Orbits and Sensors Multispectral Sensors

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 surface – coverage extends only to the mid-latitudes, no more than about 55 o

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 ~98 o ) – 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

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

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

Calculating the Field of View (FOV)

FOV = 2 H tan(scan angle) H = satellite altitude Example: SeaWIFS satellite altitude = 705 km Scan angle = 58.3

o FOV = 1410 x tan(58.3

o ) = 2282 km q FOV H

Cross-track pixel size

x = H tan( q + b /2) x 2 x 1 P c = H tan( q - b /2) = x - x 2 = H tan( q + b /2) - H tan( q - b /2) q H/cos q = H sec q H x 1 x x 2

History of the Landsat series Currently, Landsat 5 and Landsat 7 (ETM+) are in orbit

Landsat MSS 1972-present

Landsat MSS Bands and their Uses

• Band 4 (Green: 0.5 - 0.6 m 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 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 m) and Band 7 (NIR2: 0.8 1.1 m 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

Spectral Reflectance

spectral absorptance

Landsat Thematic TM 1982 - present Global positioning system antenna Attitude control module Propulsion module Power module

Thematic Mapper (TM)

High-gain antenna

Multispectral Scanner (MSS)

Solar array panel

Landsat Thematic Mapper Bands and their Uses

• Band 1 (Blue: 0.45 - 0.52 m 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 m) – water turbidity differences – sediment and pollution plumes – discrimination of broad classes of vegetation • Band 3 (Red: 0.63 - 0.69 m 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 conditions m m) – different vegetation varieties and – dry vs. moist soil – coastal wetland, swamps, flooded areas • Band 5 (NIR2: 1.55 - 1.75 m m) – leaf-tissue water content – soil moisture – snow vs cloud discrimination • Band 6 (Thermal: 10.4 - 12.5 resolution) 100 o C to +150 o C • Band 7 (NIR3: 2.08 - 2.35 (clay, mica) m m) m m) – heat mapping applications (coarse – radiant surface temperature range: – absorption band by hydrous minerals – lithologic mapping (clay zones)

Landsat 7 Enhanced Thematic Mapper (ETM+) 1999-present •15m panchromatic band •on-board calibration