2009 10 CEGEG 046 GEOG 3051 Principles Practice

2009 -10 CEGEG 046 / GEOG 3051 Principles & Practice of Remote Sensing (PPRS) 5: resolution II: angular/temporal Dr. Mathias (Mat) Disney UCL Geography Office: 113, Pearson Building Tel: 7670 05921 Email: mdisney@ucl. geog. ac. uk www. geog. ucl. ac. uk/~mdisney

Recap • Previously introduced – spatial and spectral resolution – narrow v broad band tradeoffs. . – signal to noise ratio • This week – temporal and angular sampling and/or resolution – REMEMBER: sampling NOT same as resolution, but sometimes used interchagngeably – orbits and sensor swath – radiometric resolution 2

Temporal sampling/resolution • 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: //nasascience. nasa. gov/earth-science 3

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: //nasascience. nasa. gov/earth-science 4

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 5

Orbits and swaths • Orbital characteristics – orbital mechanics developed by Johannes Kepler (1571 -1630), German mathematician – Explained observations of Danish nobleman Tyco Brahe (15461601) – Kepler favoured elliptical orbits (from Copernicus’ solar-centric system) • Properties of ellipse? 6

Ellipse • Flattened circle – – 2 foci and 2 axes: major and minor Distance r 1+r 2 = constant = 2 a (major axis) “Flatness” of ellipse defined by eccentricity, e = 1 -b 2/a 2 = c/a i. e. e is position of the focus as a fraction of the semimajor axis, a f 1 r 2 C f 2 2 b minor axis r 1 Increasing eccentricity • ecircle = 0 2 c 2 a major axis • As e 1, c a and ellipse becomes flatter From http: //mathworld. wolfram. com/Ellipse. html 7

Kepler’s laws • Kepler’s Laws – deduced from Brahe’s data after his death – see nice Java applet http: //www-groups. dcs. stand. ac. uk/~history/Java/Ellipse. html • Kepler’s 1 st law: – Orbits of planets are elliptical, with sun at one focus From: http: //csep 10. phys. utk. edu/astr 161/lect/history/kepler. html 8

Kepler’s laws • Kepler’s 2 nd law – line joining planet to sun sweeps out equal areas in equal times From: http: //csep 10. phys. utk. edu/astr 161/lect/history/kepler. html 9

Kepler’s laws • Kepler’s 3 rd law – “ratio of the squares of the revolutionary periods for two planets (P 1, P 2) is equal to the ratio of the cubes of their semimajor axes (R 1, R 2)” – P 12/P 22 = R 13/R 23 • i. e. orbital period increases dramatically with R • Convenient unit of distance is average separation of Earth from Sun = 1 astronomical unit (A. U. ) – – 1 A. 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 10

Orbits: examples • Orbital period for a given instrument and height? – Gravitational force Fg = GMEms/Rs. E 2 • G is universal gravitational constant (6. 67 x 10 -11 Nm 2 kg 2); ME is Earth mass (5. 983 x 1024 kg); ms is satellite mass (? ) and Rs. E is distance from Earth centre to satellite i. e. 6. 38 x 106 + h where h is satellite altitude – Centripetal (not centrifugal!) force Fc = msvs 2/Rs. E • where vs is linear speed of satellite (= s. Rs. E where is the satellite angular velocity, rad s-1) – for stable (constant radius) orbit Fc = Fg – GMEms/Rs. E 2 = msvs 2/Rs. E = ms s 2 Rs. E 2 /Rs. E – so s 2 = GME /Rs. E 3 11

