Calibration Status of Instruments to Measure Scattered UV

L. Flynn, with contributions from presentations by M. DeLand, G. Jaross, S. Taylor, T. Beck, L-K Huang, Q. Remund and S. Asbury, and

material from the OMI, GOME-2, TOMS, SBUV(/2) and OMPS web-sites

SBUV Solar Backscatter UltraViolet instrumentsOMI Ozone Monitoring InstrumentGOME Global Ozone Monitoring ExperimentOMPS Ozone Mapping and Profiler SuiteTOMS Total Ozone Mapping Spectrometer

Outline• What is the physical phenomenon we are

measuring?– Scattered UV radiances / Solar Irradiances

• How do we measure it?– Detectors and Optics

• What must be calibrated and how well? – Requirements– Laboratory calibration.

• How is the calibration maintained?– On-orbit calibration and trending.

• What can change? By how much? Can we tell?– Delta philosophy of characterization and trending.

Photon Penetration

325 DU 30º SZA

Ozone UV Absorption

Solar Irradiance

Earth Radiance

Earth/Solar

20%

Solar Irradiance

Earth Radiance

Earth/Solar

Factor of 1000

• Detectors– Photo-multiplier tubes and radiometers– Linear diode array detectors– CCD 2-dimensional array detectors

• Shared characteristics– Dark signals or offsets (and SAA)– Efficiency and Electronic gain ranges or settings– Variable integration times– Non-linear response– Noise

• Array detectors– Response uniformity– Cross talk, smear, blooming

• Single detectors– Hysteresis– Scanning mirror/mechanisms complications

UV Measurement Technology

Optics• Spectral elements

– Grating spectrometers and monochromators– Prism spectrometers– Filters (Flattening, Cut-Off, Dichroics)

• Shared characteristics– Radiance and Irradiance (Solar Diffusers)– Wavelength registration– Stray and scattered light (OOB and OOF)– Spectral bandpass (and resolution)– Field-of-view / Field of Regard

• Filter stability• Temperature sensitivity• Polarization sensitivity

Global Ozone Monitoring Experiment (GOME-2)

Total Ozone Mapping Spectrometer (TOMS)

Ozone Monitoring Instrument (OMI)

OMPS LP Optical DesignPrism (Quartz) Spectrometer, 2-D CCD Array

– 290-1000nm, 2-40 nm bandpass– Spectral resolution matched to ozone absorption features– Polarization compensators minimize sensor polarization sensitivity – Low stray light– High efficiency

Three 110-KM vertical slits

A three segment mirror

Six collimating mirrorsSpacecraft Platform

EntranceApertures

Polarization CompensatorsTelescope M1

Slits

Telescope M2

ApertureStops

Prism

2-Mirror Camera

Exit Port

CCD

UV/Visible Limb Scatter heritageSOLSE/LORE, OSIRIS, SAGE III, SCIAMACHY

OMPS Ozone EDR Products: Properties and Performance

Table 1. Total Column Ozone EDR Performance. Measurement Parameter Specification Horizontal Cell Size 50 KM @nadir Range 50 DU to 650 DU Accuracy 15 DU or better Precision 3 DU + 0.5% Long-term Stability 1% over 7 years

Table 2. Ozone Profile EDR Performance. Measurement Parameter Specification Vertical Cell Size 3 KM Vertical Coverage Tropopause to 60 KM Horizontal Cell Size 250 KM Range 0.1 to 15 ppmv Accuracy Below 15 KM Greater of 20% or 0.1 ppmv Above 15 KM Greater of 10% or 0.1 ppmv Precision Below 15 KM Greater of 10% or 0.1 ppmv 15 to 50 KM Greater of 3% or 0.05 ppmv 50 to 60 KM Greater of 10% or 0.1 ppmv Long-term Stability 2% over 7 years

OMPS Nadir Sensor Acceptance Test Flow

Notice the radiance, irradiance and goniometry emphasis.

