Geoscience Reference
In-Depth Information
9.1 IntRodUCtIon
Much of the geophysics equipment used today for agricultural applications has the capability to be
integrated with Global Positioning System (GPS) receivers so that positional data can be obtained
at the same time geophysical measurements are collected. Integration of GPS with resistivity and
electromagnetic induction equipment already allows soil electrical conductivity to be mapped over
large farm fields in just a few hours time. The GPS is a satellite-based, all-weather, continuous,
global radionavigation and time transfer system, designed, financed, deployed, and operated by the
U.S. Department of Defense (DOD). The system was originally intended for military applications,
but in the 1980s, the U.S. government made it available to civilian users, free of any subscription
fees or setup charges. The first GPS satellite was launched on February 22, 1978, and in 1993, the
system was declared fully operational. During the following years, the breadth and the scope of
the GPS applications exploded in the civilian market, taking advantage of the systems' full and
sustained operability. As a result, radionavigation-based positioning and tracking is currently ever-
present in a number of science, engineering, mapping, and everyday life applications.
GPS is an example of a ubiquitous technology responsible for the paradigm shift in contempo-
rary navigation, positioning, surveying, and mapping techniques. As a result of progressive innova-
tion and a significant drop in the price of the equipment in the last decade, GPS technology currently
supports a variety of applications. These applications range from precise positioning, cadastral
mapping and engineering, to remote sensing, environmental and GIS (geographic information sys-
tem) surveys, and law enforcement. GPS radionavigation signals are used to navigate spaceships,
aircrafts, and land-based vehicles, including transportation fleets and emergency cars, and to guide
and track individual pedestrians. GPS is also used to monitor wildlife, track the race cars and
paragliders, and monitor deformation of large structures. The system has also been effectively used
in traffic monitoring, location-based services (LBS) and, in recent years, in monitoring the space
weather and tropospheric conditions; and the number of new applications is still growing.
From the standpoint of agricultural geophysics, GPS is essential in providing a means for geo-
registration (geolocation) of the ground-based sensors used to monitor soil conditions and locate
subsurface features. This chapter will introduce the primary definitions, concepts, and mathemati-
cal models related to GPS positioning and sensor orientation applications. The overview of GPS
design, implementation, and modernization is provided, followed by the primary positioning modes
and associated mathematical models, including the real-time kinematic (RTK) and differential
GPS (DGPS) concepts. The primary error sources, positioning accuracy, and basic aspects of GPS
instrumentation are also addressed.
9.2
hoW GpS WoRkS
The primary concept related to navigation with satellites is triangulation in space. The GPS satel-
lites that serve as a space-based reference for the user's positioning solution transmit a continuous
signal toward Earth during their 12-hour revolution around the globe. GPS receivers use signals
from multiple satellites to determine the distances (or ranges) to the satellites that are subsequently
used to triangulate the user's position coordinates (see Figure 9.1). The range observation is recov-
ered by measuring the travel time of the signal between the satellite and the user's receiver. To
perform a positioning or a navigation task, a GPS receiver must be locked onto the signal of at least
three satellites to calculate a two-dimensional (2D) position (latitude and longitude); with four or
more satellites in view, the receiver can determine three-dimensional (3D) position coordinates
(latitude, longitude, and height) of the user. If continuous lock to multiple satellites is maintained,
the receiver can provide an uninterrupted position solution, as well as additional information, such
as speed, bearing, distance traveled, and distance to destination. The receiver can even provide
location and directions to the nearest post office or gas station if it is equipped with suitable GIS
databases and digital maps.
