Matthew Tart, one of CCL-Forensics’ cell site analysts looks at the effects that height and landscape have on cell site analysis techniques. He also looks at a scenario where, on the face of it, a suspect seems to be moving in a particular direction – but could actually be doing the opposite.
Q: It seems obvious that the landscape can affect the coverage area of cells, but just what sort of effect does it have?
A: There are two main effects. The first is that height limits coverage area by preventing the signal reaching areas in a “shadow” caused by the particular geographical feature (hills, mountains, that kind of thing). And the other is that being at height affects which mast (cell) you are able to connect to, compared to the cells which would serve at a lower level.
Q: Taking the first of those two, this is presumably about “line of sight”?
A: It is – but don’t be misled by the word “sight”. You don’t physically have to see a cell for it to provide service where you are – reflection, diffraction and other factors take care of that. Let’s consider a simplified example. The signal from the mast reduces when it travels through or around “stuff”. Whereas the walls of a building contain a relatively small amount of stuff, hills and mountains (clearly) contain a lot of stuff. It stands to reason that, if a hill is in the way, the signal will not be received on the other side.
Q: That sounds pretty obvious, but how do cell site analysts cope with this?
A: It’s all about the preparation: about knowing the landscape and the potential effects before you go out to survey (if, indeed, surveying is relevant to the investigation in question). It’s obviously wrong to be doing all your preliminary work in two dimensions, when the scene itself is in three.
Q: In terms of the other issue, i.e. height affecting which cell you connect to… just how high are we talking about?
A: We’re talking more about “artificial” height here… tower blocks and so on, rather than natural features. It’s feasible that if you’re higher than four storeys from the ground, a whole new range of cells may come into view. When you get above the rooftops of neighbouring buildings you have a whole new vista, meaning new cells can be seen. The “stuff” is no longer in the way.
A good example of this is on an estate, where a localised picocell provides coverage at street level, but twelve storeys up, a more remote macrocell provides coverage, despite being a number of miles away. Simply mapping this as a desktop exercise, you could incorrectly infer that the phone had moved a considerable distance.
Q: How misleading could this be?
A: Let’s look at an extreme example: of a valley with a mast situated high on each side, and one in the town below.
If an individual moves in the direction of the arrows shown in the diagram, i.e. from right to left through the town, he may use (in this example) mast A first, as it provides line-of-site coverage, but he is in the shadow of mast C. Next, he may use mast B as he moves through the town, as this provides localised coverage for the town only, and then as he travels back up the other side of the valley, he could use mast C, as he is in the shadow of mast A.
Plotting these on a map, and giving no consideration to the terrain, makes it look like he did the opposite. Simply looking at the mast location, and saying he used A, then B, then C, does seem to imply he is moving from left to right, rather than vice versa.
Looking at this from above…
Of course, in reality, you would more than likely be using a much larger set of data, so this is an extreme example, but it does illustrate the problems faced in not appreciating the effect of height.
In another example, let’s look at the other effect of height – that of providing service at height, which you wouldn’t expect at ground level.
In this example, the purple mast is designed to provide service at ground level (let’s say in this example it’s a picocell on the front of a nearby shop) but the orange mast would easily provide coverage at the top of the tower block, even though it is a considerable distance away. This demonstrates the importance of carrying out a survey which is in line with the requirements of the investigation.
Was the suspect likely to have been on the high levels of a building at the time of the crime? Was height a potential factor? Would a “walk survey”, using hand-held equipment, be more appropriate than a drive survey using vehicle-mounted equipment at ground level?
In summary, it’s worth remembering that a lot of network planning is focused on providing coverage at ground level, and it’s important therefore to consider the likelihood of more remote cells (at greater altitude) also providing service.
Cell Site Analyst