Issue 151 February 2003
GPS-GSM Mobile Navigator

Start • System Features • GSM Transmits Data • Accessories • Sources and PDF

With today’s stand-alone global position system (GPS) receivers, you are able to pinpoint your own position. But, what’s more useful about stand-alone GPS receivers is that they can transmit your position information to other receivers. We decided to use both of these features to create a wireless vehicle tracking and control system for the Design Logic 2001 Contest, sponsored by Atmel and Circuit Cellar.

To design the Port Navigation System, we combined the GPS’s ability to pinpoint location along with the ability of the Global System for Mobile Communications (GSM) to communicate with a control center in a wireless fashion. The system includes many GPS-GSM Mobile Navigators and a base station called the control center.

Let us briefly explain how it works. In order to monitor ships around a port, each ship is equipped with a GPS-GSM Mobile Navigator. The navigator on each ship receives GPS signals from satellites, computes the location information, and then sends it to the control center. With the ship location information, the control center displays all of the ships’ positions on an electronic map in order to easily monitor and control their routes. Besides tracking control, the control center can also maintain wireless communication with the GPS units to provide other services such as alarms, status control, and system updates.

HARDWARE

GPS became available in 1978 with the successful launch of NAVSSTAR 1. Later, in May of 2000, the U.S. government ended selective availability (SA); as a result, the GPS accuracy is now within 10 to 30 m in the horizontal plane and slightly more in the vertical plane. For more information on GPS and its accuracy, read Jeff Stefan’s article, "Navigating with GPS" (Circuit Cellar 123).

The GPS-GSM Mobile Navigator is the main part of the Port Navigation System. The design takes into consideration important factors regarding both position and data communication. Thus, the project integrates location determination (GPS) and cellular (GSM)—two distinct and powerful technologies—in a single handset (see Photo 1).

The navigator is based on a microcontroller-based system equipped with a GPS receiver and a GSM module operating in the 900-MHz band. We housed the parts in one small plastic unit, which was then mounted on the ships and connected to GPS and GSM antennas. The position, identity, heading, and speed are transmitted either automatically at user-defined time intervals or when a certain event occurs with an assigned message (e.g., accident, alert, or leaving/entering an admissible geographical area).

This information is received by the system in the dispatching or operations center, where it is presented as a Short Message Service (SMS) message on a PC monitor. SMS is a bidirectional service for sending short alphanumeric (up to 160 bytes) messages in a store­and­forward fashion. If the only data received is time and position, then the data can be displayed on a digitized map and also recorded in a database file; the recorded information can be replayed later for debriefing or evaluation of a mission.

The hardware block diagram is shown in Figure 1. The AT90S8515 microcontroller assures that all of the components work well together; it controls all incoming and outgoing messages as well as the I/O channels, serial interfaces (RS-232), peripheral devices (e.g., LCD and buttons), and all other parts. The GPS module receives the GPS signals and outputs the data to the AT90S8515 microcontroller via a TTL-level asynchronous serial (UART) interface. The microcontroller works with the GSM module by communicating with the GSM network. The interface between the GSM module and AT90S8515 is also TTL async serial. An RS-232 interface is used to exchange data with the PC.

Because the AT90S8151 has only one UART, a three-channel multiplexer is used to switch among three working modes. The location information and other data is stored in the 2-Mb serial data flash memory of the AT45D021. The flash memory stores up to 2160 pieces of location information in 12 h, because the GPS-GSM Mobile Navigator saves GPS signals every 20 s. Four buttons, an LCD, and a buzzer enable you to display the system status and information and control the navigator.