Intro to RTL-SDR for AM/FM, ISM band, LoRa transmission, and satellite tracking

Example 4: Satellite tracking

And now for the grand finale: receiving transmissions from satellites currently orbiting the Earth! This will be a bit trickier than the previous examples, mainly because unlike FM radio stations and LoRa transmitters, satellites tend to move around quite a bit. Fortunately, SDR Console has an amazing tool that will let you easily track them. Start by selecting the “View” option in the SDR Console menu ribbon, and in the last section “More Options…” press the button “Satellites”. Another window will open up, which is the satellite tracker.

Figure 9. SDR Console Satellite tracker

The first thing that you have to do in order to use the tracker is to enter your latitude and longitude so that the app can determine which satellites are visible and their position relative to yours. In the “Home” tab on the top ribbon, click the second button “Home Lat/Lon”. In the window that pops up, enter your current latitude, longitude and height above WGS84 ellipsoid. The last parameter is used to compensate for Doppler effect, if you google for a bit, you will find some tools that will get your WGS84 height from your latitude and longitude. Once you have all those information, you can search for some satellites! In the “Pass Schedule” window, press the button “Satellite List” to display all satellites that will be visible from your position in the next few hours. There are a lot of options for filtering those satellites (e.g. you can exclude all satellites with elevation less than a certain angle).

Double-click one of the satellites you want to track [1]. The main window now contains the current position of that satellite [2], its name [3] and its trajectory, as well as time until AOS (Acquisition of Signal) [4]. There’s also the current azimuth of the satellite [5] and its elevation [6] relative to your position. This information tells you where in the sky you should point your antenna in order to have the best chance of receiving whatever that satellite might be transmitting. There’s also a neat little feature in the “Passes” window that lets you view the expected satellite trajectory when it becomes visible from your location. To do this, select one of the satellites from the list and move the slider [7] below the map. It will show the satellite trajectory during its pass over you, so you know how the azimuth will change over time and whether it will actually be possible for you to receive the broadcast. Once a satellite has reached your location, click one of the receiver buttons [8], and the main SDR Console app will automatically tune in to that frequency with recommended settings for that particular broadcast.

The last thing you have to do in order to track satellites is to place your antenna in a place that has a very good view of the sky. Roof works pretty well, although you might have trouble running the signal all the way back to your PC. My antenna is placed on a terrace on the third floor of the house, so it has pretty good exposure, although if I get my hands on an active USB cable, I will probably move it to the roof and leave it there with the RTL-SDR dongle. That way, the path taken by the analog signal is much shorter.

Figure 10. My dipole antenna!

Time to listen to some satellites! Since my antenna is facing roughly south-west, it is able to pick up satellites with azimuth 120° to 320°. Elevation is a bit of an issue since, as you can see in the above image, I live in a valley. Because of that, I have the best reception on satellites with roughly 15° elevation or higher. I can receive signals from satellites coming from other directions as well, but they are weaker.

Now we need to pick a satellite we want to listen to. One of the best to start with are NOAA satellites. These satellites are operated by National Oceanic and Atmospheric Administration, part of the U.S. Department of Commerce. As the name of the agency suggests, they are used for weather prediction and climate monitoring. At the frequency of 137.1 MHz, they also transmit images in reduced resolution, and when converted to sound, you can hear characteristic beeping. The satellite I tracked was NOAA 19, launched in 2009. This satellite is on near-polar orbit, which means that its trajectory above the Earth passes near the poles. Each orbit takes about 100 minutes. The orbit is also relatively low, at approximately 850 km above the surface.

Figure 11. NOAA 19 satellite signal when it was passing over Russia. The signal was weak, but the beeping was still audible.

The first time NOAA 19 trajectory was close enough to me, it passed over Russia which is a little bit too far away from my location. The next pass about 2 hours later was much closer. When the satellite got in range, it was above North Africa, passing over Libya and the Mediterranean. Then, it passed over Italy and eventually Germany, which was closest to me then. At that point, the elevation of roughly 60° and azimuth 240° put it in an ideal place for me to record the signal. You can see and hear the recording of the entire 15-minute pass in the video here. The signal is best at around 6:00 – 6:30, you can hear the beeping and ticking very clearly. The NOAA 19 satellite continued over the North Sea and headed to the North pole.

Figure 12. NOAA 19 satellite signal 2 hours later, when it was passing over Germany. The signal is much better than during the previous pass.

Conclusion

This concludes our brief overview of the RTL-SDR. As you can see, it can achieve impressive things even with the most basic setup. Of course, there are areas for improvement. For example, we can create antennas for different modulations and frequencies to get as high signal gain as possible and to automatically reject those frequencies we aren’t interested. And using the software part of software-defined radio, the entire system can be easily modified to fulfill a variety of other roles, from “simple” stuff like receiving analog TV all the way up to observing pulsars thousands of light years away!