluding the data points identified as spikes to calculate the final standard deviation of the group.
For each detected spike, the value at the specific wavenumber is replaced by a mean of the two immediate points in the interferogram vector. The real part and the imaginary part are corrected independently.
The spike correction will always cause some distortions with respect to the original spectrum, but it has been shown that this distortion is within the radiometric accuracy requirement. Interferograms that have been corrected for spikes are flagged as such.
 

2.4.3.1.9 Detection and correction of fringe count errors

Level: 1b

Main objectives:

Detect and correct fringe count errors (FCE) in the interferograms
Specific objectives:
Specific objectives of the function are:
  • For FCE detection:
    • FFT the ZPD region of the interferogram
    • Multiply the resulting spectrum by the latest available gain (interpolated)
    • Calculate the spectral phase of the roughly calibrated spectrum
    • Perform a linear regression of the phase vs. wavenumber
    • Calculate the OPD shift
  • For FCE correction:
    • Perform the FFT of the shifted interferogram
    • Calculate the phase function necessary to correct the calculated shift
    • Multiply the spectrum with the calculated phase function
    • perform an inverse FFT on the spectrum
Organigram:
 
image
Figure 2.14
Input:
  • interferogram to inspect
Output:
  • Phase corrected interferogram
Detailed description:
The basic ground processing for MIPAS contains no explicit phase correction or compensation. For a given interferometer sweep direction, it is assumed that the gain and offset calibrations and also the scene measurements have the same phase relationship, i.e. they are sampled at precisely the same intervals. This sampling is determined by a metrology fringe counting system using a reference laser source within the interferometer subsystem, with the fringe counts forming a “ clock signal to the ADC in the on-board signal processor electronics (SPE). The fringes trigger the sampling of the interferogram. If, for any reason, a fringe is lost, then the phase of subsequent measurements will be affected and if these are calibrated using a gain or offset measurement taken before the occurrence of the fringe loss, then errors will be introduced into the final spectrum. The ground processing scheme includes a method for detecting and correcting fringe losses by analyzing the residual phase of calibrated spectra, computed from the central ZPD region of each interferograms. Hence there is no specific measurement required as part of calibration for this aspect.

The proposed approach assumes that fringe count errors occur at turn-around, i.e. between two measurements. Under this assumption, the effect of a fringe count error is to shift all measurements following the error by N points. The problem manifests itself at calibration because all the measurements involved may not have the same sampling positions, i.e. they do not have the same phase relationship.
Fringe count errors occurrence within a measurement is believed much less probable, and its effect is the same as if the error would have been at the turn-around. Thus it will be covered by the above assumption.

Fringe count errors can occur in all types of measurements done by the MIPAS instrument, except of course the LOS calibration measurements during which the sweeping mechanism is stopped. Depending on the type of measurement, the effect is not the same and therefore, the detection and correction approach will be different. Because the phase is not strictly the same for forward and reverse sweeps, the fringe count error detection and correction will be done independently for the two sweep directions. For all measurements, the fringe count reference interferogram of a given sweep direction will be the last gain interferogram of that sweep direction. The last gain interferogram can be either a deep space or a blackbody interferogram, depending on the acquisition scenario requested.

image
Figure 2.15 Fringe Count Error handling

Fringe count errors detection
The approach selected for fringe count error detection consists in a coarse radiometric calibration of the actual measurement at very low resolution, followed by an analysis of the residual phase. The radiometric calibration is done using the last available gain measurement. When the optical path difference (OPD) axis definition of the actual measurement is the same as the gain used for radiometric calibration, then the residual phase should be zero. A shift will produce a phase error increasing linearly with wavenumber.

The algorithm simply performs a linear regression on the residual phase of the calibrated spectrum to reveal an integer shift due to a fringe count error on the observed interferogram. The spectral phase is expressed as tan arctangente of the ratio of the imaginary part over the real part of the spectrum.
 

