Frequently Asked Questions

General operation

What does an OGN receiver do?

An OGN receiver listens to radio beacons from aircraft and electronic conspicuity devices, decodes them in software and forwards valid position reports to the OGN network.

The receiver normally acquires 1 MHz or 2 MHz of RF bandwidth in the local ISM band. Within that bandwidth, rtlsdr-ogn can detect and decode several systems in parallel, depending on the receiver configuration, regional frequency plan and available CPU power.

Systems being received include:

System	Frequency [MHz]	Modulation	Bandwidth	Packet [ms]
FLARM	868.2/868.4	50 kbps GFSK	250 kHz	5 ms
OGN	868.4/868.2	50 kbps GFSK	250 kHz	5 ms
ADS-L MDR	868.2/868.4	50 kbps GFSK	250kHz	5 ms
PilotAware	869.525	38.4 kbps GFSK	60 kHz	8.5 ms
ADS-L LDR	869.525	38.4 kbps GFSK	60 kHz	8.5 ms
ADS-L HDR	869.525	200 kbps GFSK	250 kHz	1.25 ms
FANET	868.2	LoRa 250 kHz SF7	250 kHz	25-30 ms

The exact frequencies depend on the region and the selected band plan.

How does the receiver select the regional frequency plan?

rtlsdr-ogn selects the appropriate ISM and GSM band plans from the GPS coordinates in the receiver configuration file.

In North America the ISM band is 902-928 MHz, centered around 915 MHz. If CenterFreq is not specified in the receiver configuration, FLARM operation there can use one of 65 channels, each 400 kHz wide.

You can check the currently selected center frequency on the receiver HTTP status page, normally:

http://receiver-address:8080

It is also visible in the file name of downloaded spectrum or spectrogram files.

Is OGN limited to one protocol?

No. OGN started in the gliding community and FLARM reception was one of the first important use cases, but the receiver software and network have expanded to support several electronic conspicuity and tracking systems.

The practical result depends on radio coverage, local frequency regulations, receiver hardware, CPU performance and configured software support.

I want my aircraft to become visible on OGN. What should I do?

The usual solution is to install or carry an OGN-compatible tracker. An OGN tracker transmits your aircraft position so that nearby OGN ground receivers can receive it and forward it to the OGN network.

For a new practical device, start with OGN Tracker and especially the list of flashable tracker modules. Modern ready LoRa/GNSS modules such as Wio Tracker, T-Echo or ThinkNode-M5 can often be flashed with OGN-compatible firmware and are now a simpler route than the old hand-built DIY tracker construction.

You can also use a ready-made commercial or community-built tracker where available. Before flying with any tracker, check that the device, firmware, frequency band, antenna installation and transmit power are legal for your country and suitable for your aircraft.

An OGN tracker makes the aircraft visible where there is compatible radio coverage. It is an electronic conspicuity and tracking aid, not a replacement for certified avionics or required collision-avoidance equipment.

What are the benefits of being electronically visible?

Electronic visibility helps other people and systems know where you are. For OGN this means that your aircraft can be received by ground stations, shown on tracking maps, included in club or competition monitoring and recorded for later flight analysis.

The safety benefit is important. If an aircraft is overdue, lands out in a field or is otherwise lost, the last received positions can greatly reduce the search area. This is especially useful for gliders, paragliders, microlights and other aircraft which may operate away from controlled aerodromes.

For outlanded gliders, visibility may still be possible even after landing. OGN trackers can relay received positions, for both OGNTP and ADS-L, so another compatible tracker flying nearby may forward the position of the glider on the ground toward an OGN receiver. This depends on radio range, antenna position, tracker firmware, relay configuration and nearby traffic, but it can be valuable when direct ground-station reception is blocked.

Electronic visibility can also help other pilots see and avoid you. Depending on the protocol, radio range, receiver coverage and integrations, your position may be visible to nearby OGN-compatible trackers, cockpit traffic displays, Stratux-style receivers, PilotAware, FLARM/FANET/ADS-L capable devices, navigation applications or services such as SafeSky.

For example, if you fly a paraglider or hang glider, being electronically visible increases the chance that another aircraft with a compatible receiver, cockpit display or connected app will notice you early enough to avoid you. It does not guarantee that every aircraft will see you, so normal lookout and airspace discipline remain essential.

