Technology to support collaborative learning has come a long way. Interactive tabletops support collaboration when correctly integrated in activity design. While these devices can now, in principle, be purchased by the general public, wide adoption in schools is hindered by their high cost and lack of mobility. In this paper, we analyze the potential technologies that could replace such devices. After developing a dozen prototypes to further test the potential of the most promising technologies, we found one robust, affordable and accurate solution: SPART (on-Surface Positioning for Augmented RealiTy) allows augmenting any flat surface (images etc.) with tablets or smartphones, addressing both affordability and mobility. This paper focuses on the technological innovations that were designed for SPART.