Over on the Saveitforparts YouTube channel, Gabe has documented how he accidentally discovered signals coming from the GPS-satellite-based US nuclear detection system.

While scanning the GPS L3 frequency around 1381 MHz with his RTL-SDR, Discovery Dish, 1420 MHz Hydrogen line Discovery Dish feed, and Discovery Drive, Gabe caught short intermittent data bursts instead of the usual navigation signals. It turns out L3 is used by the GPS constellation for nuclear detonation detection, as the satellites double as arms-control sensors. When one detects a possible event, it relays the message across the network, the only time this frequency sees activity.

Later in the video, Gabe also swaps the dish feed to compare against a regular GPS L1 signal at 1575.42 MHz, and notes that radio astronomers are reverse-engineering L3 to filter it out, since it sits close to the protected 1420 MHz hydrogen line.