1. A method for determining a location estimate for a first target object in an environment which also comprises a second target object; the method comprising: maintaining a positioning engine which is operable to model at least one location-dependent physical quantity with a data model of the environment, wherein the at least one location-dependent physical quantity is influenced by the location of the second target object, and wherein the positioning engine is operable to determine location probability distributions for the first and second target object; associating the first and second target object respectively with a co-located first and second signalling device, wherein at least one signalling device comprises means for sending a positioning-assisting signal and another signalling device comprises means for receiving the positioning-assisting signal and means for making observations from the positioning-assisting signal; the positioning engine receiving observations on the at least one location-dependent physical quantity; at least one of the signalling devices sending a positioning-assisting signal, wherein the positioning-assisting signal is received by another signalling device which makes a positioning-assisting observation set from it and sends the positioning-assisting observation set to the positioning engine; the positioning engine making a quantity observation set based on the received observations on the at least one location-dependent physical quantity; the positioning engine determining a respective first and a second location probability distribution for the first and second target object, wherein at least the second location probability distribution is based on the data model and the quantity observation set; the positioning engine determining an updated first location probability distribution based on the first location probability distribution, the second location probability distribution and the positioning-assisting observation set; the positioning engine determining the location estimate for the first target object based on the updated first location probability distribution; and the positioning engine triggering a physical action based on the location estimate for the first target object.