Fringe count errors correction
Once the OPD shift is known, the decimated interferogram must be shifted by a fractional number of points corresponding to this shift divided by the current DF. This requires some sort of interpolation. The current approach is to perform a multiplication of the Fourier transformed of the shifted IGM by the phase function obtained in the detection procedure.
With this method, no manipulation is done on the OPD axis of the interferogram, but each data point is corrected to represent the value of its desired current OPD position.
It should be mentioned that fringe count errors will affect interferograms of all bands. For the MIPAS instrument, detection is done only for bands C and D.
The approach for fringe count error detection and correction will be the same for all types of measurements. However, the implementation will be somewhat different for the different types. This is discussed below. The fringe count error detection will be performed systematically on all incoming interferograms. However, the correction procedure will be applied only if a non-zero shift is detected.

FCE handling in offset measurements
Detection and correction are done with respect to the last available gain calibration. All the offsets corresponding to one orbit are aligned to the fringe count phase of this last gain. If one or more fringe count errors occur during the computation of one orbit, the ground processing will detect the same shift for all subsequent offset interferograms and will apply the same (always recalculated) correction on these offsets until the end of the processing of the orbit.

FCE handling in gain measurements
At the beginning of a gain measurement sequence, there is no reference against which one can check for fringe count errors. Thus, there is no relation between the actual measurement and the previous fringe counting reference. This is the main reason why we start with a new gain measurement.
Fringe count errors during gain calibration are checked by comparison with the first measurement of the sequence, typically a blackbody measurement (either forward or reverse). The first step is to determine the OPD shift between that measurement and the previous gain. The same procedure as for normal error detection and correction is then followed.
This corrected gain will then be used for detection of fringe count errors on all subsequent interferograms. In principle, the calibrated spectra obtained with this corrected gain should show no additional phase until a fringe count error occurs. Then, all error-free measurements will be coadded normally. Each time a fringe count error will be detected, a new coaddition group will be formed. When the complete calibration sequence is over, then all the coadded measurements are corrected with respect to the last measurement and the remaining processing of the radiometric calibration is performed normally. Correcting the gain with respect to the last measurement presents the advantage that all subsequent error-free measurements need no correction.
After processing the data corresponding to one orbit, if one or more FCE are detected, the current gain is shifted according to the last fringe count error measured. This is done in order to avoid correcting all the offsets and scenes in subsequent orbits.

FCE handling in scene measurements
When a scene is measured, its fringe count is checked against the last available gain calibration. All the scenes corresponding to one orbit are aligned to the fringe count phase of this last gain. If one or more fringe count errors occur during the computation of one orbit, the ground processing will apply the same correction on these scenes until the end of the processing of the orbit.
After that, the gain is shifted according to the last FCE to match the offsets and scenes of subsequent orbits. This way, the worst that could happen is that all the scenes of only one orbit would need to be shifted. All the subsequent processing of the orbits to follow would not suffer needlessly of a single previous FCE event.
This approach also minimizes the accumulating of numerical error on gains, that can be modified only after successive orbits. In practice, FCE are expected to occur very infrequently during processing of one orbit; but even if this would be the case, the fact of aligning offsets and scenes to the last available gain calibration would limit the error accumulation on the gain calibration vector.
This procedure will slightly increase the throughput for the reference gains used for the ground segment computation. There will be one each time at least one fringe count is detected during one orbit. But, as fringe count errors are expected to occur infrequently, there would usually still only be one gain vector per week and, should an error occur, only the gain would be modified after the processing of the corresponding orbit. The fact of realigning gain calibration vectors between orbits should save a lot of operations, as one would otherwise be correcting every interferogram until the next gain calibration (the saving occurs independently of whether there are frequent fringe errors or not).
 

2.4.3.1.10 Correct non-linearity function

Level: 1b

Main objectives:

Correct the non-linearity of the response of the detectors of MIPAS.
Specific objectives:
Specific objectives of the function are:
  • apply the non-linearity polynomial on a detector per detector basis for each interferogram

  •  
Organigram:
 
image
Figure 2.16


Input:

  • Interferogram to correct
  • Set of non-linearity coefficient for each band/detector


Output:

  • corrected interferogram


Detailed description:

The detectors from the first three MIPAS bands (detectors A and B) are photoconductive detectors, subject to non-linearity depending on the total photon flux falling on them. Here, the non-linearity means that the response of the detector differs from a linear behavior as a function of the incoming flux. This phenomenon occurs at high fluxes.

