Automatic dependent surveillance–broadcast (ADS-B) technology enables aircraft to determine their position via satellite and periodically broadcast this location information. An ADS-B receiver system can be used to decode this broadcast signal to track airplanes.
Now let’s take a look at the various components of an ADS-B station, why they matter, and how to make improvements.
In this article we previously discussed the different ADS-B receivers on the market today. Systems like the one from Garmin are completely closed and fully integrated and there’s not much opportunity to understand how they work or improve them. The best you can do is use them as per the manufacturer’s recommendation and that should result in the best performance for this product.
Both these systems use the RTL-SDR as foundational building blocks. There’s a wealth of information online on the internal architecture and various experiments conducted on these products to understand their specifications, performance and how best to use them. There’s also a large online community to learn from and contribute to.
How Much ADS-B Signal Do You Really Need?
In general when a receiver processes signals, there is a range of optimal signal level over which it performs best. Anything lower will result in performance degradation – in other words, you will see fewer airplanes. Anything greater will have the same effect.
At the very extreme, signal levels higher than a certain level will damage the ADS-B receiver. The trick then is to optimize the signal level such that you can get to within the sweet spot of the ADS-B receiver.
A Note on Measuring Signals
In general, it is very difficult to predict actual wireless signal strengths received without making a measurement. Typically a spectrum analyzer is used to make signal measurements at any given frequency. However, not everyone can afford a spectrum analyzer or has access to one. Fortunately it is possible to use an application like SDR# with your receiver to determine how strong or weak various received signals actually are and the impact they might be having on performance.
Here is an example of signal measurement with the RTL-SDR as viewed in SDR#. The two views shown are the FFT view and the waterfall or spectrogram below it.
As you experiment with different antennas and other components below, tune across the frequency range of the RTL-SDR to check out various signals and their strengths in the waterfall view. This will give you an idea of how your changes impact performance. It will for instance tell you approximately how much gain you need to add up front. It will also tell you if you actually need any amplification at all. In general avoid doing anything that increases the RED levels as these are very strong signals.
Now let’s take a look at the various parts of the ADS-B system and how they impact performance.
The antenna is at the very front-end of your ADS-B system. You will need an effective antenna for ADS-B as it is key to good signal reception. As is often said, garbage in, garbage out. If you use a lousy antenna, a good receiver isn’t going to compensate for it. You can read our complete review of ADS-B antennas. The best antenna requires some effort for installation and optimization but it’s well worth the time spent.
If you’re looking to mount your antenna on your rooftop or on a tower for better reception, you will need to take the length of the RF cable from the antenna to your ADS-B receiver into consideration. Why is that?
As the frequency of the signal increases, so do the signal losses or attenuation due to any given length of cable. For instance, if you were to use LMR-400 cable, the loss at 30 MHz is only 0.7 dB for every 100 ft of cable. For the same length of cable, the loss at 1090 MHz is around 4 dB. This signal loss directly impacts the sensitivity of the system by 4 dB and therefore reduces the range of your ADS-B receiver by more than half relative to if you were to use a very short length of cable. That is quite significant.
Fortunately there is a way to compensate for this degradation in sensitivity due to cable loss. It involves the use of a low noise amplifier (LNA). In general, any LNA with sufficiently low noise figure (typically less than 1 dB) and the required amount of gain can be used.
So the first question is – what is the required gain?
The answer depends on several factors some of which are:
- Amount of cable loss at 1090 MHz
- Other RF signals are present in your environment – in particular strong ones
- The dynamic and operating range of your ADS-B receiver
As discussed, cable loss can be overcome with the use of an LNA. However, an LNA will amplify everything within its range of operation. It will amplify AM, FM and other signals. Consider a situation where an FM signal is stronger than the ADS-B signal of interest. The amplified signal will saturate the receiver if it exceeds the dynamic range. The signal will also possibly damage the receiver if you’re operating at signal levels that exceed the range of the receiver. The receiver will not be able to process the ADS-B signal of interest in either case.
