A method comprising: receiving, by a mobile device, a series of signals from each of a plurality of signal sources; determining a sorted list of the signal sources based on an estimated signal strength of signals from each of the signal sources, wherein a higher estimated signal strength is associated with a higher ranking in the sorted list; the method characterised by: determining a probability of false sort for each of the signal sources in the sorted list based on a signal strength separation between signals corresponding to the signal source and other signals received from the plurality of signal sources, wherein the probability of false sort of a signal source indicates a probability that a rank of the signal source is incorrect, and wherein a greater signal strength separation corresponds to a lower probability of false sort; determining a sublist of the sorted list based on a probability of false sort, wherein the sublist includes one or more signal sources each of which being associated with a probability of false sort that satisfies a threshold value; and providing the sublist of signal sources to a subsystem of the mobile device as proximity signal sources for determining a location of the mobile device. receiving, by a mobile device, a series of signals from each of a plurality of signal sources; determining a sorted list of the signal sources based on an estimated signal strength of signals from each of the signal sources, wherein a higher estimated signal strength is associated with a higher ranking in the sorted list; the method characterised by: determining a probability of false sort for each of the signal sources in the sorted list based on a signal strength separation between signals corresponding to the signal source and other signals received from the plurality of signal sources, wherein the probability of false sort of a signal source indicates a probability that a rank of the signal source is incorrect, and wherein a greater signal strength separation corresponds to a lower probability of false sort; determining a sublist of the sorted list based on a probability of false sort, wherein the sublist includes one or more signal sources each of which being associated with a probability of false sort that satisfies a threshold value; and providing the sublist of signal sources to a subsystem of the mobile device as proximity signal sources for determining a location of the mobile device. determining a probability of false sort for each of the signal sources in the sorted list based on a signal strength separation between signals corresponding to the signal source and other signals received from the plurality of signal sources, wherein the probability of false sort of a signal source indicates a probability that a rank of the signal source is incorrect, and wherein a greater signal strength separation corresponds to a lower probability of false sort; determining a sublist of the sorted list based on a probability of false sort, wherein the sublist includes one or more signal sources each of which being associated with a probability of false sort that satisfies a threshold value; and providing the sublist of signal sources to a subsystem of the mobile device as proximity signal sources for determining a location of the mobile device.