Some Concepts That Underpin GIS - Introducing Geographic Information Systems with ArcGIS - page 22

Global Positioning Systems
A global positioning system (GPS) 19 is a satellite-based system that provides users with accurate and
precise location and time information. Using NAVSTAR GPS, you can determine locations on Earth easily
within a few meters, and, with more difficulty and expense, within a few centimeters or better. Timing
within 40 billionths of a second (40 nanoseconds) is easily obtained. Timing within 10 nanoseconds is
possible.
The U.S. Department of Defense operates NAVSTAR GPS in cooperation with the U.S. Department of
Transportation. The acronym NAVSTAR stands for NAVigation System Timing And Ranging Global
Positioning System.” Informally, it is the Navigation Star.
The Russian GLONASS (Global Navigation Satellite System) operates similarly. Concerns about
U.S. control over NAVSTAR led Europe to begin development of its own independent Galileo
system in 2002, but it is just now becoming operational. China is developing the BeiDou
(Compass) GNSS.
A GPS receiver, which “remembers where it has been,” is becoming a primary method of providing data
for GIS. For example, if you drive a van with a GPS antenna on its roof along a highway, recording data
every, say, 50 feet, you will develop an accurate and precise map of the location of the highway. The
NAVSTAR GPS is discussed in Chapter 5. 20
Remote Sensing
Remote sensing probably stated with photographs taken from balloons in the 1840s. The first automated
system (not requiring human beings to be with the sensors) may have been in the 1890s in Europe when
cameras programmed to take pictures at timed intervals were strapped to pigeons!
Evelyn Pruitt probably introduced the modern use of the term “remote sensing” in the mid-1950s when
she worked as a geographer/oceanographer for the U.S. Office of Naval Research (ONR). Remote
sensing uses instruments or sensors to capture the spectral characteristics and spatial relations of objects
and materials observable at a distance, typically from above them. Using that definition, everything we
observe is remotely sensed. More practically, something is sensed remotely when it is not possible or
convenient to get closer.
We can categorize remote sensing for GIS many ways. Data can be taken from aircraft or satellite
“platforms.” The energy that the sensor “sees” can come from the objects or areas being examined as
a result of radiation emanating from them (caused by the sun or other heat or light sources) or from
radiation bounced off them by an energy source associated with the sensor (e.g., radar or lidar). The
images produced may be developed on film or produced by digital sensors. Satellites in geosynchronous
orbits can hang in a single spot over the equator. Those in near polar orbits can see different areas of the
Earth as it turns. Chapter 2 offers examples of remotely sensed data.
19 Also referred to as a positioning, navigation, and timing (PNT) system, or a global navigation satellite system (GNSS).
20 See also Prof. Kennedy's textbook The Global Positioning System and ArcGIS , Third Edition, 2010, CRC Press (Taylor &
Francis Group), 301 pages.