Fast-Trilateration + Cell-ID Method for Mobile Phones Location Estimation J. P. Martin † M. Panario † Supervisor: J. Alberto ‡ †School of Engineering - University of Buenos Aires, Argentina {jmartin, mpanario}@fi.uba.ar ‡School of Engineering - University of Buenos Aires, Argentina Professor of 66.09 Microcomputers laboratory RPIC2009 Estudiantil Abstract— This paper introduces a new model for trilateration. The aim of this sys- tem is to achieve optimal location estimation via signal attenuation of the incoming signal on a mobile phone. Calculus are simple enough for the the entire system to be implemented on a 8051-family microprocessor with an embedded web server. Keywords— Trilateration, Geopositioning, GPRS. 1. INTRODUCTION There are many strategies[10] to calculate the position of an object. Some of them use the angle from two references to the unknown position object[14], but in- formation on the angle is not available in most cases. When the positions of three reference sources and their distances to the unknown-position object are known, trilateration [11] can be used instead. A simplified method was designed[8], achieving an optimal perfor- mance and a fault-tolerance design. The tests and measurements have been done on a simulated environ- ment. 2. DEVELOPMENT 2. 1. System Overview The main system consists of a C8051F120[4] micro- processor and an ethernet card (CP2200) working as a web server[5] which trilaterates the position of the mobile phone that is being traced. The phone has a software that sends data including the strength of the signal[15] coming from 3 different cell towers, as well as their cell-ids[12]. The location coordinates of each cell tower are stored in a database built-in the micropoces- sor. After the information is recieved, the micropro- cessor trilaterates the phone’s position and creates a webpage which includes a map[2] with a marker point- ing to the estimated position of the mobile phone. 2. 2. Software / Firmware The software of the microprocessor was written in as- sembly and C languages, the TCP stack[3] and the web server[13] were taken from the Ethernet Develop- ment Kit of Actmel[6]. We use a web server because it is a simple method for the phone to send the received signal strength; the mobile phone sends a HTTP GET request to the microcontroller as http://server_ip/ ?a1=d1&a2=d2&a3=d3 where a1 indicates the Cell-ID of the first tower, and d1 states the strength of the signal coming from that tower. 2. 3. Distance Measurement Even though the distance between the cell tower and the mobile phone can not be determined with enough precision just by using the information that the mo- bile device is able to get, it is very easy to know the strength of each cell signal[7]. Knowing the signal strength, the distance can be known with an aprox- imation within the precision range expected by this method[1]. If the signal was blocked by any object, sometimes an approximation can be figured up; the signal of the three towers will be attenuated in the same proportion. Even if this does not happen, it can be demonstrated that the calculated position will be the same as if there was no attenuation. 2. 4. Aproximation In every calculus made, the unit used was distance. In other words, the x axis meant the distance between the coordinate origin and the orthogonal projection of the point in the x axis. However, all positioning systems use geographical coordinates, which may be decimal or degrees. Decimal geographical coordinates were used for the sake of system simplicity, thus introducing the need of a unit conversion. For that reason, a point of reference was taken, and as a approximation was sup- posed that the mobile phone was moving over a small area, so it was correct to make a lineal conversion; len =(ref - coord) · k (1)