Orbits and Sensors
Multispectral Sensors
Outline for 15 October 2007
Orbits: geostationary, polar
Cross-track scanners (whiskbroom sensor)
Pushbroom sensors
Field of View (“swath width”)
Pixel size
Multispectral sensors:
– Landsat
Satellite Orbits
Orbital parameters can be tuned to produce
particular, useful orbits
• Geostationary
• Sun synchronous (Polar, Low Earth Orbit)
• Geosynchronous
• Altimetric
Geostationary Orbits
• Geo orbit is stationary with respect to a
location on the earth
• Circular orbit around the equator (orbital
inclination = zero)
• Placed in high orbit (35,800 km) to match the
angular velocity of Earth
Uses of Geostationary Orbits
• Weather satellites (GOES, METEOSAT)
• Constant monitoring
• Communication satellites
Constant contact w/ground stations
Limited spatial coverage
– each satellite can only cover about 25-30% of the earth’s
– coverage extends only to the mid-latitudes, no more than
about 55o
Sun-synchronous (Polar) Orbit
• “Low Earth Orbit” (LEO) are typically about 700 km
• Precession of the satellite orbit is the same as the
angular speed of rotation of the sun
• Satellite crosses the equator at the same time each
• “Polar orbit” is very common
– Orbital inclination is “retrograde” (typically ~98o)
– Near circular orbits have period of about 98-102 minutes
Animation of GEO and LEO orbits
Polar Orbiting Satellite Tracks
Uses of Sun-Synchronous Orbits
• Equatorial crossing time depends on nature
of application (low sun angle vs. high sun
angle needs)
• Earth monitoring -- global coverage
• Good spatial resolution
Terra satellite
overpasses for
today over
North America
Getting the Data to the Ground
• On-board recording and pre-processing
• Direct telemetry to ground stations
– receive data transmissions from satellites
– transmit commands to satellites (pointing, turning
maneuvers, software updating
• Indirect transmission through Tracking and
Data Relay Satellites (TDRS)
Imaging Systems
• Cross-track scanning systems
– “whiskbroom”
• Along-track (non-scanning) system
– “pushbroom”
Cross-track Scanner
• Single detector or a linear
array of detectors
• “Back and forth” motion of the
scanner creates the orbital
• Image is built up by movement
of satellite along its orbital
track Produces a wide field-ofview
• Pixel resolution varies with
scan angle
Field of View (FOV)
• FOV is the swath width of the instrument
• It is the width of an orbital swath
• Depends on the across track scan angle of
the sensor (for whiskbroom) or the width of
the linear detector array (for a pushbroom
Along-track scanner
• Linear array of detectors (aligned
– radiance passes through a lens and
onto a line of detectors
• Image is built up by movement of the
satellite along its orbital track (no
scanning mirror)
• Multiple linear arrays are used for
multi-spectral remote sensing
– dispersion element splits light into
different wavelengths and onto
individual detectors
Dwell Time
• The amount of time a scanner has to collect
photons from a ground resolution cell
• Depends on:
satellite speed
width of scan line
time per scan line
time per pixel
• Whiskbroom scanners have much shorter
dwell time than do pushbroom scanners
Whiskbroom vs. Pushbroom
• Wide swath width
• Complex mechanical
• Simple optical system
• Shorter dwell time
• Pixel distortion
• Narrow swath width
• Simple mechanical
• Complex optical system
• Longer dwell time
• No pixel distortion
Signal Strength
• Need enough photons incident on the
detector to record a strong signal
• Signal strength depends on
– Energy flux from the surface
– Altitude of the sensor
– Location of the spectral bands (e.g. visible, NIR,
thermal, etc.)
– Spectral bandwidth of the detector
– Dwell time
Calculating the Field of View (FOV)
FOV = 2 H tan(scan angle)
H = satellite altitude
SeaWIFS satellite altitude = 705 km
Scan angle = 58.3o
FOV = 1410 x tan(58.3o) = 2282 km
Cross-track pixel size
x = H tan(q + b/2)
x2 = H tan(q - b/2)
x1 = x - x2
Pc = H tan(q + b/2) - H tan(q - b/2)
H/cosq = Hsecq
History of
the Landsat
Landsat 5
Landsat 7
are in orbit
Landsat MSS
Solar array
At titude-cont rol
Wideband recorder
elect ronics
At titude
ant enna
Re tu rn Be am
Vi di con (RB V)
cam e ras (3)
Mu l ti s pe ctral
S can n e r (MS S )
Landsat orbits
Landsat MSS Bands and their Uses
• Band 4 (Green: 0.5 - 0.6 mm)
– water features (large penetration depths)
– sensitivity to turbidity (suspended sediments)
– sensitivity to atmospheric haze (lack of tonal contrast)
• Band 5 (Red: 0.6 - 0.7 mm)
– chlorophyll absorption region
– good contrast between vegetated and non-veg. areas
– haze penetration better than Band 4
• Band 6 (NIR1: 0.7 - 0.8 mm) and Band 7 (NIR2: 0.8 1.1 mm)
– similar for most surface features
– good contrast between land and water (water is strong
absorber in near IR)
– both bands excellent haze penetration
– Band 7 good for discrimination of snow and ice
Landsat MSS Images of Mount St. Helens
September 15, 1973
May 22, 1983
August 31, 1988
Thematic TM
1982 - present
ant enna
Global positioning
system ant enna
At titude control
P ropulsion
P ower
Mu l ti s pe ctral
S can n e r (MS S )
Th e mati c
Mappe r
Landsat Thematic Mapper Bands and their Uses
• Band 1 (Blue: 0.45 - 0.52 mm)
– good water penetration
– differentiating soil and rock
surfaces from vegsmoke plumes
– most sensitive to atmospheric
• Band 2 (Green: 0.52 - 0.60 mm)
– water turbidity differences
– sediment and pollution plumes
– discrimination of broad classes of
• Band 3 (Red: 0.63 - 0.69 mm)
– strong chlorophyll absorption (veg.
vs. soil)
– urban vs. rural areas
Landsat Thematic Mapper Bands and their Uses
• Band 4 (NIR1: 0.76 - 0.90 mm)
– different vegetation varieties and
– dry vs. moist soil
– coastal wetland, swamps, flooded
• Band 5 (NIR2: 1.55 - 1.75 mm)
– leaf-tissue water content
– soil moisture
– snow vs cloud discrimination
• Band 6 (Thermal: 10.4 - 12.5 mm)
– heat mapping applications (coarse
– radiant surface temperature range: 100oC to +150oC
• Band 7 (NIR3: 2.08 - 2.35 mm)
– absorption band by hydrous minerals
(clay, mica)
– lithologic mapping (clay zones)
Landsat 7 Enhanced Thematic
Mapper (ETM+)
•15m panchromatic band
•on-board calibration

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