For each first tracker in a first subset there exists a respective second tracker in a second subset such that a distance between the first tracker and the second tracker is shorter than each distance between the second tracker and each of the other first trackers in the subset by more than a threshold distance. In a first time slot, data is wirelessly transmitted using a same first frequency from each of the first trackers in the subset of the first trackers to the respective second tracker in the subset of the second trackers. In a subsequent time slot, the data is wirelessly transmitted from each of the second trackers in the subset of the second trackers to a final destination data collector.