The TRANSIT system was the first operational satellite navigation system, and the first operational satellite system in general. The system contained, usually, ten working satellites (five for the basic constellation and five spares) in a low polar orbit, with an eventual 27 satellites total. The Navy used it for precise location of their ships and submarines; it was basically a primitive version of GPS, in function and method.
Tracking stations in New Mexico, Texas, Japan, Sicily, Antarctica, Greenland, and other places tracked the orbits of the satellites and checked their clocks for drift, obtaining ephemeris data and ensuring the system was working correctly.
The satellites broadcasted what they knew of their orbital data, and an extremely precise clock signal (this also allowed the system to function as the world’s first worldwide time keeping service). With data from clocks from several satellites, a ground station could figure out the differences and use the Doppler effect to find its location.
TRANSIT 5B-5, also known as Oscar 2, Spacecraft 1964 83D, OPS 6582, and NNS 30020 (under various codenames and designation systems) was launched on December 31, 1964 — almost 50 years ago. It was solar powered. A malfunction of unknown nature caused its navigation transmitters to shut down 19 days after it was launched, and the satellite stopped responding to command signals. Its auxilliary transmitter continued to function, broadcasting telemetry on 136.65 MHz.
The military launched another satellite to replace it, and 5B-5 was forgotten, save for the fact that it existed, never worked, and was in low polar orbit.
In 1996, Maik Hermenau read an article about receiving satellite signals with a radio scanner. Alongside the article was a table of satellite frequencies, including 5B-5's. He entered this table into his radio scanner’s memory. Several times over the years, while scanning through its entries, his scanner detected a very faint, unmodulated carrier signal on 5B-5's frequency.
In 2002, he decided to try 5B-5 again, after learning that some satellites use a modulation method called multiplexing, which transmits different signals encoded into a single FM signal. He aimed his antenna at the right point in the sky and waited for the satellite to orbit overhead, and was surprised to find that not only was it still working, but it was transmitting multiplexed information.
It can still be heard today, as someone with an RTLSDR software-defined radio discovered. The satellite’s batteries have died almost completely after fifty years in orbit, allowing the solar panels to power its circuitry directly. Time has been harsh on it; after five decades of UV exposure and micrometeorite bombardment, the solar panels barely function. It is no longer able to orient itself, and is thus tumbling on three axes, with sunlight only shining on its solar panels for brief moments; and its batteries can only store a miniscule fraction of that already-low power. It can no longer hold its position to keep its antenna aimed at the Earth, and so its signal fades in and out.
Still, despite all that, TRANSIT 5B-5 still broadcasts. If you have the right radio, the right frequency, the right antenna, and the right place and time, you can aim it into the sky and hear 5B-5's signal: “I’m here, I’m okay, tell me what to do…” until it passes into night and, after four seconds, falls silent again.