Game Development Reference
8.4.4 Global Positioning System (GPS)
Today's smart phones and tablets come with GPS receivers. These devices
calculate their position on the earth by bouncing signals off the satellites
of the global positioning system. The GPS was conceived by the American
Department of Defense (DOD) in 1973 with the first satellites being
launched in 1978. In 1993 the DOD decided to make it free for civilian
use. The official name of the system is NAVSTAR (Navigation System for
Timing and Ranging).
The GPS consists of 30 satellites orbiting the earth within a distance of 20,200
kilometers. The satellites are solar powered and circle the globe two times
each day. The configuration of the satellites is such that at any location
on the Earth's surface at least four are visible in the sky.
The GPS receiver in a mobile device calculates its position through analysis
of the high-frequency, low-power radio signals emitted by the satellites. It
determines its distance from any four of the satellites it is able to detect
and works out its position. The process is called trilateration .
Trilateration finds the intersection of a number of circles or spheres. In the
case of 3D space and the GPS system, for each detected satellite a sphere
can be imagined around each with a radius equal to the distance to the
satellite calculated by the GPS device. The location of each satellite is known,
as the DOD monitors them constantly to ensure that they are aligned. With
this information from four satellites and including the earth itself as another
sphere in the equation, the location of the GPS device can be restricted to the
area in which all spheres intersect. This process is illustrated in Figure 8.7
with just two spheres. The typical accuracy of GPS in mobile devices
is around 10 meters.
Possibly the largest GPS-based game is Geocaching . It's a worldwide
treasure hunt involving the hiding and locating of small boxes of items
placed all around the globe. The GPS positions of geocaches are registered
on the Web. Using a GPS navigation system, people scour the earth,
arriving at the correct coordinates and then searching in the vicinity for a
box, canister, or jar hidden cleverly under a pile of sticks, behind a brick,
or even inside hollow fence poles. When found, the person records his
or her presence on the log book inside and leaves a small token or gift.
More details about the geocaching phenomenon can be found at
The general genres of games that include geocaching are known as location-
based games. Most are in the format of real-world scavenger hunts, such
as visiting locations for points or virtual artifacts. Tourality ( http://tourality
.com/ ) is one such game; see Figure 8.8 . The goal is to reach as many
predefined GPS locations (stored in game sets) in the shortest amount of time.
Tourality generates game sets based on the location of the player. It is possible
to play in single player or multiplayer mode.