Mobile telephone and cell site analysis
With the advent of mobile phones came a further tool available to police in the detection of crime: phone analysis.
In its simplest form it is possible to analyse incoming and outgoing calls to and from a mobile; in more complex form it is possible to site an approximate location of a phone and, with smart phones, identify IP addresses to ascertain from where and when an email might have been sent. Each telephone has an IMEI number which can also be used as part of an investigation. In recent years we have seen a huge increase in the use of this material as part of the prosecution case against defendants, but it can equally be deployed on behalf of a defendant, or an alternative interpretation might be more persuasive.
Calls made and received can give an indication as to the frequency of contact between two or more persons. But mobile phone data can go much further than this. Cell site data is a tool that can identify the area in which a mobile phone was at a specific time, or identify the likely route that it took over time.
Mobile telephones work by transmitting their signal to the nearest aerial, or cell site. This aerial is connected to the mobile phone network that routes the call to the making or receiving phone. Each cell site has an identifier which is recorded when a phone call (or a data call) is made. Furthermore, most cell sites have three aerials each covering a 120 degree arc, called an azimuth. The relevant azimuth is also recorded. A site survey of areas within that arc will produce data showing which areas have a signal that communicates with that cell. It is important to note that cell siting can only put a phone within the arc of the azimuth, not at a specific location.
Once police have the relevant time and the telephone numbers they are investigating they can put together a schedule of calls and data. Each call has a start and end cell site, so it is possible to see whether a phone was moving or stationary, as well as whose phone it was communicating with. Tie that in with other evidence (witnesses, CCTV, etc) and a compelling picture can develop.
Police often have to conduct an attribution exercise if a phone is not registered to a specific user as the data relates to the phone and the SIM card. This can be done by looking at numbers called and whether a phone number features in another person’s phone book, perhaps recorded under their name. Sometimes phone are co-located, suggesting that an individual is operating two phones.
From the information commonly available it is open to the police to make assertions about an accused, for example that he was at a certain location at the time of a call, or that he travelled a certain route during a certain time period. Taken with other evidence, the prosecution case against an individual can appear much stronger than without it. However it is important to remember that although the data is usually unimpeachable, much of what is asserted is only interpretation of data: the data fits the story the prosecution are trying to tell. That does not mean that it is the only story: it may well be that there is an innocent explanation that is equally or more compelling than the prosecution case. The analysis of this data is critical in developing a defence case. Simply looking at the numbers is rarely sufficient: obtaining the raw data, use of filters on spreadsheets, site surveys and the use of independent experts are all tools that we use.
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