1. Location Based Services
2. What is a GPS?
3. How does it work?
4. How many Satellites orbit the earth at any given time?
5. Coordinates Systems
6. Map projections

Positioning

Location Based Services (LBS) are services,

• That are accessible with mobile devices through a mobile network
• Integrating a mobile device's location with other information so as to provide added value to the user

Classification of LBS applications

• Person-oriented - Person located can control the service
• Device-oriented - Person or object located is not controlling the service

Types of application design

• Pull Services

• Push Services

Application examples

Application areas

• Travel services
• Routing, city, hotel and restaurant guides
• Emergency
• Locating a person in need, warn persons in danger
• Fleet management
• Phone tracker
• Child watch, anti-theft tracking
• Friend finder, blind dating
• Mobile phone tariffs

Self-locating: User (his mobile device) locates himself

• Global Positioning System (GPS, GLONASS, Galileo)
• Manual position input (not smart)

External locating: Operator locates the user's mobile device

• Determining the current mobile network cell
• Signal comparison between three adjacent cells

Locating provided by third parties

• Location transponder
• Peer-to-Peer locating

Global Positioning System (GPS)

• Enables three dimensional positioning near the earth
• Measuring the runtime of signals between the satellite and the GPS-receiver, from which the distance and the position can be deduced (trilateration)
• The transmitted signal describes a circular sphere centered at the satellite on which surface the signal is received at the same time -> circular baseline of equal receiving times on earth

Global Positioning Systemis a radio navigation system developed by the US Department of Defense

Three major segments:

1. The space segment
2. The control segment
3. The user segment

A constellation of 24-32 satellites are in geosynchronous orbit around the earth, at least 24 are operational at any time

All transmit a one way signal enabling users to locate their position on earth

Six orbits

• 55° inclination each
• 20200 km altitude
• 11h 55min orbital period

From every point at earth at least 5 satellites are always visible

5 different types of satellites: Block I, Block II, Block IIA, Block IIR, and Block IIF

Almost passive ground stations to track the flight paths of the satellites

• 4 monitoring stations operated by the U.S. air force
• Since 2006 - 6 more stations operated by the NGA (National Geospatial-Intelligence Agency)
• One Master Control Station for evaluation

Every satellite can always be received by at least two ground stations

Satellites pass over these stations once every 12 hours where they receive corrections. The ground control stations time is always correct

The satellites constantly give a 3D positioning to GPS-receiver

GPS-receiver

• Composed of an antenna, receiver-processors, and a highly-stable clock
• May also include a display for providing location and speed information
• Dedicated devices or integrated into cars, mobile phones, watches, etc

Your position on earth is determined by measuring your distance from the group of satellites

The GPS receiver measures the time it takes for a radio signal to reach to it from the satellites, this depends on an accurate clock

The time is then converted to a distance and through triangulation, your position is calculated. Once there is time and velocity, distance can be calculated

The GPS receiver needs at least 3-4 signals from satellites to derive a position

There are always 4 satellites overhead at any one time. The satellites require a line of sight with the receivers to pinpoint location; this is why you generally don't get good signal indoors or under thick canopies

All satellites have the same time and they always know exact position

Every 6 seconds

• Week and time within the week
• Position

Health of the satellite

Ephemeris

• In a spherical polar coordinate system
• Updated every 2h

Almanac

• Coarse orbit and status information for all satellites
• Data related to error correction
• Updated every 24h

• Number of satellites in the sky and the arrangement of them
• Multi-path Errors - this occurs when radio signals bounce off terrestrial objects such as buildings or natural phenomenon, for example mountains, before reaching the receiver
• Atmospheric interference
• Satellite drift - Occasionally the satellites drift slightly off course. The satellites have 4 very precise atomic clocks and a microprocessor for limited data analysis of self monitoring. The ground control stations play an important part here by sending corrections to the satellites where necessary as they pass over the ground control stations
• Receiver clock errors - A receiver's built-in clock is not as accurate as the atomic clocks onboard the GPS satellites. Therefore, it may have very slight timing errors

Challenges

1. Projection of the 3D surface on 2D (paper, film, screen)
2. Selection of the spatial objects and their attributes to be displayed
3. Generalization of geometric and thematic properties (simplify, omit depending on scale)

Graticule

Geoid -> ellipsoid -> (sphere) -> latitude/longitude

Mapping the earth (sphere, ellipsoid) to a plane

Two tasks

• Conversion of geographic coordinates (longitude and latitude) to cartesian coordinates (x,y; easting,northing)
• Scaling of the map

Refers to cartesian coordinate systems based on projections

Geoid -> ellipsoid -> developable surface -> plane -> x,y

Developable surfaces: conic, cylindrical, azimuthal (plane)

Distortion: conformal, equidistant, equal area

Representation of coordinate systems in GIS

• Metadata file
• ESRI PRJ file
• EPSG codes provided by OGP - Int. Org. of Oil and Gas Producers Surveying and Positioning Committee, formerly EPSG - European petroleum survey group

EPSG

Vertical datum support often missing in GIS - specialized tools

Coordinate system definitions

Queries about this Lesson, please send them to: jmwaura@jkuat.ac.ke

*References*

• Geographic Information System Basics, 2012 J.E.Campbell & M. Shin
• Fundamentals of GIS, 2017 Girmay Kindaya
• GIS Applications for Water, Wastewater, and Stormwater Systems, 2005 U.M. Shamsi
• Analytical and Computer Cartography, 2nd ed. Keith C. Claike
• Geographic Information Systems: The Microcomputer and Modern Cartography, 1st ed. Fraser Taylor
• Land_Surveyor_Reference_Manual, 2nd ed. Andrew_L._Harbin