GPS: GPS: Working Principle and Working Principle and Interfacing Interfacing Team D1 Team D1 Ranajeet Anand(200601186) Ranajeet Anand(200601186) Shashi Kumar(200601155) Shashi Kumar(200601155)
Presentation Flow Presentation Flow Existing Technologies Existing Technologies Limitations Limitations What is GPS What is GPS Concept Concept Working Working Constraints Constraints GPS Receiver Output Protocols GPS Receiver Output Protocols Interfacing of GPS with uC Interfacing of GPS with uC Use in Embedded Systems Use in Embedded Systems References References
Existing Technologies Existing Technologies Land-based electronic navigation systems Land-based electronic navigation systems In the 1930's radio beacons were used to provide In the 1930's radio beacons were used to provide bearings from airfields bearings from airfields Radio navigation systems like - LORAN, (Long Range Radio navigation systems like - LORAN, (Long Range Aid to Navigation). Positions were determined by the Aid to Navigation). Positions were determined by the timing of signals timing of signals In 1960s the Omega system provided worldwide In 1960s the Omega system provided worldwide electronic navigation coverage for the first time electronic navigation coverage for the first time In mid-1960's: US Navy's NAVigation SATellite System In mid-1960's: US Navy's NAVigation SATellite System (NAVSAT), also known as TRANSIT, was developed to (NAVSAT), also known as TRANSIT, was developed to provide more accurate positions for ships and provide more accurate positions for ships and submarines. submarines.
Limitations Limitations Unavailability for the general public Unavailability for the general public Land based systems, so not much accurate Land based systems, so not much accurate Low frequency of updating location Low frequency of updating location Lack of 24-hour availability Lack of 24-hour availability
What is GPS What is GPS Definition: Definition: GPS stands for Global Positioning System GPS stands for Global Positioning System A “GPS System” usually means a GPS Receiver A “GPS System” usually means a GPS Receiver
What is GPS What is GPS The GPS is a constellation of 27 Earth-orbiting The GPS is a constellation of 27 Earth-orbiting satellites (24 in operation and three extras in satellites (24 in operation and three extras in case one fails). case one fails). The U.S. military developed and implemented The U.S. military developed and implemented this satellite network as a military navigation this satellite network as a military navigation system, but soon opened it up to everybody system, but soon opened it up to everybody else. else.
Concept Concept GPS Components: GPS Components: There are three Components : There are three Components : 1) ground stations that controlled the system, 1) ground stations that controlled the system, 2) a "constellation" of satellites in Earth orbit, and 2) a "constellation" of satellites in Earth orbit, and 3) receivers carried by users. 3) receivers carried by users.
Concept Concept Solar-powered satellites orbit the earth at 19,300 km Solar-powered satellites orbit the earth at 19,300 km making two complete rotations every day making two complete rotations every day Arrangement of orbits such that at least four satellites Arrangement of orbits such that at least four satellites . "visible" in the sky . "visible" in the sky Job of Receiver Job of Receiver Locate 4 or more satellites Locate 4 or more satellites figure out the distance to each, figure out the distance to each, Deduce its own location by the Trilateration Principle Deduce its own location by the Trilateration Principle
Concept Concept Principle of Trilateration Principle of Trilateration 2-D Trilateration 2-D Trilateration 2 circles intersect at 2 2 circles intersect at 2 points points Intersection with 3 Intersection with 3 rd circle rd circle => Accurate Location => Accurate Location
Concept Concept Principle of Trilateration Principle of Trilateration 3-D Trilateration 3-D Trilateration Four spheres are needed Four spheres are needed 2 spheres intersect in a 2 spheres intersect in a circle circle A 3 A 3 rd sphere intersects at rd sphere intersects at 2 points of the circle 2 points of the circle Intersection with 4 Intersection with 4 th th sphere => Accurate sphere => Accurate Position Position
Concept Concept 3 –D Trilateration 3 –D Trilateration The 3 spheres are formed by 3 satellites The 3 spheres are formed by 3 satellites The earth itself acts as the 4 The earth itself acts as the 4 th sphere th sphere To improve accuracy and provide precise altitude To improve accuracy and provide precise altitude information, receivers generally look to four or more information, receivers generally look to four or more satellites satellites
Working Working The satellites transmit high-frequency, low-power radio The satellites transmit high-frequency, low-power radio signals signals These signals are received by the GPS receiver These signals are received by the GPS receiver Better units with multiple receivers can pick Better units with multiple receivers can pick up signals from several satellites up signals from several satellites Radio waves travel at speed of light so receiver can Radio waves travel at speed of light so receiver can calculate time taken by signal to arrive calculate time taken by signal to arrive This is the distance of satellite from receiver This is the distance of satellite from receiver
Working Working The Clock Issue The Clock Issue To calculate distance, satellite and receiver clocks To calculate distance, satellite and receiver clocks should be accurately synchronized should be accurately synchronized The satellites contain atomic clocks which are extremely The satellites contain atomic clocks which are extremely accurate, and accurate with respect to each other accurate, and accurate with respect to each other So, the receiver also would need to have a highly So, the receiver also would need to have a highly accurate atomic clock accurate atomic clock This is not possible since atomic costs are very This is not possible since atomic costs are very expensive expensive
Working Working The Clock Issue: Solution The Clock Issue: Solution The receiver itself uses an ordinary quartz clock, which it The receiver itself uses an ordinary quartz clock, which it constantly resets constantly resets Incoming signals from four or more satellites used to Incoming signals from four or more satellites used to gauge its own inaccuracy gauge its own inaccuracy The receiver sets its clock at a time value such that all The receiver sets its clock at a time value such that all signals that it is receiving align at a single point in space signals that it is receiving align at a single point in space
Working Working The Clock Issue: Solution The Clock Issue: Solution Since a 4 Since a 4 th satellite is used, all 4 spheres will not th satellite is used, all 4 spheres will not intersect at 1 point. intersect at 1 point. The receiver makes necessary adjustments so that all 4 The receiver makes necessary adjustments so that all 4 spheres intersect at 1 point spheres intersect at 1 point Based on this, it resets its clock to be in sync with the Based on this, it resets its clock to be in sync with the satellite's atomic clock, thus ensuring that its clock is as satellite's atomic clock, thus ensuring that its clock is as accurate as the atomic clocks accurate as the atomic clocks
Constraints Constraints Name Description Accuracy Within 15 meters 99% of the time. Accuracy vs. User Accuracy of atleast1 meter per 22 mph. Velocity Power Consumption no more than 5 watts of power. Initial Position capable of displaying a user position Calculation Time within 4 minutes LCD Updating user position will be updated every 10 Frequency seconds.
Constraints Constraints Scope for Errors Scope for Errors : : Satellite errors Satellite errors Errors in modeling clock offset Errors in modeling clock offset Latency in tracking Latency in tracking Atmospheric propagation errors Atmospheric propagation errors Through the ionosphere, carrier experiences phase Through the ionosphere, carrier experiences phase advance advance Dependent on Dependent on Geomagnetic latitude Geomagnetic latitude Time of the day Time of the day Elevation of the satellite Elevation of the satellite
Constraints Constraints Errors due to Errors due to Multipath Multipath Receiver Noise Receiver Noise Forces on the GPS satellite Forces on the GPS satellite Earth is not a perfect sphere and hence Earth is not a perfect sphere and hence uneven uneven gravitational potential distribution gravitational potential distribution Other heavenly bodies attract the satellite, but Other heavenly bodies attract the satellite, but these are very well modeled these are very well modeled Errors due to Geometry Errors due to Geometry
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