msc remote sensing 2006-7 principles of remote sensing 5: resolution ii angular/temporal dr. hassan...
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MSc Remote Sensing 2006-7Principles of Remote Sensing 5: resolution II angular/temporal
Dr. Hassan J. Eghbali
• Previously introduced– spatial and spectral resolution– narrow v broad band tradeoffs....– signal to noise ratio
• This week– temporal and angular resolution– orbits and sensor swath– radiometric resolution
Recap
Dr. Hassan J. Eghbali
• Single or multiple observations• How far apart are observations in time?
– One-off, several or many?
• Depends (as usual) on application– Is it dynamic?– If so, over what timescale?
Temporal
Useful link: http://www.earth.nasa.gov/science/index.html
Dr. Hassan J. Eghbali
• Examples– Vegetation stress monitoring, weather, rainfall
• hours to days
– Terrestrial carbon, ocean surface temperature• days to months to years
– Glacier dynamics, ice sheet mass balance, erosion/tectonic processes
• Months to decades
Temporal
Useful link: http://www.earth.nasa.gov/science/index.html
Dr. Hassan J. Eghbali
• Sensor orbit– geostationary orbit - over same spot
• BUT distance means entire hemisphere is viewed e.g. METEOSAT
– polar orbit can use Earth rotation to view entire surface
• Sensor swath– Wide swath allows more rapid revisit
• typical of moderate res. instruments for regional/global applications
– Narrow swath == longer revisit times• typical of higher resolution for regional to local applications
What determines temporal sampling?
Dr. Hassan J. Eghbali
• Orbital characteristics – orbital mechanics developed by Johannes Kepler (1571-1630),
German mathematician– Explained observations of Danish nobleman Tyco Brahe (1546-
1601)– Kepler favoured elliptical orbits (from Copernicus’ solar-centric
system)
• Properties of ellipse?
Orbits and swaths
Dr. Hassan J. Eghbali
• 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
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
Dr. Hassan J. Eghbali
• Kepler’s Laws – deduced from Brahe’s data after his death
– see nice Java applet http://www-groups.dcs.st-and.ac.uk/~history/Java/Ellipse.html
• Kepler’s 1st law: – Orbits of planets are elliptical, with sun at one focus
Kepler’s laws
From:http://csep10.phys.utk.edu/astr161/lect/history/kepler.html
Dr. Hassan J. Eghbali
• Kepler’s 2nd law – line joining planet to sun sweeps out equal areas in equal times
Kepler’s laws
From:http://csep10.phys.utk.edu/astr161/lect/history/kepler.html
Dr. Hassan J. Eghbali
• 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)”
– P12/P2
2 = R13/R2
3
• 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
Kepler’s laws
Dr. Hassan J. Eghbali
• Orbital period for a given instrument and height? – Gravitational force Fg = GMEms/RsE
2
• 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/RsE2 = msvs
2/RsE = ms s2RsE
2 /RsE
– so s2 = GME /RsE
3
Orbits: examples
Dr. Hassan J. Eghbali
• 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 = 35.8km
Orbits: examples
Dr. Hassan J. Eghbali
• 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
Orbits: aside
r
l
Dr. Hassan J. Eghbali
• 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/
Orbital pros and cons
Dr. Hassan J. Eghbali
• 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)
Geostationary
From http://www.sat.dundee.ac.uk/pdusfaq.html
METEOSAT 0° WGOES-W 135° WGOES-E 75° W GMS 140° EIODC 63° E
Dr. Hassan J. Eghbali
• METEOSAT - whole earth disk every 15 mins
Geostationary
From http://www.goes.noaa.gov/f_meteo.html
Dr. Hassan J. Eghbali
• 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.meteo.pt/landsaf/eumetsat_sat_char.html
– 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
Geostationary orbits
From http://www.esa.int/SPECIALS/MSG/index.html
Dr. Hassan J. Eghbali
• 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
Polar & near polar orbits
From http://collections.ic.gc.ca/satellites/english/anatomy/orbit/Dr. Hassan J. Eghbali
(near) Polar orbits: NASA Terra
From http://visibleearth.nasa.gov/cgi-bin/viewrecord?134Dr. Hassan J. Eghbali
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
– 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
Dr. Hassan J. Eghbali
• 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: http://lectureonline.cl.msu.edu/~mmp/kap7/orbiter/orbit.htm
(near) Polar orbits
From http://collections.ic.gc.ca/satellites/english/anatomy/orbit/Dr. Hassan J. Eghbali
• 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)
Types of near-polar orbit
Dr. Hassan J. Eghbali
• Inclination much lower– orbits close to equatorial
– useful for making observations solely over tropical regions
• Example– TRMM - Tropical Rainfall Measuring Mission– Orbital inclination 35.5°, periapsis (near point: 366km), apoapsis (far point:
3881km)– crosses equator several times daily– Flyby of Hurrican Frances (24/8/04)– iso-surface
Near-ish: Equatorial orbit
From http://trmm.gsfc.nasa.gov/Dr. Hassan J. Eghbali
• TLEs (two line elements)– http://www.satobs.org/element.html e.g.
PROBA 1
1 26958U 01049B 04225.33423432 .00000718 00000-0 77853-4 0 2275
2 26958 97.8103 302.9333 0084512 102.5081 258.5604 14.88754129152399
• 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
Orbital location?
