UCL DEPARTMENT OF GEOGRAPHY UCL DEPARTMENT OF GEOGRAPHY GEOGG141/ GEOG3051 Principles & Practice of Remote Sensing (PPRS) Angular, temporal, radiometric resolution, sampling Dr. Mathias (Mat) Disney UCL Geography Office: 113, Pearson Building Tel: 7679 0592 Email: [email protected]http://www2.geog.ucl.ac.uk/~mdisney/teaching/GEOGG141/GEOGG1 41.html http://www2.geog.ucl.ac.uk/~mdisney/teaching/3051/GEOG3051.h tml
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GEOGG141/ GEOG3051 Principles & Practice of Remote Sensing (PPRS) Angular, temporal, radiometric r esolution, sampling
GEOGG141/ GEOG3051 Principles & Practice of Remote Sensing (PPRS) Angular, temporal, radiometric r esolution, sampling. Dr. Mathias (Mat) Disney UCL Geography Office: 113, Pearson Building Tel: 7679 0592 Email: [email protected] - PowerPoint PPT Presentation
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UCL DEPARTMENT OF GEOGRAPHYUCL DEPARTMENT OF GEOGRAPHY
GEOGG141/ GEOG3051Principles & Practice of Remote Sensing (PPRS)Angular, temporal, radiometric resolution, sampling
• Flattened circle – 2 foci and 2 axes: major and minor– Distance r1+r2 = constant = 2a (major axis)– “Flatness” of ellipse defined by eccentricity, e = 1-b2/a2 = c/a– i.e. e is position of the focus as a fraction of the semimajor axis, a
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Ellipse
From http://mathworld.wolfram.com/Ellipse.html
Increasing eccentricity
• ecircle = 0
• As e 1, c a and ellipse becomes flatter
r1 r2
f1 f2C
2a
2c
2b
major axis
minor axis
UCL DEPARTMENT OF GEOGRAPHY
• Kepler’s Laws – deduced from Brahe’s data after his death– see nice Java applet
• Kepler’s 3rd law – “ratio of the squares of the revolutionary periods for two planets (P1,
P2) is equal to the ratio of the cubes of their semimajor axes (R1, R2)”– P1
2/P22 = R1
3/R23
• i.e. orbital period increases dramatically with R
• Convenient unit of distance is average separation of Earth from Sun = 1 astronomical unit (A.U.)– 1A.U. = 149,597,870.691 km– in Keplerian form, P(years)2 R(A.U.)3
– or P(years) R(A.U.)3/2
– or R(A.U.) P(years)2/3
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Kepler’s laws
UCL DEPARTMENT OF GEOGRAPHY
• Orbital period for a given instrument and height? – Gravitational force Fg = GMEms/RsE
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• G is universal gravitational constant (6.67x10-11 Nm2kg2); ME is Earth mass (5.983x1024kg); ms is satellite mass (?) and RsE is distance from Earth centre to satellite i.e. 6.38x106 + h where h is satellite altitude
– Centripetal (not centrifugal!) force Fc = msvs2/RsE
• where vs is linear speed of satellite (=sRsE where is the satellite angular velocity, rad s-1)
– for stable (constant radius) orbit Fc = Fg – GMEms/RsE
2 = msvs2/RsE = ms s
2RsE2 /RsE
– so s2 = GME /RsE
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Orbits: examples
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• Orbital period T of satellite (in s) = 2/– (remember 2 = one full rotation, 360°, in radians)– and RsE = RE + h where RE = 6.38x106 m– So now T = 2[(RE+h)3/GME]1/2
• Example: polar orbiter period, if h = 705x103m– T = 2[(6.38x106 +705x103)3 / (6.67x10-11*5.983x1024)]1/2
– T = 5930.6s = 98.8mins• Example: altitude for geostationary orbit? T = ??
– Rearranging: h = [(GME /42)T2 ]1/3 - RE
– So h = [(6.67x10-11*5.983x1024 /42)(24*60*60)2 ]1/3 - 6.38x106
– h = 42.2x106 - 6.38x106 = 35.8x106m
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Orbits: examples
UCL DEPARTMENT OF GEOGRAPHY
• Convenience of using radians– By definition, angle subtended by an arc (in radians) = length of
arc/radius of circle i.e. = l/r– i.e. length of an arc l = r– So if we have unit circle (r=1), l = circumference = 2r = 2– So, 360° = 2 radians
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Orbits: aside
r
l
UCL DEPARTMENT OF GEOGRAPHY
• Geostationary? – Circular orbit in the equatorial plane, altitude ~36,000km– Orbital period?
