Software-Defined Radio (SDR) is a radio receiver in which traditional hardware components are replaced by software algorithms. This allows an SDR to support a variety of radio protocols and frequencies simply by changing the software rather than physically swapping hardware components.
A DVB-T stick, originally designed for receiving digital terrestrial television, can be used as an inexpensive SDR. With the RTL2832U chip, a DVB-T stick offers a flexible and cost-effective way to receive and analyze a wide range of radio signals. The combination of simple hardware and powerful software opens up a broad spectrum of possibilities.
As is typical with many low-cost components, some DVB-T sticks have less precise frequencies and exhibit deviations. This means that you need to determine the exact frequency correction to make precise adjustments. However, there are also devices that are better calibrated and facilitate entry by allowing more accurate frequency.
When purchasing a DVB-T USB stick, pay attention to the quality of the device and the type of antenna connection it has. MCX or similar connectors are better suited than standard TV connectors.
There is an active SDR (Software-Defined Radio) community constantly exploring new DIY experimentation methods. Decoding various signals and protocols (such as AIS) using a cost-effective and readily available DVB-T USB stick has become a popular DIY approach! In the end, it is the cheapest way to build your own AIS station.
Requirements
DVB-T Stick with an RTL2832U chip
AIS antenna
Computer or Raspberry Pi
Software
AIS Frequency:
AIS1 161,975 MHz (VHF 87B) Channel A
AIS2 162,025 MHz (VHF 88B) Channel B
Software
There are two ways to implement this project. On the one hand, it can be achieved by combining various Windows software, such as SDRsharp and AISmon, to decode audio. Alternatively, it can be accomplished by using AIS Catcher, which is relatively straightforward and has very interesting features. Both approaches are interesting, and it is worth taking a closer look at the details of each method.
Audio Decoding Windows
This approach combines different Windows software to tune the stick to a specific frequency and make it available as an audio output for decoding. Here, we will only cover the software available for Windows. For Linux, we recommend using AIS Catcher. Since various software is used and combined, different errors may occur.
If you use the stick for this application, you cannot use it simultaneously for DVB-T reception. You can only listen to one AIS channel at a time, not both channels simultaneously, which reduces reception.
Download the Software Defined Radio Package from AIRSPY. Extract the package and first run the
install-rtlsdr.batbatch file. This will download the Zadig driver.
Now, connect the AIS antenna to the DVB-T/SDR stick and plug it into a free USB port. Ensure you have not installed any bundled software.
Run the Zadig application found in your folder, select your device, and click on "Install Driver". It will take a moment to install the appropriate driver.
Start the SDRSharp application. If you do not have the latest .NET installed, you will be prompted to download and install it. You may need to restart SDRSharp afterward.
Select your device as RTL-SDR USB and start the software with the Play button.
Now, a lot will happen on your screen. The waterfall display at the bottom is now of interest and must be adjusted.
A waterfall display in Software Defined Radio (SDR) shows signal strength over time and frequency in a two-dimensional chart. The horizontal axis represents frequency, the vertical axis represents time, and the color intensity indicates signal strength, with brighter colors showing stronger signals. This display allows you to observe changes in the frequency spectrum over time and identify specific signals.
On the left side, set the radio to NFM, Bandwidth to 25, Filter Order to 10, and disable Squelch. Under Audio, disable Filter Audio.
In the upper section, you can see the frequency that is set. Adjust it close to the AIS frequency (161,975 MHz) by clicking on the top or bottom of the number to increase or decrease it as needed. You should see irregular stripes arranged vertically, which are AIS messages. Click on them to adjust the frequency. You can only tune one channel at a time. In many cases, the frequency does not match the AIS frequencies, which is due to the inaccuracy of your stick.
Now, we will configure the audio output. Stop the reception by clicking the Stop button at the top. Set the Output to Primary Sound Driver.
Start AISmon or, alternatively, AIS Dispatcher or Shipplotter to receive and process the audio data. Configure the settings for audio reception accordingly. You can find more details here: AISmon
AIS Catcher
Coming soon
Links