Stratux is an important open-source, low-cost cockpit receiver project, especially for ADS-B and related traffic display use. SafeSky is an app-based conspicuity service: a smartphone or tablet can share its position through the mobile network and display traffic from SafeSky and connected/integrated sources where coverage and connectivity allow.

Hardware

What computer should I use?

A Raspberry Pi 3 or Raspberry Pi 4 is the common recommendation and is easy to obtain. 512 MB of RAM is normally enough.

A Raspberry Pi 1 or Raspberry Pi Zero can still work for simpler reception, but PilotAware and FANET reception may not be possible or may require compromises.

In principle, any Linux computer can work if it has enough CPU power, USB support and SDR support. Other SBCs such as Orange Pi or Odroid boards can work. Intel or AMD Linux mini-PCs and thin clients can also be used.

What SDR receiver should I use?

An RTL-SDR compatible USB receiver is the easiest and cheapest option.

For RTL-SDR USB sticks, receivers with an R820T or R820T2 tuner are recommended. Modern RTL-SDR dongles with a TCXO are preferred because they normally need little or no frequency correction.

In principle, hardware supported by SoapySDR can also be used, but RTL-SDR remains the most common setup.

Which RTL-SDR dongles are recommended?

The RTL-SDR.com "silver" sticks are commonly recommended. They include a stable oscillator and programmable bias-T, which can power a mast-mounted LNA such as the Uputronics filtered LNA.

Other good TCXO-based RTL-SDR receivers can also work well.

Can I put the RTL-SDR dongle on a USB extension cable?

Yes, this can work and may reduce antenna feeder loss by putting the SDR closer to the antenna.

USB extension lengths such as 2 x 10 m or even 2 x 20 m have been used, but this must be tested before installing the receiver at the final site. Not every USB extension cable or active extender works reliably with an SDR.

Use a screened, good quality USB cable. A thicker cable is usually better because it reduces voltage drop. The power supply should provide a solid 5 V; in some cases 5.2 V can help compensate for cable voltage drop.

What antenna should I use?

A vertical collinear antenna for the local ISM band is recommended. In Europe this usually means 868 MHz. In North America the antenna should cover 902-928 MHz.

A gain of about 5-9 dBi is a typical target. Avoid antennas with downtilt; choose an antenna with a horizontal beam or slight uptilt.

Be careful with advertised antenna gain. It is often overstated. For example, an 8 dBi antenna that is only 50 cm long at 868 MHz is physically unrealistic. Judge antennas partly by their physical length and construction, not only by the advertised gain.

What does dBi mean?

dBi means gain in dB relative to an ideal isotropic antenna.

If an antenna gain is stated only in "dB", without a reference, the number is incomplete. dB is always a relative measure, so you need to know what it is relative to.

What coaxial cable should I use?

Use low-loss coaxial cable. H155 is acceptable for shorter runs, with about 0.25 dB/m loss around 900 MHz. For longer runs, use lower-loss cable such as H2000 Flex, around 0.15 dB/m at 900 MHz.

If a good LNA is mounted directly at the antenna, the coax loss after the LNA is much less critical. For example, a receiver can work well with a Uputronics LNA at the antenna followed by 35 m of H155 cable.

Do I need an RF filter?

Use a filter, or a filtered LNA, when strong nearby transmitters overload the RTL-SDR receiver or create intermodulation.

This is more likely near mobile phone base stations, radio/TV towers or other strong RF sources.

Do I need an LNA?

An LNA can improve sensitivity, especially when it is mounted close to the antenna.

The best configuration is often a low-noise, high-dynamic-range LNA followed by a SAW filter, mounted at the antenna and powered through bias-T. A filtered LNA can improve sensitivity and reduce intermodulation problems at the same time.

The Uputronics filtered LNA has worked well on several OGN sites and is commonly recommended.

What are intermodulation problems?

Intermodulation happens when strong RF signals overload the receiver frontend and create unwanted signals or raise the apparent noise floor.

If your site is far from strong transmitters, an RTL-SDR, Raspberry Pi and antenna may work well and receive aircraft at 50-100 km or more.

