Everyday multiple NOAA weather satellites pass above you. Each NOAA weather satellite broadcasts an Automatic Picture Transmission (APT) signal, which contains a live weather image of your area. The RTL-SDR dongle combined with a good antenna, SDRSharp and a decoding program can be used to download and display these live images several times a day.
This tutorial will show you how to set up a NOAA weather satellite receiving station, which will allow you to gather several live weather satellite images each day. Most parts of this tutorial are also applicable to other software radios, such as the Funcube dongle and HackRF and Airspy, but the RTL-SDR is the cheapest option. Hardware radio scanners can also work, provided the radio has a large IF bandwidth (30 kHz +) and a discriminator tap.
Requirements and Setup
To set up a NOAA weather satellite receive station you will need:
- A RTL-SDR dongle working with SDRSharp.
- An audio piping method.
- A right hand circularly polarized antenna tuned to 137 MHz.
- Software such as WXtoImg for decoding the APT signal.
- An LNA (Optional).
We will assume you have the RTL-SDR dongle set up and working already. If you have not bought a dongle yet, see the Buy RTL-SDR page for information and the check out the Quickstart Guide for an easy setup routine with SDRSharp. You will also need to have an audio piping method installed and set up. Audio piping will allow the audio from SDRSharp to be passed to a decoding program. You can use either windows stereo mix, VB-cable (free) or virtual audio cable (paid with trial version).
The sampling rate of your audio piping method should be set to at least 48000 samples/sec. To set this in Windows, right click your device in the Windows sound recording tab, go to properties and under the advanced tab, set the sample rate to 48000 Hz. Do the same to the same device under the Playback tab as well.
NOAA Weather Satellite Antennas
The NOAA APT weather satellites broadcast their signal at about 137 MHz, and their signals are also right hand circularly polarized (RHCP), which means you will need a right hand circularly polarized antenna to properly receive the signals. This is because as the satellites broadcast their signal, they also rotate, rotating the signal polarization. Satellite antennas are also designed to receive best from signals coming from the sky. Three options for easy home made circularly polarized satellite antennas are presented below.
Turnstile Antenna
A turnstile antenna is a circularly polarized antenna. It can be built in two modes, normal and axial. For satellite reception we want it in axial mode.
.A page showing a turnstile antenna for 137 MHz
Click the following:
Quadrifilar Helix (QFH) Antenna
A Quadrifilar Helix (QFH) is a circularly polarized antenna that can be constructed out of PVC pipe and coax cable. Most people report that the QFH antenna has slightly superior reception compared to the turnstile.
If you are into antenna theory, a good technical paper on the QFH (called QHA in the paper) can be found here.
A tutorial on building a QFH out of coax cable and PVC pipe can be found here. Another home brew QFH antenna constructed out of copper wire is here. Also, a good calculator for determining QFH antenna dimensions for 137 MHz can be
Double Cross Antenna (DCA)
Another antenna that works well with the NOAA APT satellites is the double cross antenna (DCA). It is basically four dipoles arranged in a certain way to produce circular polarization.
A good guide on DCA antennas, and constructing them can be found in this pdf guide by Gerald Martes (KD6JDJ).
Yagi antennas can also work, but since they are so directional you will need to carefully track the satellite by hand, or by using a rotor controller. Although not designed for satellite reception, you may also see limited success with other antennas such as quarter wave ground planes and discones, but you will probably discover that the signal will fade in and out.
Software Tutorial
The NOAA satellites only pass overhead at certain times of the day, broadcasting a signal. These signals appear at around ~137 MHz, and only when a satellite is passing overhead. Each satellite has a different frequency. Currently only NOAA satellites 15, 18 and 19 are operational, their frequencies are shown below.
- NOAA 15 – 137.6200 MHz
- NOAA 18 – 137.9125 MHz
- NOAA 19 – 137.1000 MHz
An example of a NOAA APT weather satellite signal is shown zoomed in and out on the frequency spectrum directly below and an example audio file of the signal is shown further below.
WXtoImg Tutorial
WXtoImg is a free weather satellite decoding program which can decode the APT signal, and also tell you the times and frequencies of the NOAA satellites passing overhead. There is also a paid version of WXtoImg which can unlock more features, however it is not required for use with RTL-SDR. To use WXtoImg and SDRSharp together follow the instructions below.
- First, download and install WXtoImg from http://adf.ly/1HlBLl.
- Next open WXtoImg, and then set your Ground Station Location, (which is the coordinates of your antenna) by going to Options -> Ground Station Location. The city you are in should suffice, but you can be more accurate by entering in an exact latitude and longitude if you want
. - In WXtoImg set your audio piping method which you have chosen. To do this go to Options -> Recording Options, and ensure the correct device is selected under the soundcard option.Also, here you can adjust the “Record only when active APT satellites are overhead” “with maximum elevation above (degrees)” and “record only when satellite is above (degrees)” settings. You may want to reduce the default values if you have an antenna with a good view of the sky and find that WXtoImg stops recording or doesn’t start fast enough even though the APT signal is present in SDRSharp.
- Now you will need to update your Kepler files. These files contain the information about satellite locations. They need to be periodically updated, because satellites drift in their orbit over time. Go to File -> Update Keplers to do this. Make sure you have an internet connection for the update.
- Now you can go to File -> Satellite Pass List, and find a time when a satellite will be passing overhead. Take note of the frequency as well.
When the time comes for the satellite to appear, open WXtoImg, and then go to File->Record, and click on Auto Record. The recording and decoding will begin when the satellite appears on your horizon, and stop when it goes out of view according to the times in the satellite pass list.
- Open SDRSharp select the audio piping method you are using under the Audio Output drop down box and then tune to the frequency that the satellite will be broadcasting at. Adjust the gain settings in SDRSharp under the Configure button so that you get good reception of the signal. Set the receive mode to WFM, filter bandwidth to 34 kHz and Filter Audio set to OFF. It may also be useful to ensure Snap to Grid is unchecked.
- As the RTL-SDR is not frequency accurate, and also due to the Doppler effect, the signal may not be at the exact frequency it should be at. Just adjust the frequency in SDRSharp until it is centered on the satellite signal. You may also increase the filter bandwidth beyond 34 kHz if there are no nearby interfering signals to cover the entire travel of the signal.
- Adjust the volume in SDRSharp and/or Windows volume settings so that the volume bar in the bottom right hand corner of WXtoImg shows a green color.
WXtoImg should now be decoding and showing the weather satellite image as it is received. You may need to periodically adjust the frequency to center the signal as the Doppler effect will cause it to move. But, with the RTL-SDR adjusting for the Doppler shift is not critical as the filter bandwidth can be simply set larger than 34 kHz (try 36 -40 kHz) so that it is large enough to receive the entire signal even as it as it shifts.Once the image has been fully received, you can play with the options under the Enhancements and Projection menu in order to add false color and enhance the received image.
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