International Space Station

The International Space Station has been humanity's home in orbit since November 2000.

Scale and Construction

ISS spans 109 meters wide, 73 meters long, and 20 meters high - larger than a football field. With a mass of 420,000 kg, it's the largest structure humans have ever placed in space. Construction required over 40 assembly flights, including 37 shuttle missions and numerous Russian launches.

Key Modules

• Destiny (US Lab): 8.5 meters long, 14,500 kg. The primary US research facility, hosting experiments in fluid physics, combustion, materials science, and biology. Contains 24 International Standard Payload Racks (ISPRs).
• Zarya (FGB): 12.9 meters long, 19,323 kg. The first ISS module launched, providing initial power, propulsion, and guidance. Now serves as storage and docking port.
• Columbus (ESA): 6.7 meters long, 12,800 kg. European laboratory module for multidisciplinary research including fluid physics, materials science, and life sciences. Features 10 ISPRs.
• Kibo (JAXA): 11.2 meters long, the largest ISS module. Japanese Experiment Module consists of the Pressurized Module (PM), Experiment Logistics Module - Pressurized Section (ELM-PS), and the Exposed Facility (EF) - a porch for external experiments.
• Zvezda (SM): Provides life support systems, crew quarters, and propulsion for attitude control and reboost.
• Harmony (Node 2): Central connecting node linking Destiny, Columbus, Kibo, and the Crew/Cargo vehicles.

Power Systems

ISS generates electrical power using 8 solar array wings (SAWs), each measuring 35 meters long × 12 meters wide. The arrays contain approximately33,000 solar cells made of gallium arsenide, producing between84 and 120 kW average power (varying with solar beta angle and degradation).

• Bus voltage: 160 V DC (regulated)
• Solar cell efficiency: ~30% (advanced triple-junction GaAs)
• Battery capacity: Nickel-hydrogen (NiH2) batteries store power for the ~35 minutes of each 90-minute orbit spent in Earth's shadow
• Power distribution: 12 channels of 160V DC, regulated to 124V for payload use

Telemetry, Tracking, and Command (TT&C)

ISS uses multiple communication systems for different purposes:

• S-band (2.0-2.3 GHz): Primary TT&C system for voice, telemetry, and commanding. Provides omnidirectional coverage for near-real-time communication with Mission Control Centers in Houston and Moscow.
• Ku-band (14.5-15.35 GHz): High-rate data link via the TDRSS constellation. Downlink rate up to 300 Mbps for video, experiment data, and payloads. Uplink at 250 kbps for file transfers.
• SSR (Solid State Recorder): 1,536 Gbit capacity for data storage when TDRSS is not available

Environmental Control and Life Support System (ECLSS)

ECLSS maintains a habitable environment for the crew, recycling air and water:

• Oxygen Generation System (OGS): Produces oxygen through electrolysis of water - splitting H₂O into H₂ and O₂. Produces approximately 5.4 kg of O₂ per day for 6 crew members.
• Carbon Dioxide Removal Assembly (CDRA): Uses zeolite beds to adsorb CO₂ from cabin air. The beds cycle between adsorption and regeneration (heating to release collected CO₂ into space).
• Water Recovery System (WRS): Recycles urine, humidity condensate, and hygiene water into potable water. Achieves 90%+ water recovery rate, recovering approximately 6,000 kg of water per year.
• Major Constituent Analyzer: Continuously monitors O₂, N₂, CO₂, H₂, CH₄, and trace contaminants

Life in Space

Crews of 6-7 astronauts live and work aboard ISS for 6-month rotations. A day includes 2 hours of exercise to prevent muscle and bone loss, 8 hours of work, and time for meals and communication with family. The station's artificial gravity is actually microgravity - objects float freely as everything orbits together.

Science and Research

• Microgravity: Protein crystal growth, cell cultures, fluid physics
• Biology: Effects of spaceflight on humans and other organisms
• Earth Observation: Monitoring climate, weather, natural disasters
• Astronomy: X-ray and gamma-ray observations without atmosphere
• Technology: Testing life support, robotics, communications for deep space

End of Life

ISS is approved to operate through 2030. When retired, NASA plans a controlled deorbit into the South Pacific ocean (Point Nemo - spacecraft cemetery). The station will break up in the atmosphere, with debris falling into a 2,700 km long debris field.

