Tools for ADS-B analysis

This is a collection of bits and pieces for ADS-B receiver analysis and monitoring, particularly on the Raspberry Pi.

All of what follows assumes that you have unpacked or cloned this repository in /usr/local (recommended to avoid collisions with distribution packages).

At present, there are four tools in this repository, all of them Munin plugins:

• dump1090 monitor
• ADS-B message distribution analysis
• UK Met Office weather
• Raspberry Pi SoC (CPU) temperature monitor

dump1090 monitor

Recent forks of antirez's excellent dump1090 ADS-B decoder, notably mutability's fork, either have the built-in web server disabled or removed completely. Instead, they write out data in JSON format which can be served using lighttpd or similar.

This plugin uses those same JSON data to generate Munin plots so you can keep an historic record of the performance of your receiver. It produces performance graphs similar to Joe Prochazka's excellent ADSB Receiver project, but without the additional overhead of that project and without requiring PHP.

It is a wildcard plugin, so it must be linked in /etc/munin/plugins/ correctly (see below for details). You don't have to enable all variations if you don't want. Note that Munin records data in 5 minutes intervals, so the data recorded in Munin will probably differ from those you see in the web interface.

NB: This plugin was tested and developed with the (then) current HEAD of mutability's fork and it expects the data provided by that fork to be present. It is not especially robustly coded and missing data will likely cause exceptions.

Supported data

• Total aircraft count and count of aircraft with position
• Average and maximum aircraft distance
• CPU utilisation (demodulation, USB and network I/O)
• Accepted message count (showing those with 1 and 2 bit errors corrected, where available)
• Signal quality problems (bad and unknown Mode S messages, and also the percentage of tracks containing only one position point)
• Signal strength (average and peak signal and noise floor)
• Total track count and single-point track count

Installation

• Make sure that you've set latitude and longitude in your dump1090 instance or else this plugin will throw an exception.
• Edit the configuration parameters at the top of /usr/local/share/munin/plugins/dump1090_. You should set your altitude and you must adjust the JSON_DATA path to where your dump1090 instance writes its JSON data.
• Install prerequisites: sudo aptitude install -R python-geopy
• Install Munin if not already installed. If you're running Raspbian, Ubuntu or Debian, the command sudo aptitude install -R munin munin-node is probably sufficient.
• Examine /etc/munin/munin.conf and possibly /etc/munin/munin-node.conf if you're security conscious. See also installing Munin on a Raspberry Pi.
• Examine the contents of /etc/munin/plugins/. You will probably want to disable most of them since every plugin enabled consumes a bit of CPU every 5 minutes. To enable this plugin, create the necessary symlinks according to your preferences:

cd /etc/munin/plugins
sudo ln -s /usr/local/share/munin/plugins/dump1090_ dump1090_ac
sudo ln -s /usr/local/share/munin/plugins/dump1090_ dump1090_cpu
sudo ln -s /usr/local/share/munin/plugins/dump1090_ dump1090_messages
sudo ln -s /usr/local/share/munin/plugins/dump1090_ dump1090_quality
sudo ln -s /usr/local/share/munin/plugins/dump1090_ dump1090_signal
sudo ln -s /usr/local/share/munin/plugins/dump1090_ dump1090_tracks
sudo systemctl restart munin-node

• Install a web server if you don't already have one running. You may already have lighttpd running if you followed Mutability's install instructions, in which case I've supplied a config file for Munin (for http://raspberry.local/munin):

sudo cp /usr/local/etc/lighttpd/conf-available/89-munin.conf /etc/lighttpd/conf-available
sudo lighty-enable-mod munin
sudo systemctl restart lighttpd

Munin usually requires two samples to begin to show anything on the plot, so you need to wait 10 minutes before you can expect to see anything in the plots. If you suspect problems, examine the Munin logs in /var/log/munin.

ADS-B message distribution analysis

Many different tools for dump1090 exist to show aircraft range and other performance data (including the above mentioned Munin plugin), but message count and maximum distance say very little about the quality of messages that your receiver is getting. Quality of messages is equally important as quantity when experimenting with antennae.

This is another Munin plugin that attempts to characterise the quality and consistency of received messages, and can be used with any SBS BaseStation data source, of which dump1090 is one example.

Operating principle

It works on the assumption that each aircraft will transmit messages at a fairly constant rate (which, according to this paper is correct: each Mode S transponder transmits a message every 400-600 milliseconds) and therefore the standard deviation of message interval (for a given aircraft) says something about how many messages you're not getting. The wider the distribution (larger the standard deviation), the less consistently you're receiving messages for each aircraft.

