The Global Positioning System consists of three parts: Earth orbiting satellites, control and monitoring stations across the Earth, and GPS receivers owned by individuals. Multiple sets of 24 satellites are orbiting the Earth every 12 hours while broadcasting their position and time. Ground-based receivers (hand-held GPS devices in watches, phones, cars, airplanes etc) listen to the signals from four or more satellites, comparing the time transmissions of each with its own clock. Given that signal travels at a known speed, the receiver can calculate the distance between the satellites and receiver. Combining the position of the satellite at the time of transmission with the distance, the receiver is able to determine its own location. After the original american GPS, other countries have developed their own versions. Europe's GPS is called Galileo, Russia's is called Glonass, and China's is called Beidou. Modern receivers can use satellites from all these systems simultaneously.

A model is simply a representation of a real thing. You have seen and used models in the past, like a globe which is a model of the earth. Geographers construct models to analyze geographic processes because the real object of study may be too large to examine, the processes which created it operate over too long of a time frame, or experimentation might actually harm or destroy it. For instance, physical geographers construct physical models like stream tables to investigate the impact of hydrological processes on the earth. A stream table is more or less like a shallow sink filled with earth material similar to the land surface of interest. Water is applied to the material to see what effect varying amounts of water have on the erosion of the surface. Models may be simple conceptual models such as a box and arrow diagram showing the flows of energy between compartments of an ecosystem. More and more, physical geographers use mathematical or numerical models. These could be complex numerical statements programmed into a computer model representing the impact of increasing carbon dioxide content of the atmosphere on global temperature or rainfall on erosion and landscape evolution. The video below shows a visualization of a global climate model output.

Figure 1.2.2.11.2.2.1\PageIndex{1} shows the rugged terrain one finds in the Gangotri Glacier. North is at the top of the photograph.