Transit 5B-5 (COSPAR object 1964-083D), a former TRANSIT navigation satellite, is the oldest satellite known to still transmit a signal. Considered "dead" for many years because its navigational systems failed after nineteen days of operation, it still emits a telemetry signal on 136.658MHz when illuminated by the sun. The signal is similar to that used by later TRANSIT satellites, but with some differences, presumably due to damage to the encoder circuits. Its distinctive, almost musical sound is the product of an old form of analog spacecraft telemetry system that represents the monitored readings as the deviation of a sub-carrier oscillator from its center frequency. The rhythmic "ticking" audible in the background is produced by the PCM portion of the signal that encodes digital data.
More recently, analysis of the digital PCM portion of the telemetry signal has revealed that it still shows some structure; among other things, there is a clearly evident frame counter. More information is available at this link.
TRANSIT 5B-5 as heard in single-sideband mode:
Occasionally, when there is not enough light hitting the solar panels, it produces the following sound (received in upper-sideband mode):
- Mike Kenny's analysis of the present telemetry signal
- An APL Technical Digest article describing the TRANSIT system
- The Wikipedia page for the TRANSIT system
- Xerbo's analysis of the PCM telemetry data