I have more and more often the occasion to deal with the location, that is those activities that aim at acquiring information about the position of mobile devices (especially smartphones). In order to carry out this activity, I use the traditional forensic tools which are appropriate for the data extraction (UFED, Oxygen etc.) and a device designed for the detection of the mobile network of the area of interest. I use TeMS (TEst Mobile System), but useful alternative products exist.

The tool scans the mobile frequencies of a provider, identifying and measuring the characteristic parameters (power, signal/noise ratio, etc.) of the serving cell for a specific place and of the so-called adjacent cells, that is those cells which could serve a specific place but lose respect to the serving cell in terms of power and signal quality or simply for the priority chosen by the telephone service provider.   

I got the idea of writing this brief article after having observed some places that, at least from the point of view of the mobile frequencies, can be defined as “singularity”. I know that the word “singularity” comes to mind Matrix (technological singularity) above all for the fans of science fiction but here I want to talk about something more concrete. The importance of this experience, that I will introduce in a little bit, concerns neither a specific place nor the usefulness that it had in this specific case. It concerns the indication that allows me to update my check list used in later similar cases.

It is worth remembering that the detection of mobile coverage is much more useful for excluding the presence of a mobile device rather than locating the same since the linkage to a cell supplies information which are, generally, too vague in terms of coverage area. I have used the adverb generally because in some cases, especially for the discrimination between indoor and outdoor building, you could acquire information which can be considered as evidence. Moreover, the detection of the mobile coverage is useful to narrow the area supplied by telephone service providers.

Given this premise, I will talk about my experience.

I worked on an assignment, conferred by an investigating magistrate (or public prosecutor), concerning the death of more than one person and it was necessary to map the centre of a medium-sized town to determine the exclusion of some mobile devices close to a specific place of interest.   

The detection was realized on different days and at different times in order to create a statistical sample reliable for our purposes. The results were presented to investigators even in graphical form by means of Google Map which also allows to represent geo-referenced values.

Amongst such values, I observed some singularities and one of these was particularly significant around an intersection as showed in the figures below.

                                            

 

 

The first three figures show the individual detection of serving cells and the indication of the geographical origin, while the last one shows the overlapping of the three detection distinguishing each cell with a different color. The cell in red was located going through from south-southwest to north-northeast and then turning left in the direction of north-northwest. The cell in blue was located going through from south-southeast to north-northwest. The cell in green was located going through from north-northwest to south-southwest and then turning to the cross toward north-northwest. So, according to the direction of the approach adopted in the same place different cells result and, at first sight, this could be a contradiction. From a visual point of view, it seems like a trail which passes through the centre. 

Actually, this phenomenon can be explained from a technical point of view considering that a mobile device uses two criteria for the selection and reselection of the serving cell. In particular, the formula for the “cell reselection” (the formula can be found here) used by providers is useful for the dynamic reselection of cells. The formula increases the algorithm used for the standard GSM which is useful, among other things, to avoid the bouncing phenomena among the serving cells that occur when two or more cells are available to serve a specific place with similar power and signal quality. The algorithm introduces a “temporary offset” which “discourages” the adjacent losing cell respect to the serving cell. At the point of this local singularity described above, a kind of continuity or hysteresis is realized which tends to create a continuous trail according to the direction from which you come.

To conclude, it is clear how this represents, around the singularity, an element of uncertainty, while it is less clear how this can represent an advantage if you want to know the way of escape of someone. By chance, unfortunately for the suspect (but fortunately for the investigators) in that time and in that position a communicative event (call, message, etc.) occurred.

Let me take this opportunity to thank the inspector Antonio Corvasce of the Italian National Police whose passion and knowledge allowed and supported this professional experience.