GPS/ Location Devices

In order to send a signal from our protective gear to an authority, we can apply existing technology, and one approach is to use GPS. One of the providers of GPS service if the US government, it offers the "Standard Positioning Service" to the users worldwide. The 95-percentile accuracy of this system is 100m, which means the actual position of the signal origin is no farther than 100m of the calculated position 95% of the time. An advantage of the GPS system is that its ground station can be a mobile unit like an ambulance so the rescuers can locate the wounded directly without contacting other authorities. And because satellites are orbiting around the earth, this system has no limited range. However, in order to apply this technology, it is necessary to implement a GPS device (see figure 1) onto our protective gear, which will increase the cost of our product.

 

Figure 1: Various GPS devices and their applications

Another way to determine our position is to apply Cellular Telephone Positioning (CTP), a system still under development. The basic structure of this system is generally the same as GPS, except that in this system we locate the source of the signal by finding its position relative to cell sites instead of to satellite, and the position of each cell site is determined by GPS. Also, we use ordinary cell phones to substitute GPS devices. Unlike satellites, cell sites are not high above the ground, so most of the time signals travel horizontally instead of vertically. As a result, the accuracy is seriously affected by physical obstructions. For example, tall buildings keep signals from travelling in a straight line, and they may also build up multipaths which will elongate the travelling distance. In order to get a more accurate result, more cell sites are needed to average out the errors, and further calculation is required to eliminate defects result from obstructions. The advantage of this system is that consumers do not have to pay extra costs for the positioning device because ordinary cell phones are used to send out signals. However, the cell sites will have to be integrated (Figure 2) in order to perform calculations required to determine the distance of a cell phone signal. And because CTP utilizes cell phones and cell sites to determine a caller's position, this positioning system will not function in areas where cell phone signals aren't available. But if the range of cell phone signal increases in the future, CTP will be highly practical even in rural areas.

Figure 3: The equipment at a cell site conducting time of arrival measurements includes a standard receiver multi-coupler, a receiver tuned to the reverse control channel, a correlator, a computer, and a GPS receiver. (Source: http://www.gpsworld.com/0898/0898fig1.html)

A field test of CTP was conducted in Calgary in 1996 and the result is showed in Figure 3. At the corners are four cell sites used to determine the position in this experiment. The asterisk in the middle of the figure in represents the actual location of the caller, and the blue dots are the calculated results of the test. The 67-petcentile accuracy is 137 m, and the result is expected to be more accurate if more cell sites are used to determine the position.

Figure 3: Relative position of the caller (red), calculation results (blue), and cell sites (green) in the experiment conducted in 1996.