Dr. Hassan J. Eghbali
• Swath describes ground area imaged by instrument during overpass
Instrument swath
one sample
two samples
three samples
satellite ground swath
direction of travel
Dr. Hassan J. Eghbali
MODIS on-board Terra
From http://visibleearth.nasa.gov/cgi-bin/viewrecord?130Dr. Hassan J. Eghbali
Terra instrument swaths compared
From http://visibleearth.nasa.gov/Sensors/Terra/
Dr. Hassan J. Eghbali
• 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
Broad swath
Dr. Hassan J. Eghbali
MODIS: building global picture
From http://visibleearth.nasa.gov/Sensors/Terra/
• Note across-track “whiskbroom” type scanning mechanism
• swath width of 2330km (250-1000m resolution)
• Hence, 1-2 day repeat cycle
Dr. Hassan J. Eghbali
AVHRR: global coverage
From http://edc.usgs.gov/guides/avhrr.html
• 2400km swath, 1.1km pixels at nadir, but > 5km at edge of swath
• Repeats 1-2 times per day
Dr. Hassan J. Eghbali
POLDER (RIP!)
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 time
From 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.
Dr. Hassan J. Eghbali
• 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
Narrow swath
Dr. Hassan J. Eghbali
Landsat: local view
From http://visibleearth.nasa.gov/Sensors/Terra/
•185km swath width, hence 16-day repeat cycle (and spatial res. 25m)
•Contiguous swaths overlap (sidelap) by 7.3% at the equator
•Much greater overlap at higher latitudes (80% at 84°)
Dr. Hassan J. Eghbali
IKONOS & QuickBird: very local view!
•QuickBird: 16.5km swath at nadir, 61cm! panchromatic, 2.44m multispectral
•http://www.digitalglobe.com
•IKONOS: 11km swath at nadir, 1m panchromatic, 4m multispectral
•http://www.spaceimaging.com/
Dr. Hassan J. Eghbali
• ERS 1 & 2– ATSR instruments, RADAR altimeter, Imaging SAR (synthetic aperture
RADAR) etc.
– ERS 1: various mission phases: repeat times of 3 (ice), 35 and 168 (geodyssey) days
– ERS 2: 35 days
Variable repeat patterns
From http://earth.esa.int/rootcollection/eeo/ERS1.1.7.htmlDr. Hassan J. Eghbali
• Wide swath instruments have large overlap– e.g. MODIS 2330km (55), so up to 4 views per day at different
angles!
– AVHRR, SPOT-VGT, POLDER I and II, etc.
– Why do we want good angular sampling?• Remember BRDF?• http://stress.swan.ac.uk/~mbarnsle/pdf/barnsley_et_al_1997.pdf
– Information in angular signal!
– More samples of viewing/illum. hemisphere gives more info.
So.....angular resolution
Dr. Hassan J. Eghbali
Angular sampling: broad swath
• MODIS and SPOT-VGT: polar plots– http://www.soton.ac.uk/~epfs/methods/polarplot.shtml
• Reasonable sampling BUT mostly across principal plane (less angular info.)• Is this “good” sampling of BRDF
Solar principal plane
Cross solar principal plane
view zenith
relative azimuth (view - solar)
Dr. Hassan J. Eghbali
Angular sampling: broad swath
• POLDER I !
• Broad swath (2200km) AND large 2D CCD array gave huge number of samples 43 IFOV along-track and
51 IFOV across-track
Dr. Hassan J. Eghbali
• Is wide swath angular sampling REALLY multi-angular?– Different samples on different days e.g. MODIS BRDF product is
composite over 16 days
– minimise impact of clouds, maximise number of samples
• “True” multi-angular viewing requires samples at same time– need to use several looks e.g. ATSR, MISR (& POLDER)
BUT.......
Dr. Hassan J. Eghbali
Angular sampling: narrow swath
• ATSR-2 and MISR polar plots• Better sampling in principal plane (more angular info.)• MISR has 9 cameras
Dr. Hassan J. Eghbali
Angular sampling: combinations?
• MODIS AND MISR: better sampling than either individually• Combine observations to sample BRDF more effectively
Dr. Hassan J. Eghbali
• Function of swath and IFOV – e.g. MODIS at nadir ~1km pixel
– remember l = r so angle (in rads) = r/l where r this time is 705km and l ~ 1km so angular res ~ 1.42x10-6 rads at nadir
– at edge of swath ~5km pixel so angular res ~ 7x10-6 rads
• Sampling more important/meaningful than resolution in angular sense...
So, angular resolution
Dr. Hassan J. Eghbali
• Had spatial, spectral, temporal, angular.....• Precision with which an instrument records EMR
– i.e. Sensitivity of detector to amount of incoming radiation– More sensitivity == higher radiometric resolution
• determines smallest slice of EM spectrum we can assign DN to
– BUT higher radiometric resolution means more data• As is the case for spatial, temporal, angular etc.
• Typically, radiometric resolution refers to digital detectors– i.e. Number of bits per pixel used to encode signal
Radiometric resolution
Dr. Hassan J. Eghbali
• Analogue– continuous measurement levels– film cameras– radiometric sensitivity of film emulsion
• Digital– discrete measurement levels– solid state detectors (e.g. semiconductor CCDs)
Radiometric resolution
Dr. Hassan J. Eghbali
• 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)
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.htmDr. Hassan J. Eghbali
• Landsat: MSS 7bits, TM 8bits• AVHRR: 10-bit (210 = 1024 DN levels)
– 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
Radiometric resolution: examples
Dr. Hassan J. Eghbali
• 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
Summary: angular, temporal resolution
Dr. Hassan J. Eghbali
• 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)
• Tradeoffs again, as a function of objectives
Summary: radiometric resolution
Dr. Hassan J. Eghbali