MIPAS Cal/Val
The MIPAS Calibration and Validation Plan was defined before the Envisat launch and was revised a few times during the MIPAS lifetime, following the major mission events.
Details of the Cal/Val activities are provided in the following documents:
- Envisat Calibration and Validation Plan: PO-PL-ESA-GS-1092 issue 1.2" 1 September 2000
- MIPAS In-Flight Characterization and Calibration Definition: PO-TN-BOM-GS-0013 issue 1B" 1 May 2001
- Implementation of MIPAS Post-Launch Calibration and Validation Tasks: PO-PL-ESA-GS-1124 issue 1B" 7 March 2002
- MIPAS Mission Plan Rationale: issue 3.5" May 2002
- MIPAS In-Flight Calibration and Processor Verification: PAPER
- MIPAS Long-Term Calibration and Characterisation Plan: ENVI-GSOP-EOPG-TN-03-0008 issue 1.0" 3 April 2003
- MIPAS In-Flight Characterisation for Modified Operations: ENVI-BOM-TN-04-0002 issue 1C" 19 October 2004
- Reviewed MIPAS Gain Calibration Scenario: ENVI-BOM-TN-04-0003 issue 1A" 17 August 2004
- Review of MIPAS L1b Microwindow Database For Spectral Calibration and ILS Retrieval: ENVI-BOM-TN-04-0004 issue 1" 18 August 2004
- Envisat Phase E Cal/Val Acquisition Plan: ENVI-SPPA-EOPG-TN-03-0008 issue 1.5" 17 March 2006
- MIPAS Mission Plan document: ENVI-SPPA-EOPG-TN-07-0073 issue 4.3" 20 January 2008
- Envisat 2010+ Mini-commissioning and Cal/Val Plan for GOMOS, MIPAS and SCIAMACHY: IDEAS-SER-SOM-PLN-0633 issue 3.1" 15 October 2010
Calibration Activities
The definition and frequency of instrument in-flight calibrations needed during the MIPAS lifetime were defined by a calibration team before launch and revised during the instrument Commissioning Phase.
Because of the Interferometer Drive Unit anomaly in 2004, the in-flight calibration strategy had been adapted to the new mission. The main changes affected the spectral resolution, the number of sweeps per scan and therefore the duration of calibrations themselves.
Instrument Settings Changes
Beside configuration changes applied to the instrument in response to the major Interferometer Drive Unit anomaly, a few other instrument settings were modified during the MIPAS lifetime. A summary of those changes are detailed below.
| Calibration | Description | Frequency |
|---|---|---|
| Offset | Offset calibration | 450 seconds |
| ILS | Instrument Line Shape | 1 orbit |
| WCC | Wear Control Cycle | 5 orbits |
| RGC (IF8) | Radiometric Gain Calibration (1 scan of 300 Black Body + 300 Deep Space sweeps) | 1 day |
| LOS | Line Of Sight measurements | 1 week |
| IF16 | Several limb scanning sequences in raw mode | 2 months |
| IF9 | Offset tangent height determination | 3 months |
| IF11 | Absence of high resolution features verification | 3 months |
| PD-IF16-IF4-IF16 | Passive Decontamination, followed by the calibration sequence IF16 – IF4 – IF16 | 6 months |
| IF4 | Generation of the non-linearity coefficients | 6 months |
| IF10 | NESR0 verification | 6 months |
| IF6 | CBB and DS SNR characterization | 1 year |
| IF14 | Field of View In-Flight Check | 1 year |
| Calibration | Description | Frequency |
|---|---|---|
| Offset | Offset calibration | 800 seconds |
| ILS | Instrument Line Shape | 1 orbit |
| WCC | Wear Control Cycle | automatically commanded by ESOC after each instrument transition to Heater |
| RGC (IF8) | Radiometric Gain Calibration (1 scan of 200 Black Body + 200 Deep Space sweeps) | 1 day |
| LOS | Line Of Sight measurements | 10 days |
| PD-IF16-IF4-IF16 | Passive Decontamination, followed by the calibration sequence IF16 – IF4 – IF16 | 6 months |
Validation Activities
Validation of the MIPAS operational datasets was performed throughout the Envisat mission lifetime. These reports provide an assessment of the product quality based on comparison with independent collocated measurements undertaken for validation campaigns.
Level 2 ORM version 8.22 dataset:
- Use of MIPAS Balloon for validation activities: Report to MS6_2: L1v8/L2v8 FM comparison for GL1-GL3
- Long-term validation of MIPAS ESA operational products using ACE v3/v4 measurements: UoL_validation_TN_dec_19-2_GS
- MIPAS ORM 8.22 profiles of T, altitude, O3, CH4, HNO3 and N2O Validation Report: TN-BIRA-IASB-MultiTASTE-Phase-F-MIPAS-ORM8-Iss1-RevB, 18/12/2020
L2 Error Analyses
Last Updated: 3-Aug-20 - Spectroscopic Errors updated
*** NOTE *** The SPECDB error has now been redefined - details below.
Note: Reference to original page of Department of Physics, University of Oxford here
[Tech Note (25-Oct-19)] describing mathematical basis of error analyses.
