A processor for use in an asynchronous wireless communication system, the system comprising one or more wireless communication units and a network of base stations (432,434); the processor being configured to: (i) receive measurements Tm(i), Tm(j) of the timing of signals received by one or more wireless communication units from one or more pairs of base stations B(i),B(j); (ii) estimate the differences dTb(i,j) between base station timing offsets Tb(i), Tb(j) for the one or more pairs of base stations B(i), B(j); (iii) iteratively correct the estimated differences dTb(i,j), to derive a final set of differences dTb(i,j), by executing the following steps: a) obtain an initial set of estimates for the differences dTb(i,j); b) estimate a difference in path lengths for signals received by a first wireless communication unit from a first pair of base stations; c) estimate differences in path lengths for signals received by the first wireless communication unit and/or by other communication units from other pairs of base stations; d) estimate the location of each wireless communication unit, on the basis of the estimated differences in path lengths, and derive a confidence score for each location estimate; e) revise the estimated differences in path lengths using the location estimates, and use the revised estimated differences in path lengths to produce a revised set of estimated differences dTb(i,j); and f) use the revised set of estimated differences dTb(i,j), weighted by the confidence scores, to iteratively correct the estimated differences dTb(i,j); and (iv) apply a synchronous geo-location technique to the final set of differences dTb(i,j), to geo-locate one or more of the wireless communication units. (i) receive measurements Tm(i), Tm(j) of the timing of signals received by one or more wireless communication units from one or more pairs of base stations B(i),B(j); (ii) estimate the differences dTb(i,j) between base station timing offsets Tb(i), Tb(j) for the one or more pairs of base stations B(i), B(j); (iii) iteratively correct the estimated differences dTb(i,j), to derive a final set of differences dTb(i,j), by executing the following steps: a) obtain an initial set of estimates for the differences dTb(i,j); b) estimate a difference in path lengths for signals received by a first wireless communication unit from a first pair of base stations; c) estimate differences in path lengths for signals received by the first wireless communication unit and/or by other communication units from other pairs of base stations; d) estimate the location of each wireless communication unit, on the basis of the estimated differences in path lengths, and derive a confidence score for each location estimate; e) revise the estimated differences in path lengths using the location estimates, and use the revised estimated differences in path lengths to produce a revised set of estimated differences dTb(i,j); and f) use the revised set of estimated differences dTb(i,j), weighted by the confidence scores, to iteratively correct the estimated differences dTb(i,j); and a) obtain an initial set of estimates for the differences dTb(i,j); b) estimate a difference in path lengths for signals received by a first wireless communication unit from a first pair of base stations; c) estimate differences in path lengths for signals received by the first wireless communication unit and/or by other communication units from other pairs of base stations; d) estimate the location of each wireless communication unit, on the basis of the estimated differences in path lengths, and derive a confidence score for each location estimate; e) revise the estimated differences in path lengths using the location estimates, and use the revised estimated differences in path lengths to produce a revised set of estimated differences dTb(i,j); and f) use the revised set of estimated differences dTb(i,j), weighted by the confidence scores, to iteratively correct the estimated differences dTb(i,j); and (iv) apply a synchronous geo-location technique to the final set of differences dTb(i,j), to geo-locate one or more of the wireless communication units.