Global Positioning Systems

GIS IFall 2011

Uses of GPS Technology

• Navigation– Marine– Automobile– Aircraft

• Wayfinding– Disaster relief– Hiking– Walking tours– Eco-burials

• Precise Timekeeping

• Military– Troop accountability– Coordination– Targeting– Selective Availability

• Surveying– Property delineation– Infrastructure

assessment– Planning– OpenStreetMap

What GPS can’t do.

• Track you. It’s a one way system. You need special equipment to collect the signals then transmit your location through other means.

• Follow you inside or underground. Just your hand can block a GPS signal. Further tracking required dead reckoning, inertial navigation, or different technology.

• Show you where to go. Consumer GPSs rely on a special purpose GIS. As you should know, it’s only as good as the data available.

GPS Systems

• GPS systems use a constellation of satellites with known orbits. Each satellite beams an encoded signal towards Earth.

• A GPS receiver can read the coded signals to determine the receiver’s location on the earth using triangulation.

• Higher end GPS units can use the code and the carrier wave for higher accuracy.(“Code vs Carrier”)

How does it work?

• The encoded message allows a receiver to determine its position on the Earth once 3 satellites (2D) or 4 or more satellites (3D) are in view.

• The encoded signal includes additional data:– Ephemeris: the precise orbital information about one

satellite– Almanac: granular information about the entire

system.

Types of GPS

• Autonomous GPS (“GPS”) relies entirely on the signals received from the satellites to determine the user’s location.

• Assisted GPS (“A-GPS”) uses an outside network (cellular, WiFi) to download the Ephemeris and Almanac, shortening the Time To First Fix.

• Differential GPS (“DGPS”) uses a static base station GPS to provide additional information about the signal, boosting accuracy.

Listening to the GPS Signal

How is the signal affected?• Position on the Earth• Health of satellites• Multipath issues• Weather disruption• Vegetation cover• Quality of receiver• Additional signals

Uncertainty in GIS Data• Just because you receive a location from a GPS

does not mean it is 100% correct.• Uncertainty influences many aspects of our work.• Remember, it is impossible to make a perfect

representation of the world, so uncertainty about it is inevitable.

Sources of Uncertainty

• Measurement error: different observers, measuring instruments

• Specification error: omitted variables• Ambiguity, vagueness and the quality of a GIS

representation• A catch-all for ‘incomplete’ representations or a

‘quality’ measure

Levels of Uncertainty

Reporting Measurements

• The amount of detail in a reported measurement (e.g., output from a GPS) should reflect its accuracy.– Stated precision of:• “14.4m” implies an accuracy of 0.1m• “14m” implies an accuracy of 1m

• Excess precision should be removed when publishing coordinates.

The term precision is often used to refer to the repeatability of measurements. In both diagrams six measurements have been taken of the same position, represented by the center of the circle. On the left, successive measurements have similar values (they are precise), but show a bias away from the correct value (they are inaccurate). On the right, precision is lower but accuracy is higher.

Positional Accuracy

• Consumer GPS receivers have a horizontal positional accuracy of ±5m (16ft).

• Multipath and scattering impacts accuracy.

• Accuracy can be improved using differential GPS.

• Stated as Dilution of PrecisionxDOP (G, P, H, V, T…)

Dilution of Precision

• Numeric value that corresponds to the degree of error in the current GPS position.

• Geometric – satellites are too close together.• Horizontal, vertical, and positional dilutions.• Time dilution – the shift from “actual” time.• Value typically from 1-50.• Lower value, lower dilution, better quality.• GPS Forecast and NANUs

Selective Availability

• Before 2000, GPS was limited to US Military use.

• Random error was introduced into the civilian signal, intentionally reducing precision.

• Now SA is turned off, but can be selectively turned on for specific regions of the world.