SOURCEFORGE » RALF » 2015 Software defined GPS receiver

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.

_*Navigation* is 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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h3. a- Visual navigation.

Seamark and landmark references > Limited range, Limited accuracy, Low availability of marks, Stars only during night and clear sky.

h3. b- Navigation with classic instruments

Magnetic compass, Sextant > Measurement of the height of stars above the horizon providing longitude inaccurately > Use of ephemeris and time reference

Improvements by combining it with Harrison's clock (Chronometer) providing latitude and longitude with acceptable accuracy.

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h2. 2 - Global Navigation Satellite System (GNSS).

h3. a- History.

- *Sputnik* in 1957 : first satellite in the world, orbit estimated using the received signal frequency : Doppler effect.

- *TRANSIT* from 1958 operational in 1964 : first satellite navigation system for the US Navy.

- *GPS* program started at 1973 providing position anywhere at anytime.

h3. b- Current definition.

_*Global Navigation Satellite System (GNSS)* is a system of satellites that provide autonomous spatial positioning on Earth with global coverage.

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. 

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.

_http://www.gps.gov/systems/gps/space/

https://en.wikipedia.org/wiki/Global_Positioning_System_

h3. a- Space Segment.

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h3. b- Ground Control Segment.

Continuously > Track GPS satellites, estimate clock and orbit, keep GPS time, upload data that describes clock & orbit for each satellite

Infrequently > Command small maneuvers to maintain orbit, small clock corrections, major relocations to compensate any satellite failure

h3. c- Delivery of Navigation messages.

Clock & Ephemeris sent by GS to SS, Spread Spectrum ranging signals & navigation data sent by SS to US & GS.

h3. d- Signals.

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h3. e- Ranging.

Propagation time : Time of transmission - Time of reception

Distance between satellite and receiver \approx Propagation time x Speed of light

How it works : attachment:"29.pdf"

_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. :_

1 measurement : 1 sphere of radius D1 centered in Sat1

2 measurements : intersection of 2 spheres  (D1, Sat1) (D2, Sat2) = Circle 

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 :* 

*[1]* M. Sahmoudi, Introduction to satellite positioning & multi-sensor navigation, 2016

*[2]* K. Borre, D. M. Akos, N. Bertelsen, P. Rinder, S. H. Jensen, A software-defined GPS and GALILEO receiver

*[3]* https://en.wikipedia.org/wiki/Satellite_navigation

*[4]* https://en.wikipedia.org/wiki/Global_Positioning_System

*[5]* http://www.gps.gov/systems/gps/