Calibration Status of Instruments to Measure Scattered UV

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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
instruments OMI Ozone Monitoring
Instrument GOME Global Ozone Monitoring
Experiment OMPS Ozone Mapping and Profiler
Suite TOMS Total Ozone Mapping Spectrometer
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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.

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Ozone UV Absorption
Photon Penetration 325 DU 30º SZA
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Solar Irradiance
Earth Radiance
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Earth/Solar
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Solar Irradiance
Earth Radiance
Factor of 1000
Earth/Solar
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UV Measurement Technology

• 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

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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

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Global Ozone Monitoring Experiment (GOME-2)
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Total Ozone Mapping Spectrometer (TOMS)
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Ozone Monitoring Instrument (OMI)
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OMPS LP Optical Design

• Prism (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 mirrors

UV/Visible Limb Scatter heritage SOLSE/LORE,
OSIRIS, SAGE III, SCIAMACHY
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OMPS Ozone EDR Products Properties and
Performance
Table 1. Total Column Ozone EDR Performance.
Measurement Parameter Specification
Horizontal Cell Size 50 KM _at_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
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OMPS Nadir Sensor Acceptance Test Flow

• 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

Notice the radiance, irradiance and goniometry
emphasis.
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OMPS Calibration Component Characterization
Calibration Component Characteristic Blocking
Filter Rejection Characteristics Xe Lamp
Spatial Stability, Output
Stability Tunable Laser Wavelength
Accuracy, Output Stability Integrating
Sphere Output Characteristics, Uniformity FEL
Lamps Output Stability Collimated Source
Bench Beam Uniformity Spectralon
Diffuser Uniformity, BRDF versus
wavelength Aluminum Diffuser BRDF versus
wavelength Nadir Flight Diffuser Uniformity,
BRDF versus wavelength Limb Flight
Diffuser Uniformity, BRDF versus
wavelength Goniometric Fixture Accuracy,
Repeatability
Most components are trace-able to NIST standards.
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Verified Requirements

• Non-linearity characterization, Non-linearity no
  more than 2 of full well, In-flight electronic
  response referred to each CCD pixel, Pre-amp gain
  variation
• Boresight
• 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 IFOV
• Shape at Nadir, Cross-track and along-track
  sizes, Response to better than 1, Cross-track
  MTFs, Pixel spatial registration
• SNR for each wavelength
• Detector temperature and stability
• Relative accuracy (wavelength dependent) of
  preflight scene-radiance calibration and solar
  irradiance calibration
• Albedo calibration (wavelength dependent and
  independent) accuracy
• Absolute accuracy for laboratory radiometric
  measurements
• Short term stability over a one-week period
• Inter-channel accuracy, Channel isolation
• Bandpass 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 sensitivity
• Response in chosen IFOV due to integrated
  out-of-field signal
• Periodic stimulation with test lamp to detect
  drifts or trends in responsivity
• Periodic stimulation during functional tests to
  detect drifts or trends in the responsivity
• Relative accuracy of pre-flight scene radiance
  and solar-irradiance calibrations

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Negotiated Nominal Databases Example LP
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Operational Mode Calibration State
Nadir Calibration Period from ta to t1 where t1
ta 118sec (47-15)sec 100sec 575sec
Term Solar Illumination Terminator

• OMPS Driving Requirements for Climate Studies
• 0.5 Long-term Albedo Calibration (l-Independent)
• Trending and Goniometry
• 0.3 Long-term Albedo Calibration (l-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

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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

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Changes from Ground to Orbit
Wave Diff 251.97 -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
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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

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Backup Material
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Long-Term SBUV and SBUV/2 Instrument Calibration
for Version 8 Ozone Data
Multiple SBUV, SBUV/2 instruments provide
overlapping data sets covering 25 years

• 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.

Matthew DeLand, Liang-Kang Huang, Steve Taylor,
Al McKay, Richard Cebula Science Systems and
Applications, Inc. (SSAI) P. K. Bhartia, Rich
McPeters NASA Goddard Space Flight Center
D-pair Total Ozone

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

Absolute Adjustments
NOAA-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. Validation
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.
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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

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

• Quantity
  Location
• Wavelength Calibration Ebert Coefficients Table
  6.1
• Standard Ozone Wavelengths Table
  6.8
• Radiance Calibration Constants Table
  12.3
• Irradiance Calibration Constants Table
  12.2
• Electronic Offsets Table
  5.1
• Non-linearity Corrections Table
  10.1
• PMT Temperature Correction Table
  8.1
• Inter-range Ratio IRR12 pg.
  53
• Inter-range Ratio IRR23A (anode mode) pg.
  53
• Inter-range Ratio IRR23C (cathode mode) Table
  9.1
• Table
  9.2
• Goniometric Corrections Table
  7.1
  
  
• Table
  7.2
• Table
  7.3
• Day-1 Solar Irradiances Table
  13.1
• Total Ozone Pair Adjustment Factors Table
  14.1

OOB Stray light characterization was delivered
later.
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Latitude Dependence 10-30 hPa
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