If the site is close to a mobile phone base station or radio/TV tower, the cheap unfiltered RTL-SDR receiver can overload. The noise floor then increases and range drops. Every 6 dB increase in noise floor roughly halves reception range, so 20 dB of extra noise can reduce range by about a factor of 10.

To diagnose this, check the noise level reported by the receiver and inspect the spectrograms. To fix it, add filtering at the antenna or use a filtered LNA.

Can an OGN receiver coexist with an ADS-B receiver?

Yes. An OGN receiver can run on the same Raspberry Pi or Linux computer as an ADS-B receiver, for example one feeding the FR24 network.

The simplest setup uses a second RTL-SDR and a second antenna.

It is possible to share one antenna, but this requires additional RF hardware such as an LNA followed by a splitter or diplexer and separate SAW filters for 868 MHz and 1090 MHz.

You may need a stronger power supply for the second SDR and enough CPU capacity for both receivers.

Configuration

Where is the OGN receiver configuration?

On service-based installations, the service configuration is normally:

/etc/rtlsdr-ogn.conf

To see which receiver configuration file is used, inspect it:

cat /etc/rtlsdr-ogn.conf

The last parameter on each task line is normally the receiver configuration file, relative to the binary path.

Some prebuilt images generate the runtime receiver configuration at startup. For example, on Sebastien's read-only image, the receiver configuration is created from OGN-receiver.conf on the /boot partition. In that case, edit /boot/OGN-receiver.conf and restart the receiver.

You can verify that changes took effect on the HTTP status pages, normally ports 8080 and 8081. These pages list most active configuration values.

What receiver name should I use?

Use a name up to 9 characters where possible. Avoid special characters. A dash may work, but simple letters and numbers are safest.

For airfields, use an international code such as EPKA when appropriate. For other locations, use a specific location name.

See also: receiver naming convention.

What position and altitude should I configure?

Configure the real receiver antenna position and altitude as accurately as practical. The receiver coordinates are used for network display, coverage analysis and automatic regional band-plan selection.

Do I need to set frequency correction?

It depends on the SDR dongle.

Older or very cheap RTL-SDR dongles can have crystal errors up to about 100 ppm. If this is not corrected, the receiver may fail to receive traffic.

Modern dongles with a TCXO, such as the RTL-SDR.com silver stick and many newer RTL-SDR models, normally have an error below about 1-2 ppm. In that case you can usually use zero correction.

How should I configure receiver gain?

The fixed receiver gain is configured as RF.OGN.Gain in the receiver configuration file.

Starting from RTLSDR-OGN version 0.3.2, automatic gain control is also available. It adjusts the tuner gain to keep the measured input noise inside the limits configured as RF.OGN.MinNoise and RF.OGN.MaxNoise, both expressed in dB.

Use fixed gain when you want a simple known setting. Use automatic gain control when the local RF environment changes or when you want the receiver to adapt within defined noise limits.

What syntax does the configuration file use?

The receiver configuration uses the libconfig format. Settings are organized into nested groups, for example:

RF:
{
  FreqCorr = 0;

  OGN:
  {
    Gain = 22.9;
  };
};

For the general format and syntax rules, see the libconfig manual: libconfig configuration file grammar.

Software operation

Which processes run the receiver?

The receiver consists of two tasks running in parallel as separate processes: ogn-rf and ogn-decode.

ogn-rf acquires RF samples and performs FFT processing. This converts sampled RF data into spectral form and splits the full RF band into the channels used by the supported protocols.

ogn-decode processes the spectra produced by ogn-rf, detects packets, demodulates and decodes them, then sends aircraft positions to the APRS server.

What is /etc/rtlsdr-ogn.conf?

/etc/rtlsdr-ogn.conf configures the process wrapper used by the service. It tells the system where the binaries are, where they run, which receiver configuration file to use and which control ports are used to monitor the tasks.

Monitoring and troubleshooting

How can I monitor my OGN receiver?

Check that both tasks are running:

top
htop

Check the standard output ports, normally 50000 and 50001:

telnet localhost 50000
telnet localhost 50001

You can also use nc instead of telnet.

Check the HTTP status pages, normally:

http://receiver-address:8080
http://receiver-address:8081

From the receiver itself, a text browser can be useful:

lynx localhost:8081

Check basic parameters, statistics, traffic counters and spectrograms.

How can I check if my receiver works as expected?

