Category: Airband

Creating a FIS-B ADS-B Weather Information Receiver for Pilots with an RTL-SDR

Recently a pilot wrote in to let us know about how he’s been using his RTL-SDR to receive real time FIS-B ADS-B weather updates in ForeFlight while flying in his plane. ForeFlight is an iOS app for pilots that helps with flight planning and provides access to important information like charts, real time weather reports and airport information. However, to access real time weather information usually you need to buy a $549 – 899 Stratus receiver

Now it is possible to use an RTL-SDR to receive the FIS-B weather information that is transmitted on the 978 MHz UAT frequency which is available only in the USA. UAT stands for Universal Access Transmitter and is similar to ADS-B transmitted at 1090 MHz, however UAT has some extra features for pilots compared to ADS-B. In addition to location information UAT provides a Traffic Information Service (TIS-B) which allows pilots to see what ground control sees on their traditional RADAR system. It also provides a Flight Information Service-Broadcast (FIS-B) which includes weather and other information. UAT is commonly used on small aircraft due to it’s lower cost and additional features. 

To receive UAT FIS-B weather information the pilot used dump978 on a Raspberry Pi 2. Dump978 is a UAT decoder, similar in operation to dump1090. He has also created a program called “Stratux“, which together with a WiFi adapter allows the decoded FIS-B data to be transmitted from the Raspberry Pi by WiFi to an iPad running ForeFlight. Running the software is as simple as piping the 978 MHz signal from rtl_fm into dump978, and then piping the decoded output of dump978 into stratux. Foreflight can then connect to the WiFi signal and work like it is connected to an expensive Stratus receiver.

In the video example below you can see some weather radar animations from FIS-B data received from the RTL-SDR shown on the ForeFlight screen at around 4:29 and onwards.

RTL-SDR, Rasperry Pi, WiFi dongle and portable battery pack for receiving UAT.
RTL-SDR, Rasperry Pi, WiFi dongle and portable battery pack for receiving UAT.
FIS-B UAT Weather Report received on Foreflight from a Raspberry Pi and RTL-SDR running dump978.
FIS-B UAT Weather Report received on Foreflight from a Raspberry Pi and RTL-SDR running dump978.

 

Using a HackRF to convert ADS-B packets into Bluetooth packets for reception on your Smartphone/Tablet

HackRF experimenter Jiao Xianjun has recently posted about his new firmware which allows a single HackRF to receive an ADS-B data packet at 1090 MHz, and then retransmit it as a Bluetooth low energy (BTLE) packet at 2.4 GHz. A smartphone or tablet can then be used to view the ADS-B data. It appears that the system works by broadcasting several fake Bluetooth peripheral names as the received flight data, so there is no way to currently view the data on a map.

The firmware needs to be flashed into the HackRF RAM or ROM, and he provides instructions for this over on his post. The video below shows the HackRF and software in action on an iPad.

ADS-B to BTLE HackRF Relay
ADS-B to BTLE HackRF Relay

Some new RF filters from Adam 9A4QV

Adam 9A4QAV is mostly known as the manufacturer of the popular LNA4ALL, a low cost low noise amplifier which is often used together with the RTL-SDR to improve reception of weak signals. He also sells an ADS-B bandpass filter and an ADS-B antenna, the latter of which we reviewed in a previous post.

Now Adam has come out with two new RF bandpass filters which are for sale. RF filters are used to block unwanted interference from other strong signals which can cause trouble, especially with low cost receivers such as the RTL-SDR. 

The first new filter that he has developed is for FLARM (FLight Alarm System). FLARM broadcasts at 868 MHz and is a protocol similar to ADS-B. It is used by Gliders and some Helicopters for collision avoidance. It is possible to decode FLARM with an RTL-SDR which allows you to track gliders on a map, as discussed in one of our previous posts.

Characteristics of Adam's FLARM Filter.
Characteristics of Adam’s FLARM Filter.

