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2009-10 CEGEG046 / GEOG3051 Principles & Practice of Remote Sensing (PPRS) 7: scanning redux, photography, lidar Dr. Mathias (Mat) Disney UCL Geography Office: 113, Pearson Building Tel: 7670 0592 Email: [email protected] www.geog.ucl.ac.uk/~mdisney Recap • Last week – storage/transmission – pre-processing stages (raw data to products) – sensor scanning mechanisms • This week – scanning mechanisms redux – photography – time-resolved signals (e.g. LiDAR) 2 Scanning mechanisms: examples • Discrete detectors and scanning mirrors – Landsat MSS, TM, ETM+, NOAA GOES, AVHRR, ATSR • Multispectral linear arrays – SPOT (1-3) HRV, HRVIR & SPOT-VGT, IKONOS, ASTER & MISR (both on board NASA Terra) • Imaging spectrometers using linear and area arrays – AVIRIS, CASI, MODIS (on NASA Terra and Aqua) From: http://ceos.cnes.fr:8100/cdrom/ceos1/irsd/pages/datacq4.htm & Jensen (2000) 3 Scanning mechanisms: examples •MODIS scan mirror http://modis.gsfc.nasa.gov/about/scanmirror.html •Continuously rotating and double-sided •SEVIRI (Spinning Enhanced Vis and IR Imager) on board MSG •Whole satellite rotates •Vertical scan plus rotation = image 4 Scanning mechanisms: continued • Image frame created by scanning detector footprint •n pixels per line, pixel size r * r nr •Along track speed v ms-1 so footprint travels distance r in r/v secs line r pixel • One line of data must be acquired in <= r/v secs •Typical v? •Orbital period T ~ 100 mins, Earth radius ~ 6.4x103m Frame •v = 2*6.4x103 / 100*60 = 6.7x103ms-1 Across track 5 Scanning mechanisms: single detector • Even if we obtain 1 line in r/v secs say..... • Significant along-track displacement from start to end of x-track scan line X-track scan (whiskbroom) Start rv Platform has moved r in rv secs 6 Scanning mechanisms: single detector • Zig-zag mechanism – active scan lasts r/2v secs – n pixels per line, so “dwell time” (seconds per pixel) is r/2nv secs/pixel – ok for low res e.g. AVHRR, as large r – But problems for mod - high res. – E.g. Landsat MSS, r = 70m, v = 7x103ms-1 n=3000 so dwell time = 70/2*3000*7x103 = 1.7secs (OK for SNR) – BUT with single detector, required length of scan cycle r/v is 10msecs (70/7x103) – = 100 scan cycles per second – TOO FAST! Active scan r/2 r flyback Speed, v 7 Scanning mechanisms: e.g. MSS • MSS has 4x6 array of receptors - 4 bands, 6 receptors per band • 6 lines scanned simultaneously – ‘footprint’ of single receptor follows a zig-zag track – ~30 cycles per second T=0 WEST T = 53ms Active scan 474m EAST T = 73.4ms 185km (swath width) 8 Scanning mechanisms: boustrophedon Active • Alternative right left, left right – 2 n line pixels scanned in r/v secs – r/2nv secs/pixel – For TM for e.g. r = 30m v = (20/3) x 103ms1 n = 6000 – dwell time 0.38 sec (not long enough for good SNR) – scan cycle ~4.5 msecs (~220 per second) – Way too fast i.e. single detector operation inadequate for TM – use 6 detectors per band (vis), and 16 lines at a time in vis, 4 at a time in thermal – 100 detectors total From: http://rst.gsfc.nasa.gov/Intro/Part2_20.html r Active Active Speed, v 9 Photography • Largely obsolete due to electromechanical sensors • Still used for – some mapping and monitoring applications • partic. aerial surveys and photogrammetry – BUT requirement to get film back and process it – Pan-chromatic (B&W) and colour (vis and some IR) but limited spectrally – Radial image distortion away from focal point • Relatively easy to correct if camera geometry known 10 Photography – E.g. Wild RC10 aerial camera + tracker software as used by NERC Airborne Research and Survey Facility – www.nerc.ac.uk/arsf – Software allows pilot to gauge coverage and overlap 11 Photography • • • • AP of Barton Bendish, Norfolk Acquired 1997 by NERC aircraft Scan of original Note flight info and fiducial marks @ corners 12 Photography: parameters • Photographic camera uses whole-frame image capture – near instantaneous snapshot of projected field-of view on ground – i.e. IFOV == whole FOV – Imaged region (A) focused by lens/mirror system onto focal plane (C) – Spectral sensitivity from 0.3 to 0.9m i.e. Uv/vis/NIR From: http://www.ccrs.nrcan.gc.ca/ccrs/learn/tutorials/fundam/chapter2/chapter2_7_e.html 13 Photography: parameters •Large and small