While silent SMSes have been used by authorities for quite some time to geolocate cell-phones, this work puts a less powerful capability into the hands of anyone. By training a ML model on the RTT from sending a silent SMS to phones in different [known] locations, a temporal map of the GSM network can be made to later classify RTTs when targeting a victim phone and approximate their location to country/region.

Without cooperation of the cell infrastructure it's pretty coarse-grained, but still a scary way to figure out where a target of interest is without alerting them.