• Advantages– See whole Earth disk at once due to large distance– See same spot on the surface all the time i.e. high temporal coverage– Big advantage for weather monitoring satellites - knowing atmos.
dynamics critical to short-term forecasting and numerical weather prediction (NWP)
• GOES (Geostationary Orbiting Environmental Satellites), operated by NOAA (US National Oceanic and Atmospheric Administration)
• http://www.noaa.gov/ and http://www.goes.noaa.gov/
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Orbital pros and cons
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• Meteorological satellites - combination of GOES-E, GOES-W, METEOSAT (Eumetsat), GMS (NASDA), IODC (old Meteosat 5)– GOES 1st gen. (GOES-1 - ‘75 GOES-7 ‘95); 2nd gen. (GOES-8++ ‘94)
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Geostationary
From http://www.sat.dundee.ac.uk/pdusfaq.html
METEOSAT 0° WGOES-W 135° WGOES-E 75° W GMS 140° EIODC 63° E
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• METEOSAT - whole earth disk every 15 mins
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Geostationary
From http://www.goes.noaa.gov/f_meteo.html
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• Disadvantages– typically low spatial resolution due to high altitude– e.g. METEOSAT 2nd Generation (MSG) 1x1km visible, 3x3km IR
(used to be 3x3 and 6x6 respectively)• MSG has SEVIRI and GERB instruments• http://www.eumetsat.int/Home/Main/What_We_Do/Satellites/
Meteosat_Second_Generation/Space_Segment/SP_1119959405658?l=en– Cannot see poles very well (orbit over equator)
• spatial resolution at 60-70° N several times lower• not much good beyond 60-70°
– NB Geosynchronous orbit same period as Earth, but not equatorial
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Geostationary orbits
From http://www.esa.int/SPECIALS/MSG/index.html
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• Advantages– full polar orbit inclined 90 to equator
• typically few degrees off so poles not covered• orbital period typically 90 - 105mins
– near circular orbit between 300km (low Earth orbit) and 1000km– typically higher spatial resolution than geostationary– rotation of Earth under satellite gives (potential) total coverage
• ground track repeat typically 14-16 days
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Polar & near polar orbits
From http://collections.ic.gc.ca/satellites/english/anatomy/orbit/
UCL DEPARTMENT OF GEOGRAPHY
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(near) Polar orbits: NASA Terra
From http://visibleearth.nasa.gov/cgi-bin/viewrecord?134
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– inclination 98.2, T = 98.8mins– http://www.cscrs.itu.edu.tr/page.en.php?id=51– http://landsat.gsfc.nasa.gov/project/Comparison.html
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Near-polar orbits: Landsat
From http://www.iitap.iastate.edu/gccourse/satellite/satellite_lecture_new.html & http://eosims.cr.usgs.gov:5725/DATASET_DOCS/landsat7_dataset.html
• ASIDE: repeat time• Orbital period is 5928s• So in this time Earth surface
moves l = r = r*(2*5928/(24*60*60))
• So if r = 6.38x106 then l = 2750km
UCL DEPARTMENT OF GEOGRAPHY
• Disadvantages– need to launch to precise altitude and orbital inclination– orbital decay
• at LEOs (Low Earth Orbits) < 1000km, drag from atmosphere• causes orbit to become more eccentric• Drag increases with increasing solar activity (sun spots) - during solar
maximum (~11yr cycle) drag height increased by 100km!– Build your own orbit:
From http://collections.ic.gc.ca/satellites/english/anatomy/orbit/
UCL DEPARTMENT OF GEOGRAPHY
• Sun-synchronous– Passes over same point on surface at approx. same local solar time
each day (e.g. Landsat)– Characterised by equatorial crossing time (Landsat ~ 10am)– Gives standard time for observation– AND gives approx. same sun angle at each observation
• good for consistent illumination of observations over time series (i.e. Observed change less likely to be due to illumination variations)
• BAD if you need variation of illumination (angular reflectance behaviour)
• Special case is dawn-to-dusk– e.g. Radarsat 98.6° inclination– trails the Earth’s shadow (day/night border)– allows solar panels to be kept in sunlight all the time)
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Types of near-polar orbit
UCL DEPARTMENT OF GEOGRAPHY
• Inclination much lower– orbits close to equatorial– useful for making observations solely over tropical regions
