Odd fluctuations in my PiAware ADS-B receivers’ coverage graphs made me wonder how AGC in dump1090-fa really worked, because information on the subject is contradictory. A common technique for gaining insight into a process is by studying its anomalies, and the following announcement offered a promising opportunity.
February 12th, 2019
For one of my receivers, air traffic over Belgium is the main source of ADS-B messages, especially in the 80-160 km range. So I was expecting to collect substantially less position broadcasts during the 24-hour strike. That proved to be the case, but I also noticed something else: for the first time since I permanently mounted the antenna outside, I was finally receiving a considerable amount of long distance (420+ km) positions again.
[Note that the bar graphs have different scales]
I recognized the above behaviour from the time when my antenna was still mobile. It always occurred immediately after moving the antenna from inside to outside the house. The effect would then gradually fade during the next couple of hours.
Conclusion so far is that many strong (relatively nearby) messages will make the AGC mechanism gradually reduce the receiver’s sensitivity. This may be beneficial for the total number of aircraft seen, but it comes at the cost of long distance reception. Once the strike was over, long distance reception dropped again within a couple of hours.
I’ve been experimenting with fixed gain values in order to receive more long distance aircraft, mainly to determine how far my receiver could reach. However, none of the allowed gain values gave results that resembled what I got during the 24-hour strike, or shortly after moving the antenna from inside to outside. Could it be that AGC does more than just adjust the receiver’s gain?
With my FlightAware feeder now permanently mounted outside, I began thinking of installing a second ADS-B/Mode-S feeder at the opposite side of my apartment. Would the extra reception coverage be worth the costs, as the second antenna could only be placed inside, blocked by some nearby obstacles. Earlier tests with a NooElec NESDR SMArt dongle and a simple antenna had been quite disappointing, but after placing the antenna behind a solid wall instead of a window, reception suddenly became interesting enough to make me buy a second RasPi + FlightAware Pro Stick Plus + 66 cm antenna.
Antenna coverage graphs of both feeders
First I had the second RasPi run just dump1090-fa (not the complete Piaware suite) and forward its decoded messages to the first feeder. A picture on the FlightAware site illustrates how processes are linked together.
[not shown is port 30004, on which dump1090-fa is listening for incoming ADS-B messages from other feeders]
I used netcat for forwarding messages from one feeder to the other:
nc -d localhost 30005 | nc -q1 <ip_of_second_Pi> 30004
Forwarding messages from dump1090-fa to the FlightAware feeder boosted its detected aircraft figure, but (inevitably) at the cost of its MLAT synchronisation/reception. So I ended up with both feeders running Piaware and showing up as two separate ‘sites’ under my flightware.com account. For visualizing detected traffic from both receivers I use Virtual Radar Server’s local webpage (http://127.0.0.1:88/VirtualRadar/desktop.html#) on a Windows laptop.
The image below shows my Virtual Radar, with combined coverage graphs of both receivers’ ADS-B & MLAT reception. The outer black circle is at 400 km from my location, so I’ve been receiving aircraft from well over 600 km distance. Occasionally, particularly when nearby traffic is low, the receivers pick up positions from even further distances (800+ km), but these are too irregular to be represented in VR’s coverage graph.
[ I could figure out which of the feeders gets most MLAT aircraft, and then forward that feeder’s ADS-B messages to the other feeder. This would stop that other feeder from receiving MLAT aircraft and make it feed FlightAware with ADS-B aircraft from both feeders. That would substantially raise its site ranking as well as my FA user ranking (ADS-B aircraft from the first feeder would be counted twice). I suspect that FA wouldn’t be pleased, so I’ll stick to operating two strictly separated feeders. ]
Here’s a well designed new display from Wemos Electronics. They call it a (D1 Mini) Shield, but instead of going on top of a Wemos D1 Mini (Pro), it lets you plug in the Mini at the back of the 2.4″ TFT touch screen display, leaving all GPIO pins accessible, thanks to double header rows. Very useful!
This table shows the internal pin mapping when used as a shield. For TFT_LED, TFT_RST and TS_CS, this mapping can be changed by closing solder bridges at the back of the display.
