ADS-B Receiving station description

This post decribes  the ADS-B receiver station as it is feeding FlightAware for some time now. I’ll try to descibe the components in detail.

Location

Location is all. With the top 4 of Europe’s busiest airports at or within 200 nm (and 7 of Europe’s top 20) the airspace around Eindhoven is busy. And with the Netherlands being flat there is plenty horizon.

Antenna

The antenna is a homemade collinear from this previous post. It consists of 8 elements and is open ended. I must have been lucky because a second 16 element version i built performed far worse. At some time i moved the antenna 2 meters up which improved overall statistics by 5%. Attached to the steel post is a 2m PVC tubing mast which gently moves in the wind at about 12m over ground.

Antenna at rooftop

Antenna at rooftop

To the east a row of large trees along the street obscure the horizon. The effect on the statistics is especially noticable at rainy periods (-10%..-20%) or in spring when fresh leaves appear (in the picture on the left with the antenna at its old lower postition)

Pnoramo to the south-east

Pnoramo to the south-east

Feeding cable

The feeding cable didn’t need to be very long (3 m) as the equipment is stored in the attic right under the roof. Just a single hole did the job. I use quite rigid SAT TV cable (Hirshmann KOKA799). Most of the equipment is hidden to improve the WAF.

The receiver

The receiver

Well hidden

Well hidden

Preamplifier

The preamp is of G4DDK design. High gain and  a very low noise figure. Since the first stage mostly determines the overall noise figure  of the system as a whole i wanted a real good preamp. The noise figure is measured at 0.4 dB and gain is 35 dB. It handles the nearby cellphone signals at 940 MHz very well. The high gain made me reduce the dongle gain to 20 dB.

Filter

To protect the poor SDR dongle from the cellphone tower at 940 MHz i designed a filter described here. Meanwhile it has been reproduced by others with success. Tuned to best ADS-B results it is probably tuned for “maximum smoke” and very narrow but luckily this is no problem since all ADS-B signals use the same frequency.

Filter response measured with BG7 wideband noise generator

Filter response measured with BG7 wideband noise generator

Dongle

With this setup i couldn’t see the difference between the R820T dongle and the R820T2 types. Running without preamp or filter this difference was bigger and the R820T2 performs better. At some time the system was very sensitive to moving things around. Then i discovered that the middle pin of a cheap MXC to BNC adapter had broken and was stuck in the dongle. So i decided to solder the pigtail right onto the board of the dongle.

Raspberry Pi2

The FA MLAT needs to run on the same Pi as dump1090 and both need quite some cpu when 250 aircraft are heard. Therfore i had to migrate to a Pi2 which i gave the same MAC address as the Pi B to continue the streak at FlightAware.

Test setup

For testing purposes a second antenna (currently a logperiodic beam) and dongle with Pi B are used. This Pi also acts as a geofencing alert for a friend. If aircraft get too close to his home a Python script wil send him an email alert with the track of the offender in gpx format.

Test receiver

Test receiver

Monitoring

A third Raspberry Pi runs collectd and receives the data from the main Pi and test Pi over the network. In fact this third Pi was a leftover from the MLAT proof of concept by Oliver Jowett. Now the main Pi is more powerfull it could as well handle the collectd stuff, but i left it as it was.

Also a small PC with Virtual Radar Server / Teamviewer is used to log tracks and have an occasional peek. It also enables remote access to the Pi’s.

Future plans

The collectd graphs show that when things are not so busy (from midnight till 5 in the morning, less than 100 aircraft seen) the percentage of received positions and the maximum distance increase. This leads to the thought that by dividing the antenna pattern in 4 areas and feed them to 4 separate receivers on a single Pi2 overall performance would increase.

Percentage positions drops as number of messages raises

Percentage positions drops as number of aircraft seen raises

The antenna in mind consists of 4 stacked Biquad antennas from these pages but stacked vertically. Each receiver would get its own stack and be placed in the 4 major directions. (a total of 16 biquad antenna’s, 4 preamps, 4 dongles and 1 Pi2). A single biquad has been built and that one worked surprisingly well.

Single biquad antenna

Single biquad antenna

However, never had it on the roof to check the directivity yet.

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