apertures in camera system •aperture compared to diameter of lens From:http://webchat.chatsystems.com/~doswell/Outdoor_Images/Photo_Basics.html 14 Photography: parameters •Focal length of photographic system •pros and cons •Amount of light v. depth of field From:http://webchat.chatsystems.com/~doswell/Outdoor_Images/Photo_Basics.html 15 Optical mechanisms: e.g. MSS • MSS optical system uses reflecting (Cassegrain) telescope – Lens with hole in centre (concave) – Convex focusing mirror Detector plane Principal plane 23cm 9cm diam mirror f = 82cm Equivalent to a lens of focal length 82cm 16 Photography: parameters • Normally adjust 4 parameters – focus - by altering position of focusing lens relative to focal plane – F-stop (f-number), defined as f/d i.e. Focal length / effective diameter of lens opening – Shutter speed • e.g. 1/2000, 1/1000, 1/500 .... 1/2, 1/1, 2/1, 4/1 seconds • Faster shutter = less motion blur, but less light – Film “speed” - exposure level over which film responds (ISO/ASA number) • Faster film responds to lower light BUT poorer spatial resolution • ISO 25-100 (slow), 200-1000 (faster) 17 Photography: parameters • General film exposure equation – E = exposure in Joules (J) mm-2, s = intrinsic scene brightness, in J mm-2s-1, d = diameter of lens opening in mm, t = time in seconds, f = lens focal length, mm – So E is measure of recorded energy • E increases with d2 , s and t • E decreases with f2 – Note that any lens system diffraction limited i.e. can’t resolve objects smaller than s/D • s = distance of object from object-side focal point; D = demagnification (Altitude/focal length i.e. D = 1/magnification = 1/s/f = f/s) 18 Photography • Historical archives of photography – many military applications now declassified – e.g. Surveillance (U2, Cuba, Bay of Pigs.....) – Vietnam, N. Korea etc. etc. ? From: Dr. S. Lewis, PhD thesis, 2003 UCL. 19 Time-resolved signals: LIDAR • Light Detection And Ranging – – – – optical wavelength analogue of RADAR active remote sensing used for laser altimetry (height measurement) but also other information Why use optical??? • Velocity of light ~ 3x108 ms-1 – one light year = 9.46 × 1015 m (10 trillion m) – used for cosmological distances BUT also useful for smaller distances – Light travels ~ 30cm in 1 nanosecond (10-9s) 20 Time-resolved signals: LIDAR • So for LIDAR – – • range of target from sensor (and source) is time of round trip for a pulse of light return pulse very weak (function of surface reflectance) & (usually) spread out LIDAR – – – – laser light from source (coherent - narrow range of wavelengths) - typically 670-700nm Spreads out as it is a wave (e.g. 10 to 100m spots on surface) Roughness variation within spot (IFOV) mean energy returns sooner from some bits than others Needs short, powerful laser pulses • safety? From: http://earthobservatory.nasa.gov/Library/VCL/VCL_2.html 21 LIDAR missions? • SLICER – Scanning Lidar Imager of Canopies by Echo Recovery – http://denali.gsfc.nasa.gov/research/laser/slicer/s licer.html • MOLA – Mars Orbital LIDAR altimeter on Mars Global Surveyor – V. Accurate info on Martian topography – Clues to geological formation • GLAS – Geoscience Laser Altimeter System on IceSAT • Altimetry uses only first and last return signal 22 ICESat (aka: Laser Altimetry Mision) The Ice, Cloud, and Elevation Satellite • Launched Jan 12, 2003 – Jan 15, 2003 Earth pointing • Measures – – – – – – • • ice sheet elevations changes in elevation through time height profiles of clouds and aerosols land elevations vegetation cover approximate sea-ice thickness. Geoscience Laser Altimeter System (GLAS) - sole instrument Combinination surface lidar with dual wavelength cloud and aerosol lidar Images and info from http://icesat.gsfc.nasa.gov/ 23 Waveform LIDAR • If we can resolve more than just first/last return – record shape of returning waveform? – Waveform LIDAR – Contains information about e.g. Vegetation canopy structure – Requires v. accurate timing information – Again, typically green or red From:http://denali.gsfc.nasa.gov/research/laser/slicer/slicer.html 24 E.g. First/last return LIDAR data • Structural information from LIDAR • Possibly in situ laser scanning • Information? – Canopy height – Canopy gap fraction and vertical profile of foliage 25 