1. V2.0 minus V2.5 RSAS Tangent Height Difference Orbit 3761
OMPS Calibration Improvements
G. Jaross1, C. Seftor2, G. Taha3, J. Warner2, G. Chen2, Z. Chen2, A. Vasilkov2, L. Moy2, M. Kowitt2, P. Bhartia1
1NASA GSFC, 2SSAI, 3GESTAR
Overview
The Ozone Mapping and Profiler Suite (OMPS) is a next-generation space-based system for monitoring global changes in stratospheric ozone and aerosols, and has been collecting science data on the Suomi NPP satellite since January, OMPS will launch on JPSS-1 in 2017 and JPSS-2 in 2022.
The NASA OMPS Core Team will soon release new versions of Level 1 data products (Nadir: v2.0, Limb: v2.5) with improved radiometric performance and altitude registration.
Nadir Radiometric Accuracy
The long wavelength (>325 nm) calibration of the Nadir Mapper was evaluated through comparisons with TOA radiance models over Antarctica. The accuracy of the model itself is no better than 2-3%, but it allows us to inter-compare with previous instruments.
Conclusions:
OMPS-NM compares well with TOMS and with Coll. 2 of the OMI Level 1B product (Not the current release product). There is no evidence for a wavelength-dependent albedo calibration error, though there is some evidence for cross-track bias.
Corrections are applied in the Level 2 O3 product, not in Level 1B.
Antarctic Ice Calibration Error (nadir)
Collection 2
nadir
Aerosol Index Wavelengths
Offset from 1.0 at nadir is likely because the Ring effect is not modeled and ozone absorption is modeled poorly.
V1.0 Aerosol Index
V2.0 Aerosol Index
Nadir Sensor Stability
Bi-weekly measurements of the Working solar diffuser provide instrument change (diffuser + telescope + spectrometer). BLUE
Working and Reference diffuser comparisons yield an estimate for Reference diffuser change. GREEN
The Ref. diffuser degradation is removed leaving an estimate for the sensor (telescope + spectrometer) change. ORANGE
The change after nearly 5 years is < 1% at 250 nm and negligibly small above 310 nm. Version 2.0 does not apply a correction for this change.
Sensor Characteristics
The suite consists of three sensors that operate concurrently.
Nadir Mapper & Profiler Sensors (NM & NP)
Wavelength range: 250 nm to 380 nm
Two grating spectrometers w/CCD detectors
Spectral sampling .41nm; resolution ~ 1 nm
Spectral channels: NM 196, NP 147
Push broom 110º FOV telescope
Mapper: 35 horizontal FOV (50km x 50km at nadir)
Profiler: 1 horizontal FOV (250km x 250km)
Two reflective aluminum solar diffusers
Limb Sensor (LP)
Spectral range 290 nm to 1000 nm
Spectral resolution: 1 nm to 40 nm
Prism spectrometer w/CCD detector
Afocal telescope with three 1.85º vertical FOVs (Left, Center, Right)
Vertical resolution 1.5 km
Two transmissive diffusers for solar calibration
Nadir Mapper
Left
Center
Right
Nadir Spectral Calibration
Limb Radiometric Accuracy
The NASA Limb retrieval algorithm minimizes its sensitivity to radiometric calibration by using altitude-normalized radiances. But calculation of the Direct Upwelling Radiation (DUR) still depends upon accurate surface reflectivity.
Comparisons between LP and NP Lambert-Equivalent Reflectivity (LER) suggests that LP radiances remain too low by 5% – 10%.
352nm
674nm
The wavelength registration of all 3 sensors is derived by regression of OMPS solar with a reference solar spectrum based on UARS SUSIM and Kitt Peak (Dobber et al., 2008; also used for Aura/OMI). Independent regressions for each spatial position include only a spectral shift value and a bandwidth scale factor (single fit window, no stretch/squeeze, no radiometric scaling). Final solar residuals for Nadir are shown below. The Earth view wavelength registration is derived from an empirical model for deviations from the solar registration.
OMPS solar irradiance residuals after spectral registration fit to KNMI reference solar spectrum
(2 parameter regression:
Pixel shift, Band width scale) NP
v 1.0
v 2.0 NM
v 1.0
v 2.0
Limb
Nadir
Future plans are to operationally derive the wavelengths on a scene-by-scene basis using the Fraunhofer structure
Version 2.0 wavelength error
Version 1.0 wavelength error
V1 mean error
V2 mean error
Independent evaluation of reported wavelength registration over Pacific ocean
Courtesy of Ball Aerospace a
(Mapper and Profiler)
Limb Tangent Height Registration
The Limb sensor absolute altitude was adjusted in V2.5 by up to 300 m using RSAS residuals (350 nm, 20km and 40km) during periods and locations of low aerosol loading. A separate technique, ARRM uses 295 nm radiance residuals at 65km to monitor changes in pointing errors. ARRM lacks absolute accuracy.
Left
Center
Right
( meters )
V2.0 minus V2.5 RSAS Tangent Height Difference Orbit 3761
ARRM Tangent Height Errors
45S 5N
35N
TH Error (meters)
Limb Stray Light
The stray light content of the measured signal is estimated using pre-launch characterizations of point spread functions. Stray light is predominantly a high altitude problem at all wavelengths. A recent empirical adjustment at VIS/IR wavelengths has improved aerosol extinction performance. The existing correction in the UV appears to be adequate when wavelengths are paired in the ozone retrieval.
Modeled Stray Light: the basis for corrections in v2.0 and v2.5
Percent
Scale Factor
Wavelength
Left
Center
Right
O2 A-band emission
V2.5 corrections use scale factors in VIS/IR to adjust the V2.0 corrections
Radiance residuals (353nm - 295nm difference) relative to calculated radiances from MERRA assimilated ozone and pressure