Introduction to MODIS Orbit, calibration and observations

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Introduction to MODISOrbit, calibration and
observations

• Contributions from
• Dr. Steven A. Lloyd (NASA GES-DISC)
• Ewa Ainsworth (SAIC/NASA-GSFC)
• P.K. Bhartia (NASA GSFC)

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NASA Earth-Observing System

• Attempt to understand the Earth as an integrated
  system
• Provide a check-up on our planet (Y. Kaufman)
• A coordinated, international, inter-agency effort
• Includes satellites

NASAs Big Blue Marble from Apollo 17 7
December 1972
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Satellite Measurement Types

• Occultation
• 1-3 km in vert, 200 km horiz, no mapping.
• Limb Emission/scattering
• 3-5 km in vert, 200 km horiz, limited mapping.
• Nadir scattering/reflection/emission
• 5 km to total in vert, 20-200 km horiz, daily
  mapping.
• Radar/LIDAR
• Very high vert, 10 km horiz, limited mapping.

P.K. Bhartia
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NASAs A-Train
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Some Satellite Orbit Basics
Low Earth Orbit (LEO) Orbiting at an altitude of
600-1,000 km.
Path of Satellite
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NASA Earth-Observing Satellites
Low Earth Orbit Orbiting at an altitude of
600-1,000 km.
Path of Satellite
Ascending Orbit The satellite is moving South to
North when that portion of the orbit track
crosses the equator.
Ascending Orbit The satellite is moving South to
North when that portion of the orbit track
crosses the equator.
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NASA Earth-Observing Satellites
Low Earth Orbit Orbiting at an altitude of
600-1,000 km.
Descending Orbit The satellite is moving North
to South when that portion of the orbit track
crosses the equator.
Descending Orbit The satellite is moving North
to South when that portion of the orbit track
crosses the equator.
Ascending Orbit The satellite is moving South to
North when that portion of the orbit track
crosses the equator.
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NASA Earth-Observing Satellites
Low Earth Orbit Orbiting at an altitude of
600-1,000 km.
Ascending Orbit The satellite is moving South to
North when that portion of the orbit track
crosses the equator.
Descending Orbit The satellite is moving North
to South when that portion of the orbit track
crosses the equator.
Ascending vs. descending orbits are like night
and day!
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NASA Earth-Observing Satellites
Low Earth Orbit Orbiting at an altitude of
600-1,000 km.
Ascending Orbit The satellite is moving South to
North when that portion of the orbit track
crosses the equator.
Sun-Synchronous The satellite is always in the
same relative position between the Earth and Sun.
Descending Orbit The satellite is moving North
to South when that portion of the orbit track
crosses the equator.
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NASA Earth-Observing Satellites
Low Earth Orbit Orbiting at an altitude of
600-1,000 km.
Ascending Orbit The satellite is moving South to
North when that portion of the orbit track
crosses the equator.
Period A typical polar, Sun-synchronous LEO
satellite takes about 90 minutes to completely
circle the Earth. This gives it about 16 orbits
per day.
Period A typical polar, Sun-synchronous LEO
satellite takes about 90 minutes to completely
circle the Earth. This gives it about 16 orbits
per day.
Descending Orbit The satellite is moving North
to South when that portion of the orbit track
crosses the equator.
Sun-Synchronous The satellite is always in the
same relative position between the Earth and Sun.
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NASA Earth-Observing Satellites
Low Earth Orbit Orbiting at an altitude of
600-1,000 km.
Ascending Orbit The satellite is moving South to
North when that portion of the orbit track
crosses the equator.
Equatorial-Crossing Time The local apparent
solar time when the satellite crosses the
equator. Example Terra has an equatorial
crossing time of 1030 am, and is called an AM
or morning satellite.
Period A typical polar, Sun-synchronous LEO
satellite takes about 90 minutes to completely
circle the Earth. This gives it about 16
orbits per day.
Descending Orbit The satellite is moving North
to South when that portion of the orbit track
crosses the equator.
Sun-Synchronous The satellite is always in the
same relative position between the Earth and Sun.
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NASA Earth-Observing Satellites
UARS
Nimbus-7
SORCE
TRMM
Earth Probe
Aura
CloudSAT
SeaWIFS
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Satellite Inclination
Low Inclination Orbit (often near 57º-- Space
Shuttle) no polar coverage
High Inclination or Polar Orbit (near
90º) virtually complete global coverage
Equator
Inclination The
position of the orbital plane relative to the
equator. For near-polar orbits, typically about
97º.
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MODerate-resolution Imaging Spectroradiometer
(MODIS)

