A method of locating positions of m mobile devices in the same vicinity, wherein each of said mobile devices comprises a receiver (102, 104) to receive location signals from Global Navigation Satellite System, GNSS, satellites, wherein each of said location signals comprises timing data enabling a range (110, 112, 114, 116) to a satellite (106, 108) of said GNSS satellites to be determined, and each of said mobile devices further comprises a transmitter to transmit said timing data to a remote receiver at a position processor, the method comprising: using said m mobile devices to determine timing data from a set of n satellites of said GNSS at each of said m mobile devices to provide m.n measurements of said timing data; communicating said m.n measurements of said timing data to said position processor, wherein m and n satisfy a constraint such that 4.m + n ≤ m.n; and using, at said position processor, said m.n measurements in combination with 3D positions for said n satellites at a time of each of said measurements to co-determine, based on resolution of a set of simultaneous equations: a corrected position in space for each of said m mobile devices, a local clock offset for each of said m mobile devices, and a common space segment error for each of said n satellites, said common space segment error for a satellite being common to said m mobile devices due to that said m mobile devices are in the same vicinity, and including a satellite clock error for the satellite, and wherein a said corrected position in space comprises a said position in space corrected for said common space segment errors. using said m mobile devices to determine timing data from a set of n satellites of said GNSS at each of said m mobile devices to provide m.n measurements of said timing data; communicating said m.n measurements of said timing data to said position processor, wherein m and n satisfy a constraint such that 4.m + n ≤ m.n; and using, at said position processor, said m.n measurements in combination with 3D positions for said n satellites at a time of each of said measurements to co-determine, based on resolution of a set of simultaneous equations: a corrected position in space for each of said m mobile devices, a local clock offset for each of said m mobile devices, and a common space segment error for each of said n satellites, said common space segment error for a satellite being common to said m mobile devices due to that said m mobile devices are in the same vicinity, and including a satellite clock error for the satellite, and wherein a said corrected position in space comprises a said position in space corrected for said common space segment errors. a corrected position in space for each of said m mobile devices, a local clock offset for each of said m mobile devices, and a common space segment error for each of said n satellites, said common space segment error for a satellite being common to said m mobile devices due to that said m mobile devices are in the same vicinity, and including a satellite clock error for the satellite, and wherein a said corrected position in space comprises a said position in space corrected for said common space segment errors. said common space segment error for a satellite being common to said m mobile devices due to that said m mobile devices are in the same vicinity, and including a satellite clock error for the satellite, and wherein a said corrected position in space comprises a said position in space corrected for said common space segment errors.