• DORIS, GPS, Galileo etc.– DORIS: Doppler Orbitography and Radiopositioning Integrated by Satellite– Tracking system providing range-rate measurements of signals from a dense
network of ground-based beacons (~cm accuracy)– GPS: Global Positioning System– http://www.vectorsite.net/ttgps.html– http://www.edu-observatory.org/gps/tracking.html
• Swath describes ground area imaged by instrument during overpass
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Instrument swath
one sample
two samples
three samples
satellite ground swath
direction of travel
UCL DEPARTMENT OF GEOGRAPHY
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MODIS on-board Terra
From http://visibleearth.nasa.gov/cgi-bin/viewrecord?130
UCL DEPARTMENT OF GEOGRAPHY
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Terra instrument swaths compared
From http://visibleearth.nasa.gov/Sensors/Terra/
UCL DEPARTMENT OF GEOGRAPHY
• MODIS, POLDER, AVHRR etc.– swaths typically several 1000s of km– lower spatial resolution– Wide area coverage– Large overlap obtains many more view and illumination angles
(much better termporal & angular (BRDF) sampling)– Rapid repeat time
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Broad swath
UCL DEPARTMENT OF GEOGRAPHY
• Note across-track “whiskbroom” type scanning mechanism• swath width of 2330km (250-1000m resolution)• Hence, 1-2 day repeat cycle
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MODIS: building global view
From http://visibleearth.nasa.gov/Sensors/Terra/
UCL DEPARTMENT OF GEOGRAPHY
• 2400km swath, 1.1km pixels at nadir, but > 5km at edge of swath• Repeats 1-2 times per day
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AVHRR: global view
From http://edc.usgs.gov/guides/avhrr.html
UCL DEPARTMENT OF GEOGRAPHY
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POLDER (RIP!): global view
From http://www-loa.univ-lille1.fr/~riedi/BROWSES/200304/16/index.html
• Polarisation and Directionality of Earth’s Reflectance– FOV ±43° along track, ±51° across track, 9 cameras, 2400km swath, 7x6km
resn. at nadir– POLDER I 8 months, POLDER II 7 months....
Each set of points corresponds to given viewing zenith and azimuthal angles for near-simultaneous measurements over a region defined by lat 0°±0.5° and long of 0°±0.5° (Nov 1996)
Each day, region is sampled from different viewing directions so hemisphere is sampled heavily by compositing measurements over timeFrom Loeb et al. (2000) Top-of-Atmosphere Albedo Estimation from Angular Distribution Models Using Scene Identification from Satellite Cloud Property Retrievals, Journal of Climate, 1269-1285.
UCL DEPARTMENT OF GEOGRAPHY
• Landsat TM/MSS/ETM+, IKONOS, QuickBird etc.– swaths typically few 10s to 100skm– higher spatial resolution– local to regional coverage NOT global– far less overlap (particularly at lower latitudes)– May have to wait weeks/months for revisit
• Bits per pixel– 1 bit (0,1); 2bits (0, 1, 2, 3); 3 bits (0, 1, 2, 3, 4, 5, 6, 7) etc.– 8 bits in a byte so 1 byte can record 28 (256) different DNs (0-255)
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Radiometric resolution
• 1 to 6 bits (left to right)– black/white (21) up to 64 graylevels (26) (DN values)– human eye cannot distinguish more than 20-30 DN levels in grayscale
i.e. ‘radiometric resolution’ of human eye 4-5 bits
From http://ceos.cnes.fr:8100/cdrom/ceos1/irsd/pages/dre4.htm
– TIR channel scaled (calibrated) so that DN 0 = -273°C and DN 1023 ~50°C • MODIS: 12-bit (212 = 4096 DN levels)• BUT precision is NOT accuracy
– can be very precise AND very inaccurate– so more bits doesn’t mean more accuracy
• Radiometric accuracy designed with application and data size in mind – more bits == more data to store/transmit/process
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Radiometric resolution: examples
UCL DEPARTMENT OF GEOGRAPHY
• Coverage (hence angular &/or temporal sampling) due to combination of orbit and swath– Mostly swath - many orbits nearly same
• MODIS and Landsat have identical orbital characteristics: inclination 98.2°, h=705km, T = 99mins BUT swaths of 2400km and 185km hence repeat of 1-2 days and 16 days respectively
– Most EO satellites typically near-polar orbits with repeat tracks every 16 or so days
– BUT wide swath instrument can view same spot much more frequently than narrow
• Tradeoffs again, as a function of objectives
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Summary: angular, temporal resolution
UCL DEPARTMENT OF GEOGRAPHY
• Number of bits per pixel– more bits, more precision (not accuracy)– but more data to store, transmit, process– most EO data typically 8-12 bits (in raw form)