Procedures • Electrical Isolation • Thermal Vacuum Test• Functional Test• Goniometry• Irradiance Method #1• Boresight• Vibration Test• Spectral Scale• Radiance & Field Of View• Polarization Sensitivity• Stray Light and Bandpass• Irradiance Method #2

OMPS Calibration Component Characterization

Calibration Component Characteristic

Blocking Filter Rejection CharacteristicsXe Lamp Spatial Stability, Output StabilityTunable Laser Wavelength Accuracy,

Output StabilityIntegrating Sphere Output Characteristics, UniformityFEL Lamps Output StabilityCollimated Source Bench Beam UniformitySpectralon Diffuser Uniformity, BRDF versus wavelengthAluminum Diffuser BRDF versus wavelengthNadir Flight Diffuser Uniformity, BRDF versus wavelengthLimb Flight Diffuser Uniformity, BRDF versus wavelengthGoniometric Fixture Accuracy, Repeatability

Most components are trace-able to NIST standards.

Verified RequirementsNon-linearity characterization, Non-linearity no more than 2% of full well, In-flight

electronic response referred to each CCD pixel, Pre-amp gain variationBoresight

Cube surface orthogonality to mounting surface, Unit to unit cube-to-boresight / cube-to-baseplate alignment variation, Per axis allowed alignment change, Aligned FOV center pixels

FOV and IFOVShape at Nadir, Cross-track and along-track sizes, Response to better than 1%, Cross-track MTFs, Pixel spatial registration

SNR for each wavelengthDetector temperature and stabilityRelative accuracy (wavelength dependent) of preflight scene-radiance calibration and

solar irradiance calibrationAlbedo calibration (wavelength dependent and independent) accuracyAbsolute accuracy for laboratory radiometric measurementsShort term stability over a one-week periodInter-channel accuracy, Channel isolationBandpass and wavelength scale

Wavelength calibration, Bandpass limits and spectral response functions, Spectral data range and resolution, Out-of-band signal to expected signal ratio, Thermal design to limit spectral shifts between weekly on-orbit solar calibrations and spectral shift variability

Linear polarization sensitivityResponse in chosen IFOV due to integrated out-of-field signalPeriodic stimulation with test lamp to detect drifts or trends in responsivityPeriodic stimulation during functional tests to detect drifts or trends in the responsivityRelative accuracy of pre-flight scene radiance and solar-irradiance calibrations

Negotiated Nominal Databases Example LP

Operational Mode – Calibration State

Sensor Measurement When Required Window in Orbit ta + Cal. Time Required

Solar Working Diffuser - Weekly Reference Diffuser - 6 mo.

Term* -10 to Term* +10

118 seconds

Dark Current Weekly 100 seconds Total

Column

Linearity Weekly Term*+10 to

Term*+67 575 sec onds

Solar Working Diffuser - Weekly Reference Diffuser - 6 mo.

At center TC diffuser position

47 seconds (15 second overlap with Total Column)

Dark Current Weekly 100 seconds Nadir

Profile

Linearity Weekly Term*+10 to

Term*+67 575 seconds * Term = Solar Illumination Terminator

Nadir Calibration Period from ta to t1 where t1 = ta + 118sec + (47-15)sec + 100sec + 575sec

OMPS Driving Requirements for Climate Studies• 0.5% Long-term Albedo Calibration Independent)

– Trending and Goniometry

• 0.3% Long-term Albedo Calibration (Dependent)– Trending and Solar SNR

• 0.01 nm Wavelength Monitoring Accuracy– Solar SNR and bandpass

• 0.5% Pixel-to-pixel Radiometric Calibration– Solar, Dark and LED

On-orbit Calibration Systems and Monitoring• Solar Diffusers (Working and Reference)

– Diffuser versus Detector throughput changes• Lamps

– Spectral (wavelength scale, diffuser)– White (flat fielding, diffuser)– Monochromatic – LEDs (nonlinearity, flat fielding)

