Ham radio and space, amateur operators have long dreamed of communicating beyond Earth. With the International Space Station (ISS), that dream became a reality. The ISS hosts an active amateur radio station, allowing operators on Earth to make direct voice and digital contacts with astronauts.
These opportunities bring together technology, science, and human connection, they remain one of the most exciting aspects of the hobby. However, making contact requires careful preparation, frequency knowledge, and an understanding of orbital dynamics.
The ARISS Program
Amateur Radio on the International Space Station, or ARISS, organizes and manages ham activity aboard the station. Through ARISS, astronauts use amateur frequencies to talk with schools, clubs, and individuals worldwide. Furthermore, ARISS schedules educational contacts that allow students to ask astronauts questions directly. Beyond school events, general operators can attempt casual contacts when the ISS passes overhead.
Frequencies for ISS Contacts
The ISS transmits on VHF and UHF amateur bands. Because it moves quickly across the sky, frequencies include uplink and downlink pairs, with Doppler shift requiring slight adjustments. The most commonly used frequencies are:
- Voice Downlink (FM): 145.800 MHz
- Voice Uplink (FM): 145.990 MHz (usually for scheduled contacts)
- Packet Radio (APRS Digipeater): Uplink 145.825 MHz, Downlink 145.825 MHz
- SSTV (Slow Scan Television): Downlink 145.800 MHz
- Cross-Band Repeater (when active): Uplink 145.990 MHz (PL 67 Hz), Downlink 437.800 MHz
Because the ISS crew may not always operate the voice station, availability changes. Therefore, operators should monitor activity reports and listen before transmitting.
Understanding Uplink and Downlink
Every space contact involves two frequencies: one for transmitting from Earth to space (uplink) and one for receiving from the ISS (downlink). For example, if the ISS transmits on 145.800 MHz, your radio must listen there.
Meanwhile, you transmit to the ISS using its designated uplink frequency. Because the ISS travels over 17,000 miles per hour, Doppler shift alters frequencies by several kilohertz. Consequently, operators must tune slightly higher at the start of a pass and slightly lower as it moves overhead.
Equipment Needed for ISS Contacts
Making contact with the ISS does not require elaborate equipment. Many operators succeed with handheld radios and directional antennas. At minimum, you need:
- A VHF/UHF transceiver that supports FM mode
- A suitable antenna, such as a handheld Yagi or a high-gain vertical
- A way to track passes, either with tracking software, mobile apps, or websites
- Optional: a full-duplex radio setup that allows simultaneous transmit and receive
Although simple equipment can work, stronger antennas and higher power improve reliability. Furthermore, pre-programming Doppler-corrected frequencies into your radio makes the process smoother during fast-moving passes.
Tracking the Space Station
Since the ISS orbits Earth every 90 minutes, you must know when it passes overhead. Several free tools calculate these passes using orbital elements. By entering your location, the software provides times, directions, and elevation angles. Additionally, many smartphone apps give real-time notifications. When tracking, aim for passes above 30 degrees elevation because lower passes often suffer from interference and shorter contact windows.
Making a Voice Contact
To attempt a voice contact, begin by listening on the ISS downlink frequency, usually 145.800 MHz. If you hear astronaut activity, wait for a pause and call with your call sign. A simple format works best: “NA1SS, NA1SS, this is [your call sign].” Speak clearly and keep transmissions short. Because passes last only about 10 minutes, efficiency matters. Furthermore, avoid calling continuously; give others a chance to make contact.
Using the Digipeater and APRS
When the crew is not active on voice, the APRS digipeater often runs automatically. To use it, configure your packet station to transmit on 145.825 MHz and listen on the same frequency. Set your packet path to “ARISS.” When the ISS passes, your packets may digipeat through the station and return to Earth. Additionally, online services display live maps of stations that successfully digi-peated through the ISS, letting you confirm your success.
Receiving SSTV Images
From time to time, the ISS transmits SSTV images on 145.800 MHz. Operators receive these using any FM radio and a computer or smartphone with decoding software. During SSTV events, images typically follow a schedule, and they arrive as slow-scanned pictures lasting about two minutes each. Furthermore, these transmissions allow anyone, even without a license, to receive and enjoy images directly from space.
The Cross-Band Repeater
On certain occasions, the ISS activates its FM cross-band repeater. This mode allows operators to use the station as a relay, much like a terrestrial repeater. For example, you may transmit on 145.990 MHz with a PL tone of 67 Hz and receive on 437.800 MHz. Because Doppler shift is greater on UHF, frequent adjustments become necessary. However, this mode creates exciting opportunities for operators to communicate through the ISS even when astronauts are not on the air.
Practical Tips for Success
To maximize your chances of success, prepare in advance. First, pre-program frequency memories into your radio with Doppler-corrected steps. Second, use a good antenna and aim it as the ISS moves across the sky. Third, practice before real passes by monitoring terrestrial satellites to understand Doppler effects. Additionally, record your attempts, since signals fade quickly and reviewing later may confirm a successful contact.
Challenges of Contacting the ISS
Although contacting the ISS excites many operators, it is not always easy. Competition for astronaut contacts is high, especially during voice events. Moreover, the rapid movement of the station demands precise timing. Weather, interference, and local obstructions can also reduce signal strength. Despite these challenges, even hearing the ISS signal represents a rewarding experience. Furthermore, each attempt teaches valuable lessons in satellite communication.
Educational and Inspirational Value
ISS contacts inspire curiosity about science, engineering, and space. Schools participating in ARISS programs often report increased student interest in STEM careers. For individual hams, the experience of hearing your own signal relayed from space leaves a lasting impression. Because the ISS represents international cooperation, making contact also emphasizes the unifying spirit of amateur radio.
Ham Radio and Space
Ham radio and space create a unique partnership, allowing everyday operators to connect with astronauts orbiting Earth. With basic equipment, careful preparation, and knowledge of frequencies, operators can enjoy voice contacts, APRS digipeating, SSTV reception, and cross-band repeater activity.
Furthermore, each contact demonstrates the power of radio to cross boundaries and inspire imagination. Ultimately, whether you succeed in speaking with an astronaut or simply hear the ISS beacon overhead, the experience connects you to the frontier of human exploration and the shared spirit of amateur radio.