Recently the company Stratux released a new ADS-B/UAT diplexer PCB. This is useful if you have a single antenna and want to feed two RTL-SDR dongles, with one receiving 1090 MHZ ADS-B and the second receiving 978 MHz UAT. The filter consists of a splitter and two SAW filters.
ADS-B is short for Automatic Dependant Surveillance Broadcast and is used to help track aircraft in the sky. It is transmit at 1090 MHz and the signal contains aircraft data such as the location, speed, altitude and aircraft call sign. ADS-B is utilized worldwide.
UAT is short for Universal Access Tranceiver and is transmit at 978 MHz. Like ADS-B it is used to keep track of aircraft, however UAT is only available in the USA and only for aircraft that fly below 18,000ft. It is a little cheaper and unlike ADS-B, UAT transmissions can also contain weather and traffic data.
US aircraft owners/operators that fly below 18,000ft can choose to install either UAT or ADS-B transmitters in their aircraft, so in the US a complete monitoring solution needs to monitor both 1090 MHz and 978 MHz.
For some time now, small aircraft pilots who don’t have access to expensive ~$1000+ ADS-B gear have been successfully using an RTL-SDR and Raspberry Pi combination to receive ADS-B and UAT to display aircraft and weather data on an iPad. The first time we posted about this was back in August 2015.
The full implementation uses two RTL-SDR dongles to receive both 1090ES aircraft position information and 978 UAT to receive weather radar information. Both dongles are used on a Raspberry Pi mini computer that runs a program called Statrux. Stratux takes the ADS-B information received by the RTL-SDR’s and re-transmits the data out via WiFi. Then an iPad running special pilot navigation aid software such as ForeFlight can interface with the WiFi signal and receive the ADS-B and weather information.
Assembly of a Stratux box requires the purchase of each individual component or a Raspberry Pi kit that includes the stratux software image on an SD card, RTL-SDR and WiFi adapter. However, setting up a Stratux box may be a little difficult for pilots who do not have any electronics DIY skills.
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.
In most of the world aircraft use the ADS-B standard for location tracking which transmits at a frequency of 1090 MHz. However, in the USA there is the option for aircraft to instead use the Universal Access Transceiver (UAT) protocol which transmits at 978 MHz.
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. It seems that most small aircraft in the USA prefer to use the UAT system due to it’s lower cost and additional features.