E.g. Waveform LIDAR data • Canopy height AND density information – intensity of return related to density – from http://ltpwww.gsfc.nasa.gov/eib/projects/airborne_lidar/slicer.html 26 Time-resolved signals: LIDAR • VCL didn’t get launched – NASA budget cuts – http://earthobservatory.nasa.gov/Library/VCL/VCL.html – http://www.geog.umd.edu/vcl/vcltext.html • But being applied in airborne projects – rapid way to generate information on standing biomass – Wood volume per hectare • Used in carbon studies • useful for forestry, inventory etc. etc. From: http://earthobservatory.nasa.gov/Library/VCL/VCL_2.html 27 E.g. Waveform LIDAR modelling • Use Monte Carlo Ray Tracing to model LIDAR signal of GLAS ICEsat • Images courtesy of U. Heyder 28 Ground-based laser scanning? • Tripod-mounted LIDAR – developed for surveying – BUT has uses for collecting information on forest density and structure – Typically records point cloud from several known locations then use software to reconstruct scene in 3D From: http://www.geospatialonline.com/geospatialsolutions/article/articleDetail.jsp?id=65014&pageID=4 29 The next generation! ECHIDNA • Scanning (multi-beam) ground-based LIDAR – Developed by Jupp et al. at CSIRO (Aus.) specifically for vegetation From talk by D. Jupp at ISPMSRS, Beijing, October 17-19 2005. 30 ECHIDNA From talk by D. Jupp at ISPMSRS, Beijing, October 17-19 2005. 31 ECHIDNA •Generalise hemispherical information •But much more than for photography (discriminate canopy compnents) From talk by D. Jupp at ISPMSRS, Beijing, October 17-19 2005. 32 ECHIDNA From talk by D. Jupp at ISPMSRS, Beijing, October 17-19 2005. 33 LIDAR sounding (up/down) • For studying atmospheric aerosols, clouds etc. – – – – – Use backscatter properties of atmosphere e.g. LITE (1994 Shuttle mission) Upward looking? e.g. ELF Coherence of laser gives narrow beam better azimuthal sampling than thermal, RADAR From:http://alg.umbc.edu/elf/elf.html 34 Ground-based: GPR • Ground penetrating RADAR – gives v. accurate information on sub-surface density and structure – Use for surveying hidden pipes for e.g. – Archaeology • Hidden graves • dinosaur tracks! – Geophysics • ice and snow density & movement – Hidden objects? • Landmines... From:www.geomodel.com && http://www.du.edu/~lconyer/picketwire_canyonlands_dinosaur_.htm 35 Summary • Sensor scanning mechanisms – Limitations (dwell-time/SNR, scan rate) – Striping of detector lines and arrays – CCD • Photography – Becoming less widely-used but still some applications • Time-resolved: LiDAR – For altimetry AND imaging (veg. structure) – higher vertical resolution than RADAR • Ground-based – Upward-looking for atmospheric studies – GPR for sub-surface surveying: archaeology, geophysical dynamics 36 Revision problems: Planck’s Law •Fractional energy from 0 to F0? Integrate Planck function •Note Eb(,T), emissive power of bbody at , is function of product T only, so.... Radiant energy from 0 to E0 , T Eb , T F0 , T d , T 4 5 T T 0 T Total radiant energy for =0 to = 37 Revision: Planck’s Law example •Q: what fraction of the total power radiated by a black body at 5770 K fall, in the UV (0 0.38µm)? •Need table of integral values of F0 •So, T = 0.38m * 5770K = 2193mK T (mK x103) •Or 2.193x103 mK i.e. between 2 and 3 2 3 4 5 6 8 10 12 14 16 18 20 •Interpolate between F0 (2x103) and F0 (3x103) F00.38 , T F00.38 2 x103 2.193 2 0.193 F00.38 3x103 F00.38 2 x103 3 2 F00.38 , T 0.067 0.193 0.273 0.067 F0(T) (dimensionless) .067 .273 .481 .634 .738 .856 .914 .945 .963 .974 .981 .986 •Finally, F00.38 = 0.193*(0.273-0.067)+0.067 = 0.11 •i.e. ~11% of total solar energy lies in UV between 0 and 0.38m 38 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 From:http://csep10.phys.utk.edu/astr161/lect/history/kepler.html 39 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: geostationary altitude? 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 40 Orbits: examples • 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: show separation of successive ground tracks ~3000km – – – – – Earth angular rotation = 2/24*60*60 = 7.27x10-5 rads s-1 So in 98.8 mins, point on surface moves 98.8*60*7.27x10-5 = .431 rads Remember l =r* for arc of circle radius r & in radians So l = (Earth radius + sat. altitude)* = (6.38x106 +705x103)* 0.431 = 3054km 41