Interposer Helps GPS Receiver Overcome Its Age

We return to [Tom Verbeure] in his workshop, where he's been tinkering with Symmetricom GPS receivers again. Last time, he tackled the issue of active antenna voltage compatibility, but this time, the problem is more complex - and the solution remains the same: hardware hacking.

The GPS receiver chip has a built-in internal rollover date set for September 18th, 2022. We've long since passed that date, but the receiver's firmware hasn't been updated to handle it yet. So, what's the solution? Building an interposer, of course.

Interposers have proven themselves to be a reliable way to work around firmware bugs in the past, and this latest project is no exception. By bringing the module up to date with some extra init commands sent during bootup, [Tom] was able to breathe new life into an aging GPS receiver.

To achieve this, [Tom] required just a few simple components: an RP2040 board, a custom PCB, semi-bespoke connectors, and a handful of zero-ohm resistors. With these tools at his disposal, he set about crafting an interposer that would allow him to bypass the outdated firmware.

The process was straightforward: [Tom] designed a circuit that would connect the GPS receiver chip to a miniature computer (the RP2040 board) during bootup. This allowed him to send in some extra init commands, effectively "rebooting" the receiver's firmware.

Once this process was complete, [Tom] installed a MITM (Modem In The Middle) firmware on the receiver. This clever piece of software would allow it to pass all incoming traffic through without raising any suspicions - the system would suspect nothing, unaware that its internal workings had been tweaked to accommodate an outdated operating system.

As is often the case with open-source projects, [Tom]'s interposer was made available for others to use and modify. This not only provides a reliable workaround for future GPS chipset-to-receiver comms issues but also demonstrates the power of community-driven innovation in overcoming seemingly insurmountable technical challenges.

The revived S200 has now taken its place alongside [Tom]'s other trusty projects, serving as a testament to the versatility and effectiveness of interposers. Whether you're dealing with an aging GPS receiver or simply looking for a creative solution to a hardware-based problem, this DIY approach can be a game-changer.