4.1.3 Global positioning system (GPS)

These days, it is possible to buy a device known as a global positioning system (GPS) to tell you where you are. Receivers are made for aircraft, ships, ground vehicles, and (as the one shown in Figure 6) for carrying in the hand.

Figure 6 The Lowrance iFinder, an example of a hand-held GPS receiver for use by sports people

Examples of applications for GPS are:

• navigation;
  
• surveying, and establishing the shortest distance between two points (a line of sight along the ground is no longer necessary for precise positioning, so greater distances, with features such as hills obscuring the line of sight, can be surveyed much more easily);
  
• plate tectonic studies (seeing how large areas of the earth's surface move relative to each other).
  
  

The worldwide GPS is funded and controlled by the US Department of Defense (DOD) but its standard positioning service is used by many thousands of civilian users worldwide. More expensive receivers, such as those used in aircraft, are more accurate than the standard service used by most recreational receivers.
The system involves a network of satellites in orbit around the earth, which provide specially coded signals that can be processed in a GPS receiver. Signals from four GPS satellites enable the computer in the receiver to compute the receiver's position in three physical dimensions (i.e. latitude, longitude and altitude), the receiver's velocity, and a highly accurate time. (Velocity is speed plus direction.)

The satellite system for GPS

The GPS satellite system consists of at least 24 satellites in orbit at any one time. The placement of the satellites is such that a user's ground-based receiver can receive signals from between five and eight satellites from any point on the earth's surface. Satellite orbits are calculated and controlled from a series of ground stations, one of which (in Colorado in the US) is the master station.

Most leisure users of GPS receivers want to relate position, and perhaps movement, to particular places and features in the landscape. It may be no use to a hiker (trying to get to the next refuge) to come to the foot of a high cliff believing that she's travelling in the right direction; she needs to know the cliff is in the way of her direct line of travel. This is where the geographical data that goes into making maps comes into its own.
Most GPS systems (other than very simplest) allow users to superimpose their position information onto a map created from a collection of geographical data that can be loaded into the GPS receiver. As the user moves through an area, the map is constantly updated keeping the user at the centre of the map. What transforms the data into a map (information) which can be understood is a small computer contained within the receiver device.
Figure 7 shows a GPS receiver manufactured by the American company Garmin for BMW motorcycles, and gives an example of how the GPS receiver's position is ‘placed’ at the centre of the map.

Figure 7 An example of a GPS receiver storing geographical data for producing maps; the small arrow at the centre of the map indicates the current position and direction of the motorcycle carrying the GPS receiver

What happens when a map is wholly inappropriate, e.g. when navigating in the dark, smoke or fog, or if the user has a visual impairment? In such situations oral directions are needed. These use words like left, right, straight ahead, crossroads, junction, and so on to describe both surroundings and the ‘features’ of a journey. While rare, it is possible for something like a GPS receiver to be programmed to give audible directions as an alternative (or supplement) to a map: ‘Proceed straight ahead for 100 metres before turning left 90 degrees into Porchester Road …’.
Such devices are being further developed as hand-held aids for:

• those with a visual impairment to navigate campuses, shopping malls and buildings;
  
• members of the emergency services who may have to find their way around an unfamiliar building quickly in thick smoke and darkness.
  

Since, in the case of buildings, GPS signals don't work to a small enough scale and don't penetrate into such structures, they can be supplemented by small local signal beacons or even bar codes on doors and in corridors.
Even without a GPS receiver, you may still benefit from the way computers can represent geographical data. Motoring organisations offer a free route-planning service over the web that gives directions from one place to another. This works out a route on which it's possible to apply constraints, such as taking the fastest route or taking the shortest route or avoiding road works. I typed in my location and a friend's, and received a printable map with the route highlighted (not shown) and a set of directions shown in Figure 8. Even if I couldn't read a map, I could use these directions.

Figure 8 A route between Biddenham and Leagrave in Bedfordshire, obtained from a route-planning service on the web. Taken from http://www.theaa.com. AA recommended.

There are three important themes in this case study on maps that will recur throughout this unit.

1. The right sort of data, properly used, is a very powerful aid in creating appropriate information (e.g. generating maps from geographical data).
   
2. It is possible to present information in a variety of ways to meet different requirements (e.g. a map for a hiker or directions for those who find maps daunting).
   
3. A computer can transform data into information in ways not previously thought possible (e.g. the information on a GPS receiver).
   

Here are some further examples of how a computer system can use the right sort of data to generate useful information in an appropriate way.

• A computer in a microwave oven transforms the pulses of an electronic clock into a time display that shows how long until the cooking is finished.
  
• A computer in a satellite television control box obtains the signals emitted by a transmitter satellite and converts them into a television picture and sound for the attached television set.
  
• The computers in a nuclear power station monitor signals produced by pressure sensors and other devices to provide a moment-by-moment summary of the state of the reactor.
  
• A computer in a car turns the pressure of the driver's foot on the brake pedal into fine control movements of each wheel's brake so as to prevent the car from skidding.
  
• A powerful PC turns signals from a scanner into a representation on the computer's screen of the item scanned. The PC can then accept commands from the user to modify that image.
  

It is transformations like these that lie at the heart of this unit.

Exercise 9

Consider a computer in a modern cooker.

1. What kinds of data might it require and where would these originate?
   
2. What kinds of information might it present to the cook?
   

Discussion

1. The data originates either from the cook (pressing buttons to set a timer, for example) or from signals from the cooker's clock or its temperature sensor inside the oven.
   
2. A small display might show the time on the clock, how much time is left on the timer, and the oven temperature.
   

SAQ 4

1. What is the role of the computer with respect to the data given to it?
   
2. How should requirements (such as the need for a user's attention to be focused on a complex task like driving) affect the presentation of information?
   
3. What, in your own words, is the meaning of the term parameter?
   

   Answer

   1. The role of the computer is to transform data into information.
      
   2. The presentation of information needs to be fit-for-purpose and, in the example given, presented in a way that lets the user keep their primary focus of attention on the task.
      
   3. A parameter is a property or characteristic of something that is measurable or quantifiable.