Voice Communications (UCS)

The primary voice and command link between ISS and ground uses the Unified S-Band (UCS)system, operating in the 2.0–2.3 GHz range. This system carries all crew-to-ground voice traffic, commanding from Mission Control, and critical telemetry. The S-band signal is received by NASA's Tracking and Data Relay Satellite System (TDRSS) constellation of geosynchronous satellites, then relayed to ground stations at White Sands, New Mexico. This gives near-continuous coverage as ISS orbits overhead, though brief blackouts occur during handovers between TDRSS satellites.

For ham radio operators, ISS carries amateur radio equipment through the ARISS (Amateur Radio on the ISS) program. The callsign used for voice contacts is NA1SS. Scheduled contacts typically occur between school groups and the ISS crew. Operators on the ground use standard VHF/UHF amateur radio gear, communicating on 2 meters (145.825 MHz for APRS, 145.800 MHz for SSTV) or 70 cm (437.800 MHz for packet radio).

• UCS frequency band: 2.0–2.3 GHz (S-band), near-continuous via TDRSS
• Ham voice callsign: NA1SS
• Ground contact methods: Direct VHF/UHF (limited pass windows), or ARISS scheduled school contacts
• Typical ARISS contact duration: 10 minutes maximum
• Required license: Technician class or higher for US operators (Technician for VHF, General for HF)

ARISS (Amateur Radio on the ISS)

ARISS is a volunteer program that installs and maintains amateur radio equipment aboard the ISS, enabling the crew to make direct contacts with schools, scout groups, and the general public. It is a collaboration between AMSAT (Radio Amateur Satellite Corporation), ARRL (American Radio Relay League), NASA, ESA, JAXA, and Roscosmos.

Equipment Aboard ISS

• Kenwood TM-D710E: Dual-band VHF/UHF transceiver (144/430 MHz, 50W output) — primary ham radio for packet radio, APRS, voice, and crossband repeater operation
• Acom 1010 HF amplifier: 1 kW solid-state amplifier used for HF contacts during ARISS events
• Astro-Hatch laptop: HP laptop running HamRadio Deluxe software for rig control, logging, and digital mode operation
• Packet/APRS digipeater: Forwards APRS packets on 145.825 MHz (Columbus NA1SS) and 437.825 MHz (Zvezda RS0ISS), extending packet radio range worldwide
• HamTV: 2.395 GHz DATV transmitter from Columbus for school ARISS video contacts

Requesting an ARISS School Contact

• Schools and educational organizations apply through the ARISS website (ariss.org) during open application windows, typically twice per year
• Applications require an educational outreach plan, a ham radio mentor, and pre-contact educational activities
• Contact windows are scheduled 2–3 months in advance based on ISS crew schedule and orbital pass geometry
• During the contact, the crew uses the Kenwood TM-D710E on scheduled frequencies; the school's ground station uses a similar setup
• Contacts are also streamed live on NASA TV and the ARISS YouTube channel

Telemetry and APRS

The ISS is one of the most accessible amateur radio satellites for packet radio and APRS. APRS (Automatic Packet Reporting System) is a digital communications protocol used by amateur radio operators to transmit position, status, and short messages. The ISS acts as a digipeater — it receives packets from ground stations and retransmits them, extending their range across the station's 28,000 km/h orbital ground track.

