What Is ISS Ham Radio and Why It Matters
ISS ham radio describes the entire ecosystem of amateur radio activity involving the International Space Station — receiving voice and SSTV transmissions, exchanging APRS packets via the onboard digipeater, and, in rarer cases, making a direct two-way voice contact with a licensed astronaut crew member. The first and longest-running educational outreach program on the space station is ISS Ham Radio, operated by an organization known as Amateur Radio on the International Space Station, or ARISS.
Brief History of Amateur Radio Aboard the ISS
Crew members on the space shuttle Columbia first used an amateur radio to communicate with people on Earth in 1983. That program, the Shuttle Amateur Radio Experiment (SAREX), ended in 1999. The leap to the ISS came quickly after. The first amateur radio equipment was delivered to the International Space Station in September 2000, and Commander William Shepherd, KD5GS, made the first amateur contacts in November of that year.
ARISS first went on the air on November 13, 2000, when the ISS Expedition 1 crew made the inaugural ham radio contact using an Ericsson VHF radio. That same year, the first scheduled school contact linked ISS Commander Bill Shepherd, who had call sign KD5GSL, with students at Luther Burbank School in Burbank, Illinois.
The FCC issued ham radio call sign NA1SS for ISS operations. Today, that callsign is one of the most coveted in the world to work. Each year, the program hosts about a hundred contacts. It has now directly connected over 100 crew members with more than 1 million student participants from 49 U.S. states, 63 countries, and every continent.
Why Astronauts Use Ham Radio in Space
Most of the astronauts on the International Space Station are licensed radio amateurs and sometimes during their spare time they talk to other radio amateurs back on Earth. The station's amateur radio gear serves multiple roles simultaneously. The ARISS amateur radio gear on the ISS provides added value in its STEM educational mission. The beneficial side effect for amateur radio operators is that the ARISS station remains available for general amateur radio usage when it is not engaged in educational contacts.
Because the ARISS program supports the testing and installation of amateur radio stations aboard the ISS, astronauts have the equipment available to also make unscheduled ham radio contacts with radio amateurs all around the world on a one-to-one basis during their personal time. For many crew members, operating the ham radio station is a welcome mental break from the rigors of life aboard the station.
The ARISS Program Explained
ARISS is an international educational outreach program partnering the participating space agencies — NASA, Russian Space Agency, ESA, CNES, JAXA, and CSA — with the AMSAT, ARRL, and IARU organizations from participating countries.
ARISS is an international program that lets students use amateur ham radio to talk directly with crew members living and working on the International Space Station. The ham radio organizations' volunteer efforts provide the equipment and operational support to enable communication between crew on the ISS and students around the world using amateur radio. Today, ARISS typically connects about 200,000 students, educators, and enthusiasts every year with people in orbit.
ISS Ham Radio Frequencies and Modes
Before you can do anything with the ISS on the air, you need to know where to tune. The ISS operates across several frequencies and modes, and understanding which frequency is active and when is the foundation of every successful ISS ham radio session.
ISS Downlink and Uplink Frequencies
The following frequencies are currently used for amateur radio ISS contacts: Voice Downlink: 145.80 MHz (Worldwide); Voice Uplink: 144.49 MHz for ITU Regions 2 and 3 (The Americas, and the Pacific and Southern Asia); Voice Uplink: 145.20 MHz for ITU Region 1 (Europe, Russia and Africa); VHF Packet Uplink and Downlink: 145.825 MHz (Worldwide); UHF Packet Uplink and Downlink: 437.825 MHz; VHF/UHF Repeater Uplink: 145.99 MHz (PL 67 Hz); VHF/UHF Repeater Downlink: 437.80 MHz; SSTV Downlink 437.550 MHz with Robot36 mode and two minutes between each image.
Most ARISS operations are split-frequency, meaning each station uses separate receive and transmit frequencies. The downlink is the earth station's receiving frequency. The uplink is the earth station's transmitting frequency. Earth stations can listen to the downlink frequency and transmit on the uplink frequency when the ISS is in range and crew members are on the air.
Voice Contacts: FM Simplex Operations
Two channels on the 2-meter radio band support voice operations — 145.80 down/144.49 up for ITU Regions 2 and 3, and 145.80 down/145.20 up for ITU Region 1. It is necessary to use two uplink frequencies to operate in accordance with region-to-region IARU band plan differences. The crew switches between one frequency and the other; scanning is not used. For example, if a crew member begins a QSO over the US, they can track US stations until they hit the Atlantic, and then they will quickly lose US stations. They can then switch over to the other frequency and pick up stations in Europe or Africa.
APRS and Packet Radio via the ISS
There is one radio on the ISS that operates as a packet digipeater. The Columbus D710GA can support those operations at about 10 watts and uses the callsign NA1SS. It will respond to the alias "ARISS."
The ISS keeps the packet station on 145.825 MHz when it is available. This lets the packet downlink from ISS operate with other APRS satellites there, and its downlink is collected by the established global network of APRS Internet-Gateway stations feeding data to the ARISS-APRS web page.