The default pin mapping leaves I2C pins D1 and D2, as well as D4, A0, TX and RX available for other purposes. With my Wemos Mini Pro v2 plugged in, this shield has become my first choice for battery powered esp8266 projects that require a display.
But there’s more. All pins are also broken out to a breadboard-friendly header and to a TFT connector. Connecting the display to a D32 Pro (that has the same TFT connector) now requires only one (special) cable! The picture shows the connector’s pinout.
The 320×240 display uses the well-proven ili9341 chip, for which I already wrote a lot of sketches, both for esp8266 and esp32 boards. The XPT2046 library can be used to drive the touch screen.
I tested several sketches on a plugged-in Mini Pro, as well as a TFT cable-connected D32 Pro, and everything worked fine. Example sketches on the Wemos site use Adafruit’s ili9341 library, but I prefer the much faster TFT_eSPI library.
This is a well-thought product that makes wiring very convenient and stable. Recommended!
After two months of experimenting with a generic SDR dongle, a dedicated ADS-B dongle and a couple of antennas, here’s my experience so far.
- FlightAware Pro Stick Plus
- NooElec NESDR SMArt
- FlightAware antenna (66 cm)
- Magnetic ground plane antenna
- Telescopic FM antenna
- Raspberry Pi 3B+ (Piaware image)
In order of importance, expressed in an (approximated) percentage of influence on my ADS-B reception, this is what helped to improve my FlightAware feeder statistics:
[70%] – Antenna placement. Outside is better than inside (obviously), especially for houses with energy saving windows (metal coated glass reduces signal strength ~40%). Higher is better, in general, but it pays off to experiment:
- Raising my outside antenne by 80 cm, as well as increasing its view angle by 2 degrees, made it receive more aircraft – but at the cost of most of its >400 km reception…
- My inside antenna turned out to perform much better after moving it from the window to a wall, where an obstacle outside did no longer block a high air traffic area.
- Since ADS-B is basically a ‘line of sight’ transmission, it’s clear that obstacles can block signals. But they can also act as rear view mirrors! A large apartment building that blocks my long range reception in one direction compensates for this by bouncing signals from an otherwise completely blocked direction.
[20%] – Antenna type & quality. The FlightAware antenna isn’t cheap, but its 5 dB gain will extend your reception range compared to simple 1/4 or 1/2 wavelength antennas.
[5%] – Dongle. The FlightAware Pro Stick Plus squeezed a bit more out of all tested antennas. And it didn’t get as hot as the NooElec.
[5%] – Cables & connectors. Use quality cables and keep them as short as possible. Also: periodically check your SMA and/or N-type connections (they can get loose over time).
Tip 1: The amazing Virtual Radar Server (VRS) software is extremely useful for finding the best antenna position. It can create receiver range plots over time, even for different altitude ranges. This allows for a visual comparison of different antenna positions and choosing the one that fits your needs (i.e. best coverage of a particular area, best long range reception, etc.)
Tip 2: Experiment with your receiver’s gain. This is a setting in dump1090 (the demodulator/decoder software component). Default setting in the Piaware image is ‘AGC’ (Automatic Gain Control), that is supposed to dynamically adjust gain in order to avoid ‘clipping’ of the strongest signals. However, some reliable sources claim it will set gain to ‘max’ (49.6 dB) …
If you have ssh enabled on Piaware, here’s the commands to read the current gain setting:
sudo systemctl status dump1090-fa -l
To set gain to, for instance, 40.2 dB:
sudo piaware-config rtlsdr-gain 40.2
Make sure to restart the dump1090 process for your changes to come into effect:
sudo systemctl restart dump1090-fa
Possible gain values are: 0.0, 0.9, 1.4, 2.7, 3.7, 7.7, 8.7, 12.5, 14.4, 15.7, 16.6, 19.7, 20.7, 22.9, 25.4, 28.0, 29.7, 32.8, 33.8, 36.4, 37.2, 38.6, 40.2, 42.1, 43.4, 43.9, 44.5, 48.0 and 49.6
Note: supplied values will be rounded to the closest value in the list. A value of -10 will set the receiver in AGC mode.