The non-linearity can be a source of significant radiometric errors if it is not properly handled (as much as 40% in band A). As explained in ( Ref. [1.9 ] ), the non-linearity produces a change in the effective responsivity as well as the apparition of spectral artifacts. The present method corrects for the decrease of responsivity with DC photon flux in the radiometric calibration, within the required radiometric accuracy. The approach is the following:
A characterization must first be performed on ground, and then in space at specific intervals, at instrument level, of the total height of the unfiltered and undecimated interferogram with the on-board calibration blackbody at different pre-selected temperatures. These values will be used during the characterization phase for a computation of the non-linear responsivity coefficients. These values will be used to correct for the non-linearity of the detectors by means of a specific algorithm called the Adaptive Scaling Correction Method (ASCM).
Although they are intended to be combined in a single band, the optical ranges of the detectors A1 and A2 are not the same. They will then exhibit a different behavior with respect to photon flux. As a result, they will require different non-linearity corrections. Because of this, the signals from these detectors are not equalized and combined on board the instrument in the SPE. This operation is instead performed by the ground processor following non-linearity correction. The other two PC detectors, B1 and B2, are not combined in any case as they produce the bands AB and B. Other than the need to keep A1 and A2 separate in the baseline output set up at the SPE, the non-linearity measurements and correction has no impact upon the calibration scenario.
The important effect of detector non-linearity is on the radiometric accuracy performance. The present radiometric error budget allocated to the non-linearity in the 685 “ 1500 cm“1 (where the detectors are the most non-linear) shall be better than the sum of 2 x NESR and 5% of the source spectral radiance, using a blackbody with a maximum temperature of 230K as source.
A polynomial correction is then applied on each incoming interferogram, at the very beginning in the processing chain, with the purpose of compensating for the global effects of responsivity.
For the measured responsivity curves of the MIPAS engineering and demonstration model (EDM), the correction of the non-linearity error due to the change of effective responsivity and from the cubic artifacts have shown to lead to an accuracy within the allocated budget.

The polynomial used to correct the non-linearity is of the form:

k = 1 + do F + d1 F 2 + d2 F 3 + d3 F 4 eq 2.2
where F is the total flux on the detector (estimated as the difference between the maximum and minimum values of the digitized interferogram by the ADC before filtering and decimation and multiplied by the amplification gains) and the di are coefficients determined by the non-linearity characterisation for each detector. In absence of non-linearity effect, these coefficients are 0. The interferograms are corrected by dividing them by this polynomial.
 

2.4.3.1.11 Responsivity scaling

Level: 1b

Main objectives:

Scale interferograms acquired with different gains to a common baseline.
Specific objectives:
Specific objectives of the function are:
  • Scale interferograms acquired with different gains to a common baseline.

  •  
Organigram:
 
image
Figure 2.17


Input:

  • Interferograms (gain, offset and scene measurements)
  • gains scaling factors


Output:

  • Scaled interferograms


Detailed description:

The gains of MIPAS are adjusted depending on the relative intensity of the target so as to maximize the dynamic range of the instrument. For instance the gains are not the same for deep space measurement and for CBB measurements. Since interferograms acquired with different gains will be combined during the radiometric calibration processing, it is necessary to scale these interferograms to a common baseline.

In practice, three scaling items need to be considered, as a result of the pre-amplifier warm (PAW) system:
1) A scaling to account for a commanded gain change
The gains are predefined and are commanded by an 8-bit word sent via the instrument control unit. Since different gains may be commanded, a data scaling in the ground segment to equalize performance must be foreseen. The commanded gain is available in the auxiliary data stream and so this is a simple scaling effect based on the extracted word.

2) A temperature dependent scaling to account for changes in responsivity of the detectors.
The detector units are specified to provide a stable response based upon assumed knowledge of their temperature (i.e. the responsivity may vary but it must be well characterized). For this reason, a correction of performance with time/temperature must be foreseen. This is made based on the measured detector temperature (available via thermistor values in the auxiliary data) and using characterization curves generated during characterization tests on ground.

3) A temperature dependent scaling (gain & possibly phase) to account for the variations in the performance of the electronics of the PAW and the SPE around the orbit.
At present, it is not thought necessary to correct for these effects around the orbit as predictions show the variations will not cause the units to drift out of specification. The In-Flight Calibration Plan Ref. [1.1 ] foresees to make around orbit measurements during Commissioning Phase to check whether there are any such variations.
 
 


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