1. Orbits and Sensors Multispectral Sensors

2. 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

3. Satellite Orbits
Orbital parameters can be tuned to produce particular, useful orbits
Geostationary
Sun synchronous (Polar, Low Earth Orbit)
Geosynchronous
Altimetric

4. 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

5. 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 55o

6. 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 minutes

7. Animation of GEO and LEO orbits

9. Polar Orbiting Satellite Tracks

10. 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

11. Terra satellite overpasses for today over North America
See

12. 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)

13. Imaging Systems Cross-track scanning systems
“whiskbroom”
Along-track (non-scanning) system
“pushbroom”

14. 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

15. 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)

16. 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

17. 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

18. 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

19. 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

20. 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 q H
FOV

21. 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) q H
H/cosq = Hsecq x1 x2 x

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

23. Landsat MSS
1972-present

25. Landsat orbits

27. Landsat MSS Bands and their Uses
Band 4 (Green: mm)
water features (large penetration depths)
sensitivity to turbidity (suspended sediments)
sensitivity to atmospheric haze (lack of tonal contrast)
Band 5 (Red: mm)
chlorophyll absorption region
good contrast between vegetated and non-veg. areas
haze penetration better than Band 4
Band 6 (NIR1: mm) and Band 7 (NIR2: 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

28. Landsat MSS Images of Mount St. Helens
September 15, 1973
May 22, 1983
August 31, 1988

29. Landsat Thematic TM
present

30. Landsat Thematic Mapper Bands and their Uses
Band 1 (Blue: mm)
good water penetration
differentiating soil and rock surfaces from vegsmoke plumes
most sensitive to atmospheric haze
Band 2 (Green: mm)
water turbidity differences
sediment and pollution plumes
discrimination of broad classes of vegetation
Band 3 (Red: mm)
strong chlorophyll absorption (veg. vs. soil)
urban vs. rural areas

31. Landsat Thematic Mapper Bands and their Uses
Band 4 (NIR1: mm)
different vegetation varieties and conditions
dry vs. moist soil
coastal wetland, swamps, flooded areas
Band 5 (NIR2: mm)
leaf-tissue water content
soil moisture
snow vs cloud discrimination
Band 6 (Thermal: mm)
heat mapping applications (coarse resolution)
radiant surface temperature range: -100oC to +150oC
Band 7 (NIR3: mm)
absorption band by hydrous minerals (clay, mica)
lithologic mapping (clay zones)

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