SSTV Transmissions from the ISS
Slow Scan Television images can be transmitted from the International Space Station. An SSTV system is an integral part of one of the ARISS ham radio stations, NA1SS/RS0ISS, in the Service Module. It transmits and receives JPEG still images using the Kenwood D700 and D710 radios and the ARISS antennas mounted on the Service Module.
ISS SSTV images are transmitted using the PD-120 mode on 145.800 MHz. During special events, the station has also used the 437.550 MHz frequency with the Robot36 mode. The International Space Station transmits SSTV images several times a year to commemorate special space-related events.
Equipment You Need to Hear the ISS
One of the most appealing aspects of ISS ham radio is how little equipment is required to get started. Receiving the downlink is genuinely achievable with gear that fits in your shirt pocket.
Minimum Receiver and Antenna Requirements
The amateur radio station on the ISS can be received using very simple equipment. For stations in the ISS footprint, the RS0ISS signal should be easy to copy on a handheld transceiver and a quarter-wave whip. The simplest antenna to use to contact the space station is a quarter-wave vertical antenna. However, for transmitting to the ISS, more power and a better antenna make a significant difference. A typical ground station for contacting the ISS station includes a 2-meter FM transceiver and 25–100 watts of output power.
Best Handheld Transceivers for ISS Reception
For a two-way contact, you'll need a VHF/UHF dual-band handheld transceiver that can transmit and receive on 2-meter and 70 cm bands. Some recommended transceivers for ISS contacts include the Yaesu FTM-400XDR, Icom IC-2730A, and Kenwood TH-D74A. The Kenwood TH-D74A and Yaesu FT5DR are particularly popular because they have built-in APRS hardware, allowing you to decode incoming packets from the ISS digipeater without a separate computer or TNC. For receive-only work during SSTV events, even an inexpensive dual-band HT paired with a simple antenna will get the job done on a good overhead pass.
Using an SDR Dongle to Receive ISS Signals
A cost-effective option for receiving ISS signals — including SSTV, APRS, and voice — is a Software Defined Radio (SDR). An SDR dongle connects to a computer and uses software like SDR# or GQRX to tune and demodulate signals. An RTL-SDR V3 dongle paired with a simple dipole or whip antenna and a low-noise amplifier (LNA) is more than sufficient for receiving the 145.800 MHz and 145.825 MHz downlinks. SDR users should set receiver bandwidth to at least 30 kHz to capture the signal even as it drifts due to Doppler shift. This is an especially practical approach for SSTV events, where the receive-only nature of the operation means no license is required.
Directional vs. Omnidirectional Antennas
A Yagi antenna focuses your signal, increasing success. A rotatable Yagi is best for tracking the ISS. Even a simple outdoor vertical antenna can receive strong signals. For listening only, an outdoor omnidirectional antenna such as a 5/8-wave vertical or a basic turnstile typically gives you enough margin to copy the ISS during high-elevation passes. For transmitting, a handheld or fixed Yagi pointed toward the ISS dramatically increases your chances of being heard. As the angle lowers toward the horizon, a handheld beam antenna is useful for increasing the antenna gain when transmitting and receiving.
How to Make a Two-Way Contact with the ISS
A direct two-way voice contact with an astronaut is one of the rarest and most rewarding achievements in amateur radio. Understanding how the ISS moves and how to position yourself for a pass is the first step.
Understanding ISS Pass Timing and Orbital Windows
The ISS orbits about 248 miles above Earth and travels at around 17,150 miles per hour. This means its range for radio contact is constantly changing. The theoretical limit for any pass is about 10 minutes maximum, with an average of 5–7 minutes usable per pass. The ISS crosses overhead multiple times per day from any given location, but not every pass offers a usable contact window — low-horizon passes that peak below 10 or 15 degrees elevation give very little time and produce weak signals. Target passes that peak above 30 degrees elevation for the best results.
Using Tracking Software: Heavens-Above, Gpredict, and ISS Detector
You can check websites like AMSAT.org or Heavens-Above.com to find out when the ISS will be visible from your location and in range for radio contact. Gpredict is a free, real-time satellite tracking and orbit prediction application for Unix-like systems, including Linux, macOS, and Windows. On mobile, ISS Detector (available for both Android and iOS) is a straightforward choice that gives you pass predictions, elevation, and azimuth data in a simple interface. Enter your latitude and longitude or 6-digit grid square, and the number of passes into the future you want data for. You'll get complete information on when passes start and stop, what azimuth the satellite will appear and disappear over the horizon, and the highest elevation the satellite will achieve during a specific pass.
TLE data is critical for accurate predictions. The ISS is a significant exception to standard satellite tracking norms. Because of its low orbit, it experiences significant aerodynamic drag which over a few days introduces noticeable errors. Additionally, its orbit is periodically raised by thrusters. These combined factors require frequent updates to ensure accurate predictions. Always refresh your TLE data before a planned contact attempt.
Doppler Shift Correction Explained
Doppler shift is the change in received frequency caused by the relative motion between the ISS and your station. As the ISS is moving and transmitting, you have to adjust your receive/downlink frequency — it is the same phenomenon as a passing train blowing its whistle, where you hear the tone of the
