TRT800 ADS-B out
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Picture: left fin antenna right prone to breakage rod antenna |
An aircraft radio transponder is a device installed in the aircraft which transmits and receives data. In Germany, this data (not radiotelephony) is used by German air traffic control (DFS) to identify aircraft. Which data is transmitted is determined by the set mode. Entry into transponder mandatory airspace is only permitted in Mode S (ACS), whereby the squawk, the flight level and the ICAO 24-bit address (AA) of the aircraft are transmitted. The AA allows FIS (the DFS flight information service) to request specific data from the transponder. The use of a transponder is generally recommended, even for microlight aircraft. FIS can give air traffic informations by radio as soon as a potential danger becomes apparent. Without a transponder or with the transponder switched off, aircraft can only be recognized via radar, so targeted warnings to the aircraft's registration number would not be possible. In other countries there are similar systems for air traffic control established.
First, some information about the antenna. Devices that transmit and receive (aircraft radio and transponders) each require an antenna that is tuned to the frequency of the frequencies to be transmitted and received. The picture shows a transponder fin antenna and next to it a standard simple transponder rod antenna. Both types work as expected. They are mounted on the outside of the aircraft. The simple rod antenna has a filigree design (internal bore weakens the rod - cannot be seen from the outside) and can therefore be easily damaged/broken off, as practical experience has shown. If the metal rod breaks (as in the picture), it is still clamped in its white plastic sleeve, so the point of breakage is not immediately recognizable. Resourceful bodgers glue the broken rod back into the sleeve with a contact adhesive. Such faults (sporadic no transponder activity) are usually only discovered after a long search. New rod antennas of the same design can break during installation or at the latest during careless cleaning, another practical experience. In short, this design is prone to breakage, with a fin antenna (non TSO is sufficient) there are no more problems when cleaning.
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Picture: GlobalSat Serial GPS Receiver BR-355 |
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Picture: D-DUB 15 Male and Female with solder cup and safety brackets |
By connecting a suitable GPS receiver to the correspondingly setted transponder (TRT800 V4.8), it can transmit ADS-out (dataset with GPS coordinates). How and with what is described in the corresponding manual (operation and installation). This means that it is also possible to locate an aircraft without radar if the aircraft with equipped with the appropriate receiver and display devices. This type of localization is not requested by FIS in Germany - they use radar for localization and the transponder data for identification.
The advantage of sending coordinates is that it also enables aircrafts without radar (onboard radar is only used in military aviation) to locate other aircrafts. This is possible by a professional TCAS system (receives transponder data and sends TCAS collision avoidance data via the transponder). This system is too expensive for small aircraft, but there are already affordable collision warning devices that use transponder data (e.g. Stratux - receives Flarm and ADS-B). Nearby air traffic can be displayed on an external display device (e.g. tablet with navigation software). Localization essentially serves to avoid collisions and is therefore a sensible safety measure.
The transponder manual describes which additional equipment is required. The corresponding connector for the GPS input (TRT800EMSS) can of course also be purchased from the transponder manufacturer. Suitable GPS receivers are recommended. However, there are also cheaper solutions, some of which are ready-made or can be built yourself. What is needed?
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Graphic: linear voltage transformer 12 to 5 V |
1. An adapter plug for the input of the GPS signal (if only the simple TRT800EM connector is installed). Opening and retrofitting the plug connector is not permitted, as it contains the memory module for the aircraft-relevant transponder data.
2. A GPS receiver that sends the data as the transponder can process it. In the case of the TRT800, the following are relevant: serial interface, NMEA 0183 and 4800 baud. There are GPS receivers (not to be confused with simple GPS antennas that look similar) that are programmable, but it is better that the GPS receiver already has the values/protocols set by default because the programming may not work or the set values may be "forgotten" again and switched to standard (default). The above values/protocol are mandatory, otherwise the data transmission will not work.
3. Voltage transformer for the GPS receiver. This is required if the 12V on-board voltage cannot be used directly for the GPS receiver (marine GPS receivers are usually designed for 12V). Common inexpensive GPS receivers are usually designed for 5 V.
Re 1. Adapter plug
Ready-made or partially ready-made adapter plugs on the market work. But what if the space behind the transponder is limited? A D-SUB 15 angled plug provides a temporary solution. Adapter plug (on circuit board), angled plug and the original transponder plug hang on the fuse bracket/transponder socket, weight relief with cable ties would be necessary - if possible. Alternative: solder simple D-SUB 15 plug (male) and socket (female) directly together at all solder cups with except for the pin 12 to be tapped.
A cable is soldered to the solder cup of pin 12 - Female (female is plugged into the transponder) a cable (as a data cable RS232 input - RX) is soldered and routed to the outside. This is then later soldered to the green data cable RS232-output (TX) of the GPS receiver. Pinch off the solder cup of the pin 12 male. Solder a 2nd cable, the ground cable, to pin 1 (pin 9 is also possible) and lead to the outside. Do not cut the male/female connection (pin 1), just solder it on. This means that only 2 cables lead to the outside. Screw the whole thing together with threaded bushes between male/female and screw on the safety brackets on the male side. This ensures that female/male are firmly connected and do not "hang" at the soldering points. To secure the open solder joints, they can be filled with hot-melt adhesive, for example. Ambitious hobbyists can tap additional pins with cables to connect other external external devices with the GPS and transponder data (e.g. pin 5 RS232-output). By soldering male and female directly together, you have the most compact adapter plug.
Re 2. GPS-receiver
Various GPS receivers in the "Operation and installation" manual for the TRT800 are recommended, either no longer available or too expensive. The universal NMEA format with 4800 baud is also offered as an alternative. The GlobalSat Serial GPS Transceiver BR-355 fulfills all relevant requirements as standard (serial RS232 protocol, NMEA data sets and 4800 baud transmission rate). If it is not possible to program the GPS in such a way (if at all possible) that only the RMC ($GPRMC data sets) are transmitted (recommendation), practice has shown that the transponder can still cope with the unnecessary data records, it ignores the non-RMC data records. To connect to the transponder, disconnect the GPS plug, expose the wires and connect the TX cable (green) of the GPS is connected to the RX cable (pin 12) coming out of the adapter plug. As the selected GPS receiver is operated with 5 volts, a voltage transformer is required for its power supply. If you were to use another GPS receiver from the marine range with 12V, no voltage transformer would be necessary. + 5V of the GPS (red) is connected to the +5V voltage transformer, -5V of the GPS (black) is connected to the -5V voltage transformer and also to the ground wire of the transponder, which comes from the adapter plug (pin 1).
Re 3. Voltage transformer
The selected GPS receiver requires a voltage transformer to 5V. We strongly advise against using a ready-made step-down voltage transformer. In practice, these generate high-frequency radiation which can block aircraft radio reception on certain frequencies. The interference is not audible. Reception on a particular frequency is simply interrupted. No noise, no other signs. Only FIS reports that there is no interference on this frequency. But the response from FIS does not reach the radio - "check your radio" - again a uselessly long search - but the fault is not in the radio. That is why a simple linear voltage transformer is recommended, as shown in the graphic. You can "borrow" 12V from a fused instrument. An additional ground cable from the linear voltage transformer must be connected to the ground cable of the adapter plug. This electronic component can be cleverly soldered together and wrapped in heat-shrink tubing.
Transponder setting
Finally, the settings (NMEA and 4800 baud) must be made on the transponder (TRT 800 V4.8). They are not easy to find and may differ slightly from the manual description. The test shows a perfect function, no blocking of a frequency. The flight is displayed on flightradar24.com. ADS-B out therefore successfully retrofitted.
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