So you think today’s handheld, SDIO and cellphone-based GPS units are small? Get ready for a new generation of even smaller GPS devices, ranging from wristwatches to slim cellphones to — and we just know this is coming — implants. That’s the promise held forth by what is being billed as the world’s smallest GPS receiver, which was developed by New Zealand-based Rakon, a company affiliated with GPS-make Navman. According to Rakon, the chip is about the size of a baby’s fingernail, and should be available in a range of devices within the next two years.