Orbits: examples • Orbital period T of satellite (in s) = 2 / – (remember 2 = one full rotation, 360°, in radians) – and Rs. E = RE + h where RE = 6. 38 x 106 m – So now T = 2 [(RE+h)3/GME]1/2 • Example: polar orbiter period, if h = 705 x 103 m – T = 2 [(6. 38 x 106 +705 x 103)3 / (6. 67 x 10 -11*5. 983 x 1024)]1/2 – T = 5930. 6 s = 98. 8 mins • Example: altitude for geostationary orbit? T = ? ? – Rearranging: h = [(GME /4 2)T 2 ]1/3 - RE – So h = [(6. 67 x 10 -11*5. 983 x 1024 /4 2)(24*60*60)2 ]1/3 - 6. 38 x 106 – h = 42. 2 x 106 - 6. 38 x 106 = 35. 8 km 12

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 l r 13

Orbital pros and cons • Geostationary? – Circular orbit in the equatorial plane, altitude ~36, 000 km – 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/ 14

Geostationary • Meteorological satellites - combination of GOES-E, GOES-W, METEOSAT (Eumetsat), GMS (NASDA), IODC (old Meteosat 5) – GOES 1 st gen. (GOES-1 - ‘ 75 GOES-7 ‘ 95); 2 nd gen. (GOES-8++ ‘ 94) GOES-E 75° W GOES-W 135° W METEOSAT 0° W From http: //www. sat. dundee. ac. uk/pdusfaq. html IODC 63° E GMS 140° E 15

Geostationary • METEOSAT - whole earth disk every 15 mins From http: //www. goes. noaa. gov/f_meteo. html 16

Geostationary orbits • Disadvantages – typically low spatial resolution due to high altitude – e. g. METEOSAT 2 nd Generation (MSG) 1 x 1 km visible, 3 x 3 km IR (used to be 3 x 3 and 6 x 6 respectively) • MSG has SEVIRI and GERB instruments • http: //www. eumetsat. int/Home/Main/What_We_Do/Satellites/Meteosat_Sec ond_Generation/Space_Segment/SP_1119959405658? l=en – 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 17

Polar & near polar orbits • Advantages – full polar orbit inclined 90 to equator • typically few degrees off so poles not covered • orbital period typically 90 - 105 mins – near circular orbit between 300 km (low Earth orbit) and 1000 km – 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 / 18

(near) Polar orbits: NASA Terra From http: //visibleearth. nasa. gov/cgi-bin/viewrecord? 134 19

Near-polar orbits: Landsat – inclination 98. 2 , T = 98. 8 mins – – http: //www. cscrs. itu. edu. tr/page. en. php? id=51 http: //landsat. gsfc. nasa. gov/project/Comparison. html • ASIDE: repeat time • Orbital period is 5928 s • So in this time Earth surface moves l = r*(2 *5928/(24*60*60)) • So if r = 6. 38 x 106 then l = 2750 km From http: //www. iitap. iastate. edu/gccourse/satellite_lecture_new. html & http: //eosims. cr. usgs. gov: 5725/DATASET_DOCS/landsat 7_dataset. html 20

(near) Polar orbits • Disadvantages – need to launch to precise altitude and orbital inclination – orbital decay • at LEOs (Low Earth Orbits) < 1000 km, drag from atmosphere • causes orbit to become more eccentric • Drag increases with increasing solar activity (sun spots) - during solar maximum (~11 yr cycle) drag height increased by 100 km! – Build your own orbit: http: //lectureonline. cl. msu. edu/~mmp/kap 7/orbiter/orbit. htm From http: //collections. ic. gc. ca/satellites/english/anatomy/orbit/ 21

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 ~ 10 am) – 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) 22

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: 366 km), apoapsis (far point: 3881 km) – crosses equator several times daily – Flyby of Hurrican Frances (24/8/04) – iso-surface From http: //trmm. gsfc. nasa. gov/ 23

Orbital location? • TLEs (two line elements) – http: //www. satobs. org/element. html e. g. PROBA 1 1 26958 U 01049 B 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 24

Instrument swath • Swath describes ground area imaged by instrument during overpass direction of travel satellite ground swath one sample two samples three samples 25

MODIS on-board Terra From http: //visibleearth. nasa. gov/cgi-bin/viewrecord? 130 26

