Weather-satellite images (coming soon)

Chapter under construction. The goal is ambitious and deserves doing properly — here's the promise and the theory already.

This is the turning point for many SDR beginners: receiving, with a makeshift antenna and a $30 dongle, the image from a real satellite passing overhead. Not abstract data — a photo of the clouds, taken at 800 km altitude, streamed down live.

What makes it magic (and doable)

The polar-orbiting NOAA weather satellites (15, 18, 19) transmit APT (Automatic Picture Transmission) on 137 MHz — right within an RTL-SDR's reach. The signal is slow, robust, designed in the 70s to be decoded with almost nothing.

Modulation: a 2400 Hz audio subcarrier, amplitude-modulated, itself FM on the carrier. Demodulate as wide FM, recover an audio signal, and each pixel's brightness is that subcarrier's amplitude.
Cadence: 2 lines per second. A full pass lasts ~15 minutes and yields an image strip thousands of km long.
The Russian successor Meteor-M transmits LRPT (digital, QPSK): colour image, prettier, a touch more demanding.

The real challenge: knowing when to look up

A polar-orbiting satellite is only visible for a few minutes, during a pass. So you must predict its passes from its orbital parameters (the TLEs, Two-Line Elements, published and updated regularly) and your position. That brick — pass prediction + Doppler tracking — is what needs the most work, and why this chapter comes after the others.

The antenna is everything

Polarisation is circular (the satellite spins): a plain vertical receives it poorly. The classics:

V-dipole (two ~52 cm legs in a 120° V), simple and effective.
QFH antenna (quadrifilar helix), the omnidirectional reference for 137 MHz.

In the meantime

You can prepare everything already: understand the modulation, tune your antenna, master the Le waterfall (cascade temporelle) to recognise the APT signal (a scrolling "ladder"). When the pass decoder is ready, you'll be set.