Complete ISS Amateur Radio Frequency List

All amateur radio operations on ISS use the NA1SS callsign for the Columbus module and RS0ISS for the Zvezda module. Frequencies below are confirmed active as of July 2026:

• Packet rate: 1200 baud AFSK, standard APRS protocol
• Crossband repeater: Uplink 145.990 MHz → Downlink 437.800 MHz, CTCSS 67 Hz required on uplink
• HamTV: 2395 MHz (2.395 GHz) — digital amateur television, test signal from Columbus

Receiving ISS APRS with a Handheld Radio

You can receive ISS APRS packets with a basic FM handheld radio and no special equipment beyond an antenna. No transmit license is required for receive-only operation. The ISS APRS digipeater has two active frequencies — primary VHF and backup UHF:

• Primary: Set your HT (Baofeng UV-5R, Yaesu FT-60, Kenwood TH-D72, etc.) to 145.825 MHz FM. The digipeater alias is ARISS — set your callsign accordingly and any packet with via ARISS will bedigipeated by ISS.
• Secondary (Zvezda): 437.825 MHz FM — receives as RS0ISS
• Use a decoding app on your phone or computer connected to the radio's audio output
• Run a pass prediction tool (Heavens-Above, N2YO, or ISS Detector app) to know when ISS is above your horizon
• Point your antenna toward the ISS during the pass — a vertical whip works, but a 5/8 wave or turnstile antenna improves reception significantly
• Packets decode as short text strings containing callsign, position, altitude, and status

ISS SSTV (Slow Scan Television)

Periodically, ISS crew members transmit SSTV images on 145.800 MHz FM. SSTV encodes images as audio tones that a receiving station decodes into a picture. ISS SSTV events are typically scheduled for special occasions (Cosmonautics Day in April, ARISS anniversaries, etc.).

• Frequency: 145.800 MHz FM
• Common modes: PD120 (120 seconds per image, 320×240 resolution) and PD180 (180 seconds, 640×480 resolution)
• Software: MMSSTV (Windows), SDR#, or mobile apps like Robot36 (Android/iOS)
• Procedure: Tune HT to 145.800 MHz, connect audio to decoding software, listen for the characteristic warbling SSTV tones during a pass, and decode the image in real time

Tracking the ISS

To communicate with or receive signals from ISS, you must know exactly when it will be above your horizon. ISS orbits Earth approximately every 92 minutes, at an altitude of 370–460 km and an inclination of 51.6 degrees. Its orbital parameters change over time due to atmospheric drag, so up-to-date Keplerian elements (Two-Line Elements or TLEs) are essential for accurate predictions.

Keplerian Elements and TLEs

• NORAD Catalog Number: 25544
• Current TLE (July 2026):

  ISS 1 25544U 98067A   26206.44322024  .00008631  00000-0  16367-3 0  9991 2 25544  51.6316 110.7757 0006909 336.1751  23.8916 15.49160253577680

• TLE updates: Refresh your TLE data every 1–2 weeks from celestrak.org/NORAD/elements or space-track.org for accurate predictions
• TLE format: Two lines of 69 characters each containing epoch, inclination, eccentricity, mean motion, and perturbation terms
• Propagation: SGP4/SDP4 orbital propagator computes ISS position from TLEs — all prediction software uses this algorithm
• Orbital parameters: Period ~92 min, Altitude 370–460 km, Inclination 51.63°, Semi-major axis 6,793 km

Prediction Software

• Heavens-Above: Free web-based pass predictions with sky charts — enter your location and see ISS passes for the next 7 days
• N2YO: Real-time ISS tracking with ground track visualization and pass alerts
• Stellarium: Free planetarium software with satellite plugin — shows ISS in the sky map in real time
• ISS Detector (Android): Mobile app with push notifications for upcoming passes, includes signal strength estimates
• Gpredict (cross-platform): Open-source satellite tracking with Doppler correction for radios

Pass Prediction and Doppler Correction

• Maximum pass duration: ~10 minutes (but most passes are shorter, 2–6 minutes)
• Minimum elevation for reception: 10° above horizon — below this, terrain and buildings block the signal
• Doppler shift: Up to ±10 kHz on VHF as ISS approaches and recedes from your location — tune your receiver down 5 kHz as ISS rises, then up 5 kHz as it sets
• Azimuth: Compass direction from your location toward ISS (0°–360°) — ISS rises from a specific direction on the horizon
• Elevation: Angle above your horizon (0°–90°) — a 45°+ pass gives the strongest signal and longest contact window

Downlink Frequencies Reference

The following table lists all publicly known ISS downlink frequencies accessible to amateur radio operators and signal enthusiasts.

