The ability of visually impaired individuals, especially when blind, to locate themselves in an urban or rural environment with respect to road intersections, prominent landmarks, etc. has always been a serious problem. In addition, and of equal importance, is the inability to communicate their location to a central control facility in the event of their needing any additional assistance.

With the advent of high accuracy global positioning many applications, which heretofore had been considered highly futuristic and discussed only in the most tentative terms, are now being researched. One such application is the use of differential Navstar GPS and its capability as an aid to the visually impaired. The concept envisions a small device that can be worn by the user. This device will have the means for providing Navstar GPS phase information accurate enough to be converted to a specific street location. Inherent in the concept is means by which the users can be instantly apprised of their location and/or can communicate with a central location for other purposes.

To establish feasibility of such an application, it was necessary to understand the limitations of Navstar GPS. In particular those that may result from its use in an urban environment. Accuracies of the Navstar GPS system tend to depend on the sophistication of the receiver to be used. At the time there was some question as to future developments of a small enough package for the intended use. Along with this was the question of the suitability of differential positioning for increased accuracy. This article deals with the initial field tests that were run and the conclusions drawn from this work.

Similar work was previously accomplished in 1985 by the US Department of Transportation, Transportation Systems Center. The referenced effort included both the use of Navstar GPS as well as Loran-C which our effort did not but did confirm a number of our findings relating to the limitations of Navstar GPS in an urban environment and attenuation in the presence of foliage, it did not centre on differential Navstar GPS as did our work.