1. Radiometric inter-comparison of IASI
IASI-A / IASI-B,
IASI / AIRS, IASI / CrIS
Denis JOUGLET
(Successor of Denis Blumstein for the IR sounders inter-calibration)
with J. Chinaud, X. Lenot, D. Blumstein, E. Pequignot

2. Introduction Objectives of our inter-comparison study Overview
External validation of the IASI radiometric calibration, complimentary to the performance monitoring done by the IASI TEC
Checks the long term data quality, in particular for climatology
CNES participation to the GSICS
Overview
IASI / AIRS inter-comparison
Prototyped by D. Blumstein
Development of the SIC tool, with industrial development rules
Under exploitation in routine mode
Still evolving
IASI-A / IASI-B inter-comparison
Based on the SIC tool with adaptations
Will be activated during the IASI-B commissioning phase
IASI / CrIS inter-comparison
To develop, with SIC as a basis

3. Reminder on IASI, AIRS, CrIS
AIRS/Aqua (since 2002), IASI/Metop-A (since 2006), CrIS/NPP (since 2011), IASI/Metop-B
Sun-synchronous polar orbits, local time: 1h30 am/pm, 9h30 am/pm, 1h30 am/pm
High resolution TIR spectrometers, designed for atmospheric studies
Spectral range : ~ cm-1 ( µm)
AIRS (grating) : 2378 channels, 3 separate bands, variable spectral resolution cm-1
IASI (FTS) : 8461 channels, continuous coverage, constant spectral resolution 0.5 cm-1
CrIS (FTS): 1305 channels, 3 separate bands, variable spectral resolution cm-1

4. Status of IASI-A and IASI-B
IASI-A is performing very well after almost 6 years in orbit
No redundancy used
All mission requirements are met : both instrument and processing
The instrument is extremely stable : radiometry, spectral, geometry
Mechanisms (Cube Corner, Scan) show no evolution in orbit
Radiator (passive cooling) show no evolution in orbit
IASI-B Current CAL/VAL planning:
METOP-B launch : September 17th 2012
First science data : October 24th 2012
L1 early dissemination (end of phase A) : January 15th 2013
L1 Operational dissemination (end of phase B) : March 12th 2013
End of CAL/VAL (end of phase C) : April 2013

5. IASI / AIRS : methodology 1/2
Work with CNOs (Common Nadir Observations)
= Simultaneous Nadir Overpasses with a tolerancy on the time delay between IASI & AIRS up to 20min
 Same scene and similar observation conditions
 Increase the amount of data: ~ 34 CNOs every 3 days, ~10000 events in 2007 – 2011
 always at high latitudes
Spatial match:
Pixels are not colocated
Zonal averaging for IASI and AIRS around the tracks crossing
Warning about heterogeneous scenes
Geographical location of CNOs

6. IASI / AIRS : methodology 2/2
Spectral match:
Work with IASI L1C, AIRS L1B
33 broad pseudo-bands (PBs) = summation of elementary channels (most widths: 23 to 63 cm-1)
 Reduces noise and spectral resolution differences
 Takes AIRS spurious channels into account through a weighted summation of the IASI channels
Other methods under progress
similar channels (statistical similar behavior)
convolved channels
Comparison of ΔT = TIASI - TAIRS in each PB
Instrumental functions of one PB for AIRS (including spurious channels), for IASI without weighting in the channels summation and for IASI with weighting

7. IASI / AIRS : some results
SIC operational for systematic download and comparison of data
AIRS data from NASA website
IASI data from Eumetsat FTP or UMARF
Monitoring of calibration with time
ΔT = f(date) by PB
 IASI and AIRS radiometric calibration are stable and very close
 No seasonal or inter-annual trends
1 dot = 1 scene
Monthly means

8. IASI / AIRS : some results
2011
Study of biases and st dev
 AIRS & IASI radiometric calibration very close (|NeDT| < 0.1K)
 Small biases under investigation
Dependency of ΔT with
Scene temperature (surface or atmo)
 larger dispersion at high Tb
 Very small trend = small non linearities
Dependency of | ΔT | with
Scene temperature heterogeneity
IASI / AIRS temporal delay …
2010
Abs(T(AIRS) – T(IASI))

9. IASI-A / IASI-B : common observations
Metop-B : same orbit as Metop-A with 180° shift
 IASI-A & -B have no Simultaneous Nadir Overpasses
Relative location of the tracks :
 IASI-A & -B have many common observations
For each IASI-A line, with the IASI-B previous and following tracks
Possible for all latitudes
Off-nadir observations (nadir at the poles)
50 min systematic delay between IASI-A and IASI-B

10. IASI-A / IASI-B : selection of scenes
We choose to use these common observations, focusing on:
The IASI columns of similar viewing absolute angles (same atmospheric thickness from both obs) = opposite SNs
The most relevant regions
= homogeneous and stable in 50 min
Pre-selection based on:
IASI engineering data
Differences in IIS/AVHRR A & B temperatures
Inter- and intra-pixel IIS/AVHRR homogeneity
Oceans
Amounts of snow, clouds
ECMWF analysis data:
NRT processing of T and H fields
Daily Δfield, converted in daily Δspectrum through 4AOP jacobians
Daily NeDT and ranking
“Geophysical” daily NeDT
0.74K
19.92K

11. IASI-A / IASI-B : methodology
Data processing: based on the IASI / AIRS software
IASI 1C data from Eumetsat FTP
Spatial match: Zonal averaging for IASI-A and IASI-B spectra
Spectral match, ΔT comparison:
at IASI elementary channel
at pseudo-band level
 reduces noise and avoids any possible spectral bias
An atmosphere correction by double differences will be available if necessary, using meteo profiles
Tool development is finished
Validated with IASI-B simulated data
 Tool ready for cal/val and routine
Test of the tool with simulated data

12. IASI / CrIS : under construction
We want to perform the same job for IASI / CrIS
Comparison to IASI-A and to IASI-B
Method: same as for IASI / AIRS because same orbital configuration
Work with zonal averaging
ΔT comparison at pseudo-band level
ΔT comparison of convolved channels (construction of CrIS-like spectra by convolution from IASI)
 Tool under construction
CrIS data downloaded from EumetCast
Reader for BUFR format
Need for CrIS ISRFs
Should be ready by october

13. Conclusions Status of the tools:
IASI / AIRS: under exploitation, a publication is to come
 Very close cross-calibration, very stable with time
IASI-A / IASI-B: the tool is ready for cal/val and routine
IASI / CrIS: start of implementation
Feel free to come and see my poster! Nr 96 (session 5)
Contact for any questions: