A method for determining positions of one or more target receivers (122, 124), wherein the one or more target receivers and two or more reference receivers (116, 118, 120) are configured to receive a plurality of signals transmitted at unknown times from a plurality of sources (102, 104, 106, 108, 110, 112, 114) that are located at unknown positions and not synchronized with each other, the method comprising: obtaining times of arrival of the plurality of signals at the one or more target receivers at which each of the one or more target receivers receives at least a portion of a signal transmitted from each of the plurality of sources; obtaining, from the two or more reference receivers, a plurality of times of arrival of the plurality of signals at the two or more reference receivers associated with when the two or more reference receivers receive a corresponding portion of the plurality of signals as measured according to local clocks of the two or more reference receivers, wherein the two or more reference receivers are located at a plurality of known reference positions and not synchronized with each other or with the plurality of sources; and determining the positions of the one or more target receivers by computing, based on the plurality of times of arrival at the respective two or more reference receivers, the plurality of reference positions, and the times of arrival at the one or more target receivers, the method characterised by comprising approximating one or more signal propagation wavefronts in neighborhoods of the one or more target receivers and the two or more reference receivers using a curve or a straight line, wherein an orientation of the curve or the straight line is related to a direction of each of the plurality of signals from the plurality of sources, and wherein a curvature of the curve or the straight line is used in determining the positions of the one or more target receivers. obtaining times of arrival of the plurality of signals at the one or more target receivers at which each of the one or more target receivers receives at least a portion of a signal transmitted from each of the plurality of sources; obtaining, from the two or more reference receivers, a plurality of times of arrival of the plurality of signals at the two or more reference receivers associated with when the two or more reference receivers receive a corresponding portion of the plurality of signals as measured according to local clocks of the two or more reference receivers, wherein the two or more reference receivers are located at a plurality of known reference positions and not synchronized with each other or with the plurality of sources; and determining the positions of the one or more target receivers by computing, based on the plurality of times of arrival at the respective two or more reference receivers, the plurality of reference positions, and the times of arrival at the one or more target receivers, the method characterised by comprising approximating one or more signal propagation wavefronts in neighborhoods of the one or more target receivers and the two or more reference receivers using a curve or a straight line, wherein an orientation of the curve or the straight line is related to a direction of each of the plurality of signals from the plurality of sources, and wherein a curvature of the curve or the straight line is used in determining the positions of the one or more target receivers.