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MSc Remote Sensing 2006-7 Principles of Remote Sensing 5: resolution II angular/temporal. Dr. Hassan J. Eghbali. Recap. Previously introduced spatial and spectral resolution narrow v broad band tradeoffs.... signal to noise ratio This week temporal and angular resolution

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msc remote sensing 2006 7 principles of remote sensing 5 resolution ii angular temporal

MSc Remote Sensing 2006-7Principles of Remote Sensing 5: resolution II angular/temporal

Dr. Hassan J. Eghbali

slide2

Recap

  • 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

Dr. Hassan J. Eghbali

slide3

Temporal

  • 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?

Useful link: http://www.earth.nasa.gov/science/index.html

Dr. Hassan J. Eghbali

slide4

Temporal

  • 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

Useful link: http://www.earth.nasa.gov/science/index.html

Dr. Hassan J. Eghbali

slide5

What determines temporal sampling?

  • 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

Dr. Hassan J. Eghbali

slide6

Orbits and swaths

  • 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?

Dr. Hassan J. Eghbali

slide7

r1

r2

2b

minor axis

f2

f1

C

  • ecircle = 0
  • As e 1, c a and ellipse becomes flatter

Increasing eccentricity

2c

2a

major axis

Ellipse

  • 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

From http://mathworld.wolfram.com/Ellipse.html

Dr. Hassan J. Eghbali

slide8

Kepler’s laws

  • 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

From:http://csep10.phys.utk.edu/astr161/lect/history/kepler.html

Dr. Hassan J. Eghbali

slide9

Kepler’s laws

  • Kepler’s 2nd law
    • line joining planet to sun sweeps out equal areas in equal times

From:http://csep10.phys.utk.edu/astr161/lect/history/kepler.html

Dr. Hassan J. Eghbali

slide10

Kepler’s laws

  • 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/P22 = R13/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

Dr. Hassan J. Eghbali

slide11

Orbits: examples

  • Orbital period for a given instrument and height?
    • Gravitational force Fg = GMEms/RsE2
      • 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 = msvs2/RsE = ms s2RsE2 /RsE
    • so s2 = GME /RsE3

Dr. Hassan J. Eghbali

slide12

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 /42)T2 ]1/3 - RE
    • So h = [(6.67x10-11*5.983x1024/42)(24*60*60)2 ]1/3 - 6.38x106
    • h = 42.2x106 - 6.38x106 = 35.8km

Dr. Hassan J. Eghbali

slide13

l

r

Orbits: aside

  • 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 = 2r = 2
    • So, 360° = 2 radians

Dr. Hassan J. Eghbali

slide14

Orbital pros and cons

  • 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/

Dr. Hassan J. Eghbali

slide15

GOES-E 75° W

GOES-W 135° W

METEOSAT 0° W

IODC 63° E

GMS 140° E

Geostationary

  • 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)

From http://www.sat.dundee.ac.uk/pdusfaq.html

Dr. Hassan J. Eghbali

slide16

Geostationary

  • METEOSAT - whole earth disk every 15 mins

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

Dr. Hassan J. Eghbali

slide17

Geostationary orbits

  • 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

From http://www.esa.int/SPECIALS/MSG/index.html

Dr. Hassan J. Eghbali

slide18

Polar & near polar orbits

  • 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

From http://collections.ic.gc.ca/satellites/english/anatomy/orbit/

Dr. Hassan J. Eghbali

slide19

(near) Polar orbits: NASA Terra

From http://visibleearth.nasa.gov/cgi-bin/viewrecord?134

Dr. Hassan J. Eghbali

slide20

Near-polar orbits: Landsat

  • 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

From http://www.iitap.iastate.edu/gccourse/satellite/satellite_lecture_new.html & http://eosims.cr.usgs.gov:5725/DATASET_DOCS/landsat7_dataset.html

Dr. Hassan J. Eghbali

slide21

(near) Polar orbits

  • 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

From http://collections.ic.gc.ca/satellites/english/anatomy/orbit/

Dr. Hassan J. Eghbali

slide22

Types of near-polar orbit

  • 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)

Dr. Hassan J. Eghbali

slide23

Near-ish: Equatorial 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

From http://trmm.gsfc.nasa.gov/

Dr. Hassan J. Eghbali

slide24

Orbital location?

  • 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

Dr. Hassan J. Eghbali

slide25

direction of travel

satellite ground swath

one sample

two samples

three samples

Instrument swath

  • Swath describes ground area imaged by instrument during overpass

Dr. Hassan J. Eghbali

slide26

MODIS on-board Terra

From http://visibleearth.nasa.gov/cgi-bin/viewrecord?130

Dr. Hassan J. Eghbali

slide27

Terra instrument swaths compared

From http://visibleearth.nasa.gov/Sensors/Terra/

Dr. Hassan J. Eghbali

slide28

Broad swath

  • 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

Dr. Hassan J. Eghbali

slide29

MODIS: building global picture

  • Note across-track “whiskbroom” type scanning mechanism
  • swath width of 2330km (250-1000m resolution)
  • Hence, 1-2 day repeat cycle

From http://visibleearth.nasa.gov/Sensors/Terra/

Dr. Hassan J. Eghbali

slide30

AVHRR: global coverage

  • 2400km swath, 1.1km pixels at nadir, but > 5km at edge of swath
  • Repeats 1-2 times per day

From http://edc.usgs.gov/guides/avhrr.html

Dr. Hassan J. Eghbali

slide31

POLDER (RIP!)

  • 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.

From http://www-loa.univ-lille1.fr/~riedi/BROWSES/200304/16/index.html

Dr. Hassan J. Eghbali

slide32

Narrow swath

  • 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

Dr. Hassan J. Eghbali

slide33

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°)

From http://visibleearth.nasa.gov/Sensors/Terra/

Dr. Hassan J. Eghbali

slide34

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/

IKONOS & QuickBird: very local view!

Dr. Hassan J. Eghbali

slide35

Variable repeat patterns

  • 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

From http://earth.esa.int/rootcollection/eeo/ERS1.1.7.html

Dr. Hassan J. Eghbali

slide36

So.....angular resolution

  • 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.

Dr. Hassan J. Eghbali

slide37

relative azimuth (view - solar)

view zenith

Cross solar principal plane

Solar principal plane

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

Dr. Hassan J. Eghbali

slide38

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

slide39

BUT.......

  • 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)

Dr. Hassan J. Eghbali

slide40

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

slide41

Angular sampling: combinations?

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

Dr. Hassan J. Eghbali

slide42

So, angular resolution

  • 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...

Dr. Hassan J. Eghbali

slide43

Radiometric 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

Dr. Hassan J. Eghbali

slide44

Radiometric resolution

  • Analogue
    • continuous measurement levels
    • film cameras
    • radiometric sensitivity of film emulsion
  • Digital
    • discrete measurement levels
    • solid state detectors (e.g. semiconductor CCDs)

Dr. Hassan J. Eghbali

slide45

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

Radiometric resolution

  • 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)

From http://ceos.cnes.fr:8100/cdrom/ceos1/irsd/pages/dre4.htm

Dr. Hassan J. Eghbali

slide46

Radiometric resolution: examples

  • 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

Dr. Hassan J. Eghbali

slide47

Summary: angular, temporal resolution

  • 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

Dr. Hassan J. Eghbali

slide48

Summary: radiometric 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

Dr. Hassan J. Eghbali