It's become clear that I need to figure out some way to get to the diagnostic header while the board is in its chassis. I've also decided that the battery backup clip for the GPS module is worthless. It takes the OCXO long enough to come up to temperature that the extra time it takes the GPS module to get a first fix is completely moot. Instead, a mini DIN 4 connector will go there, bringing the diagnostic header out to the back panel. To make room for it, the rear BNC jack will have to go, but one of the BNC jacks was bound to go anyway because the number of outputs has been reduced by one because of the phase detector.

The four pins of the diagnostic port are a buffered PPS signal (rising edge sync), two TTL serial lines and ground. With the debug firmware, one of the serial lines is output from the microcontroller, and the other is NMEA output from the GPS module. With non-debug firmware, the microcontroller output pin becomes an input pin for the GPS module, allowing you to use the GPS for other duties if you like.

The debug firmware output format is my own invention, but it is human-readable. It's designed to help me tune and debug the firmware more than anything else. The firmware doesn't make any attempts to get time or location information from GPS. The debug firmware fetches the PDOP value to share on the diagnostic port and makes sure the GPS has at least a 3D position fix at all times.