FM Repeater Passes
FM repeater satellites like the ISS, SO-50, and TEVEL act as orbiting FM repeater stations. When you listen to a recording, you'll hear operators calling CQ with their callsign and grid square, brief exchanges, and the distinctive sound of the repeater tail — a short squelch burst after each transmission. The audio quality varies with elevation — high passes sound clear, while low passes have more fading and noise as the signal path skims the horizon.
CW Beacons
CW (Morse code) beacons are the heartbeat of amateur satellites. Each satellite transmits its callsign and sometimes telemetry data in Morse at regular intervals. The tone drifts slightly during a pass due to Doppler shift — you'll hear the pitch drop as the satellite recedes. Recordings of CW beacons are useful for calibrating your receiver and learning to identify satellites by their keying pattern and speed.
SSTV Transmissions
Slow-Scan Television (SSTV) produces a distinctive warbling sound — a series of tones that encode an image line by line. The ISS occasionally transmits SSTV commemorative images on 145.800 MHz. These sound otherworldly and can be decoded back into images using free software like MMSSTV or Robot36. The raw audio itself has a sci-fi quality that makes it some of the most interesting content in the archive.
APRS Packets
APRS (Automatic Packet Reporting System) packets sound like brief bursts of static — a warbling screech lasting about a second. The ISS runs an APRS digipeater on 145.825 MHz that repeats packets from ground stations. Each burst is a data packet containing a callsign, position, and short message. Decoding software like Direwolf or APRSdroid can extract the data from the audio.
Recording Tips
For the best recordings, connect your radio's audio output to your computer's line input using a 3.5mm cable. Record in WAV format for maximum quality, then convert to MP3 for upload. Trim the recording to the active portion of the pass — dead air before and after acquisition of signal (AOS) and loss of signal (LOS) isn't useful. For SDR users, you can record the IQ baseband and demodulate later, which preserves the full signal for analysis.
To find upcoming passes for recording, use the amateur radio pass predictor. For live satellite tracking, try the live globe. Learn more about orbital mechanics in the Orbital Academy.