OGN receiver hardware and software

An OGN receiver is a ground station which listens for aircraft electronic-conspicuity and tracking signals, decodes them in software and forwards valid position reports to the OGN network.

The usual low-cost receiver is built from:

1. a suitable outdoor antenna for the local ISM band,
2. an RTL-SDR compatible USB receiver,
3. a Linux computer, most often a Raspberry Pi or similar small board,
4. coaxial and USB cabling,
5. optionally a mast-mounted LNA, RF filter and outdoor enclosure.

The most important design goal is not the computer. It is the RF installation: antenna position, antenna quality, cable loss, filtering and receiver overload margin.

Why the RF installation matters

Aircraft tracking signals in the 868/915 MHz ISM bands are weak. Transmit powers can be low, and aircraft antennas are often small quarter-wave antennas installed in non-ideal locations. Gliders, paragliders and light aircraft also fly much lower than airliners, so antenna height and a clear horizon matter more than for ADS-B reception.

For this reason, a receiver with a good antenna in a clear position can outperform a more expensive receiver installed behind obstacles or with lossy cable.

A practical target is:

• clear view of the horizon,
• vertical polarization,
• low-loss coaxial cable or the SDR/LNA placed close to the antenna,
• filtering when strong nearby transmitters are present,
• reliable power and network connection.

Minimal receiver build

A simple and effective receiver normally consists of:

In principle, other SDR hardware can be used when supported by the software stack, but RTL-SDR remains the common low-cost path.

Proven parts

The following parts have been proven in real OGN receiver installations and are good starting points when building a station. This is not a complete shopping list; it is a short list of known-good choices.

NOTE: The RTL-SDR Blog V4 is not the recommended default for OGN receiver stations at this time. Although it is a valid SDR product, OGN experience has shown it can perform clearly worse in this application and it may require different driver support. Use the V3 unless the V4 has been specifically tested with the intended OGN receiver software and RF setup.

Antenna

Use a vertical omnidirectional antenna for the local ISM band. In Europe this is normally 868 MHz. In North America the antenna should cover 902-928 MHz.

A collinear antenna with a realistic gain around 5-9 dBi is a good target for most permanent stations. Avoid antennas with downtilt; choose a radiation pattern which covers the horizon or has slight uptilt.

Be careful with advertised gain. Many cheap antennas overstate it. Judge an antenna partly by its physical size and construction. For example, a very short antenna advertised with high gain around 868 MHz is suspicious.

Build or buy?

Buying a ready outdoor antenna is usually the easiest route. Building an antenna can work well if you can measure or carefully reproduce a proven design.

OGN 165 cm / Chinese 9 dBi antenna

The well-known 165 cm fiberglass "Chinese antenna" was made specifically for OGN ground stations and has performed very well in many installations. It is a practical reference antenna for serious 868 MHz OGN receivers: long enough to have real 9 dBi-class gain, vertically polarized, omnidirectional and suitable for outdoor mast mounting.

It has also been sold in Europe as the OGN groundstation antenna by Segelflugbedarf24. Check current availability before ordering, but this antenna should remain listed because it is a proven OGN-specific choice rather than a generic short "high gain" antenna.

• OGN groundstation antenna 165 cm

Useful references:

• Build your own high performance antenna by Pawel
• 5.5 dBi 868 MHz collinear antenna
• 868 MHz ground-plane antenna
• 5.5 dBi 915 MHz collinear antenna
• ARRL Antenna Book

Directional antennas such as Yagis can be useful for experiments or targeted coverage, but a normal OGN ground receiver usually uses an omnidirectional antenna.

Coaxial cable and placement

At 868/915 MHz, coaxial cable loss is significant. Thin cable such as RG174 is unsuitable for long runs. H155, RF-5, Aircell-5 or similar cable is acceptable for moderate lengths; lower-loss cable is better for long outdoor runs.

The loss before the first low-noise amplifier matters most. Good options are:

• put the SDR close to the antenna and run USB or Ethernet down,
• put a filtered LNA directly under the antenna and run coax after it,
• use very low-loss coax for long antenna runs.

When a Uputronics filtered LNA, or a similar good mast-head LNA, is powered by bias-T and mounted directly at the antenna, the coax after the LNA is much less critical. In this configuration, even 30-40 m of H155-class cable has worked without noticeable loss of OGN receiver performance. The important point is that the LNA must be at the antenna; placing it next to the RTL-SDR after a long lossy cable does not recover the weak signal already lost in the cable.

If the SDR is placed near the antenna, use a good screened USB cable and test the cable length before final installation. Some long USB extension cables work well and some do not. A stable power supply is important because voltage drop on USB cables can make the SDR unreliable.

SDR receiver

The common receiver is an RTL-SDR compatible USB dongle based on the RTL2832U chipset and an R820T/R820T2 tuner.

Modern RTL-SDR dongles with a TCXO are preferred. They normally need little or no frequency correction, so the receiver configuration can often start with:

FreqCorr = 0;

Older or very cheap dongles can have large crystal frequency errors. Those receivers may need GSM calibration or manual frequency correction before they can receive aircraft packets reliably.