Terra instrument swaths compared From http: //visibleearth. nasa. gov/Sensors/Terra/ 27

Broad swath • MODIS, POLDER, AVHRR etc. – – swaths typically several 1000 s 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 28

MODIS: building global view • • • Note across-track “whiskbroom” type scanning mechanism swath width of 2330 km (250 -1000 m resolution) Hence, 1 -2 day repeat cycle From http: //visibleearth. nasa. gov/Sensors/Terra/ 29

AVHRR: global view • • 2400 km swath, 1. 1 km pixels at nadir, but > 5 km at edge of swath Repeats 1 -2 times per day From http: //edc. usgs. gov/guides/avhrr. html 30

POLDER (RIP!): global view • Polarisation and Directionality of Earth’s Reflectance – FOV ± 43° along track, ± 51° across track, 9 cameras, 2400 km swath, 7 x 6 km resn. at nadir – POLDER I 8 months, POLDER II 7 months. . Each set of points corresponds to given viewing zenith and azimuthal angles for nearsimultaneous 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-lille 1. fr/~riedi/BROWSES/200304/16/index. html 31

Narrow swath • Landsat TM/MSS/ETM+, IKONOS, Quick. Bird etc. – – – swaths typically few 10 s to 100 skm higher spatial resolution local to regional coverage NOT global far less overlap (particularly at lower latitudes) May have to wait weeks/months for revisit 32

Landsat: regional to local view • 185 km swath width, hence 16 -day repeat cycle (and spatial res. 25 m) • 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/ 33

IKONOS & Quick. Bird: very local view! • IKONOS: 11 km swath at nadir, 1 m panchromatic, 4 m multispectral • Quick. Bird: 16. 5 km swath at nadir, 61 cm! panchromatic, 2. 44 m multispectral • http: //www. spaceimaging. com/ • http: //www. digitalglobe. com 34

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/ERS 1. 1. 7. html 35

So. . . angular resolution • Wide swath instruments have large overlap – e. g. MODIS 2330 km ( 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? • See Barnsley et al (1997) paper – Information in angular signal we can exploit! – Or remove BRDF effects when combining observations from different times/angles – More samples of viewing/illum. hemisphere gives more info. 36

Angular (BRDF) effects • • Can look like noise over time BUT plotted as a function of angle we see BRDF effect So must account for BRDF if we want to look at changes over time 37

Angular sampling: broad swath relative azimuth (view - solar) view zenith Solar principal plane • MODIS and SPOT-VGT: polar plots – • • Cross solar principal plane 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 38

Angular sampling: broad swath • • POLDER I ! Broad swath (2200 km) AND large 2 D 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 ~1 km pixel – remember l = r so angle (in rads) = r/l where r this time is 705 km and l ~ 1 km so angular res ~ 1. 42 x 10 -6 rads at nadir – at edge of swath ~5 km pixel so angular res ~ 7 x 10 -6 rads • SAMPLING more important/meaningful than resolution in angular sense (as for temporal) 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); 2 bits (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/ceos 1/irsd/pages/dre 4. htm 46

Radiometric resolution: examples • Landsat: MSS 7 bits, TM 8 bits • 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 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°, h=705 km, T = 99 mins BUT swaths of 2400 km and 185 km 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 49

ASIDE: 2 nd ESA Explorer launched 2/11/09 • SMOS (Soil Moisture and Ocean Salinity) probe – Interferometric radiometer – Global maps of soil moisture every three days within an accuracy of 4% at a spatial resolution of 50 km – Global maps of sea-surface salinity down to 0. 1 practical salinity units for a 30 -day average over an area of 200× 200 km • http: //www. esa. int/SPECIALS/smos/SEMNEYAOE 1 G_0. html • AND Proba-2 (PRoject for On-Board Autonomy • SEMINAR: tomorrow (5 th) at 5 pm in room 305 50