msc remote sensing 2006-7 principles of remote sensing 5: resolution ii angular/temporal dr. hassan...

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


• 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


• 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


• 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?


• 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


• 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


• 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


• 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


• 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


• 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


• 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


• 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


• 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


• 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


• METEOSAT - whole earth disk every 15 mins

Geostationary

From http://www.goes.noaa.gov/f_meteo.html


• 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


• 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


• 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


• 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?


http://www.edu-observatory.org/gps/tracking.html

• Swath describes ground area imaged by instrument during overpass

Instrument swath

one sample

two samples

three samples

satellite ground swath

direction of travel


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/


• 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


MODIS: building global picture


• Note across-track “whiskbroom” type scanning mechanism

• swath width of 2330km (250-1000m resolution)

• Hence, 1-2 day repeat cycle


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


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.


• 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


Landsat: local view


•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°)


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/


• 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


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)


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


• 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.......


Angular sampling: narrow swath

• ATSR-2 and MISR polar plots• Better sampling in principal plane (more angular info.)• MISR has 9 cameras


Angular sampling: combinations?

• MODIS AND MISR: better sampling than either individually• Combine observations to sample BRDF more effectively


• 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


• 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


• Analogue– continuous measurement levels– film cameras– radiometric sensitivity of film emulsion

• Digital– discrete measurement levels– solid state detectors (e.g. semiconductor CCDs)



• 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)


• 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


• 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


• 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


msc remote sensing 2006-7 principles of remote sensing 5: resolution ii angular/temporal dr. hassan...

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