The second filter is for amateur radio astronomers who wish to detect the Hydrogen Line at 1420 MHz. Hydrogen molecules in space occasionally emit a photon at 1420 MHz. A single emission can’t be easily detected, but space and the galaxy is full of Hydrogen and the net result is an observable RF power spike at 1420 MHz. This can be detected with a high gain antenna, LNA, RF filter and radio like the RTL-SDR. The Hydrogen line can be used to measure things like the rotation and number of arms in our galaxy. Filters are very important for radio astronomy work as man made interference can easily drown out the relatively weak cosmic signals.

Characteristics of Adam's Hydrogen Line Filter.
Characteristics of Adam’s Hydrogen Line Filter.

Adam sells all his fully assembled filters for 20 euros, plus 5 euros worldwide shipping.

One of the ADS-B/FLARM/HLine Filters by Adam 9A4QAV.
One of the ADS-B/FLARM/HLine Filters by Adam 9A4QAV.

Tutorial on using an RTL-SDR for ADS-B on a BeagleBone Black from Make Magazine

Make magazine has recently released a tutorial and uploaded a video showing a nice overview on how to get an RTL-SDR set up for ADS-B decoding on a BeagleBone Black embedded Linux computer. In the tutorial and video they show you the parts you will need and show you how to compile and install the RTL-SDR drivers and dump1090 ADS-B decoder on the BeagleBone.

ADS-B decoding allows you to receive GPS and other information from aircraft in your vicinity. We also have a tutorial about ADS-B decoding available here.

The BeagleBone Black is a small embedded Linux computer, similar to the Raspberry Pi. It has enough computational power to run the RTL-SDR and ADS-B decoder. 

Monitoring FBI Surveillance Aircraft with ADS-B and an RTL-SDR

After reading an article by the Washington Post about FBI surveillance aircraft spotted in the air after the West Balimore riots, John Wiseman decided to look for more information about these aircraft. Fortunately, John had on his hands a database of about 2 months of ADS-B data that was collected by his continuously running RTL-SDR + BeagleBone Black ADS-B decoder set up.

From reports on the internet John found out that FBI aircraft squawked with 4414 or 4415 codes, and used call signs like JENNA or JENA. With this information John decided to take a look through his ADS-B logs to see if if he could find anything similar. Out of 15,000 aircraft he had tracked, he found 9 aircraft in his logs that matched the criteria, and saw that they did exhibit suspicious behaviour such as circling over LA for hours at a time. Then by looking up their FAA records of the tail numbers of the suspicious aircraft, he was able to discover that these aircraft where licensed to companies with names like NG Research, OBR Leasing, Aerographics Inc. and PXW Services which are suspected Department of Justice front companies. John also writes:

If you Google the tail numbers of aircraft registered to those companies, you start to find forum and mailing list posts (often at sites that tilt toward paranoid/conspiracy/right wing, but not always) with people discussing these specific tail numbers and linking them to the FBI. Some of the supposed evidence includes details of radio communications that people have heard, e.g. talking about “being on station” or using callsigns that start with JENNA, JENA or ROSS, which are supposedly used by the FBI. Other posts claim that DOJ/FBI surveillance aircraft often squawk 4414 or 4415 on their transponders.

An article from the startribune talks about the surveillance planes and says:

The planes use “persistent wide-area surveillance” to photograph large areas for hours at a time, Stanley said. The captured images allow authorities to go back in time, if necessary, to trace pedestrians and vehicles who come to their attention.

Other devices known as “dirtboxes,” “Stingrays” or “IMSI catchers” can capture cellphone data. Stanley said it’s still unclear what technologies have been used in the surveillance flights.