How to Receive ISS Signals

Equipment Needed

• FM handheld radio (HT): Baofeng UV-5R (~$25), Yaesu FT-60 (~$150), or Kenwood TH-D72A (~$300) — any FM HT with 2m capability works for receive
• Antenna: Stock rubber duck antenna works for close passes, but a 5/8 wave whip, turnstile antenna, or Arrow satellite antenna (dual-band Yagi) dramatically improves reception
• Audio interface: 3.5mm audio cable from radio speaker output to computer/phone mic input for decoding software
• Decoding software: SDR#, Dire Wolf, Multipsk, or mobile apps (Robot36 for SSTV, APRSdroid for packet)
• Optional SDR receiver: RTL-SDR dongle ($25) connected to a computer gives you a wideband receiver with waterfall display — use with SDR# or GQRX for real-time signal visualization

Receiving APRS Packets — Step by Step

• Step 1: Check ISS pass predictions using Heavens-Above or an app — note the rise time, direction (azimuth), maximum elevation, and set time
• Step 2: Program your HT to 145.825 MHz FM with CTCSS tone disabled (open squelch or use tone squelch if you prefer)
• Step 3: Connect radio audio output to your computer or phone running APRS decoding software (Dire Wolf, APRSdroid)
• Step 4: When ISS rises above your horizon (~10° elevation), begin recording/decoding — point your antenna toward the satellite's predicted position
• Step 5: As ISS passes overhead, you will receive short data bursts decoded as APRS packets containing callsigns, positions, and messages
• Step 6: Log the packets and upload to APRS-IS (aprs.fi) to see your reception reported on the global APRS network

Receiving SSTV Images — Step by Step

• Step 1: Check the ARISS SSTV event schedule at ariss.org — events are announced 1–2 weeks in advance and typically run for 2–3 days
• Step 2: Install MMSSTV (Windows), Robot36 (Android/iOS), or similar SSTV decoding software on your computer or phone
• Step 3: Connect your HT's audio output to your computer or phone via 3.5mm cable, or use the built-in microphone pointed at the radio speaker
• Step 4: Tune your HT to 145.800 MHz FM before the scheduled pass — set wide squelch so the audio is always open
• Step 5: During the pass, listen for the distinctive warbling SSTV tones — the decoding software will automatically detect the mode (PD120/PD180) and render the image in real time
• Step 6: Each pass typically contains 1–2 images depending on pass duration and elevation. Save your decoded images and compare with other operators' results online

Video and Television

ISS has carried multiple television systems throughout its history. Early video downlinks usedNTSC (analog, US/Japan) and PAL (analog, Europe/Russia) formats, transmitted via the S-band (2.0 GHz) video system. These analog systems were replaced with digital HD-SDI (High-Definition Serial Digital Interface) video feeds capable of 1080p resolution, routed through the Ku-band TDRSS link for high-bandwidth transmission.

The analog S-band video system was historically the primary method for live television from the station, used for crew activities, spacewalks, and docking operations. The digital transition allowed for higher quality, multi-camera feeds, and simultaneous transmission of multiple video streams alongside scientific data and crew communications.

How SSTV Works

SSTV (Slow Scan Television) encodes a still image as a continuous audio signal usingfrequency-shift keying (FSK) of audio subcarriers. Each scan line of the image is represented by a sequence of audio tones where the frequency corresponds to pixel brightness. Common SSTV modes used on ISS:

• PD120: 120 seconds per image, 320×240 pixels, uses frequency modulation from 1500 Hz (black) to 2300 Hz (white) — each line takes about 0.5 seconds
• PD180: 180 seconds per image, 640×480 pixels, higher resolution but longer transmission time
• Scottie 1 (SC1): 110 seconds, 320×256 pixels, used on some ISS contacts
• Sync tones: Each line begins with a 1200 Hz sync pulse, followed by video tones encoding color channels (red, green, blue) sequentially
• FM deviation: ±4 kHz frequency deviation on the FM carrier, centered on the pass frequency