Global Positioning Systems - GPS

See powerpoint here

What is GPS?

Constellation of 24 satellites launched by the United States (NAVSTAR), in part for military purposes. This constellation of satellites allows GPS receivers to communicate to establish a fixed geographic location in a specified spatial reference system. There are other GPS systems, including a Soviet system (GLONASS) and a proposed EU system. Most receivers are configured to work with the US system.

Locational positions are established by calculating the distance from the field GPS receiver to a selected set of satellites (three or four).

The distance to each satellite is established by establishing the length of time it takes a signal to travel from each satellite to the GPS receiver. Each GPS satellite has a highly accurate (but not totally accurate) clock that allows this time measurement to be made.

This time/distance measurement establishes that the GPS receiver is located somewhere along a sphere surrounding the GPS satellite. Once the distance to a set of satellites is established, a single location can be established by intersecting the spheres from several identifying a single possible locational position.

Can be used to acquire a fixed point location, to trace a route, or bound a polygon in the field.

Reception of GPS signals relies on line-of-sight. GPS receivers can not geo-locate in buildings, underground (subways) or in dense forest canopy.

GPS data can be input into a GIS for integration with other datasets. Key is to ensure data are collected in a known and desired spatial reference system or at least can be transformed to another spatial reference system

Coordinates can be captured and recorded in various user-specified projections, coordinate systems and datums

What accuracies are possible with GPS receivers?

Consumer grade - 10-15m
Consumer grade WAAS capable - ~3m
Consumer grade with differential correction - ~4-5m
Survey grade with real time differential correction- 1-2cm

GPS Applications

Utilities - geo-locating services such as fire hydrants, light poles...

Route tracking and management - GPS devices on trucks, buses, trains

Navigation - used by airlines, marine applications

Field data collection - forest plot surveys, training samples for remote sensing imagery

Differential Correction - correction of GPS signals based on use of a fixed and stationary known location collecting data for the same period as your roving GPS receiver

Real-time differential correction
Post-processing differential correction

Sources of Error (no differential correction, with differential correction):

Clock Errors (1.5m, 0)
Orbit/Ephemeris Errors (2.5m, 0)
Multi-path Errors (0.6m, 0.6m)
Ionosphere (charged particles), troposphere (water vapor) (5m, 0.4m; 0.5m, 0.2m)
Dilution of Precision (DOP, PDOP)

Dispersion of satellites on horizon

Good DOP

Poor DOP

Some GPS receivers allow you to use a DOP mask, where the GPS does not report a location if the DOP is above a user set threshold

Position measurements can be taken once every second. Data over a series of seconds can be averaged to establish a more accurate position than based on a single second of data.

WAAS - Wide Area Augmentation System

Similar to differential correction, establishes errors in GPS signals based on a series of ground receiving stations. Errors are transmitted to two orbital satellites, WAAS capable GPS receivers can receive information from these WAAS satellites allowing GPS signals received from the NAVSTAR satellites to be corrected. WAAS is not certified yet but reportedly yields accuracies of about 2-3m.