Location estimation for a first target object assisted by a second target object, which have co-located signalling devices is provided. A positioning engine employs a data model of a location-dependent physical quantity and determines location probability distributions for the target objects. One signalling device sends positioning-assisting signals to the other which makes observations from it. The positioning engine uses observations on the physical quantity and the positioning-assisting signals to make a quantity observation set, and determines location probability distributions for the target objects. The positioning engine determines an updated first location probability distribution based on the location probability distributions and the positioning-assisting observation set. The positioning engine determines the location estimate for the first target object based on the updated first location probability distribution and triggers a physical action based on the location estimate for the first target object.