A solution to this problem is to use a Filtered LNA. This is an LNA with a filter integrated along with it as shown in the picture below.
The LNA is usually the first component and it is followed by a filter specific to the band of interest – in this case 1090 MHz for ADS-B. This product will reject all signals outside the ADS-B frequency range. For instance it will reject strong FM signals, 4G signals from cellular base stations, signals at 2.4 GHz from your Wi-Fi access point and more.
Here are a couple of ADS-B filtered LNAs that will work depending on your setup.
Use this Filtered LNA with 30 dB gain if you have a very long cable run and need to compensate for significant signal loss. Using the calculation for LMR400 above, this Filtered LNA can compensate for 700 ft of cable loss.
Use this Filtered LNA that provides 15 dB gain, if you have a shorter cable run.
How to Install an LNA
It is best to place the LNA very close to the antenna. This will help minimize the noise figure and improve the sensitivity of your ADS-B receiver system. Since it’s difficult to get a power source or supply at the top of an antenna, a bias tee system can be used. A bias tee at the receiver is used to send the DC voltage up to the LNA over the RF cable connecting the two. At the LNA there is a reciprocal bias tee that splits the DC and RF signals. The DC is used to power the LNA. The block diagram below shows a situation where the SDR has a built-in bias tee. This is indeed the case with the RTL-SDR.
Sometimes when an LNA is connected the results look like in the following picture – amazing results with a tripling of the number of planes observed!
At other times, the addition of an LNA degrades performance and you will not be able to see any airplanes. Why does this happen? Let’s assume that your LNA is connected properly with the RF input end connected to the antenna and that it is powered correctly. There are then two possibilities:
- Too much cable loss and the amount of LNA gain cannot help overcome this degradation. In this situation more gain is required and you can simply add another LNA. Remember to place the first LNA as close to the antenna as possible so as to minimize noise figure. Remember also to check signal levels so you can adjust the amplitude to be within the optimal range.
- Too much gain and you are likely saturating your receiver with strong signals. Note that even when using a filtered LNA, if the FM signals in your area are very strong, the out-of-band attenuation due to the filter might not be adequate to prevent receiver saturation. Once again you can check what’s going on with the spectrum analyzer view in SDR#.
How to Optimize the Signal Levels into the RTL-SDR
There are three methods by which you can optimize signal levels going into the RTL-SDR after the addition of a Filtered LNA.
In the event that you have strong out-of-band signals, they should be attenuated to a reasonable level. The easiest way to do this is by adding an additional ADS-B bandpass filter after the Filtered LNA.
Another way to optimize gain is to reduce the signal level across the band with an attenuator. Connect an inline fixed attenuator to the output of the LNA. You can experiment with different values such as 30 dB, 20 dB, etc. until you start seeing output by way of planes.
The third method is to adjust the gain in the Flightaware stick digitally. The Flightaware stick is essentially an RTL-SDR and utilizes an R802T chipset. Below is a block diagram of the R802T.
As you can see the user has the ability to modify the Variable Gain Amplifier (VGA). The range over which this can be varied is nearly 50 dB. The FlightAware powers up with maximum amplification in the system. So you essentially have to adjust it to a point where you get the best reception. Here are instructions on how to modify the gain.
Many experiments have been conducted and reported on the Flightaware discussion site. Here is one such discussion. In the plot below, user BartJr has reported an improvement in reception with increase in amplification.
By contrast below are the results reported by user ieand who has used an LNA prior to a Flightaware stick. In this situation there is clearly an optimum gain setting of around 37 and anything higher or lower resulted in fewer planes detected.
In this post we have presented a few different ways to improve the reception performance of your ADS-B receiver. No two systems will provide identical results due to the variable and often unpredictable nature of the RF environment. The best way to learn and achieve great results is to experiment with various settings in your ADS-B system.
Read our post on the Best Software-defined Radios.