Positif is not actively maintained any more, as its functionality is now part of MiXiM. Questions regarding Positif will usually result in a pointer to the new MiXiM simulator.
Positif is a framework for localization algorithm testing. It was the simulation framework used for 'Distributed localization in wireless sensor networks: a quantative comparison' by Koen Langendoen and Niels Reijers, as well as 'Refined Statistic-based Localisation for Ad-hoc Sensor Networks' by TomParker and Koen Langendoen. It includes documentation on how to add your own algorithms and some scripts to process the output.
It is available from the Consensus project software page