The International Space Station carries amateur radio gear, and you can use it from the ground with equipment you probably already own. This is the single most achievable thing in amateur satellite work - no dish, no rotator, no tracking mount.
But the phrase “contact the ISS” hides a misunderstanding that disappoints a lot of newcomers, so let’s clear it up first.
Three different things
1. Digipeating a packet. The station carries an APRS digipeater on 145.825 MHz simplex. It hears your packet and repeats it. Easiest of the three, and it works with modest gear.
2. Talking through the repeater. There is a cross-band FM repeater aboard. You transmit on 2 m, the station retransmits on 70 cm, and other hams on the ground hear you. This is a real contact, logged and confirmed like any other.
3. Talking to the crew. This is what most people picture, and it is the one you cannot plan. Scheduled ARISS contacts are school events - you may listen, but calling in is not done. Astronauts do occasionally operate casually, but it is unpredictable and rare.
So: yes, hams work the ISS constantly. They are using it as a repeater to reach each other, not chatting with astronauts. Go in expecting the first two and you will have a good time.
The frequencies
| Mode | Uplink | Downlink | Notes |
|---|---|---|---|
| APRS digipeater | 145.825 FM simplex | Same frequency both ways. Worldwide. | |
| FM voice repeater | 145.990 FM, 67.0 Hz tone | 437.800 FM | Cross-band. Tune the downlink for Doppler. |
| SSTV | - | 145.800 FM | Receive only, during scheduled events. |
These change. The radio gets reconfigured, switched between modes, and sometimes switched off entirely for spacewalks or dockings. Check the ARISS status page before you plan an evening around it. ChatterBack cannot show you which mode is live - there is no feed for it, and we would rather say so than guess.
What you need
- A dual-band handheld. Any 2 m / 70 cm HT will do for a start.
- A better antenna than the one it came with. This is the part that matters. A small handheld Yagi turns a frustrating evening into a working one. The stock rubber duck will get you APRS on a high pass and not much else.
- Full duplex, if you can. Hearing yourself come back from orbit tells you instantly whether you are getting in. Two radios works. One radio means transmitting blind.
Picking a pass
Elevation is everything. A pass that peaks at 15° above the horizon is a fight against terrain and noise; one that peaks at 60° is comfortable. Wait for the good ones - there will be several a day.
A full horizon-to-horizon ISS pass lasts about ten minutes, of which maybe five to eight are usable. The station is roughly 400 km up and moving at about 17,500 mph, which is why none of this is leisurely.
The satellite page shows your next passes with rise and set times, the compass bearings to swing between, and the maximum elevation - brighter rows are the better passes.
Doppler, without the maths
The station is closing on you at several kilometres a second, then receding. That shifts the frequency you hear. On the 70 cm downlink the total swing is around ±10 kHz across a pass - far more than an FM receiver will tolerate.
The rule is simple: start high, end low. Begin a few kilohertz above 437.800 as it rises, sit on the nominal frequency at closest approach, and step down as it departs. Leave your 2 m uplink alone; the shift there is small enough that FM copes.
If you would rather not do it by ear, the satellite page computes the corrected downlink live while the station is above you, so you can read the number off and dial it in.
Etiquette, which matters more here than anywhere
The footprint is enormous - a couple of thousand kilometres across - and it is one channel. Everyone under it is sharing a single repeater, and the pass is over in minutes.
- Listen first. Do not transmit into an existing contact.
- Keep it short. Callsign and grid square, confirm the other station, done. Ten seconds is a complete contact.
- No ragchewing. There are dozens of people waiting and the window is tiny.
- Do not call repeatedly. If you are not getting in, the problem is usually your antenna or the pass elevation, not your persistence.
Start with APRS
If you have a radio that does packet, aim there first. Set the path for the ISS digipeater, transmit on a high pass, and look yourself up afterwards on one of the ARISS gateway sites. Seeing your own callsign digipeated from orbit is a good first win, and it requires no timing, no duplex and no conversation.
Nothing heard? Check the pass elevation, check ARISS status, and check your antenna before you blame the radio. In that order - it is almost always one of the first two.