PART0 » History » Version 19

COLIN, Tony, 03/13/2016 09:20 PM

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h1. PART 1 : An introduction to Navigation.
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p((. Before anything else, it is necessary to tackle the origin of navigation and its applications, the creation of the current GNSS with a comparison of different systems and finally, describing the main principles of GPS.
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h2. 1 - History of Navigation.
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_*Navigation* is defined as a field of study that focuses on the process of monitoring and controlling the movement of a craft or vehicle from one place to another.[1] The field of navigation includes four general categories: land navigation, marine navigation, aeronautic navigation, and space navigation._
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A brief evolution of Navigation through history is suggested in the following. 
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*Figure 1* : Lighthouse __________________________ *Figure 2* : Magnetic compass ________________________ *Figure 3* : Sextant __________________ *Figure 4* : Chronometer _______________ *Figure 5* : TomTom GPS
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h3. a- Visual navigation.
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Seamark and landmark references *[Figure 1]* where among the first entities to help to find a direction. However, this system works with a limited range, a limited accuracy, a low availability of marks.
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Note that stars has been used for centuries, but are only available during night and clear sky.
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h3. b- Navigation with classic instruments.
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Then, come the Magnetic compass *[Figure 2]*, the Sextant *[Figure 3]* as navigation tools.
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The last one allows the measurement of the height of stars above the horizon providing longitude inaccurately i.e. it was the first use of ephemeris and time reference.
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Improvements has been done by combining it with Harrison's clock (Chronometer) *[Figure 4]* providing latitude and longitude with acceptable accuracy.
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h2. 2 - Global Navigation Satellite System (GNSS).
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h3. a- History.
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- *Sputnik* in 1957 : first satellite in the world, orbit estimated using the received signal frequency via Doppler effect.
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- *TRANSIT* from 1958 operational in 1964 : first satellite navigation system for the US Navy.
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- *GPS* program started at 1973 providing position anywhere at anytime.
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h3. b- Current definition.
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_*Global Navigation Satellite System (GNSS)* is defined as a system of satellites that provide autonomous spatial positioning on Earth with global coverage.
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It allows small electronic receivers to determine their location (longitude/latitude/altitude) to high precision (within a few meters) using time signals transmitted along a line of sight by radio from satellites. 
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Global coverage for each system is generally achieved by a satellite constellation of 20–30 medium Earth orbit (MEO) satellites spread between several orbital planes. The actual systems vary, but use orbital inclinations of >50° and orbital periods of roughly 12 hours._
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h2. 3 - Comparison of systems.
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h2. 4 - Our system : GPS details.
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_http://www.gps.gov/systems/gps/space/
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https://en.wikipedia.org/wiki/Global_Positioning_System_
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h3. a- Space Segment.
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h3. b- Ground Control Segment.
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Continuously > Track GPS satellites, estimate clock and orbit, keep GPS time, upload data that describes clock & orbit for each satellite
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Infrequently > Command small maneuvers to maintain orbit, small clock corrections, major relocations to compensate any satellite failure
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h3. c- Delivery of Navigation messages.
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Clock & Ephemeris sent by GS to SS, Spread Spectrum ranging signals & navigation data sent by SS to US & GS.
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h3. d- Signals.
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h3. e- Ranging.
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Propagation time : Time of transmission - Time of reception
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Distance between satellite and receiver \approx Propagation time x Speed of light
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How it works : attachment:"29.pdf"
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_In geometry, *trilateration* is the process of determining absolute or relative locations of points by measurement of distances, using the geometry ofcircles, spheres or triangles. :_
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1 measurement : 1 sphere of radius D1 centered in Sat1
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2 measurements : intersection of 2 spheres  (D1, Sat1) (D2, Sat2) = Circle 
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3 measurements : intersection of 3 spheres (D1, Sat1) (D2, Sat3) (D2, Sat3) = 2 points (1 often aberrant) 
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h3. f- GNSS Receivers.
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h3. g- GNSS Measurements.
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*References :* 
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*[1]* M. Sahmoudi, Introduction to satellite positioning & multi-sensor navigation, 2016
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*[2]* K. Borre, D. M. Akos, N. Bertelsen, P. Rinder, S. H. Jensen, A software-defined GPS and GALILEO receiver
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*[3]* https://en.wikipedia.org/wiki/Satellite_navigation
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*[4]* https://en.wikipedia.org/wiki/Global_Positioning_System
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*[5]* http://www.gps.gov/systems/gps/