Authors- Ashok Ghildiyal and Ashish Joshi, Indian Institute of Remote Sensing, ISRO Dehradun

NOAA weather satellites continuously transmit Automatic Picture Transmission (APT) signals that carry live images of the Earth’s surface. These signals can be received using a software-defined radio (SDR) such as an RTL-SDR dongle, paired with a suitable antenna and decoding software. The NOAA satellites, particularly NOAA-18 and NOAA-19, operate in polar sun-synchronous orbits at an altitude of approximately 800 kilometres. These satellites pass over the same region of the Earth at roughly the same local solar time each day. They transmit signals on VHF frequencies, with NOAA-18 broadcasting at 137.9125 MHz and NOAA-19 at 137.1000 MHz. Both use FM modulation with a bandwidth of around 50 kHz and right-hand circular polarization (RHCP).

A low-cost weather satellite receiving station has been developed for capturing and decoding real-time NOAA-18 weather data. It utilizes free and open-source tools like SDRSharp for signal processing, Gpredict for satellite tracking, and WXtoImg for image decoding and enhancement. A Quadrifilar Helix Antenna (QFH) has been  developed for receiving signals around 137 MHz. The antenna is constructed using PVC pipes, coaxial cables, and basic connectors, making it both effective and affordable. The signal from the antenna is processed using SDRSharp software. After installing the necessary drivers via Zadig, SDRSharp is configured in wideband FM mode with a bandwidth setting of approximately 35 to 40 kHz. Gpredict is used alongside to track satellite orbits and to predict passes over the station. It also helps compensate for Doppler shift by adjusting the tuning frequency in real-time. The output audio from SDRSharp is piped to WXtoImg using virtual audio cable software like VB-Cable. WXtoImg decodes the audio signal into images, offering enhancements such as contrast adjustment, overlays, and thermal or multispectral representations.

Proper placement and orientation of the antenna are critical for good signal reception. Adjustments to software settings and regular updates to satellite orbital data (Keplerian elements) help maintain accuracy and improve image quality. Once operational, the system tracks the satellite’s pass, tunes the frequency accordingly, and records the transmission. The audio is then processed to generate good quality weather imagery.The overall setup includes the low cost QFH antenna connected to the RTL-SDR dongle, which plugs into a PC running SDRSharp and WXtoImg. Gpredict provides pass predictions and Doppler corrections, while VB-Cable routes audio between software applications. This arrangement allows seamless integration and continuous monitoring of NOAA satellite signals. Block diagram and processed NOAA1-8 satellite images are shown in the figure1.

Figure 1– Overview of NOAA-18 Ground Station at IIRS, Dehradun

All software components—SDRSharp, WXtoImg, Gpredict, and VB-Cable—are freely available. Upon successful implementation, the station can regularly capture images from NOAA-18 and NOAA-19 satellites. The setup provides an excellent educational tool for schools and institutions looking to promote interest in space science, atmospheric studies, and remote sensing. This project demonstrates how accessible SDR technology, combined with simple hardware and free software, can create a fully functional satellite receiving station. It offers valuable hands-on experience and promotes STEM education by bringing space science directly into the classroom or lab environment.

Reference:

1. RTL-SDR Quick Start Guide- https://www.rtl-sdr.com/rtl-sdr-quick-start-guide/
2. NOAA Satellite Signals with a PVC QFH Antenna and Laptop – Spacemanlabs, Instructables Workshop- https://www.instructables.com/NOAA-Satellite-Signals-with-a-PVC-QFH-Antenna-and-/