Rendezvous with NOAA Polar-Orbiting Weather Satellites
by Rob Rothfarb • March 13, 2017
A key mission objective of the National Oceanic and Atmospheric Administration (NOAA) is to monitor and provide research data about weather and atmospheric conditions that affect the United States. After the initial success by NASA of launching and operating weather satellites during the 1960s, NOAA, established in 1970, has operated a weather satellite program that includes both polar-orbiting and geostationary spacecraft. These satellites send back images of clouds and storms as well as other data, including sea surface temperatures, aerosol gases in the atmosphere, and data about radiation from the sun. The POES (Polar Operational Environmental Satellites) program includes a fleet of satellites that orbit the earth at a height of approximately 530 miles above sea-level, passing close to the poles in north-south and south-north orbital paths. Currently, there are three active polar-orbiting satellites, the most recent of which, NOAA-19, was launched in 2009. Their relatively close height above the earth's surface allows them to deliver high-resolution images.
False-color visible light image from NOAA 19 on February 2, 2017 on 137.1 Mhz. The location of the Exploratorium along San Francisco Bay is marked in yellow. The location marker, latitude and longitude lines, and the map overlay are added to the received image by WxtoImg, a software program that decodes the audio signal received from the satellite on its designated frequency and converts the signal to an image.
Ground stations for receiving communications from weather satellites are typically something that require specialized radio equipment and antennas or satellite dishes. But, a new generation of radios, called Software Defined Radios (SDRs), now allows scientists, educators, and radio hobbyists to communicate with weather satellites and receive images and other data from them. SDRs are small, general purpose digital radio receivers that can be programmed to be different kinds of radios and tune into different wavelengths of the radio frequency spectrum. They're a great tool for learning about radio communications and for experimentation to tap into a wide range of frequencies.
NOAA 19 weather satellite visible light and infrared images from February 1, 2017.
I've been experimenting with an SDR recently and, using information available on the internet about how to use one to tune into NOAA's polar-orbiting satellites, have been able to receive dynamic images of weather along the west coast of the U.S. during the recent period of frequent storm activity. Paul Dancstep built a quadrifilar helix antenna, a type of a dipole antenna arranged in a helix shape, that's tuned to receive circularly-polarized radio signals at 137-138 Mhz—the frequency range used for transmission by NOAA's satellites. It's possible to build a qfh antenna as we did or another type of antenna that receives circularly-polarized signals. Another option would be to purchase a suitable antenna. The components needed to set up a ground station for receiving images from these satellites includes the SDR, the tuned dipole antenna, and software for both SDR receiving and for decoding an audio recording of the satellite's transmission to an image. We used CubicSDR for the SDR receiver software, WxtoImg for decoding the audio signal, and Soundflower for routing the audio output of the SDR software to the decoder. The various software components needed are freely available and we found that the setup works well. The SDR that we used is an inexpensive (< $20.00) RTL-SDR USB dongle, commonly sold as a digital television tuner. It takes a few minutes to receive an image during a satellite pass, and we found that we have the best reception when a pass is close to our longitude and at an maximum elevation of between 60 and 90 degrees.
The Cubic SDR software with a RTL-SDR USB receiver tuned to the NOAA satellite signal.
To help you plan your reception from a satellite, the WxtoImg software has a great feature that fetches the latest orbital information and displays upcoming passes for all three NOAA satellites flying over your location. Just enter a latitude and longitude and the software will display a printable list of satellite passes for up to a month. You can also use a site like N2YO.com to see the paths for each satellite.
With this setup, you can determine when any of the three NOAA orbiting satellites will pass by your location, put up the antenna which connects directly to the SDR receiver, run the software programs, and rendezvous with a weather sensing spacecraft as it flies overhead—no internet connection needed. While you can easily download similar images from these satellites directly from NOAA's website, there's something magical about using a radio to receive a transmission directly from a spacecraft that has a camera pointed at the earth. It's like receiving a fax from space through the airwaves!