Useful links:

• DVB-T / RTL-SDR dongles
• rtl-sdr project
• manual installation guide

LNA and filtering

An LNA can improve sensitivity, especially when installed close to the antenna. The best practical arrangement is often:

1. antenna,
2. cavity filter, when needed for strong out-of-band signals,
3. low-noise, high-dynamic-range LNA,
4. coaxial cable,
5. SDR receiver.

Many practical installations use a combined filtered LNA at the antenna and power it through bias-T. Some RTL-SDR dongles include software-controlled bias-T support.

Filtering becomes important when the site is close to strong transmitters such as mobile-phone base stations, radio/TV towers, TETRA, amateur radio or other high-power RF sources. A cheap unfiltered SDR can overload; the result is a raised noise floor and much shorter range.

As a rule of thumb, every 6 dB increase in effective noise floor roughly halves the radio range. A 20 dB overload/noise problem can reduce the range by about a factor of 10.

Near cellular base stations, radio towers or TV transmitters, a good cavity filter can be a crucial element. It reduces out-of-band signals before they reach the LNA or SDR and can prevent cross-modulation and frontend overload.

Proven 868 MHz cavity filter

The sysmocom 868 MHz cavity filter has proven to work well for OGN receiver sites affected by strong nearby transmitters. It covers the 863-870 MHz ISM band, has low insertion loss and is reasonably priced with delivery from Germany.

• sysmocom 868 MHz cavity filter, 863-870 MHz

Useful details from the supplier:

Because it is not an outdoor-rated part, mount it inside a protected box if it is placed near the antenna.

Examples of other useful hardware families include filtered LNAs for 868/869 MHz, 915 MHz filtered LNAs for North America, SAW filters and cavity filters. Check current availability, band, connectors, bias voltage and weatherproofing before buying.

Computer / CPU board

The receiver software runs on Linux. A Raspberry Pi 3 or Raspberry Pi 4 is a common choice and is powerful enough for a normal receiver configuration. A Raspberry Pi 1 or Zero can still work for simpler reception, but may not have enough CPU for all supported protocols or wider bandwidth.

Other Linux computers can work as long as they have:

• enough CPU power,
• reliable USB for the SDR,
• stable power,
• network connectivity,
• enough storage for logs and temporary files.

Small Intel/AMD mini-PCs, thin clients and other SBCs can be useful where more CPU margin is desired.

Outdoor enclosure

If equipment is installed near the antenna, protect it from water, condensation, heat, UV and debris. Provide strain relief for cables and avoid putting connectors where water can run into them.

Outdoor boxes should be ventilated or designed to avoid condensation. Do not rely on an indoor USB dongle or Raspberry Pi case for mast installation.

Example reference:

• Outdoor PipePi enclosure

Receiver sensitivity

Measured RTL-SDR based OGN receiver sensitivity is discussed in the FAQ. In laboratory measurements, the receiver reached approximately:

The practical result from those measurements is that error correction matters: about 4 dB gain for ADS-L at 10% packet error rate, and about 5.5 dB total coding gain for OGNTP because of its stronger LDPC FEC.

Installed station performance can be much better or much worse than the lab number depending on antenna, cable, LNA, filtering, local noise and overload.

Compared with the RTL-SDR-only laboratory baseline described in the note, a good Uputronics-type filtered LNA mounted at the antenna can typically improve effective receiver performance by about 3-5 dB, provided the receiver is not overloaded by strong out-of-band signals. At difficult RF sites, the cavity filter before the LNA may be needed to obtain this benefit.

See:

• Frequently Asked Questions
• Bit error correction and receiver sensitivity measurements (PDF)

Software

The OGN receiver software is normally installed on Linux. The receiver consists of two main processes:

The receiver may acquire 1 MHz or 2 MHz of RF bandwidth depending on configuration and CPU capacity. Within that bandwidth it can decode several systems in parallel, depending on regional frequency plan and enabled protocol support.

Installation options

The usual options are:

• ready-to-use Raspberry Pi image, where available,
• manual installation on Raspberry Pi or another Linux computer,
• advanced/manual deployment for custom hardware or multiple receivers.

Start here:

• downloads
• manual installation guide
• receiver configuration file
• software troubleshooting guide

Configuration topics

Important receiver configuration topics include:

• receiver name and position,
• regional frequency plan,
• SDR frequency correction, often zero for TCXO dongles,
• RF gain or automatic gain control,
• optional bias-T power,
• enabled protocols,
• APRS/network settings.

Starting from RTLSDR-OGN version 0.3.2, automatic gain control can adjust the tuner gain to keep the measured input noise within configured limits:

RF:
{
  OGN:
  {
    MinNoise = ...;
    MaxNoise = ...;
  };
};

The configuration file uses libconfig syntax. For syntax rules, see:

• libconfig manual

Useful related pages

• Frequently Asked Questions
• manual installation guide
• receiver configuration file
• receiver naming convention
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• firewall rules
• adding e-mail notifications
• Raspberry Pi installation guide
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