satellites and sensors: how they work - · pdf filehow satellites work •launch -- how do...
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Satellites and Sensors:How they work
18 April 2003
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Outline
• Satellite and their orbits
• Sensor typeswhiskbroom scanners
pushbroom scanners
• Example of pixel size calculation
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How Satellites Work
• Launch -- how do they get up there?
• To put a satellite into a stable orbit,need to overcome gravitationalattraction and the resistance of thelower atmosphere
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A rocket of initial mass, Mi, burning a mass offuel Mf will increase its velocity, DV, by
where U is the velocity of the exhaust gases withrespect to the rocket.U = 2.4 kms-1
orbital velocity of a satellite is about 7kms-1
Rocket must be about 97% fuelPayload can only be about 3% of rocket mass
†
DV = U ln Mi Mi - M f( )( )
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Terra launch
Vandenberg AFB
18 December 1999
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Description Satellite Orbits
• Determined by Kepler’s Laws– perturbed by gravititational irregularities, friction,
solar pressure, etc.– ranging and repositioning is required for satellites
to maintain their orbit
• The orbital period is the time it takes for thesatellite to circle the earth
• Easy to compute if assume the earth is asphere but earth is an oblate ellipsoid– creates an orbit that precesses (it rotates around
the polar axis)
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Special Orbits
Orbital parameters can be tuned toproduce particular, useful orbits
• Geostationary
• Geosynchronous
• Sun synchronous
• Altimetric
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Geostationary Orbits
• Orbit is stationary with respect to a locationon the earth
• Circular orbit around the equator (orbitalinclination = zero)
• The orbital period is equal to the earth’srotation (for a sidereal day, rotation withrespect to the Sun)
• Orbital altitude must be about 36,000 kmabove the equator
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Uses of Geostationary Orbits
• Weather satellites (GOES, METEOSAT)• Telephone and television relay satellites
Constant contact w/ground stationsLimited spatial coverage
– each satellite can only cover about 25-30% of theearth’s surface
– coverage extends only to the mid-latitudes, nomore than about 55o
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Geosynchronous Orbits
• Orbital period = earth’s rotation• Orbital inclination ≠ zero
– traces a figure eight
– half the time, the orbit is above(below)where it needs to be
– highly eccentric versions of this sort of orbitare possible but not widely used
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Sun-synchronous Orbit
• Precession of the satellite orbit is thesame as the angular speed of rotationof the sun
• Satellite will cross the equator at thesame time each day
• Orbital inclination is retrograde (typically~98o)
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Uses of Sun-Synchronous Orbits
• Equatorial crossing time depends onnature of application (low sun angle vs.high sun angle needs)
• Earth monitoring -- global coverage
• Orbital altitude typically between 600and 1000km -- good spatial resolution
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Altimetric Orbits
• Ascending and descending orbitsshould cross at 90o
– Designed so that orthogonal componentsof surface slope will have equal accuracy
• Orbital inclination depends on locationof altimetric needs
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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 Trackingand Data Relay Satellites (TDRS)
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Imaging Systems
• Cross-track scanner
• Whiskbroom scanner
• Pushbroom sensor
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Cross-track Scanner
• “back and forth” motion of the foreoptics
• scans each ground resolution cell one-by-one
• Instantaneous Field of View (IFOV) ofinstrument determines pixel size
• Image is built up by movement of satellitealong the orbital track and scanning across-track
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Dwell Time
• the amount of time a scanner has to collectphotons from a ground resolution cell:
(scan time per line)/(#cells per line)
depends on:– satellite speed
– width of scan line
– time per scan line
– time per pixel
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Dwell time example
(down track pixel size / orbital velocity)
(cross-track line width / cross-track pixel size)
dwell time =[(30m / 7500 m/s)/(185000m / 30m)]
=6.5 x 10-7 seconds/pixelThis is a very short time per pixel -- low SNR
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Along-track scanner(“Pushbroom)
• Linear array of detectors (aligned cross-track)– reflected radiance passes through a lens and onto
a line of detectors
• Image is built up by movement of the satellitealong its orbital track (no scanning mirror)
• Area array can also be used for multi-spectralremote sensing– dispersion used to split light into narrow spectral
bands and individual detectors
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Dwell Time Example
(down track pixel size / orbital velocity)
(cross-track line width / cross-track pixel size)
• denominator = 1.0
• dwell time = 4.0 x 10-3 seconds/pixel
• but different response sensitivities in each detectorcan cause striping in the image
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Whiskbroom Sensor
• Linear or area array of detectors
• Image is built up by movement ofsatellite along its orbital track and bycross-track scanning using a mirror– wide field of view (FOV)
– pixel resolution varies with scan angle
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Whiskbroom vs. Pushbroom
• Wide swath width
• Complex mechanicalsystem
• Simple optical system
• Filters and sensors
• Shorter dwell time
• Pixel distortion
• Narrow swath width
• Simple mechanicalsystem
• Complex optical system
• Dispersion grating andCCDs
• Longer dwell time
• No pixel distortion
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Calculating the Field of View(FOV)
FOV = 2 H tan(scan angle + b/2)
H = satellite altitude
Example:
SeaWIFS satellite altitude = 705 km
Scan angle = 58.3o
FOV = 1410 x tan(58.3o) = 2282 km
H
q
FOV
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Computing pixel size
tan(angle)= opposite/adjacentb = IFOV
Pn = pixel size at nadir
H = altitude of satellitetan(b/2) = (Pn/2) / H
Pn= 2 H tan(b/2)
bH
Pn
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x2x1
x
x = H tan(q + b/2)x2 = H tan(q - b/2)x1 = x - x2Pc = H tan(q + b/2) - H tan(q - b/2)
HH/cosq = Hsecq
q
Cross-track pixel size