When did you last see your phone? And where was it on the night in question?

Cell site analysis is the process of determining where a mobile phone may have been when certain calls were made.

In just 10 years, it has gone from being a barely understood forensic technique to being a key investigative tool which is used to support a growing proportion of criminal cases.

The process of undertaking a cell site analysis is aided both by the way in which mobile networks gather billing information and by the physics that underpins the operation of the phone networks themselves.

The main plank of evidence that supports cell site analysis is provided by mobile network billing records – known in technical jargon as ‘call detail records’ or CDRs.

If an investigative team manages to attribute a mobile phone number to a suspect they are able – through the provisions of the Regulation of Investigative Powers Act (RIPA) – to request copies of that phone’s CDRs from the relevant network operator.

A typical CDR provides details of all calls made or text messages sent by a phone; some networks also provide details of calls and texts received by the phone, too. A CDR record shows the date and time the transaction took place, the numbers of the phones involved, the call duration and, importantly, the identity of the mobile phone ‘mast’ or cell site that carried the call.

A typical mobile phone cell site incorporates a radio device known as a ‘base station’ that supports three radio antennas. Each antenna sends and receives radio traffic over an area that covers just one third of the geographical area surrounding the mast. These subdivisions of a cell’s coverage are known as ‘sectors’ and each sector on each mast belonging to each mobile phone network is allocated a unique identity number, known as a ‘cell ID’. The cell ID used for a call is captured in each call detail record.

As each sector covers only a limited geographical area, it is possible to gain a reasonably clear idea of where a user and their phone may have been located when calls were made. The implication of this is that not only is Big Brother watching; he knows where your mobile phone is, too!

If we at Focus are instructed to undertake a cell site investigation – by either prosecution or defence – we begin with the call records obtained for significant mobile phone numbers. These provide details of the cell IDs of the masts used by the significant phone at around the time the offence being investigated occurred.

Once we have a list of cells to analyse we use the provisions of RIPA again to request details from the relevant network operators that show the exact location of each mast and other significant operational details.

The main question asked in a cell site analysis is: “Is it possible for a mobile phone call to have been made using a particular mast at a significant location?” The only way to definitively answer that question is to visit the location and take a series of ‘network readings’, which allow us to see whether the mast in question provides a signal there.

To make that determination, Focus engineers use radio measurement equipment that collates details of the set of mobile phone masts that provide signals at a given location and the received strength of each of those signals. The strongest signals measured at a location are those that a mobile phone is most likely to choose to use if asked to make a call; these are known as the ‘serving’ or ‘active’ signals for that location.

The information that forms the basis of a cell site analysis includes:
• A phone’s CDRs.
• A time and a location.
• Network readings.

If the target mobile phone made calls at around the time of an offence using a cell site that provides a serving signal at the offence location, we are able to conclude that it is possible for that phone to have been at that location at that time. If, on the other hand, the target phone was using a cell site that provided no signal at the significant location we could conclude that it is not possible for the phone to have been there.

Cell site analysis is undoubtedly a useful tool but it has some limitations.

Firstly, cell site data can show where a particular mobile phone may have been when calls were made; it cannot provide details of who was using the phone at that time.

Secondly, cell site analysis is not an exact science. The fact that a call was made using a cell that serves at a significant location only shows that it is possible for the phone to have been there when the call was made; it doesn't mean that the phone was definitely there.

All we can say with certainty is that the phone was somewhere in the serving coverage area of a cell when it was used; pinpointing the exact location of the phone is beyond the capabilities of cell site analysis.

There is therefore a degree of uncertainty related to cell site evidence, which means that it works best when supporting other evidence rather than being the key piece of evidence itself.

Cell site analysis is an evidential tool increasingly being used by the prosecution: how many defence cases would benefit from access to the same techniques?