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GPS Simplified How Does GPS Work? Introduction Location Finding Trilateration Almanac and Ephemeris A-GPS GPS Routing- Finding best route

Gps simplified

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Page 2: Gps simplified

GPS - Introduction • GPS or Navstar provides location and time information.

• Originally developed by US government for military navigation.

• Now any GPS device can use its services.

• GPS does not need telephonic or internet connection for position finding.

• Telephonic or internet reception enhances usefulness of GPS positioning information.

• GPS is mainly used for

1. Location finding

2. Optimum route finding

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GPS - Introduction • GPS is a network of about 31

satellites orbiting Earth at an

altitude of 20,000 km.

• Satellites orbit Earth with period

of 12 hours (two orbits

per day) at 14,000kph.

• 24 satellites are major, with

4 satellites each orbiting in 6 equally spaced orbit planes.

• On Earth, at least four GPS satellites must be ‘visible’ at any time at a point.

• India among five nations to have own navigation and positioning system with launch of IRNSS-1G, country’s seventh navigation satellite.

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Location Finding • Each satellite transmits information at regular intervals

about its--

• exact position

• current time.

• Information travelling at speed of light is received by our GPS receiver.

• GPS calculates how far away each satellite is based on how long it took for the messages to arrive.

• Total Time = Received time -Transmission time indicated in signal.

• Distance = Speed of Light * Total time (Speed=distance/time)

• GPS receiver can pinpoint our location using a process called trilateration.

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Trilateration

• Sphere surface intersects circle circumference at two points.

• To be on third circle as well as on circle circumference, you can be on these two possible points.

• Still there is ambiguity regarding your exact position.

P1

P2

r2 r1

Intersection of circle border and

sphere surface at two points

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Almanac and Ephemeris • The satellites broadcast two types of data,

• Almanac- Course orbital parameters for all SV (satellite vehicle).

• Each SV broadcasts Almanac data for all SVs.

• Almanac data not very precise and considered valid for up to several months.

• Ephemeris- Very precise orbital and clock correction for each SV.

• Ephemeris necessary for precise positioning.

• EACH SV broadcasts ONLY its own Ephemeris data.

• Ephemeris data valid for about 30 minutes.

• Ephemeris data broadcasted every 30 seconds.

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A-GPS • GPS fixes on location within few seconds to several

minutes depending on when it was used last.

• Needs time to download current Almanac, Ephemeris, time etc. from satellite.

• If used recently, it uses last data and fixes soon.

• Cell-phone GPS units get a fix almost immediately.

• They use Assisted GPS (A-GPS) using a data connection to a server.

• Server supplies Almanac, Ephemeris to cell-phone GPS.

• GPS doesn't wait to receive them from satellites.

• Server can also send an approximate location derived from cell-phone towers, allowing an immediate fix.

• In some cases the A-GPS device may send incomplete GPS data to the server for processing into a fix.

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GPS ROUTING Is Internet required?

• Location Finding-

– Only GPS used.

– Internet connection not required.

– If internet connection available, GPS finds your location much faster.

• Navigation-

– Plans and tracks your movement from A to B.

– GPS is used only to find your location.

– navigation apps like Google Maps require internet connection to access map data, compute directions, look up traffic details, search for points of interest, etc.

– Apps are available that don't require Internet connectivity for navigation.

– Data as directions, turn-by-turn navigation, POIs, can be stored on device SD Card.

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Navigation- A* Algorithm Finding best route

• GPS uses A* algorithm for finding shortest path.

• It is a variation to Dijkstra's algorithm.

• A digital map divides a Broad Street into hundreds of road segments, with nodes at intersections.

• GPS navigation app looks at the entire road network as a graph.

• Routing is explained with example:-

• To find route between A and P.

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Navigation- A* Algorithm Finding best route

• A human would mentally draw a straight line from start to destination and pick roads that are close to that line.

• A* algorithm does something similar when it is at an intersection (node) with multiple possibilities.

• It picks node that gives the shortest total route length as if it could go directly from that node to the destination.

• Virtual Direct path between A to P is A-F-K-P .

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Navigation- A* Algorithm Finding best route

• Eventually, it makes its way to P.

• Along the way it remembers several routes, and discard others, always following the shortest total path.

• Several shortest paths.

• Arbitrary decision made at several places to arrive at final path.

• But would never pick A-B-C-D-H-L-P, even of same length, as it takes it away from direct line from A to P.

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Dead Ends

• Remembered routes used to backtrack from dead ends.

• At dead end point K, its only option is to go to G as it is coming from J.

• However, it already remembers a path through G that is shorter.

• Since there's no other path, it discards the route through K and examines J and G again.

• And so on….

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Limitations

• Grid is useful way to understand graph traversal algorithms, but real world is not grid.

• Time-taken and not the distance is important factor in choosing a route.

• GPS searches for shorter route even if it is narrow or crowded.

• Riders prefer highways to winding roads even if longer.

• Real-time traffic condition is very important in finding optimum route.

• Problems regarding CPU and memory.

• A* search too exhaustive to be used on long distance travels like inter-city.

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Remedy

• Digital maps classify roads based on their suitability for long-distance travel.

– Top category are generally interstate highways,

– Lowest are neighbor-hood roads.

– In middle are various types of highways and arterial roads.

• For long-distance routes, GPS finds shortest path from you to nearest arterial and highway network.

• Once on arterial and/or highway network, it gets you as close to your destination as possible on highway.

• Then it steps down, from highway, to arterial, to neighbourhood roads, until finding your destination.

• The goal is to find the destination within a hundred or so road segments.