Many amateur radio operators would like to decode weather pictures sent down to the earth by the polar orbiting NOAA weather satellites. I have been doing this for the past 25 years. It is not real difficult and is very rewarding. To accomplish this task, you will need the following items:
- A receiver capable of working with an FM radio signal that is 38 KHz wide. An SDR is perfect
- An antenna that can receive signals in the 137 MHz band, preferably having a circular polarization. A 2 meter antenna is close enough
- An average computer running Windows 10 or 7. The faster the processor the better
- WxtoImg software (FREE)
- Virtual USB Cable software (FREE)
- SDR Console software to decode what the SDR receives (FREE)
- A method to determine when the weather satellites will be visible at your QTH (FREE or at a cost)
I’ll first discuss a little bit about each of the items above, then show a diagram and explain how they all fit together. You will be able to obtain several weather satellite pictures, in different wavelengths (IR Visible and false color) from one pass of a weather satellite.
A very versatile receiver available to amateur radio operators is the Software Defined Radio (SDR). This modern marvel is incorporated in most newer amateur transceivers. There are a multitude of these devices available in all prices ranges from tens of dollars to a couple hundred dollars. Since this is the most important part of your satellite receiving station you need to remember, “you get what you pay for”.
I have several SDR’s but my favorite is the FUNcube Dongle Pro Plus. I originally bought this to receive telemetry from the amateur radio satellites. I use it for so much more. This receiver is perfect for the Weather satellites. It covers the frequency range of the current U.S., Chinese, and Russian weather satellites. It will do any mode, super wide bandwidths, and will allow you to adjust the AGC along with many other parameters. The key is to be able to receive a 38 KHz wide FM signal. If you use a smaller bandwidth, you will loose information and details in the received picture. Most SDR’s are great for listening to all types of signals from almost DC to daylight
A good antenna is needed to receive the signal from the weather birds. Since all satellites spin or tumble while orbiting the Earth, the received signal will change polarity. To avoid deep fades you want an antenna that is circularly polarized. I use an Eggbeater type antenna. I started out using a turnstile antenna. A 2 meter amateur radio antenna will get a picture, with fades. I know people who have used a 2 meter vertical on the roof of a car with fair results.
The person who wrote WxtoImg “freeware“, software, no longer supports it. There are some links to it on the internet that are broken. If you are interested in this software, I can give it to you. Send an email to me via my address listed on Queen Romeo Zulu dot Charlie Oscar Mike. This is the software that actually takes the decoded radio signal coming from the NOAA satellites and converts it to a picture.
A virtual USB cable allows you to connect audio between two different programs running on your computer. A great example is taking audio from a receiver fed into your soundcard and connecting to slow scan television software like MMSSTV. The virtual USB cable is easy to install on your computer. It shows up in the list of your audio sources, both inputs and outputs. You will feed the output of your SDR receiving software to the input section of the WxtoImg software that creates the weather satellite images.
There are many pieces of software that will take the digital output of an SDR, in any mode and bandwidth, and turn it into a water fall with all the signals present in the reception bandwidth of your SDR. When you click on any signal in the water fall, you will hear it, You can also direct this audio via a virtual cable to the input of software that will create a slow scan TV image, a weather satellite picture, packet reception including APRS, NAXTEX, and digital audio via DMR, etc. The software I use to do all of this is SDR Console. I have been using this free soft ware for years. There are many others on the market. If you already have a favorite, you can probably use it.
Important! When receiving any signals from a satellite, you must correct for the Doppler Effect. I’m sure as a siren from an ambulance approached you, you could hear the pitch or frequency get higher. As it moved away from you it got lower. The only time you heard the correct frequency of the siren is when it was right beside you. The very same thing happens with the radio frequency of a speeding satellite. They move at 17,500 MPH. This causes the reception frequency of the satellite to increase as it approaches your station and decreases as it moves away from you. You need to keep tuning your receiver to stay at the correct frequency to properly decode the weather satellite picture. SDR Console does this for you! The satellite mode will keep changing the downlink frequency you receive to keep up with the Doppler frequency change.
There are several apps available for smart phones that do a great job tracking satellites and predicting when they pass over your location. SatSat is a good one for the iPhone. You can also use Heavens-Above, a website that will predict when the International Space Station will be visible in your sky, and when all the amateur radio satellites, including the NOAA weather satellites pass you location. Make sure you tell any of the programs your exact location to have accurate results.
You will be able to decode the pictures coming from 3 NOAA weather satellites currently in operation. Most weather satellites produce two types of radio signals, analog and digital. The analog signals are the easiest type to decode. The digital signal has a much larger bandwidth and baud rate, as well as being transmitted on a microwave frequency. The digital signals require a small dish for reception. The analog signals we wish to decode are called APT signals, Automatic Picture Transmission. This is how the data sounds on a typical weather satellite transmission. It is actually an AM signal on top of the FM carrier.
Frequencies currently used by the active NOAA weather birds
- NOAA 15 launched in 1998, APT transmission on 137.620 MHz
- NOAA 18 launched in 2005, APT transmission on 137.9125 MHz
- NOAA 19 launched in 2009, APT transmission on 137.100 MHz
Eventually these satellites will stop operating. I started receiving pictures from NOAA 9. The latest polar orbiting weather satellite is NOAA 20. It only transmits digital data. I plan to move to reception of the digital data from the geosynchronous satellites using a dish. I understand you can fill a fairly large hard drive with data after a couple of days!
Looking at the block diagram above, to receive a weather satellite picture from one of the NOAA satellites, we need to have a circularly polarized antenna to capture the FM signal. The signal is fed into an SDR (software defined radio) which is capable of receiving all modes and bandwidths of data. This includes the newest method of sending health telemetry via DUV, Data Under Voice, which consist of very low audio frequencies below the normal frequencies of the human voice. These frequencies are normally cut off in all receivers right after the IF frequency conversion.
The SDR feeds data into a computer which is decoded by software such as SDR Console. That digital data is recovered audio information which needs to be fed via a Virtual Audio Cable installed on your computer to Software that converts the audio frequencies and timing data into a usable picture of the Earth from space. A typical program is WxtoImg.
The following videos show each step of the process
Feel free to contact me on the air, or via email listed at my Queen Romeo Zulu dot Charlie Oscar Mike page if you have any questions. If you are interested in pursuing receiving NOAA weather satellite pictures, I will help you in any way I can. The most difficult portion of this process is setting up the WxToImg software. We can easily do this together over the phone. I will tell you what settings you need in each section of the software so you are successful the very first time you try to receive your first weather satellite picture. If you are anything like me, you’ll keep that one forever!