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8.1.2What Mobility Tools and Traditional Maps are Available for the Blind?
The most common mobility tools used to aid in obstacle avoidance are:
•Long cane.
•Guide dog.
•Sighted guide.
•Tactile maps.
The long cane and guide dog allow the blind pedestrian to explore their environment independently. A sighted guide would make sure the blind traveller travels from point A to point B without running into obstacles. Ultrasonic devices like the Miniguide augment these other mobility tools.
In this chapter, we are focussing on electronic orientation technology, specifically that which uses the GPS, as opposed to obstacle avoidance devices for mobility like the long cane or electronic travel aids (ETAs).
When it comes to maps, blind people are not only in the dark, they are in the dark ages. Internet-based navigation, like Yahoo!® maps and MapQuest®, could be useful if the blind person has a computer screen reader but again these mapping systems are designed with one thing in mind the “sighted” mass market. These systems will try to find the most direct driving route using highways, and obeying the rules of the road. This may be a great solution if you have someone to drive you, but is useless for a blind pedestrian.
While tactile maps are great for giving an overview of a country, state, or city, they are not as good for representing detailed streets and landmarks. A physical overview plays an important role in developing a mental geographical picture. The fundamental size limitation of tactile resolution also limits its utility for quickly displaying detailed street information. A good tactile geographical overview does help to put the detailed digital map information into context if those details are accessible.
Blind travellers with ingenuity and excellent orientation and mobility skills have learnt to use their memory and other senses to access a small percentage of this location information. Specific routes are memorized including the occasional landmark to remind the blind traveller where to turn or conclude the route. It has been stated that visually impaired people have no less potential than the sighted for developing a fully integrated representation of space (Millar 1988). Notice the use of the word potential in that statement. Blind people can get around effectively with proper training, experience and tools.
8.2 Principles of Global Positioning Systems
8.2.1 What is the Global Positioning System?
The GPS is a worldwide radio-navigation system formed from a constellation of 24 satellites with three active spares and their ground stations (Figure 8.1). The
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Figure 8.1. GPS satellites in orbit (image courtesy of Peter H. Dana, The Geographer’s Craft Project, Department of Geography, The University of Colorado at Boulder)
Figure 8.2. GPS satellite (image courtesy of Peter H. Dana, The Geographer’s Craft Project, Department of Geography, The University of Colorado at Boulder)
satellites orbit the earth in six orbital planes at 55◦, 12-h orbits, at an altitude of 20,350 km (12,644 miles) in space, weigh 862 kg (1900 lbs.), are 5.18 m (17 ft) in length, and last for about 7.5 years before they need to be replaced (Figure 8.2).
The ground stations (also known as the “Control Segment”) verify that the GPS satellites are functioning properly and keep track of their exact position in space. If there are any discrepancies between the satellites and the ground stations, the master ground station will transmit the corrections to the satellites themselves. This will ensure that the GPS receivers have the correct data from the satellites (TNL 2001). The concept behind GPS is to use satellites in space as reference points for locations here on earth. Picture yourself as a point on earth with the satellites circling above you in space. The GPS software measures the distance from the satellites to your GPS receiver. This is calculated by measuring the time it takes for the signal to travel from the satellite to the receiver. By gathering data from at least three satellites (also called trilateration; Figure 8.3), the GPS receiver is able to calculate your position on earth and sends the latitude and longitude coordinates to whatever device (including a computer) that can make use of them.
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Figure 8.3. Illustration for the trilateration of GPS satellites
Figure 8.4. Illustration for latitude (left) and longitude (right)
Latitude lines run horizontally across the globe and longitude run vertically. Every point on the planet has a latitude and longitude and the GPS means this point can be given a meaningful name (Figure 8.4).
8.2.2 Accuracy of GPS: Some General Issues
On average, commercial GPS receivers are accurate within 9.15 m (30 feet). So, instead of viewing GPS position as a pinpoint, consider it as a bubble of radius 9.15 m around your position (Figure 8.5). However, various general factors can make the satellite information more or less accurate.
For a GPS receiver to work properly, it needs to have a clear view of the satellites. That means that GPS receivers do not work in places where all the satellites can be blocked, such as indoors, in tunnels, or in subways, for example. Poor GPS reception is also known to occur on streets in big cities when some satellites are screened by skyscrapers (also called urban canyons), and in areas surrounded by very tall mountains or forests.
Another situation where a GPS receiver might not be as accurate would be when all the satellites are coming from the same direction. Due to the significant distance between the satellites and the GPS receiver, the satellites need different angles to
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Figure 8.5. Range of GPS accuracy illustrated by man in bubble
Figure 8.6. Satellite positions for accuracy: poor satellite positioning resulting in poor accuracy (left); good satellite positioning resulting in good accuracy (right)
decipher the precise location of the receiver (Figure 8.6). So, the receiver might be out in an open valley, but if all the satellites are right above it, the accuracy will diminish (Dana 2001).
Another factor that could influence GPS accuracy is the need for national security. The Department of Defense (DOD) developed GPS for military purposes. Since this technology is available worldwide, anyone can use GPS for location information. Originally, the government implemented a system called selective availability (SA) to scramble the signals from the satellites. This caused inaccuracies of 100 m. From May 2, 2000, U.S. President Clinton turned off selective availability but this decision could be reversed at any time (NGS 2000). However, this seems unlikely as the proliferation of civilian applications of the GPS permeate our society. Two new satellite systems are being implemented to augment the accuracy of GPS. The so-called Wide Area Augmentation System (WAAS) and the system known as EGNOS are geosynchronous satellites, which work in conjunction with ground stations to correct for some of the 9.15 m average error in the GPS system. These systems offer up to 3 m accuracy when the signals can be intercepted. Unlike GPS,