The following table shows the linear error analyses for MIPAS L2 products. These errors have been evaluated for 5 different atmospheric conditions.
| DAY | Mid-Latitude day-time (similar to US Standard Atmosphere) |
| NGT | Mid-Latitude night-time |
| SUM | Polar Summer day-time |
| WIN | Polar Winter night-time |
| EQU | Equatorial day-time |
Click on 'Data' for the numerical data that has been plotted. In the plots, the same symbols are used for each error source (explained below) throughout, and listed in the key in the approximate order of significance for that plot. Only the most significant errors are plotted. Click on the atmosphere or species for plots of the atmospheric profiles assumed.
List of errors considered
- TOT
- Total Error. Root sum square of all SYS and RND components
- RND
- Random Error. Due to the propagation of instrument noise through the retrieval.
- NB: A more accurate assessment of this component is included in the L2 product
- NONLTE
- Non-LTE error. Due to assumption of local thermodynamic equilibrium when modelling emission in the MIPAS forward model. Based on calculations using vibrational temperatures supplied by M.Lopez-Puertas, IAA, Granada.
- SPECDB (updated August 2020)
- Spectroscopic database errors. Due to uncertainties in the strength, position and width of infrared emission lines, but only for the target species in each microwindow (CO2 for pT microwindows). For line parameters, each parameter is perturbed by the 1s uncertainty indicated by the associated error in the HITRAN record (not the HITRAN data itself but the version adapted for MIPAS v4.43), subject to a maximum strength uncertainty error of 3%. For cross-section molecules (eg CFCs) a fixed strength uncertainty of 5% is assumed, which is also intended to include interpolation errors from the tabulated (p,T) data points.
- GAIN (updated July 2019)
- Radiometric Gain Uncertainty. Based on an internal study by A. Kleinert, this is assumed to have a value of 2.5% in band A, 2% bands AB and B, and 1% in bands C and D. It is also assumed to be fully correlated within each band, but uncorrelated between bands.
- SPREAD
- Uncertainty in width of apodised instrument line shape (AILS). A value of 0.2% has been assumed based on likely variations in apodised instrument line shape from modelled.
- SHIFT
- Uncertainty in the spectral calibration. The design specification of ±0.001cm-1 has been used, and is consistent with the 1st derivatives signatures in the residual spectra.
- CO2MIX
- CO2 line-mixing. Due to neglecting line-mixing effects in the retrieval forward model (only affects strong CO2 Q branches in the MIPAS A and D bands)
- CTMERR
- Uncertainty in gaseous continua. Assumes an uncertainty of ±25% in the modelling of continuum features of H2O (mostly), CO2, O2 and N2.
- HIALT
- Uncertainty in high-altitude column. Retrieval assumes a fixed-shape of atmospheric profile above the top retrieval level. Effect is calculated assuming `true' profile can deviate by climatological variability.
- PT
- Propagation of pT retrieval random covariance into VMR retrieval. Note that since July 2019 other errors from the pT retrieval are now propagated and contribute to the various systematic errors (so, for example, there is a component of the Band A gain error from the pT retrieval even for species which are retrieved without using any microwindows in Band A)
- [species]
- Uncertainties in assumed profiles of contaminant species. For species which are not retrieved this is taken from the climatological 1-sigma variability profiles provided by J Remedios (U.Leicester). For retrieved species it is the optimally-weighted combination of the climatological uncertainty with the retrieval random error, i.e. smaller than either component.
Use of Systematic Errors
The definition of 'systematic error' here includes everything which is not propagation of the random instrument noise through the retrieval. However, to use these errors in a statistically correct manner for comparisons with other measurements is not straightforward. Each systematic error has its own length/time scale: on shorter scales it contributes to the Bias and on longer scales contributes to the SD of the comparison.
Fortunately, two of the larger systematic errors (PT and SPECDB) can be treated properly:
The pT propagation error (PT) is uncorrelated between any two MIPAS profiles (since it is just the propagation of the random component of the pT retrieval error through the VMR retrieval) so contributes to the SD of any profile comparison.
Spectroscopic database errors (SPECDB) are constant but of unknown sign, so will always contribute to the Bias of any comparison, but note that the magnitude of these errors is very uncertain.
Of the other significant errors, the calibration-related errors (GAIN, SHIFT, SPREAD) should, in principle, be uncorrelated between calibration cycles however analysis of the residuals suggests that these errors are almost constant so could be included in the Bias.
The high altitude column (HIALT) and contaminant gas errors ([species]) are likely to be correlated over small areas (1000km) or times (weeks), hence contribute to the Bias for localised comparisons, but as the comparison datasets are extended these errors will contribute more to the SD.
Line mixing errors (CO2MIX) are also contribute towards the Bias but in principle the sign of these errors is known (unlike spectroscopic errors) so this bias could be removed. Non-LTE errors (NONLTE) should also, in principle, contribute a known Bias but these are highly variable (especially diurnally) so care has to be taken to make sure that representative conditions for the comparison are used.