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CU-Boulder GEOG 5093 - PLATFORMS FOR REMOTE SENSING

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GEOG/GEOL 4093/5093 263. PLATFORMS FOR REMOTE SENSINGThe main platforms for remote sensing are• Aircraft• SatellitesOther useful platforms are• Towers• Balloons• Kites• Very convenient platform• Range of heights: meters to several kilometers• Speed: 0 – 300 m s-1• Capacity: 50 kg to many tons3.1.1. Operational aspectsHeight determines scale, coverage, linear resolutionSpeed determines linear sampling rate3.1.2. NavigationElectronic navigation system give the aircraft’s true position• LORAN surface based radio transmitter• OMEGA surface based radio transmitter• SATNAV satellite based radio transmitter• GPS satellite based radio transmitter(cm range accuracy)3.1.3. Ground control points• natural features of know location• markers fixed to the ground (MW, Radar)3.1. AIRCRAFTGEOG/GEOL 4093/5093 27GLOBAL POSITIONING SYSTEMThe Global Positioning System, usually called GPS (the USmilitary refers to it as NAVSTAR GPS - Navigation SignalTiming and Ranging Global Positioning System), is the onlyoperational satellite navigation system. For a list of otherproposed and partially developed systems, including Russia'sGLONASS and Europe's Galileo, see satellite navigation system.The Global Positioning System (GPS) is a satellite-basednavigation system made up of a network of 24 satellites placedinto orbit by the U.S. Department of Defense. GPS was originallyintended for military applications, but in the 1980s, thegovernment made the system available for civilian use. GPSworks in any weather conditions, anywhere in the world, 24hours a day. There are no subscription fees or setup charges touse GPS.GPS satellites circle the earth twice a day in a very precise orbitand transmit signal information to earth. GPS receivers take thisinformation and use triangulation to calculate the user's exactlocation. Essentially, the GPS receiver compares the time asignal was transmitted by a satellite with the time it was received.The time difference tells the GPS receiver how far away thesatellite is. Now, with distance measurements from a few moresatellites, the receiver can determine the user's position anddisplay it on the unit's electronic map.3.2. WHAT IS GPS3.3. HOW DOES IT WORK?GEOG/GEOL 4093/5093 28GLOBAL POSITIONING SYSTEMThe 24 satellites that make up the GPS space segment are orbitingthe earth about 12,000 miles above us. They are constantly moving,making two complete orbits in less than 24 hours. These satellites aretravelling at speeds of roughly 7,000 miles an hour.GPS satellites are powered by solar energy. They have backupbatteries onboard to keep them running in the event of a solareclipse, when there's no solar power. Small rocket boosters on eachsatellite keep them flying in the correct path.Here are some other interesting facts about the GPS satellites (alsocalled NAVSTAR, the official U.S. Department of Defense name forGPS):• The first GPS satellite was launched in 1978.• A full constellation of 24 satellites was achieved in 1994.• Each satellite is built to last about 10 years. Replacements areconstantly being built and launched into orbit.• A GPS satellite weighs approximately 2,000 pounds and isabout 17 feet across with the solar panels extended.• Transmitter power is only 50 watts or less.3.4. GPS SATELLITESGEOG/GEOL 4093/5093 29PLATFORMSGEOG/GEOL 4093/5093 30Advantages of satellites in orbit are:• Increased platform speed• Continuity of mission• Better data coverage• Homogeneous data set for +5 years• No legal problems with political boundaries3.5.1. Launch of satellitesΔV increase in velocityU velocity of exhaust gas (2.5 km s-1)Mi initial mass of rocketMf mass of fuelVelocity of satellite in low orbit: 7 km s-1• A rocket capable of reach this orbit consists of 97 % fuel• Single stage rockets are only capable of placing smallsatellites in orbit• Multiple-stage rockets can put a few tons into low earthorbit• Space shuttle can place 30 tons into 400 km orbit, or 6tons in a geostationary orbit3.5. SATELLITESfiiMMMUV−=Δ lnGEOG/GEOL 4093/5093 313.6.1. Description of orbits• All satellite have an elliptical orbit with the earth's center asone focusSatelliteb r Apogee a PerigeeEarth3.6.2. Orbital timeThe orbital time depends only on the orbital height of thesatellite.T0 orbital time in secondsRp planet radius (6380 km for Earth)H orbit altitudegs gravitational acceleration at the surface 0.00981 km s-23.6. ORBIT GEOMETRY)()()(22PsPPoRgHRHRT++=πGEOG/GEOL 4093/5093 323.7.1. Geostationary• Circular orbit above the equator (35,800 km)• Nodal period is not equal 24 hours• The earth rotates in 86,400 s once with respect to the sun,but need only 86,164 s (sideral day = in Latin star day) torotate once with respect to a fixed star.• Useful coverage up to 55° for quantitative work, and 65°for qualitative work• METEOSAT, GOES3.7.2. Geosynchronous• Orbit traces its figure-of-eight at an approximately uniformrate• For half a day in the NH and visa versa• Nodal time equals 24 hours3.7.3. Sun-synchronous• The orbital plane rotates about the polar axis with theangular speed of 1.991 10-7 s-1(1 year)• The satellite will cross the a given latitude at the samesolar time every day• Orbital height = 1500 km, inclination > 96°• Landsat, SPOT, NOAA, DMSP, ERS-13.7.4. Altimertic•Ascending and descending orbits should cross at 90o3.7. ORBITSGEOG/GEOL 4093/5093 333.7.5. Decay of orbits• Due to the atmospheric friction the LANDSAT 5 satellitedescends by about 5 m/day3.7.6. Summary of the important satellite orbits• PN = nodal period• a=semimajor axis of elliptical orbitGEOG/GEOL 4093/5093 344. AERIAL AND SPACE PHOTOGRAPHYMore than 100 different models of aerial film cameras arecurrently in use. They can be divided into the following fourcategories4.1.1. Single-lens frame camera• Most common camera in use today• Used for mapping ⇒ metric or cartographic camera• Extremely high geometric image quality• Film size commonly square 230 mm• Wild RC-10 (airplane)• Large-Format Camera (LFC), Metric Camera (space)4.1.2. Multi Lens frame camera• Photographs taken from same object with different filterand film combinations•


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