How to hear satellites with a $40 SDR: a beginner's guide

What an SDR is, and why it changed this hobby

A software-defined radio (SDR) is a small USB receiver that hands raw radio samples to your computer. The tuning, filtering and demodulating that used to need a cabinet of equipment now happen in software, so one stick covers everything from about 25 MHz to 1.7 GHz.

The best-known kind started life as a cheap TV dongle. Improved versions made for radio listeners, such as the RTL-SDR Blog V4, cost around 40 US dollars and are more than good enough for satellites. Pricier receivers (Airspy, SDRplay, HackRF) have less noise and wider bandwidth, but nobody needs one to start.

Satellites are a perfect first target because their signals are predictable. You know to the second when one will rise, where it will be in the sky and what frequency it sends on. You are not scanning and hoping; you are waiting for an appointment.

What you can actually hear

All of these work with a basic SDR and a simple antenna:

• The International Space Station on 145.800 MHz FM: crew contacts with schools, and on event weekends slow-scan TV pictures (SSTV) you can decode into images.
• ISS packet radio (APRS) on 145.825 MHz: short bursts that decode into position reports from amateur stations on the ground.
• Amateur FM satellites around 145 MHz and 435–438 MHz: radio amateurs talking to each other through a repeater in orbit, often across 2,000 km or more.
• CW and telemetry beacons from CubeSats on 435–438 MHz: Morse code or data packets that report the satellite's battery, temperature and health.
• Russian Meteor-M weather satellites on 137 MHz: a digital signal that becomes a photo of the clouds below you (see our weather satellite guide).

The shopping list

• An SDR dongle with a TCXO (a stable clock), such as the RTL-SDR Blog V3 or V4. A stable clock matters because satellite signals are narrow and already drifting with Doppler.
• An antenna for the band you want. A V-dipole made from two metal rods works for 137 MHz; for 145 and 435 MHz a small handheld Yagi or a turnstile does far better than the telescopic whip that comes in the box.
• Short, decent coax. At 435 MHz, ten metres of thin cable can throw away half the signal before it reaches the radio. Put the dongle near the antenna and run a USB extension instead.
• Optional: an FM broadcast-band filter. Strong local FM stations around 88–108 MHz can overload a cheap receiver and drown out weak signals on 137 and 145 MHz.

Free software you will use

For listening, SDR++ (Windows, macOS, Linux) is simple and fast; SDR# and SDRangel are popular too. They show a waterfall: a scrolling picture of the radio spectrum where a satellite appears as a bright line that slowly slides sideways.

For decoding, SatDump turns weather-satellite signals into images, MMSSTV or QSSTV turn ISS slow-scan TV into pictures, and Direwolf decodes APRS packets. For knowing when to listen, use a pass predictor: our pass list and radar show the time, direction and the Doppler-corrected frequency for every second of the pass.

Doppler: why the signal slides

A satellite in low orbit moves at about 7.5 km per second. As it approaches, its signal arrives slightly higher in frequency; as it leaves, slightly lower. On 145.8 MHz the shift is up to about 3.5 kHz either way, and on 437 MHz up to about 10 kHz.

For wide FM voice you can often leave the radio on one frequency and only miss the edges of the pass. For narrow signals such as CW beacons or SSB, follow the line on the waterfall by hand, or let software steer the tuning for you. Start a few kHz above the published frequency and drift down as the satellite crosses.

Your first pass, step by step

• Pick a high pass: a maximum elevation above 40° gives a strong signal for several minutes. Low passes skim the horizon through buildings and trees.
• Go outside ten minutes early, start the software and tune to the frequency. Set the gain to around 30–40 dB; turn it down if the whole waterfall turns bright.
• Point the antenna toward where the satellite will rise. With a Yagi, follow it across the sky; with a V-dipole or turnstile, just leave it lying flat facing up.
• Watch the waterfall. Within a minute or two of the rise time, a faint line should appear and strengthen. Record the audio or raw I/Q so you can decode it again later.
• Write down what you heard and when. Comparing passes teaches you more about your antenna and location than any manual.

When nothing shows up

Most failed first passes have one of four causes: the wrong frequency (some satellites change mode or switch off; check recent reports), the satellite was below 15° for most of the pass, local noise from the computer, LED lights or a phone charger is masking it, or the receiver is overloaded by nearby FM or mobile-phone transmitters. Moving the antenna a few metres away from the house and the laptop fixes more problems than buying a better radio.

Is it legal to listen?

In most countries, receiving amateur and weather satellite signals is legal and openly encouraged: these downlinks are meant to be heard by anyone, and many satellites ask listeners to send in their telemetry. Transmitting is different and needs an amateur radio licence. Rules on other kinds of signals, and on sharing what you hear, vary by country, so check your local radio law before going beyond amateur and weather bands.