Decode NOAA Weather Satellite Images

~120 minBeginner

Build a wire antenna, predict satellite passes, and decode real weather imagery from NOAA polar-orbiting satellites at 137 MHz.

Prerequisites
Gather these before starting

Hardware

  • •RTL-SDR dongle (any model works)
  • •~110 cm of stiff wire (coat hangers work)
  • •SMA connector and short coax cable
  • •Outdoor antenna placement with clear sky view

Software / Knowledge

  • •SDR++ or GQRX for reception
  • •noaa-apt decoder (free, open source)
  • •Satellite tracking app or website
  • •No prior SDR experience required
Step 1 of 714% complete
Step 110 min
Understanding APT Signals

NOAA weather satellites broadcast Automatic Picture Transmission (APT) continuously on VHF frequencies. Each satellite circles Earth in a polar orbit approximately every 100 minutes, transmitting two-channel analog imagery at 137 MHz.

NOAA 15

137.620 MHz

Oldest, occasional signal issues. Still decodable.

NOAA 18

137.9125 MHz

Best performance. Primary choice for most hobbyists.

NOAA 19

137.100 MHz

Launched 2009. Good signal strength globally.

APT signal characteristics

ModulationAM on FM carrier (APT subcarrier at 2400 Hz)
Subcarrier2400 Hz AM, modulated with image data
Line rate2 lines/second (120 lines/minute)
Image resolution4 km/pixel (visible and IR channels)
Frame width2080 pixels total (2 × 909 pixel image channels + sync)
Bandwidth~34 kHz occupied (FM deviation ±17 kHz)

NOAA 15, 18, and 19 are the three active birds. Check their status at celestrak.org — NOAA 15 has had intermittent issues since 2019 but still transmits.

Understanding APT Signals