Detecting so called "silent SMS"


Believe it or not, but in most cities the main form of propagation is actually reflection.
Metal sheets can be great reflectors, but even plain old walls will reflect signals.

In cities you are actually very unlikely to have a line-of-sight signal and most communication will be by reflected and refracted signals.

Mobile networks overwhelmingly use cross-polarized (X-pol) antennas which can allow to detect reflections, however, it is quite impractical.
This all changes with beamforming in 5G where reflections are actively used for beam-steering, and 5G base-stations will actively try to reach you by reflecting the beam of buildings.

Also note, to the antenna discussion: changing your devices antennas gains very little, especially since the mechanism of silent SMS is not to triangulate you based on the received signals of the towers, but by querying your phone for its measurements, which it then sends to the network.


That’s a different kind of message, a so-called RRLP (radio resource location protocol) request. Those are what ask the phone’s baseband “hey, give me your position”, which then takes its measurements and replies. There are 2 ways of doing this:
-GPS (assisted, normal, or just sending the raw received data back along the line for the other end to process) - something which won’t be possible on the Librem 5 unless you, the user, explicitly allow it (because the modem can’t talk directly to the GPS).
-Time of flight detection, where all the base stations in the area broadcast a message at the same time, and the modem records how long it takes for each one to arrive. If you can only see one base station (either because you’re in the middle of nowhere, or because you’re using highly directional antennas), all you get is a large circular area rather than a single point.

Silent SMS messages (the so-called “empty paging”) actually won’t be affected by highly directional antennas, but for a different reason. All they do is cause a location update, telling the network which cell tower you are attached to right now.


For clarification: the GPS on the modem card will (to my understanding) not even be functional. To use the separate GNSS chip, the modem firmware (and OS software) would certainly have to be adapted, which is merely a hypothetical possibility.


News post from Nicole that touches on this subject.