Learn / Guide 10

From space: ISS, radiosondes and satellites

APRS and SSTV from the Space Station on 145.825 and 145.800 MHz, weather balloons from 400 to 406 MHz, and what is left of weather satellites.

intermediate 6 min read

Signal coming from above is easy

A high transmitter sees far. There are no buildings or hills in the way, and a few watts are enough. That is why signals from space and the upper atmosphere are among the first that a beginner can receive with simple equipment: a RTL-SDR or a handheld radio and a small antenna, outdoors.

The difficulty is time. Low Earth orbit objects pass quickly, and you need to know when.

The Space Station

The ISS flies at about 400 km altitude and circles the Earth every 92 minutes or so. The orbit is inclined 51.6 degrees, which covers all of Brazil. A good pass lasts up to 10 minutes, from horizon to horizon, and generally there are a few per day at changing times.

On board there is an amateur radio station maintained by the ARISS program. The frequencies of interest to listeners:

Not everything is turned on at the same time. ARISS publishes equipment status and the schedule.

Doppler effect. The station moves at more than 7 km per second. On 145.800 MHz the signal arrives about 3.5 kHz higher at the start of the pass and 3.5 kHz lower at the end. In FM, with a wide filter, you can leave it parked on 2 meters. On 437.800 MHz the shift gets close to 10 kHz each way, and you need to track it.

APRS on 145.825

APRS is an amateur radio system to send short packets: position, text messages, telemetry. The ISS repeats the packets it receives, and with that a small station reaches a few thousand kilometers.

In the speaker, they are half-second bursts sounding like an old modem. A decoder program turns the audio into text: callsign of the sender, position, and message. When the ISS repeats it, its callsign, RS0ISS, appears in the packet path.

SSTV on 145.800

SSTV is Slow Scan Television: a still image transmitted as audio, line by line. A few times a year ARISS schedules events where the ISS transmits a series of commemorative images for a few days.

The mode used is usually PD120, where each image takes about two minutes. A pass can fit two or three. You record the audio and run it through a decoder program on a computer or mobile phone. Because the transmission is in strong FM, even a handheld radio with the original antenna receives it if the pass is high.

Dates are released in advance on the ARISS website. Outside of events, the frequency is used for school contacts, where you can hear the astronaut answering.

Radiosondes

Once or twice a day, weather stations around the world release a balloon with a small instrument box hanging underneath. That is the radiosonde. It measures pressure, temperature, and humidity on the way up and transmits everything by radio, along with GPS position. Wind is derived from the balloon's movement.

• Time: soundings are referenced to 0h and 12h UTC, and the balloon ascends a little earlier. In Brazil this falls close to 9h and 21h Brasília time.
• Frequency: between 400.15 and 406 MHz. Each station uses a fixed frequency.
• Flight: the balloon ascends for about an hour and a half, up to about 30 km, pops and the probe descends by parachute.
• Signal: a narrow carrier with a continuous data hum, one transmission per second.

Power is minimal, but at 30 km height the line of sight is huge. You can track a radiosonde hundreds of kilometers away.

The open source radiosonde_auto_rx program sweeps the band, detects the radiosonde, decodes it, and sends the data to SondeHub. The SondeHub map shows flights in real time, with trajectory and landing prediction. Even without a receiver, opening the map near 12h UTC is a way to see which Brazilian stations are launching.

Weather satellites: what changed

For over twenty years, the first project for anyone buying an RTL-SDR was to receive analog APT images from NOAA-15, 18, and 19 satellites on 137 MHz. That is over. NOAA-18 was decommissioned in June 2025, and NOAA-15 and NOAA-19 in August of the same year. The tutorials still circulating about them are outdated.

What continues, and calls for a check before setting up an antenna:

• Meteor-M (Russia): polar orbit satellites transmitting digital images at 137 MHz in LRPT format. Received with an RTL-SDR and a simple antenna, and decoded with programs like SatDump. Each satellite's status changes, so check the current state.
• GOES (United States): the geostationary satellite covering the Americas transmits images of the entire Earth disk at 1,694.1 MHz. It does not move: it stays fixed in the sky. On the other hand, it requires a satellite dish of about one meter and its own amplifier.

Amateur radio satellites

There are dozens of satellites built by and for radio amateurs. Some are FM repeaters in orbit: you uplink on one band, downlink on another. AMSAT publishes the list of active ones, and Brazil has its own section, AMSAT-BR.

To know the time of the pass

Everything in low orbit requires prediction. Open-source tracking software like Gpredict calculates passes for your location from public orbital data. For each pass, three numbers matter:

1. The start time, in UTC or local.
2. Maximum elevation. Above 30 degrees is a good pass. Below 10, the satellite barely clears the horizon.
3. The direction where it rises and sets, to know if there is an obstacle.