As well as inter-message arrival times, it also performs the same analysis on the displacement between position reports. Rather than analyse actual distance in feet or nautical miles, it analyses the ratio of the actual distance between reports and the distance the aircraft would have travelled in one second at its last reported ground speed. This is to allow for the fact that even in a perfect environment, a plane cruising at 450 kts will go much further between position reports than the same plane that has slowed to Vref on final. As a result, raw displacements don't say a great deal about the consistency of ADS-B message reception.

Naïvely, in a perfect situation and in both cases, the mean interval would be about 500 ms (or a displacement ratio of 0.5) and the standard deviation would be about the same because the aircraft transmits positional messages every 0.5 sec ± 0.1 sec.

There are several reasons why it'll never work out that way:

• Position and velocity are transmitted in separate ADS-B messages, so while the mean interval might be 500 ms, the position messages are probably transmitted no more frequently than every second — so the mean displacement ratio is more likely to be 1. Additionally, just like the Heisenberg Principle, this means you can never know the precise position and velocity of an aircraft at any given moment (though the error is likely to be small most of the time).
• There are at least 8 different types of ADS-B message, some of which are triggered by ground radar and TCAS rather than emitted periodically. This will disturb the interval distribution, although this plugin tries to mitigate these by considering only message subtypes 3 and 4.
• Worse still, all transponders transmit on the same frequency without any means to prevent collisions of messages from other aircraft. Nor does ADS-B implement CSMA/CD (Carrier Sense Multiple Access/Collision Detection) as old-school Ethernet did. Instead, aircraft transmit periodic messages at a random interval of 500 ms ± 100 ms. Even so, message collisions are guaranteed, particularly near busy airports, and will often be received as a garbled message.
• However good your antenna, not every message from aircraft at the outer edges of your range will be received without error. Unless a range limit is imposed, the standard deviation of messages from distant aircraft will be larger than those from aircraft nearer. This plugin does not currently implement any such range limit, partly because doing so would require knowing how close messages can reliably be received but mostly because collisions will still make the standard deviation larger even if they're within that limit.

The best that can be hoped for, then, is some sort of index based on standard deviation compared with the standard deviations from other stations. Future versions might weight nearer aircraft more heavily than distant aircraft.

Since this is experimental code, it presently reports only the statistical mean and standand deviation for each of message interval and displacement, and discussion is welcome.

Installation

• Install Munin as per the dump1090_ plugin instructions.
• Edit the configuration parameters at the top of /usr/local/share/munin/plugins/adsb-msg-dist.
• Install prerequisites: sudo aptitude install -R python-geopy python-daemon
• Install the systemd service description and start the collector daemon:

sudo cp /usr/local/etc/systemd/system/adsb-msg-dist.service /etc/systemd/system/
sudo systemctl enable adsb-msg-dist
sudo systemctl start adsb-msg-dist

• Enable the plugin:

cd /etc/munin/plugins
sudo ln -s /usr/local/share/munin/plugins/adsb-msg-dist
sudo systemctl restart munin-node

UK Met Office weather

This uses the public UK Met Office hourly site-specific observations API to record current basic weather data in Munin. These data are only updated hourly so they're fairly coarse, but they should be fairly reliable.

Installation

• Apply for a UK Met Office DataPoint API key if you don't already have one.
• Install Munin as per the dump1090_ plugin instructions.
• Edit the configuration parameters at the top of /usr/local/share/munin/plugins/ukmo_wx_config.py:
  • You must configure the API key for this plugin to work.
  • Optional, but recommended, set your latitude, longitude and altitude. As written, you can input your altitude in feet but it must end up in kilometers. That's what the multiplier fraction is for.
  • Configure what stations you want to record. Don't configure too many or Munin's graphs will be indecipherable. If you've configured your location, you can get a ready-made list of the stations within, say, 60 miles by executing the command: /usr/local/share/munin/plugins/ukmo_wx list 60
• Enable the plugin:

cd /etc/munin/plugins
sudo ln -s /usr/local/share/munin/plugins/ukmo_wx
sudo systemctl restart munin-node

Raspberry Pi SoC (CPU) temperature monitor

This is a very simple Munin plugin to monitor the temperature of the SoC IC of your Raspberry Pi. The Pi is not sold with a heatsink and the Pi 3, at least, can get quite hot especially when doing a lot of I/O or doing a lot of computational tasks — and especially in summer if your Pi, like mine, is in the loft.

The SoC will automatically throttle core speed when it reaches 82°C, so between the system load plugin and this plugin, you can monitor the performance of your Pi and add a heatsink (and perhaps miniature fan) if its temperature is getting too close to that limit.

Installation

• Install Munin as per the dump1090_ plugin instructions.
• Enable the plugin:

cd /etc/munin/plugins
sudo ln -s /usr/local/share/munin/plugins/SoC_temp
sudo systemctl restart munin-node