 

Possible FBI Surviellance Aircraft Path from flightradar24.com
Possible FBI Surviellance Aircraft Path from flightradar24.com

Building a Passive Radar System with RTL-SDR Dongles

Back in 2013 we posted about Juha Vierinen’s project in which he created a passive radar system from two RTL-SDR dongles, two Yagi antennas, and some custom processing code. Passive radar can be used to detect flying aircraft by listening for signals bouncing off their fuselage and can also be used to detect meteors entering the atmosphere. The radar is passive because it does not use a transmitter, but instead relies on other already strong transmitters such as FM broadcast radio stations. Juha writes:

A passive radar is a special type of radar [that] doesn’t require you to have a transmitter. You rely on a radio transmitter of opportunity provided by somebody else to illuminate radar targets. This can be your local radio or television station broadcasting with up to several megawatts of power. 

How passive radar works
How passive radar works

His previous write up was brief, but now over on Hackaday Juha has made a detailed post about his RTL-SDR passive radar project. In the post he explains what passive radar is, shows some examples of his and others results, shows how it can be done with an RTL-SDR dongle, and finally briefly explains the signal processing required. In his next post Juha aims to go into further detail on how passive radar works in practice.

Below we show a video that shows an example of one of his passive radar tests that was performed with a USRP software defined radio and two Yagi antennas. 

This video shows a lot of airplanes around the New England area detected using a simple passive radar setup, consisting of: one USRP and two yagi antennas, a quad core linux PC. Every now and then an occasional specular meteor echo is observed too.

In his other tests shown on YouTube Juha also used two RTL-SDR dongle’s with a shared clock and was able to get similar results.

How coax cable loss affects ADS-B reception

Over on YouTube user Adam Alicajic has uploaded a video showing how coax cable loss affects the frame rate when receiving ADS-B. To do this test Adam uses a precision attenuator in between his ADS-B antenna and RTL-SDR dongle to simulate attenuation from coax cable loss. His results show that for every 1 dB of attenuation the frame rate drops by about 10%.

Coax cable loss for common type of cable can be estimated with calculators available at http://www.net-comber.com/cable-loss.html and http://www.arrg.us/pages/Loss-Calc.htm. RG-6 cable has a low loss at 1090 MHz of about 0.23 – 0.32 dB per meter, whereas RG58 has a loss of about 0.5 – 0.6 dB per meter and RG174 (stock antenna cable on most RTL-SDR units) has a greater loss of about 1.2 dB per meter.

New ADS-B Filter with Built in Bias Tee Available

Adam who is the manufacturer of the popular LNA4ALL low noise amplifier (LNA) that is commonly used with the RTL-SDR has come out with a new product for ADS-B enthusiasts. The product is an ADS-B filter with a built in bias tee for providing phantom power. Adam previously sold an older version of the ADS-B filter that came without the bias tee.

The bias tee allows you to inject DC power into the coaxial cable in order to easily power an LNA (like the LNA4ALL) or other device that is placed near the antenna. The antenna could be far away from a power source, such as on your roof or up a mast. It ensures DC power reaches the LNA, but at the same time does not enter the RTL-SDR dongle, as DC current on the antenna input could destroy the RTL-SDR. For best performance it is recommended to use an LNA near the antenna, especially if you have a long run of coaxial cable between the antenna and RTL-SDR.

The filter uses Low Temperature Co-fired Ceramics (LTCC) type components as opposed to the seemingly more commonly used SAW and microstrip filters. Adam writes that each type of filter has its tradeoffs, but he believes the LTCC filter is the best for this application.

Comparison between different filter types.
Comparison between different filter types.

The insertion loss of the filter in the pass band is about 2.4 dB and the filter will significantly attenuate broadcast band FM, TV stations, WiFi and 1.8 GHz+ cell phones. However, it does not do so well with 950 MHz cell towers and possible radar on 1.2-1.3 GHz as the LTCC filter is not as sharp as a SAW filter. In Adams own tests he shows that the addition of the filter improves ADS-B decoding performance by about 20%, but the improvement you see will vary greatly with your RF environment.

The filter is currently selling for 20 Euros + 5 Euros shipping (~$28 USD).

ADS-B LTCC Filter with Bias Tee
ADS-B LTCC Filter with Bias Tee