1

Remote Sensing and Image Processing: 8

Dr. Mathias (Mat) DisneyUCL Geography

Office: 301, 3rd Floor, Chandler HouseTel: 7670 4290 (x24290)

Email: mdisney@geog.ucl.ac.ukwww.geog.ucl.ac.uk/~mdisney

2

Recap

• Last week introduced– spatial and spectral resolution– narrow v broad band tradeoffs....

• This week– temporal and angular resolution– orbits and sensor swath

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/index.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/index.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

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

8

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

9

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/P2

2 = R13/R2

3 i.e. orbital period increases dramatically with R

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

10

Orbital pros and cons

• Geostationary?– Circular orbit in the equatorial plane, altitude ~36,000km– Orbital period, T?

• 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/

11

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)

METEOSAT 0° WGOES-W 135° WGOES-E 75° W GMS 140° EIODC 63° E

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

12

Geostationary

• METEOSAT - whole earth disk every 15 mins

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

13

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

14

Polar & near polar orbits

• Advantages– full polar orbit inclined 90 to equator

• typically few degrees off so poles not covered• orbital period, T, 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/

15

(near) Polar orbits: NASA Terra

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

16

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

17

(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/

18

• Swath describes ground area imaged by instrument during overpass

Instrument swath

one sample

two samples

three samples

satellite ground swath

direction of travel

19

MODIS on-board Terra

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

20

Terra instrument swaths compared

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

21

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

22

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/

23

MODIS: building global picture

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

24

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

25

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/

26

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/

27

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