Making Your First ISS Contact

There's something genuinely extraordinary about making a radio contact through a spacecraft carrying a human crew, traveling at 28,000 km/h just 400 km above your head. The ISS is visible to the naked eye, passes overhead multiple times a day, and can be worked with surprisingly modest equipment. It's also one of the most unpredictable satellites to plan around — which keeps things interesting.

What the ISS Offers Ham Radio Operators

The ISS has an active ham radio program called ARISS (Amateur Radio on the International Space Station), which has been running since the station was first occupied in 2000. The station carries licensed amateur radio equipment and several crew members hold ham licences.

For most operators, the practical option is the ISS FM repeater, which operates on:

• Uplink: 145.990 MHz (67 Hz CTCSS required)
• Downlink: 437.800 MHz

When active, this works exactly like a terrestrial FM repeater — transmit on the uplink, listen on the downlink. The footprint of the ISS at 400 km altitude is enormous, so during a single pass you may find yourself in a QSO with stations hundreds or thousands of kilometres away.

The station also carries a packet radio digipeater on 145.825 MHz (1200 baud AFSK) that is active more consistently than the voice repeater, and occasionally operates in cross-band FM voice mode.

The Catch: It's Not Always On

Here's the honest truth about the ISS repeater: it operates on a schedule determined by the crew and ARISS, and it is frequently off. Sometimes for days, sometimes for weeks. The crew are busy astronauts — ham radio is a secondary activity.

Before planning an ISS pass, always check current status at ariss.org. The ARISS team posts updates when the repeater is active or when scheduled school contacts (ARISS School Contacts) are planned. When a school contact is scheduled, the downlink is often audible to anyone in the footprint even if you can't get in.

Don't get discouraged if your first several attempts produce nothing. That's normal with the ISS. When it is active, though, it's a genuine thrill.

Equipment You Need

The ISS passes at a relatively high altitude compared to some amateur satellites, which means signal levels are generally good. You can work it with:

• Any dual-band VHF/UHF handheld — a basic HT with the rubber duck will often work on high-elevation passes
• 67 Hz CTCSS tone programmed on the uplink — this is mandatory, the repeater won't open without it
• A small directional antenna — a handheld yagi or Arrow antenna dramatically improves your chances, especially on lower passes

One common mistake: forgetting to enable the CTCSS tone on the uplink. You'll hear everything fine on the downlink but nobody will hear you. Double-check before the pass starts.

Planning and Working the Pass

Use Ham Sat Tracker to find upcoming ISS passes from your location. Look for passes with maximum elevation above 30° — these give you the most time in range and the strongest signals. Passes below 20° max elevation are shorter and often blocked by obstructions.

The ISS moves fast. A typical overhead pass lasts 8–10 minutes from AOS to LOS, but the usable window — where the signal is strong enough to reliably access the repeater — may be only 4–6 minutes. Be ready before AOS.

Doppler shift on the ISS downlink (437.800 MHz) is about ±9 kHz from AOS to LOS. Ham Sat Tracker gives you the corrected frequency at AOS, TCA, and LOS. For a quick pass you can often get away with tuning to the TCA frequency and leaving it — the shift is more gradual near closest approach. For a longer high-elevation pass, tune down gradually from the AOS value.

When the repeater is active and busy, keep transmissions short. Give your callsign, your grid square, and listen. It can be chaotic during a good pass with many stations trying to get in, but with a good signal and short calls you'll make it through.

The Packet Digipeater

If you have a TNC or sound card interface, the ISS packet digipeater on 145.825 MHz is often a more reliable option than the voice repeater. It operates in APRS mode — you can send position packets through the digipeater and they'll appear on aprs.fi with the path via ISS. Many operators' first ISS "contact" is actually a packet digipeat rather than a voice QSO.

To use it: configure your TNC or APRS software with 145.825 MHz simplex, 1200 baud AFSK, and a path of ARISS. Send a position beacon a minute or so before AOS and watch aprs.fi for confirmation during and after the pass.

Hearing the Crew

Occasionally crew members operate the ham radio equipment directly, calling CQ on 145.800 MHz (downlink) or conducting scheduled school contacts. These are announced on the ARISS website well in advance. Even if you can't make contact, monitoring the downlink during a school contact and hearing an astronaut answer questions from a classroom is an experience worth planning for.

Check ARISS upcoming contacts to see if a school contact is scheduled from your region — the downlink is usually audible throughout a large footprint.

73 de VE3AKK