Transcript
Cell Site Analysis Cell site analysis is a relatively new specialty that deals with the geographical location of a cell phone at the time particular calls were made. As part of an investigation Call Data Records (CDRs), which record the start and end time of a telephone conversation or SMS message and the cell site (the mast) on which the call was made, are obtained either from the police or from a cell phone operator such as O2, Vodafone etc. For most operators this includes both the starting and finishing cells and an approximation of the direction of the phone from the mast. Mobile phone coverage in a given area is provided by a collection of overlapping areas of different sizes that are referred to as cells. Typically each cell radiates in a 120 degree arc from the cell site mast, although this does vary with some cells having omni directional antennas. Some, for instance on motorways, may just radiate in either direction along the direction of traffic. A simplified example of typical coverage from a number of cells is shown to the left. In this example, each mast has three 120 degree cells each radiating in a different direction and overlapping with cells from adjacent masts. There is a small localised cell, known as a pico-cell that may for instance serve a shopping centre, that is completely encompassed by a larger cell (a macro cell) A cell phone operating on the 2G network by specification monitors a number of different cells and as the phone moves (or as determined by the network) the phone will hop from cell to cell in order to maintain a good quality signal. Based on the information from the CDRs a cell site survey is undertaken using handsets with specially modified software to monitor the radio signals in the vicinity of the relevant masts so determining the “radio footprint” of the cell or cells in the area. The results of this survey in conjunction with the CDRs can be used to determine a range of locations that the telephone may have been in when the call was made. It should always be emphasised in any investigation utilising cell site analysis that it is usually only possible to place a handset in a particular geographical area, in urban sites this could be anywhere from a few hundred yards to a few miles from the cell site mast. In the countryside a particular cell can cover many miles. However, cell site analysis can often be used with confidence to say that a telephone could not be located in a particular area.
As an example, consider a scenario where a witness has claimed that immediately after a crime a suspect was observed entering an alleyway in the centre of a town to make a telephone call. Analysis of the call logs shows that the call both started and completed on the same cell ID. Records were obtained from the service provider and it was determined that this particular cell was a small footprint low power pico-cell that was commissioned to handle the heavy telephone traffic associated with the town centre. After the test handset was locked to the particular cell and a survey carried out in the immediate vicinity of the cell, and in particular in the alleyway where the call was said to have been made, it was determined that the signal strength of that particular cell at that location was not sufficient to make a telephone call. A graph depicting the signal strength of the cell as the test handset travels into the alleyway shows that it decreases below the strength required for a successful call indicating that the evidence provided by this witness may not be reliable. In a second example, it was claimed that because a particular call started on one cell and completed a few seconds later on a different cell that the telephone must have been travelling between the areas covered by the two cells. The defendant claimed that he was at home at the time the call was made. A network survey and subsequent examination showed that the defendants house was located in an area covered by both cells and best serving cell for any given call was determined by the location of the handset within the property. Although the simple explanation given earlier of the standard coverage of a cell helps us to understand the concepts of cell coverage, in practice of course it is more complex than this. In the example alongside, the yellow dots show locations where the associated cell was received by the handset as a serving cell. The picture clearly shows that the cell radiating to the North can be received and utilised by a handset located immediately behind the cell.
At Sanderson Forensics we use the latest hand portable equipment with GPS based location technology that allows us to undertake drive-by and walk-by surveys of the most inaccessible areas. Our measurement equipment and analysis software have been developed by a leading provider of test equipment to the mobile telephone industry and is designed around a standard mobile phone. It can be used to undertake surveys and analysis on both the 2G (GSM) and 3G (UMTS) networks. Where required we can decode the underlying logs and provide bespoke reports - such as this example showing the decay of signal strength of the neighbouring cells, below that necessary to sustain a telephone call on the GSM network, as the phone descends into a valley. Our investigators have been working in UK forensics since the early 90’s and have worked on many hundreds of cases including some of the UK’s most high profile civil and criminal investigations. In addition to Cell Site Analysis Sanderson Forensics can perform forensic analysis of mobile telephones providing detailed forensic reports on the content of the telephone and depending upon the make and model of the phone this can include deleted SMS messages, call logs and contact lists. Of course, we also provide traditional computer forensic services where we continue to provide reports to the highest of standards. Computer forensic investigations Mobile phone forensics and Cell Site Analysis Tribunals Civil disputes Litigation support Civil Search Orders Criminal Search Warrants
Accredited CPD providers to the Bar Standards Board and Solicitors Regulatory Authority For more information Contact Sanderson Forensics at: 01869 325667
[email protected] or visit our web site www.sandersonforensics.com
