NOAA weather satellites in low Earth orbit transmit analog APT imagery on the 137 MHz band. With a low-cost SDR and a simple V-dipole antenna, you can receive those passes and decode the audio into weather images.
APT (Automatic Picture Transmission) is an analog image transmission system similar in concept to a slow-scan fax. Instead of digital files, the satellites broadcast an analog audio signal that contains the scan-line data for the image.
NOAA 15
137.620 MHz
NOAA 18
137.9125 MHz
NOAA 19
137.100 MHz
Reception quality depends on three physical factors that change every minute as the satellite orbits at 17,000 mph:
Doppler & Polarization View
The best beginner antenna for 137 MHz
Use two stiff wire elements (like copper or aluminum) cut to about 53 cm each for the 137 MHz band.
Arrange elements in a V-shape at a 120° angle. This helps match the signal's circular polarization.
Mount the antenna with a clear, unobstructed view of the sky. Aim the V toward the satellite's path.
Connect the elements to your SDR via a stable coax feedline. The center conductor to one leg, the shield to the other.
RTL-SDR v4
Primary Receiver
V-Dipole
DIY ~53cm Legs
WXtoImg / noaa-apt
Analog Decoder
Pass Elevation
High Angle is Best
At orbital velocities (~17,000 mph), the signal frequency shifts significantly during a pass. This simulation demonstrates how to track the center frequency to maintain a phase lock.
The Automatic Picture Transmission (APT) protocol uses an AM-modulated subcarrier. Below is a recreation of the rhythmic "tick-tock" audio and its conversion into scan-line data.
NOAA_19_PASS.wav
Click to route audio
Test what you learned about NOAA satellite reception
What frequency do NOAA weather satellites transmit APT images on?