Testing the synchronized GPS watches
Summary
Testing the synchronized GPS watches is the culminating demonstration from the episode "GPS Has a Secret Job," where Sean Hodgins and Jeff Geerling attempt to verify that their newly assembled GPS PPS Watches can achieve perfect timing synchronization. After completing the soldering and assembly of both watches, they connect them with a BNC cable to test whether the devices can lock onto GPS satellites simultaneously and blink in perfect unison—proving the concept that two watches anywhere on the planet can be synchronized through GPS timing signals alone.
Details
The test begins at 19:25 after both watches have been fully assembled. Sean Hodgins explains the fundamental concept to Jeff Geerling: "So, the point was that you can take two of these, or if there's two people... And you can sync them up with like a BNC cable. So, we're going to try that right now, and we'll see how off the signal is."
The initial attempt proves challenging due to the indoor workshop environment. Both watches struggle to acquire GPS signals simultaneously, with Sean noting, "I think one of them's getting a signal, and then it's sending it over, but they're not both getting a signal like that right now." The middle LED on the watches is designed to blink green only when the two timing signals are out of sync—essentially acting as a visual indicator of any timing discrepancy.
To improve their chances of acquiring satellite lock, Sean suggests they move: "Do you want to go back by the window for a minute?" Jeff Geerling attempts to help by holding his watch up to get better reception. After repositioning and waiting for both devices to lock onto GPS satellites, the test finally succeeds.
The results exceed expectations. Sean observes that "there's no green visible to the human eye," meaning the synchronization is so precise that no timing discrepancy can be detected visually. He confirms, "technically to us they are perfectly in sync." The moment of success is marked by Jeff's simple declaration: "We are locked."
The successful demonstration proves the core concept of the GPS PPS Watch project—that GPS timing signals are accurate enough to synchronize devices anywhere on Earth to within imperceptible margins, connected not by proximity or communication between the devices, but by their shared reference to the atomic clocks aboard GPS satellites.
People Involved
- Sean Hodgins — Designer and builder of the GPS PPS Watch, who orchestrates the test and explains how the synchronization detection works through the middle LED indicator
- Jeff Geerling — Collaborator who helps with the test by holding his watch up for better GPS reception and confirms when both devices achieve satellite lock
Why It Came Up
This test represents the payoff moment of the entire episode—after walking through the design, explaining the GPS PPS (Pulse Per Second) concept, and completing the hands-on assembly with Jeff Geerling soldering his first surface mount circuit board, the duo needed to demonstrate that their watches actually work as intended. The synchronization test validates the precision timing capabilities that were the entire motivation for building the GPS PPS Watch project in the first place. It's the proof-of-concept that transforms the technical discussion and assembly process into a working demonstration of GPS timing technology.