It’s a common question asked of cell site analysts. How does, for example, rain affect the validity of cell site analysis techniques? Iain Brodie looks at weather and its effect – or otherwise – on cell site measurements, surveying and interpretation.
Q: This sounds like a question which regularly comes up in court. Is it?
A: In short, yes. I’ve regularly been asked about the effects, particularly of rainfall, on the coverage of mobile phone masts. It’s a very common challenge as to the admissibility of cell site evidence. The simple answer that is it does make a difference, BUT… not a significant one.
Q: So what difference does it make?
A: As mentioned earlier, very little. In the UK, even the heaviest rain pulses never come in bursts of more than 40mm per hour. Even at this intensity, the level of attenuation (reduction in signal strength) is less than one decibel per kilometre – or to put that into English, not very much at all. Remember also, that unless the weather conditions in question are very localised, it is likely that all relevant cells will be affected by a similar amount – hence cancelling out any major effect.
It could only really become an issue (and still not a very major one) when a cell covers a very large distance – typically for rural cells.
Q: What about cells which are linked together by radio link, rather than underground cables?
A: It’s a relatively common configuration that rural masts are linked together by a microwave link, rather than being connected directly to the network by traditional copper or fibre-optic cable. It’s been questioned in court as to whether rainfall or other meteorological conditions can affect this, hence “knocking out cells” for a period of time.
These microwave links operate on a much higher frequency than the waves used to connect individual calls – and that makes them much more susceptible to heavy rain – BUT… the power with which they operate is high enough that it would only cause an issue in a tiny minority of cases. It HAS happened – just not very often. Plus, the skill of the cell site analyst is called into play here, as knowledge of how the network is configured can be crucial.
For example, certain network operators tend to use microwave links between sites – others tend not to. Knowing this, puts this issue – and any potential challenges from the other side – into context.
Q: Is there a general rule of thumb about the technical effects of the weather?
A: In summary, you can never say weather has no effect – but any small effect it does have is likely to have no significant impact on the validity of the cell site analysis of the case in question.
Remember that even if a particular cell WAS to be affected by weather at the time of the offence, a good cell site analyst would also pick up other (neighbouring) cells which might also serve at each location surveyed. So if a cell had been inoperative either at the time of the alleged crime, and a different cell was serving at the scene, our survey process would generally detect the other cell anyway.
Clearly a cell could also be off air at the time of the survey (although I don’t tend to like surveying in really foul weather!) so we wouldn’t know for sure if it would have served or not. However, if we suspected that the cell being off air was temporary we would survey on repeat occasions. Also we retain all our survey data from all investigations so often we can see any changes to networks over time from previous surveys in the area.
Q: Are there any other weather related effects? Surely radio towers get hit by lightning and equipment rooms are prone to flooding? Does snow ever build up on the antennas and affect their performance?
A: Firstly, radio towers are always built with lightning conductors and the antennas themselves are insulated from the metallic structure of the tower. Radio equipment rooms tend to be sealed and mounted on small platforms so as to avoid being flooded. As for snow – the antennas are vertical, and since they do transmit energy, there is also a very slight warming effect. Thus they are highly unlikely to ever be affected by a build-up of snow. Of course I cannot say for sure that faults have never occurred in extreme weather – but I can say that even in the most extreme weather – such faults are still highly unlikely.
Q: So, to wrap up, if you were challenged in court about the effect of the weather on cell site analysis, what would you say?
A: Simply that you can never say weather has NO effect, but the amount of variation it causes is not sufficient to discredit cell site analysis as a science. Remember that the more data you have, and the more analysis you do, over a significant timescale – the more robust the evidence… and a drop or two of rain isn’t going to affect that.