Sensors: 2

1
Sensors 2

• Dr. Mathias (Mat) Disney
• UCL Geography
• Office 301, 3rd Floor, Chandler House
• Tel [phone] (x24290)
• Email mdisney_at_ucl.geog.ac.uk
• www.geog.ucl.ac.uk/mdisney

2

Recap

• Last week 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

3

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/ind
ex.html
4

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
• Months to decades

Useful link http//www.earth.nasa.gov/science/ind
ex.html
5

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

6

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?

7

Ellipse

• Flattened circle
• 2 foci and 2 axes major and minor
• Distance r1r2 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
8

Keplers laws

• Keplers Laws
• deduced from Brahes data after his death
• see nice Java applet http//www-groups.dcs.st-and.
  ac.uk/history/Java/Ellipse.html
• Keplers 1st law
• Orbits of planets are elliptical, with sun at one
  focus

Fromhttp//csep10.phys.utk.edu/astr161/lect/histo
ry/kepler.html
9

Keplers laws

• Keplers 2nd law
• line joining planet to sun sweeps out equal areas
  in equal times

Fromhttp//csep10.phys.utk.edu/astr161/lect/histo
ry/kepler.html
10

Keplers laws

• Keplers 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

Fromhttp//csep10.phys.utk.edu/astr161/lect/histo
ry/kepler.html
11

Orbits examples

• Orbital period for a given instrument and height?
  
• Gravitational force Fg GMEms/RsE2
• where 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

Fromhttp//csep10.phys.utk.edu/astr161/lect/histo
ry/kepler.html
12

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?(REh)3/GME1/2
• Example polar orbiter period, if h 705x103m
• T 2?(6.38x106 705x103)3 / (6.67x10-115.983x10
  24)1/2
• T 5930.6s 98.8mins
• Example geostationary altitude? T ??
• Rearranging h (GME /4?2)T2 1/3 - RE
• So h (6.67x10-115.983x1024 /4?2)(246060)2
  1/3 - 6.38x106
• h 42.2x106 - 6.38x106 35.8km

13

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 (r1), l
  circumference 2?r 2?
• So, 360 2? radians

14

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/
  

15

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
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Geostationary

• METEOSAT - whole earth disk every 15 mins

From http//www.goes.noaa.gov/f_meteo.html
17

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
18

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/englis
h/anatomy/orbit/
19

(near) Polar orbits NASA Terra
From http//visibleearth.nasa.gov/cgi-bin/viewreco
rd?134
20

Near-polar orbits Landsat

• inclination 98.2?, T 98.8mins
• http//www.cscrs.itu.edu.tr/page.en.php?id51
• http//landsat.gsfc.nasa.gov/project/Comparison.ht
  ml

From http//www.iitap.iastate.edu/gccourse/satelli
te/satellite_lecture_new.html
http//eosims.cr.usgs.gov5725/DATASET_DOCS/landsa
t7_dataset.html
21

(near) Polar orbits

• Disadvantages
• need to launch to precise altitude and orbital
  inclination
• orbital decay
• at LEOs (Low Earth Orbits) lt 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/englis
h/anatomy/orbit/
22

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 Earths shadow (day/night border)
• allows solar panels to be kept in sunlight all
  the time)

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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/
24

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

25

Instrument swath

• Swath describes ground area imaged by instrument
  during overpass

26

MODIS on-board Terra
From http//visibleearth.nasa.gov/cgi-bin/viewreco
rd?130
27

Terra instrument swaths compared
From http//visibleearth.nasa.gov/Sensors/Terra/
28

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 BRDF sampling)
• Rapid repeat time

29

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/
30

AVHRR global coverage

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

From http//edc.usgs.gov/guides/avhrr.html
31

POLDER (RIP!)

• Polarisation and Directionality of Earths
  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 00.5
and long of 00.5 (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
32

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

33

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/
34

IKONOS QuickBird very local view!
35

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
36

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.

37

Angular sampling broad swath

• MODIS and SPOT-VGT polar plots
• http//www.soton.ac.uk/epfs/methods/polarplot.sht
  ml
• Reasonable sampling BUT mostly across principal
  plane (less angular info.)
• Is this good sampling of BRDF

38

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

39

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)

40

Angular sampling narrow swath

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

41

Angular sampling combinations?

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

42

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...

43

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

44

Radiometric resolution

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

45

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.fr8100/cdrom/ceos1/irsd/pag
es/dre4.htm
46

Radiometric resolution examples

• Landsat MSS 7bits, TM 8bits
• AVHRR 10-bit (210 1024 DN levels)
• TIR channel scaled (calibrated) so that DN 0
  -273C and DN 1023 50C
• MODIS 12-bit (212 4096 DN levels)
• BUT precision is NOT accuracy
• can be very precise AND very inaccurate
• so more bits doesnt mean more accuracy
• Radiometric accuracy designed with application
  and data size in mind
• more bits more data to store/transmit/process

47

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, h705km, 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

48

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