Orbits and SensorsMultispectral 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 stationsLimited spatial coverage

– each satellite can only cover about 25-30% of the earth’s surface

– 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 altitude

• 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 day

• “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

See http://www.ssec.wisc.edu/datacenter/terra/

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 “swath”

• Image is built up by movement of satellite along its orbital track Produces a wide field-of-view

• 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 sensor)

Along-track scanner (Pushbroom)

• Linear array of detectors (aligned cross-track)– 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

system• Simple optical system• Shorter dwell time• Pixel distortion

• Narrow swath width• Simple mechanical

system• 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– IFOV– Dwell time

Calculating the Field of View (FOV)

FOV = 2 H tan(scan angle)

H = satellite altitude

Example:

SeaWIFS satellite altitude = 705 km

Scan angle = 58.3o

FOV = 1410 x tan(58.3o) = 2282 km

H

FOV

x2x1

x

x = H tan(/2)x2 = H tan(/2)x1 = x - x2

Pc = H tan(/2) - H tan(/2)

HH/cossec

Cross-track pixel size

History of the Landsat series

Currently, Landsat 5 and Landsat 7 (ETM+) are in orbit

Landsat MSS1972-present

Attitude-control subsystem

Wideband recorder electronics

Attitude measurement

sensor

Multispectral Scanner (MSS)Return Beam

Vidicon (RBV) cameras (3)

Data collection antenna

Solar array

Attitude-control subsystem

Wideband recorder electronics

Attitude measurement

sensor

Multispectral Scanner (MSS)Return Beam

Vidicon (RBV) cameras (3)

Data collection antenna

Solar array

Landsat orbits

• Band 4 (Green: 0.5 - 0.6 m)– 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 m)– chlorophyll absorption region– good contrast between vegetated and non-veg. areas– haze penetration better than Band 4

• Band 6 (NIR1: 0.7 - 0.8 m) and Band 7 (NIR2: 0.8 - 1.1 m)– 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 Bands and their Uses

Landsat MSS Images of Mount St. Helens

September 15, 1973 May 22, 1983

August 31, 1988

Landsat Thematic TM1982 - present

High-gain antenna

Multispectral Scanner (MSS)

Solar array panel

Thematic Mapper

(TM)

Global positioning system antenna

Attitude control module

Propulsion module

Power module

High-gain antenna

Multispectral Scanner (MSS)

Solar array panel

Thematic Mapper

(TM)

Global positioning system antenna

Attitude control module

Propulsion module

Power module

• Band 1 (Blue: 0.45 - 0.52 m)– good water penetration– differentiating soil and rock

surfaces from vegsmoke plumes– most sensitive to atmospheric

haze

• Band 2 (Green: 0.52 - 0.60 m)– water turbidity differences– sediment and pollution plumes– discrimination of broad classes of

vegetation

• Band 3 (Red: 0.63 - 0.69 m)– strong chlorophyll absorption (veg.

vs. soil)– urban vs. rural areas

Landsat Thematic Mapper Bands and their Uses

• Band 4 (NIR1: 0.76 - 0.90 m)– different vegetation varieties and

conditions– dry vs. moist soil– coastal wetland, swamps, flooded

areas

• Band 5 (NIR2: 1.55 - 1.75 m)– leaf-tissue water content – soil moisture– snow vs cloud discrimination

• Band 6 (Thermal: 10.4 - 12.5 m) – heat mapping applications (coarse

resolution)– radiant surface temperature range: -

100oC to +150oC• Band 7 (NIR3: 2.08 - 2.35 m)

– absorption band by hydrous minerals (clay, mica)

– lithologic mapping (clay zones)

Landsat Thematic Mapper Bands and their Uses

Landsat 7 Enhanced Thematic Mapper (ETM+)1999-present

•15m panchromatic band•on-board calibration