If you are forced to cross fortified border installations such as walls or fences at national borders, you should switch off your smartphone and all electronic devices completely. Remove the battery to make detection less likely.
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Border fences and walls have been technically upgraded for some years now. This is happening at the EU borders, but also at the US border with Mexico, for example. We have to assume that, in addition to camera technology, so-called radio frequency analyzers are also being used. These are technical receivers that are able to recognize different radio signals. Because these devices are located at different points along border installations, they can determine the exact position of people or groups of people by measuring the transit times of the radio signals. It is therefore important to deactivate mobile internet, Bluetooth or WiFi. It is best to switch off the devices completely and remove the battery for safety reasons. This should also be done a few kilometers before a border facility. If you are traveling in a group, this advice should be followed by everyone.