Characteristic• Spectral scale and bandpass width

– Spectral features in solar and earth views, line source lamps• Stray light

– Filling in of solar features (additive errors)– Correlation with scene brightness

• Dark or Offset (night side or closed aperture)• Nonlinearity

– Integration time– Bright scenes

• Absolute calibration from Earth reflectivity– Maxima and minima and Ice radiances

• Relative calibration from D-Pair and Ascending/Descending

Changes from Ground to Orbit

Wave Diff251.97 -9.4%273.64 -12.7%283.10 -7.1%287.67 -7.1%292.30 -5.9%297.58 -5.1%301.97 -5.6%305.84 -6.6%312.61 -4.4%317.54 -4.5%331.25 -1.6%339.86 -2.3%378.62 -7.6%

Key Attributes / Lessons Learned• Relative measurements are nicer

– Rad/Irrad, Pairs, Height Normalization

• Stable orbits make trending easier– Solar repeatability, ascending/descending comparisons

• Changes in Day-1 versus Ground can be large• Time dependent changes must be tracked

– Delta philosophy; X changes smoothly across the board

• UV contamination difficulties– Diffuser degradation– Throughput degradation

• Occultation instruments make direct extraterrestrial measurements for self-calibration

• Limb measurements need accurate pointing

Backup Material

SCIENCE GOAL: Monitor changes in stratospheric ozone (total column, profile) over multi-decade timescales.

•Accurate data from individual instrument requires knowledge of absolute calibration, time-dependent changes.

•Inflight calibration typically uses both specifically designed measurements (“hard” calibration) and carefully chosen science data (“soft” calibration) to determine instrument characterization.

Multiple SBUV, SBUV/2 instruments provide overlapping data sets covering 25 years

•D-pair total ozone (305.8, 312.5 nm) has good sensitivity to ozone abundance, low sensitivity to -dependent errors. Avoid profile shape effects choose small path length, low total ozone data (equatorial latitudes, SZA < 60 o).

•Calculate D-pair and B-pair total ozone, assume D-pair correct, evaluate residues. Results provide correction for B-pair calibration (317.5 nm). Consistent with onboard calibration system for NOAA-11, NOAA-14.

Long-Term SBUV and SBUV/2 Instrument Calibration for Version 8 Ozone Data

Matthew DeLand, Liang-Kang Huang, Steve Taylor, Al McKay, Richard Cebula

Science Systems and Applications, Inc. (SSAI)

P. K. Bhartia, Rich McPetersNASA Goddard Space Flight Center

D-pair Total Ozone

Absolute AdjustmentsNOAA-11 SBUV/2: Identify coincident measurements with SSBUV instrument from multiple flights. No correction applied at 340 nm based on reflectivity comparisons.

Nimbus-7 SBUV: Use overlap with adjusted NOAA-11 data to define initial changes. Reflectivity over ice indicated need to adjust calibration at non-absorbing wavelengths.

NOAA-16 SBUV/2: Uniform shift to prelaunch calibration (+5.7%) determined from snow/ice radiance comparisons. Wavelength-dependent variations are minimal.

NOAA-16 comparisons: Use microwave, lidar data for SBUV/2 wavelengths corresponding to useful altitudes of external data. V alidation tests for microwave results are not sensitive to derived linear wavelength dependence.

NOAA-9 SBUV/2: Normalize to NOAA-11 in 1993, when both instruments observe at similar solar zenith angles.

Solar irradiance agrees with reference spectrum within calibration uncertainty over 200-400 nm

Solar irradiance measured over same wavelength range used for ozone measurements

Onboard calibration system tracks diffuser reflectivity changes

SOFT CALIBRATION [selected techniques]•Analyze normal on-orbit science measurements to understand instrument characterization.

•Restrict data based on specific measurement parameters [e.g. wavelength, geographic location, signal level] such that results are sensitive to instrument calibration or performance.