• NASA, Terra Aqua
• launched 1999, 2002
• 705 km polar orbits, descending (1030 a.m.)
  ascending (130 p.m.)
• Sensor Characteristics
• 36 spectral bands ranging from 0.41 to 14.385 µm
• cross-track scan mirror with
• 2330 km swath width
• Spatial resolutions
• 250 m (bands 1 - 2)
• 500 m (bands 3 - 7)
• 1000 m (bands 8 - 36)

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Not one, but two MODISs!
MODIS-Aqua (ascending orbit)
MODIS-Terra (descending)
1 August 2007
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Aquas Orbit

• Near-polar, sun-synchronous, orbiting the Earth
  every 98.8 minutes, crossing the equator going
  north (daytime ascending) at 130 p.m. and going
  south at 130 a.m.
• The orbit track changes every day but will repeat
  on a 16 day cycle. This is true for both Aqua and
  Terra.

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MODIS Orbit in 3D
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Daytime Orbits
Aqua - Ascending
Terra - Descending
When looking at an image of a piece of the orbit
the two sensors will have opposite tilts.
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MODIS is advanced and complex
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MODerate-resolution Imaging Spectroradiometer
(MODIS)

• NASA, Terra Aqua
• launched 1999, 2002
• 705 km polar orbits, descending (1030 a.m.)
  ascending (130 p.m.)
• Sensor Characteristics
• 36 spectral bands ranging from 0.41 to 14.385 µm
• cross-track scan mirror with
• 2330 km swath width
• Spatial resolutions
• 250 m (bands 1 - 2)
• 500 m (bands 3 - 7)
• 1000 m (bands 8 - 36)

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MODIS Reflected Solar Bands
250 M
500 M
1 KM
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MODIS Thermal Bands
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MODIS is calibrated

• Direct calibration
• Pre-launch sensor is calibrated in a laboratory
  (thermal vacuum)
• On-orbit regular solar, deep-space, and lunar
  observations track changes in sensor response
  (possible additional on-board calibrators)
• Absolute radiometric accuracy
• reflective solar bands (0.41 2.1?m) 2 in
  reflectance and 5 in radiance
• thermal emissive bands (3.7 14.4?m) 1 for
  most bands
• Vicarious calibration uses sources independent
  of the primary direct calibration in situ
  measurements

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Sensor characterization and calibration concerns

• Mirror degradation, Response Versus Scan-angle
  (RVS), two mirror sides
• Detector calibration changes
• Polarization sensitivity
• In-band and out-of-band response
• Instrument and focal plane temperature effects
• Electronic cross-talk
• Stray-light contamination
• Solar Diffuser stability

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Low Level MODIS Data

• ATT EPH
• spacecraft attitude
• spacecraft position

• Level 0
• raw digital counts
• native binary format

• GEO
• geolocation
• radiant path geometry

• Level 1A
• raw digital counts
• HDF formatted

• Level 1B
• calibrated
• radiances
• converted
• telemetry

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Native satellite view vs. map projection
cylindrical isotropic projection
BowTie effect
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True Color Image Terra April 19,
2000(composite of Red, Green and Blue)
Note gaps in the orbit at the equator
Orbits overlap at the poles
Mark Gray
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Global Level-1B Composite Image
R 0.65 µm G 0.56 µm B 0.47 µm
granule coverage (5 min)
May 28, 2001
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MODIS Products

• Ancillary data
• analyses (O3, H2O, winds, etc)
• Maps (land/ocean), etc

L1A ATT EPH Geo

Level 1B

• Retrieval algorithms
• Theory
• RadiativeTransfer/Lookup tables

Level 2 (products)_
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What makes MODIS unique?

• Well calibrated.
• High spatial resolution.
• Large number of spectral bands.
• A great variety of products.

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