How a signal crosses hundreds of kilometres of space, and how you can listen to UNNE-1B from home
A satellite only hears you while it is above your horizon: about 10–15 minutes per pass. Press play and watch how distance, delay and frequency change. Then switch on repeater mode and find out why two stations must see the satellite at the same time to talk.
Simplified simulation: circular orbit of ~500 km and a pass right over both stations. The Earth and the altitude are drawn at different scales so they can be seen; the numbers are calculated with the real distances. The sound is indicative.
Every radio system has two ends: the transmitter (TX), which generates and sends the signal, and the receiver (RX), which picks it up and extracts the information. In between, the signal travels through space as an electromagnetic wave.
On UNNE-1B, the on-board computer generates the telemetry packets, a modulator turns them into FSK tones (or relays your voice in FM) and an amplifier feeds the signal to the antenna.
The satellite antenna is omnidirectional (dipole or monopole): it radiates in all directions so any ground station can receive it during the pass.
On the ground, the antenna picks up the signal (extremely weak after travelling 500 km), the SDR digitises it and the software processes it: it demodulates the signal, decodes the packets and extracts the telemetry.
The satellite moves at ~7.6 km/s. At 436.888 MHz the apparent frequency shifts by up to about ±10 kHz during a pass: it rises as it approaches and falls as it moves away. You can see it in the simulator waterfall.
The antenna converts electrical energy into electromagnetic waves (TX) and vice versa (RX). Its size is proportional to the working wavelength. A well-designed antenna makes the difference between hearing the satellite perfectly or receiving nothing.
An optimal dipole is λ/2 long. At 436.888 MHz (UNNE-1B): λ = 68.6 cm → 34.3 cm dipole. You can make one from copper wire in 30 minutes.
| Antenna type | Gain | Pattern | Ideal for... |
|---|---|---|---|
| Dipole / Monopole | 2.1 dBi | Omnidirectional | First home-made antenna. Works for satellites high above the horizon. |
| Yagi-Uda | 6–16 dBi | Directional | Pointing at satellites. Better reception, but you have to track. |
| Helical | 10–15 dBi | Conical + circular | Satellites: circular polarisation avoids orientation losses. |
| Turnstile / QFH | 3–4 dBi | Omnidirectional, circular | Automatic reception of the whole pass without pointing. Ideal for UNNE-1B. |
| Parabolic dish | 20–50 dBi | Highly directional | GEO satellites, radio astronomy, radar. |
When a wave travels through free space, its power spreads over a growing sphere. This is called free-space path loss (FSPL) and it is the biggest challenge in satellite reception.
6 picowatts is 0.000 000 000 006 watts. An RTL-SDR can detect signals like that because its own noise level is very low. The key is not power: it is the signal-to-noise ratio (SNR).
Go to Heavens-Above.com → enter your city → search for "UNNE-1B" (it will appear once it is catalogued, a few days after release). You will see the times of the next passes with their maximum elevation (the higher the better).
Plug the RTL-SDR into your computer, install the drivers and open your SDR software. Tune to 436.888 MHz in NFM (narrow FM) mode. Turn on the AGC (automatic gain control).
When the satellite rises above the horizon you will hear the signal. You will need to adjust up to ±10 kHz to compensate for the Doppler effect. Some software corrects it automatically.
Route the audio from your SDR software to the AMSAT-EA ground station software (with a virtual audio cable such as VB-Cable). It will decode the 200 bps FSK packets: power, temperatures, status… and the Nebrija game packet.
Upload it to our data registry and share it with the AMSAT-EA community. Every reception helps monitor the satellite. SatNOGS.org also collects receptions from all over the world.