Check the noise level and traffic on the HTTP status page, normally port 8081.

If traffic is being received, check the sensitivity indication and the distance to received aircraft. Do not judge performance from a very short observation period, especially when local traffic is low.

How can I check if the RTL-SDR receiver works?

Check the output of ogn-rf on port 50000. Problems with the RTL-SDR device are usually reported there.

Also check:

• CPU usage: very low CPU use can mean that no RF data is being acquired,
• spectrograms: look for typical ISM band signals,
• dmesg: check for USB or RTL-SDR errors after connecting the dongle.

How can I judge receiver sensitivity?

Check the traffic and distances to received aircraft on the 8081 HTTP page.

Also check the sensitivity measure expressed as SNR at 10 km distance. A good receiver is often around 20 dB at 10 km. With a good LNA, 23-26 dB at 10 km is typical.

This measure is heuristic. It can be inaccurate when traffic is low, aircraft geometry is poor or the observation period is short. Do not jump to conclusions too soon.

What is the measured sensitivity of an OGN receiver using an RTL-SDR dongle?

Laboratory measurements were made with a signal generator, attenuator and an OGN receiver based on an RTL-SDR USB dongle. A constant 25 Hz packet stream was transmitted while the input signal level was reduced, and the receiver recorded packet error rate, SNR and the number of corrected bits.

In this measured setup, ADS-L packets with CRC24-based error correction reached about 10% packet error rate at roughly -113 dBm input level. OGNTP packets with LDPC forward error correction reached the same order of packet error rate at about -114 dBm to -115 dBm. Without error correction, ADS-L and OGNTP behaved nearly the same and required roughly 4 dB stronger signal for similar packet error rate.

The important practical result is that error correction extends the useful sensitivity. The measured gain was about 4 dB for ADS-L at 10% packet error rate, while OGNTP gained about another 1.5 dB because its 48-bit LDPC code is stronger, for about 5.5 dB total coding gain at 10% packet error rate.

This does not mean every installed receiver will achieve the same range. The final station performance also depends on antenna gain and placement, coax loss, LNA and filtering, receiver gain setting, local noise, intermodulation and whether the RTL-SDR frontend is overloaded.

For details, see the second part of:

• Bit error correction and receiver sensitivity measurements (PDF)

What are spectrograms?

A spectrogram shows one reception timeslot, about 850 ms long.

The vertical axis is time. The start of the slot is at the top of the image and the end is at the bottom.

The horizontal axis is frequency. Lower frequencies are on the left, higher frequencies are on the right, and the center corresponds to the selected RF center frequency. The file name normally contains the center frequency.

The image width corresponds to the selected RF bandwidth, usually 1 MHz or 2 MHz.

What is the spectrogram pixel scale?

At 1 MHz sample rate, spectrogram files are typically 2048 x 831 pixels and 512 x 3321 pixels.

At 2 MHz sample rate, they are typically 4096 x 831 pixels and 1024 x 3321 pixels.

The image width divided into the sample rate gives the frequency width represented by each horizontal pixel. For example:

1000000 Hz / 2048 pixels = 488.281 Hz per pixel

The height represents time. In the 831-pixel-high spectrogram, one vertical pixel represents about 1.0 ms. In the 3321-pixel-high spectrogram, one vertical pixel represents about 0.25 ms.

On port 8080, spectrogram pixels are about 1.95 kHz x 0.25 ms. On port 8081, they are about 0.49 kHz x 1.0 ms.

How do signals look on spectrograms?

A centered vertical line corresponds to a carrier at the selected center frequency, for example 868.8 MHz depending on the band plan.

A horizontal line usually corresponds to a short pulse or interference burst. Short pulses are wideband, so they spread across frequency.

An OGN packet is about 5 ms long, so it takes about 20 vertical pixels on the 8080 spectrogram. It is also about 250 kHz wide, so it takes about 120 horizontal pixels on the same spectrogram.

You can use the spectrogram to estimate how long a signal lasts from its vertical size and how wide it is in frequency from its horizontal size.

Installation

Where are the installation instructions?

See the manual installation guide: manual installation guide.

Open questions

If you have a question which is not on the list, put it here and it should be answered at some point.

Remember this is a Wiki: anybody from the OGN group can edit this page.