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with different visual impairments. When asked to walk to pre-determined outdoor and indoor landmarks, participants experiencing a central vision loss and total vision loss, for instance, asked more questions relating to side streets, steps, distance, and temporary obstacles, than participants with a peripheral vision loss. Such differences in the form and structure of spatial knowledge will need to be addressed if way-finding technologies (such as those illustrated in the next section) are to be usable by visually impaired people.

7.3 Overview of Existing Technologies

In the 1940s, the long cane was adopted as the primary mobility aid within the blind community (Farmer 1980). Since then, electronic travel aids (ETAs) have been developed in order to provide more detailed feedback regarding obstacle avoidance and navigation within the immediate environment (see Chapters 5 and 6 for more discussion on obstacle avoidance issues). However, in order to support independent mobility of visually impaired people to unknown destinations along unfamiliar routes, these devices will need to be supported with distant navigation technologies. This is the topic of the next section in which both indoor and outdoor technologies will be illustrated. The last section will describe output technologies for communicating information to the visually impaired traveller.

7.3.1 Technologies for Distant Navigation

Within the last decade, the development of ETAs has been guided more to the challenging topic of supporting distant navigation and independent mobility. One approach is to use a network of location identifiers that can be remotely sensed by the visually impaired traveller within either indoor or outdoor environments. Some systems use infrared transmitters installed throughout the environment to transmit digital speech about the location (see Chapter 10), while others use radio frequency (RF) beacons to wireless transfer digitally coded and compressed speech, e.g. Kemmerling and Schliepkorte (1998). The main drawback of this technology is the cost of installation. In addition, contextual information is limited (vital information regarding temporary obstacles is not included) and generic for all users. Individual requirements such as those described in Figure 7.1 would not be supported.

Global positioning system (GPS)-based navigation aids, combined with geographical information systems (GIS), have been used by many researchers to assist navigation for visually impaired people in outdoor environments (see Chapter 8). Here, signals picked up from satellites orbiting the earth’s atmosphere are used by a mobile device to convert latitude and longitude coordinates to a geographical location using a digital map. Good examples of research-based GPS systems (other than Chapter 8) include: (i) the MOBIC Travel Aid which integrates differential correction from ground base stations (DGPS) for greater location precision, a compass worn on the body for heading direction, and mobile telecommunication

facility (Strothotte et al. 1996), and (ii) a navigation system for the blind that uses a telephone connection to transmit the user’s GPS coordinates to a central server, which sends back digitised speech to the visually impaired traveller (Makino et al. 1997). There are, however, privacy issues associated with the latter system, concerning the transmission of location information, that would need to be given further investigation.

While GPS capability offers great potential, its accuracy and reliability is still not good enough for safe and effective navigation by visually impaired people since DGPS is not available in many locations, and the signal from satellites can become blocked in narrow built up areas. In addition, most GPS-based systems have based the level of service solely around the GPS function, and have not considered other contextual information in the visually impaired person’s environment that could facilitate and enhance independent mobility and navigation within unfamiliar environments (discussed further in Section 7.4). Helal et al. (2001), for instance, describe how many GPS-systems ‘lack dynamic query capabilities and support for dynamically changing environments’ and that ‘context-awareness is not well supported’.

Unfortunately GPS is ineffective inside buildings and so most location-aware systems within indoor environments depend on relative positioning using various technologies. Active Badge, for instance, is a small wearable device which transmits a unique infrared signal every 10 s (Want et al. 1992). These signals are detected by one or more networked sensors, which are used to determine the location of the badge on the basis of information provided by these sensors. There is also evidence to suggest that location can be inferred using other methods by integrating information from accelerometers, magnetometers, temperature and light sensors (Golding and Lesh 1999) or using wireless networks (see Section 7.4).

7.3.2 User Interface Output Technologies

Most existing mobile GPS systems transmit information to the user by a visual display (or screen) only. However, those designed specifically for visually impaired users normally give information by speech audio. There are, however, notable problems using such an approach. Franklin (1995) highlights the difficulties of interpreting spatial relations from common speech, Pitt and Edwards (1996) indicate how speech interfaces are slow to use and more demanding on memory than vision or touch, and Strothotte et al. (1996) demonstrate how headphones used to transmit audio messages may mask/distort environmental sounds that visually impaired people use to avoid hazards. Other styles of communicating information include:

•Non-speech output. This is where audio signals, such as beeps, are used to direct the user to a landmark without the use of speech. Some researchers have studied the use of panning to indicate spatial relations, where contrasting tones indicate direction and the rapidity of sounds indicate distance (Holland and Morse 2001).

•Haptic feedback. Vibration alerts can be transmitted using tapping interfaces to indicate when the user needs to turn left or right. Refreshable Braille displays