The weak GPS signal-processing work described here was part of the development of an innovative GPS tag - TrackTag™. The technology is aimed at animal tagging applications and therefore has particular challenges. These primarily relate to designing a tag to be small enough in physical size and weight so as not to impact the behaviour of the animal under study as well as robust enough to operate wherever the animal goes. TrackTag's architecture differs from that of a conventional GPS receiver. The key differences are that very short (typically 24 ms) snapshots are used and there is no processing intelligence on the tag itself. Such a short length of ON time, with no need for longer warm or cold starts, results in very little energy being used. This, in turn, allows the design of the tag to be made very small with a minimal battery size requirement and capable of being used over far greater periods than conventional GPS tags. Processing of the data is done on a PC following the download of the tag's memory. The GPS signal-processing using such short snapshots must not compromise the accuracy of the positions calculated and they should be comparable or better than normal GPS receivers. Weak signal processing is also a requirement for tracking signals in harsh environments involved with tagging animals in their natural habitat. This research has demonstrated that weak-signal processing coupled with the short snapshots, and careful integration of a suitable antenna, allows tracking under rainforest canopy and marine environments. Conventional GPS tends not to work well under those conditions. © The Royal Institute of Navigation 2008.