•Uncertainties typically determined from external validation and comparisons.

Reflectivity•Antarctica, Greenland have high surface reflectivity, year-to-year stability evaluate absolute calibration, time-dependent drift. Solar zenith angle dependence also provides information about linearity.

•NOAA-11 results produce long-term instrument characterization after solar diffuser failure in 1994.

CONCLUSIONS•“Soft” calibration techniques using science data supplement and validate onboard calibration system.

•Combination of multiple techniques provides absolute, long-term calibration if onboard calibration system is unavailable.

•Estimated long-term uncertainty is approximately 3% in V8 profile ozone over duration of single data set from D-pair, residue methods. This is consistent with results of Ahn et al. [paper #200].

•All V8 SBUV and SBUV/2 profile ozone data are available at this meeting on DVD.

Residue Analysis•Residue measured radiance values minus calculated radiance values for each sample.

•Changes can represent calibration drift or instrument performance or geophysical change. Require final residues < ±0.5% over complete data set for V8 calibration.

•NOTE: Typical final residue range for previous profile ozone product [V6] was ±1.5%.

HARD CALIBRATION•SBUV/2 instruments measure radiance and irradiance with same optical system.

•Solar diffuser is only element not common to both measurements track reflectivity changes to get accurate albedo calibration.

•Onboard calibration system provides baseline for long-term instrument characterization.

•Solar measurements and onboard system also monitor wavelength calibration.

SBUV/2 Product Consistency1% TOZ, 5% Profile

• a. Reflectivity – ice, average, maxima, minima• b. Total Ozone (TOZ) – zonal means, absolute, pairs• c. Profile Ozone – zonal means

– c.i. Day-1 albedo calibration– c.ii. Stray light identification– c.iii. Ascending/descending– c.iv. Seasonality and SZA effects– c.v. Initial and final residual for V6 and V8– c.vi. Inter-channel calibration– c.vii. Non-linearity– c.viii. Dark current

• d. Time dependent changes– d.i. Calibration lamp – d.ii. Diffuser reflectivity– d.iii. Wavelength scale– d.iv. Inter-range ratios– d.v. Cathode/anode

Internal and Soft Calibration and Validation Sequence for Total Ozone

1. Check 331-nm reflectivity channel calibration by using global distributions of reflectivity – minimum ocean (4%) and land (1%) reflectivity, maximum global reflectivity and ice radiances (Greenland and Antarctica).

2. Check agreement between 360-nm reflectivity and 331-nm reflectivity for scenes with reflectivity greater than 80%.

3. Compute total ozone for nadir measurements from B-pair (317.5-nm and 331-nm) in the tropics and compare to expected values.

4. Check agreement of other ozone sensitive channels/pairs with the B-pair results.

5. Check agreement between zonal means at each satellite view angle and the nadir zonal means.

6. Compare ozone and reflectivity results for different channels and pairs as functions of solar zenith angles and reflectivity.

Methods developed at NASA GSFC over the last 30 years.

Parameters & Tables from NOAA-17 SBUV/2 Activation and Evaluation Report

Quantity LocationWavelength Calibration Ebert Coefficients Table 6.1Standard Ozone Wavelengths Table 6.8Radiance Calibration Constants Table 12.3Irradiance Calibration Constants Table 12.2Electronic Offsets Table 5.1Non-linearity Corrections Table 10.1PMT Temperature Correction Table 8.1Inter-range Ratio IRR12 pg. 53Inter-range Ratio IRR23A (anode mode) pg. 53Inter-range Ratio IRR23C (cathode mode) Table 9.1 Table 9.2Goniometric Corrections Table 7.1

Table 7.2 Table 7.3Day-1 Solar Irradiances Table 13.1Total Ozone Pair Adjustment Factors Table 14.1OOB Stray light characterization was delivered later.

Latitude Dependence 10-30 hPa