Software
Data-Base of Carleton
Refer to Appendix A for the computerized data-base of the streets of Carleton.Figure 5) GPS Map of Carleton
With the GPS Pathfinder Basic (a GPS receiver), I walked around the streets of Carleton collecting data points every 10 seconds. This took approximately one hour to complete. Once the data points were collected, these points were differentially corrected to increase the accuracy from + 100m up to + 3m. This was accomplished with help from SurNav, a company which deals in GPS equipment.
Once the points were differentially corrected they were plotted to scale, and overlaid on an actual map of Carleton to verify the accuracy of the GPS receiver (Fig. 5). As seen in Figure 5, there are some slight discrepancies at points A and B. These were caused by loss of a satellite. Three satellites were in view at these two points, which introduced errors into the measurements.
After obtaining the entire electronic map of Carleton, I then proceeded to key the street intersections of Carleton into the data-base. These were taken directly off the differentially corrected map.
Software
Data-Base of Carleton
Refer to Appendix A for the computerized data-base of the streets of Carleton.
Figure 5) GPS Map of Carleton
Communication
In order for this software to communicate with the GPS receiver the receiver's comm port must be set to TSIP (Trimble Standard Interface Protocol).
The software I am using was developed by Trimble and is called Toolkit. The specific source I am using is called pktmon, which is a C-language program. Pktmon was stripped down to protocol communication between the GPS receiver and the computer. Afterwards, it was rebuilt with additional features, such as: a help menu, search algorithms, and satellite strength information.
Before communication can commence, an initialization code must be sent to the receiver so that the receiver will be sending correct information to the computer. That is, the GPS receiver is to send the positional information, only upon request, in a format which can be directly compared with the internal data-base. Refer to Appendix B for the I/O Options sent to the GPS receiver.
Determining the Street Intersection
Upon being queried by the user, the computer will send a command to the GPS receiver asking for the current position. After obtaining this position the computer will then search the data-base for the shortest distance (within a given radius), between the user's current position and all intersections. The radius depends on the accuracy of the positional information being obtained via the satellites. A small radius indicates fairly accurate positional information, where as a large radius could indicate an accuracy of + 300m. Refer to Appendix C for the intersection search algorithm.
Programming the Intex Talker
The Intex Talker is a text-to-phonetics converter. These phonetics are amplified and then sent to the speaker. After a match between the user's current location and an intersection has been made, this text string containing the names of the two intersecting streets is sent to the Intex Talker, which in turn audibly relays the information to the user. Refer to Appendix D for the communication between the computer and the Intex Talker.
Hardware Interfaces
GPS Receiver - Computer
The GPS receiver is connected to the computer through the comm ports of both devices. The signal from the GPS receiver is in RS-422 format. The computer's comm port is RS-232 but it will accept RS-422 signals.
Computer - Intex Talker
The printer port of the computer is connected to the parallel port of the Intex Talker via a ribbon cable. With reference to the schematics of the LapTop computer and the Intex Talker, I made up the cable needed to connect these two devices together.
The Intex Talker was originally run off 15v a.c.; with some help from Mike Kelly I was able to get the Intex Talker to run off a 12v d.c. gell cell.
