Geography Reference
In-Depth Information
NAVSTAR was developed and implemented by
the US Department of Defense (DoD), which is
responsible for the maintenance of the system.
The first of the GPS satellites was launched in
1978, and by mid-1994 all twenty-four satellites
were in operation. Each satellite weighs
approximately 860 kg and is 8.7 m in height
(Kennedy 1996). The satellites are solar-powered,
and only twenty-one of the twenty-four satellites
transmit positional information. The three
remaining satellites are spare units that can be
brought into immediate use should the need
arise. The twenty-four satellites are arranged on
six orbital planes, with four satellites on each
plane. Each plane is inclined to the equator by
55°. This design ensures complete coverage over
the entire surface of the Earth and that a GPS
receiver can see multiple satellites from the
majority of locations (van Sickle 1996). For many
locations, it is often possible for a receiver to see
ten or more satellites at particular times of the
day. There is, however, a recognised problem in
the polar regions, where the poor geometrical
alignment of the satellites at certain periods
reduces positional accuracy.
The NAVSTAR satellites have a twelve hour
orbit time and pass over control stations during
each orbit. This allows close monitoring of the
orbit and position. This locational information can
be transmitted from the monitoring stations to the
satellite so that each satellite has up-to-date
information concerning its own location. The
control stations also monitor the accuracy of
atomic time clocks on board the GPS satellites and
transmit information regarding the accuracy of
these clocks to the satellite. It is essential that each
NAVSTAR satellite provides complete and
accurate information regarding position and that
the accuracy of its atomic clock is known.
A large number of national and regional GPS
base station networks exist that monitor satellite
positions over time. These provide continuous
information about the positional accuracy of the
GPS signals at specific geographic locations. Many
of these networks transmit this information to
users of stand-alone GPS receivers. The
information from base station networks allows
users to improve the positional accuracy of the
data from their own GPS receivers.
GPS receivers are capable of detecting the
signals transmitted from the NAVSTAR satellites
and converting this information into location
information. A wide range of receivers are now
available on the market at prices from a few
hundred dollars to well over $10,000. The cheapest
receivers will usually have an accuracy, when used
individually, of 100 m for 95 per cent of the time.
Receivers costing over $10,000 should be capable
of at least centimetre and in some cases millimetre
accuracy. No matter what the cost or positional
accuracy a receiver can achieve all receivers have
the following components in common (adapted
from Kennedy 1996; and van Sickle 1996):
•
a microcomputer, which processes the
information from the GPS satellites and uses
this information to determine position;
•
an aerial and associated electronics, which
receives the signal from which position is
calculated;
•
a user interface (usually an LCD screen and
key pad), which allows the user to program
the receiver and view the output); and
•
computer memory, which allows the user to
record positional information. More
expensive receivers have greater memory and
allow the user to record additional
information, for example, what is present at a
given location.
The last component of the GPS framework is the
user. Historically, the main users of GPS were the
military. Today, however, civilian applications of the
technology far exceed military use. The majority
of civilian users fall into one of two categories,
although the boundary between is fuzzy. The first
group uses GPS for navigation. The second group
uses the technology to record, map and survey the
position of features such as the location of an oil
well, the boundary of a forest or the footprint of a
new building. From a geographer's perspective, it
is this latter application that is perhaps the most
interesting. Therefore, the remainder of this
chapter focuses on the use of GPS technology for
recording survey locations and mapping.
