Hi Tom,
Wojciech Burzyński (PL) gave a talk on this at last year’s ESOP in Freiberg. The PDF of his presentation can be found here.
Using a 1PPS GPS receiver module they found that their Arduino-based VTI gave timing accuracies as good as the off-the-shelf IOTA-VTI and GPSBOXSPRITE2 models used by many occultation observers around the world.
Happy assembly!
Alex.