The International Space Station carries amateur radio equipment, operated by astronauts and supported by ARISS (Amateur Radio on the International Space Station), an international group of volunteer radio operators, national amateur societies and the space agencies. Its flagship activity is school contacts: students ask astronauts questions over an amateur radio link, live, from the classroom.
How a pass looks
Elevation of the station during one pass (illustrative: 410 km, Earth's rotation ignored). Faint curves: a low and an overhead pass.
Above the horizon10.1 min
Above 10°5.9 min
The ISS is low and fast, so everything happens in a few minutes. A pass that goes nearly overhead gives about 10 minutes above the horizon, a low pass far less. A scheduled school contact is built around the best pass for the school and must squeeze its questions into the useful part of it.
The Earth turns beneath the orbit, so each lap's ground track lands about 23° of longitude further west than the last.
Ascending = heading north. Descending = heading south. East and west do not matter.
Because of the 51.6° tilt, the station is never directly overhead at latitudes beyond that. Radio passes work in daylight or darkness, whether or not you can see the station.
What you can do
Listen. On an ordinary VHF or UHF receiver you may hear school contacts, voice, or occasionally packet or image transmissions, as they happen. A simple antenna and a handheld radio can be enough.
Talk to the station. Sometimes there is a repeater or digipeater on board that any licensed operator can use, subject to the crew's schedule.
Join an event. Amateur-radio image (SSTV) events and school contacts run on a schedule posted by ARISS.
Equipment, frequencies and schedules change. Always check the ARISS website for what is currently available before you try to transmit.
Good habits
Predict the pass with a tracking program and note the elevation: low passes are hard.