Navspark has some affordable ($80) modules that do "raw" (carrier phase) output. Also RTK modules for $50 that can give you real-time centimeter accuracy and GPS derived attitude and bearing between two units. Alas, they don't seem to do L1/L2 or accept an external clock. They claim +/- 10 ns timing accuracy and also have a timing receiver that claims 6 ns. At one time not too long ago carrier phase / RTK capabilities would set you back at least a couple of grand. ----------------------- So far I have not seen one with the “right options” enabled for under $800.