Global Positioning System (GPS)GPS – Space Segment (Satellites)GPS – User Segment (Receivers)Passive vs. Active Remote SensingSolar RadiationSolar Electromagnetic RadiationSatellite Imagery - 4 ResolutionsTemporal ResolutionSpectral ResolutionRadiometric ResolutionImage Pre-ProcessingImage EnhancementsSpatial EnhancementsSpectral EnhancementsClassificationAirborne Remote SensingAir Photo ScaleOrthophotographsLandsat Platforms and their SensorsLandsat Orbits‘Wiskbroom’ SensorsSPOT Characteristics‘Pushbroom’ SensorsIkonosQuickbirdGeostationary OrbitAVHRR CharacteristicsAVHRR BandsMODIS CharacteristicsImprovement Over Old Global DEMsNexrad Doppler Weather RADARCONUS Hourly Nexrad RainfallDavid Tenenbaum – GEOG 070 – UNC-CH Spring 2005Global Positioning System (GPS)• A space-based 3-dimensional measurement and positioning system that operates using radio signals from satellites orbiting the earth• Created and maintained by the US Dept. of Defense and the US Air Force• The system as a whole consists of three segments:– satellites (space segment)– receivers (user segment)– ground stations (control segment)• Note: Russia and a European consortium are implementing similar systems.David Tenenbaum – GEOG 070 – UNC-CH Spring 2005GPS – Space Segment (Satellites)• 24 NAVSTAR satellites in the GPS constellation• orbit the Earth every 12 hours• ~11,000 miles altitude (a very high orbit)• positioned in 6 orbital planes (4 per plane)• orbital period & planes are designed to keep 4-6 satellites above the horizon at any time everywhere on the planet • controlled and monitored by five ground stations around the globeDavid Tenenbaum – GEOG 070 – UNC-CH Spring 2005David Tenenbaum – GEOG 070 – UNC-CH Spring 2005GPS – User Segment (Receivers)• Ground-based devices that can read and interpret the radio signals from several of the NAVSTAR satellites at once•Use timing of radio signals to calculate the receiver’s position on the Earth's surface • Calculations result in varying degrees of accuracy that depend on:• quality of the receiver • user operation of the receiver• local & atmospheric conditions• current status of systemDavid Tenenbaum – GEOG 070 – UNC-CH Spring 2005Passive vs. Active Remote Sensinghttp://www.ccrs.nrcan.gc.ca/ccrs/learn/tutorials/fundam/chapter3/chapter3_1_e.htmlPassive sensors receive solar energy reflected by the Earth’s surface (2), along with energy emitted by the atmosphere (1), surface (3) and sub-surface (4)Active sensors receive energy reflected from the Earth’s surface that originally came from an emitter other than the SunDavid Tenenbaum – GEOG 070 – UNC-CH Spring 2005Solar RadiationElectromagnetic radiation energy: Wave-particle dualityparticleWavelength (λ)• EMR energy moves at the speed of light (c): c = f λ• f = frequency: The number of waves passing through a point within a unit time (usually expressed per second)• Energy carried by a photon: ε = h f [h=Planck constant (6.626×10-34Js)]• The shorter the wavelength, the higher the frequency, and the more energy a photon carries. Therefore, short wave ultraviolet solar radiation is very destructive (sunburns)David Tenenbaum – GEOG 070 – UNC-CH Spring 2005Solar Electromagnetic Radiation•The sun emits EMR across a broad spectrum of wavelengths:Atmospheric windowsBut the atmosphere blocks much of the energy before it reaches the surfaceDavid Tenenbaum – GEOG 070 – UNC-CH Spring 2005Satellite Imagery - 4 Resolutions• Satellite imagery can be described by four resolutions:– Spatial resolution: area on ground represented by each pixel, e.g.• Landsat Thematic Mapper - 30m• Advanced Very High Resolution Radiometer (AVHRR) and Moderate Resolutions Imaging Spectrometer (MODIS) - 1km• SPOT - 10m panchromatic /20m multispectral• IKONOS - 1m panchromatic /4m multispectral– Temporal resolution: how often a satellite obtains imagery of a particular area– Spectral resolution: specific wavelength intervals in the electromagnetic spectrum captured by each sensor (bands)– Radiometric Resolution: number of possible data values reportable by each sensor (how many bits)IKONOS panchromatic image of Sydney Olympic Park - 1mSpatial ResolutionDavid Tenenbaum – GEOG 070 – UNC-CH Spring 2005David Tenenbaum – GEOG 070 – UNC-CH Spring 2005Temporal Resolution• Number of days between overhead passes - satellite orbit– Landsat - 16 days– AVHRR & MODIS - daily– IKONOS - 1 to 3 daysDavid Tenenbaum – GEOG 070 – UNC-CH Spring 2005Spectral Resolution• Number, spacing and width of sampled wavelength bands (Landsat: 7 bands, AVIRIS: 224 bands!)• Multispectral vs. Panchromatic• Higher resolution results in more precision in the representation of spectral signaturesDavid Tenenbaum – GEOG 070 – UNC-CH Spring 2005Radiometric Resolution• Number of possible data values reported by the sensor, which determines how many levels of brightness it can distinguish• Range is expressed as 2npower– 8-bit radiometric resolution has 28values, or 256 values - range is 0-255 (e.g. Landsat TM data)– 16-bit resolution has 216 values, or 65,536 values- range is 0-65535 (e.g. MODIS data)• The value in each pixel is called the– Digital Number (DN)– Brightness Value (BV)David Tenenbaum – GEOG 070 – UNC-CH Spring 2005Image Pre-Processing• Radiometric Corrections– changing the image data BVs to correct for errors or distortions from a variety of sources:• atmospheric effects• sensor errors• Geometric Corrections– changing the geometric/spatial properties of the image data so that we can accurately project the image, a.k.a.• image rectification• rubber sheetingDavid Tenenbaum – GEOG 070 – UNC-CH Spring 2005Image Enhancements• Image enhancements are designed to improve the usefulness of image data for various applications:– Contrast Enhancement - maximizes the performance of the image for visual display– Spatial Enhancements - increases or decreases the level of spatial detail in the image– Spectral Enhancements - makes use of the spectral characteristics of different physical features to highlight specific featuresDavid Tenenbaum – GEOG 070 – UNC-CH Spring 2005Spatial Enhancements• Filters - used to emphasize or de-emphasize spatial information– Low-pass filter -emphasize large area changes and de-emphasize local detail– High-pass filter -emphasize local detail and