Do you remember the 11st September 2001 ? In the USA some aircraft disappeared from the screens of the air traffic controllers (ATC). The transponders of threes aircraft had been switched off. I asked myself: don't they have radar in the US?
Large areas of the US-airspace are not covered by radar. The ATC depends on "secondary radar". A ground station (interrogator) transmits an interrogation to a special receiver inside the aircraft, a transmitter in the aircraft answers by sending a replay back to the ground station. If the ground station employs a directional antenna and measures the time delay from interrogation to the answer, then it can predict direction and range to the aircraft - like a real radar. Of course this works only, if the aircraft has the necessary hardware (receiver-transmitter, called transponder) and if this transponder is switched on.
A secondary radar ground station is much cheaper then a real radar. Another feature is, that the answering aircraft can insert additional helpful information into the answer.
For several years a new generation of transponders is in use, they support the new Mode-S. This kind of transponder sends more data (ADS-B-data) then the previous types. And they transmit data even if they are not interrogated. This feature is called squitter. Everybody can receive this squitter-information and create an own virtual radar picture with the position (and additional information) of all squittering aircraft. About 70 ... 80 percent of all aircraft transmit such information. Thus the virtual radar gives you a good idea about the air traffic in your neighborhood.
Why should somebody do this? Because it's fun.
To observe air traffic one needs:
|The transponder signals are
vertical polarized. A vertical polarized antenna tuned to 1090 MHz is
required to receive this signals. The simplest solution is a vertical
wire or metal stick of 13 cm length , this is the half wavelength of
the 1090 MHz signal. Such an antenna is an electric dipole and receives
signals from all directions.
To improve the antennas sensitivity multiple electric dipoles can be combined. But if they are placed side by side, then the antenna would not be an omnidirectional antenna anymore. Consequently the individual dipoles have to be placed about each other. Finally the dipoles have to be interconnected, but upper and lower end of each dipole oscillate with 180 degree phase shift. To connect them 130mm long horizontal loops have to be used. They work as 180 degree phase shifter.
Dipoles and loops should be bended from one long piece of wire. Its diameter should be large enough for the necessary stability.. The lowest point of this dipole-group has to be connected to the middle wire of an 50-ohms coaxial cable.
Now we need a "dummy-ground", that has to be connected to the shield of the coaxial cable. A round piece of sheet metal (13 cm radius) would be great, but some radial wires can be used instead. At least 4 wires (90 degree interspaced) should be used (groundplane-antenna). "Dummy-ground" and dipole don't have to touch each other!
This is just a basic design. The impedance of the antenna is not matching the impedance of the cable (but can be modified by bending the "dummy-ground"-wires downwards. Longer or shorter connection loops between the dipoles can change the elevation angle of the antenna ....... Antenna design requires knowledge, experience and luck.
|To receive ADS-B-signals a 1090
MHz receiver is needed. I use the direct detection receiver miniadsb from jetvision.de. It can be ordered as
kit at http://miniadsb.web99.de/
for 45 Euro. To build the kit together requires experience in
SMD-soldering. The whole receiver has the size of a textmarker (2cm x
2cm x 8cm). The input is a coax connector for the antenna cable. The
output for the received signals is an analog output.
Beside the antenna connector, the decoder has 3 connection-wires only.:
The analog output can deliver up to 0.4 mA. It should never be connected to any voltage, because it can be easily damaged.
The value of the supply voltage influences the receivers sensitivity. The optimum is 4V. Voltages larger then 5V or voltages of wrong polarity will destroy the receiver. For safety reasons i placed a Schottky diode inside the receiver. It is in line with the power supply wire (prevents wrong polarity) and increases the optimum voltage at the power supply wire to 4,5V. This voltage will be produced by my decoder.
|The picture at the right site
shows the whole decoder on a universal board. It is made up from a
small number of parts. With SMD-parts the
decoder fits on a 4cm x 4cm PCB (single sided).
The one and only active part is the PIC microcontroller (PIC18F2455 or PIC18F2550). Its comparator converts the analog signal into a digital signal. The comparator needs a reference voltage. This reference voltage is produced by the decoder itself.
The PIC18F2550 searches in the digitized signal for ADS-B-data and reads this data.
Transponders send many different ADS-B-frames. The most valuable frames ate DF17-frames. DF18 and DF19 would be of same value, but are rarely transmitted. The decoder can forward all data to the PC or only DF17/18/19-frames.
|ADS-B frames contain a CRC
checksum to identify faulty data. The decoder can check the correctness
of DF17/18/19-frames and reject broken frames. (However, I suggest not
to use this feature.)
If the decoder is connected to the PC, then it will be identified as additional serial port (e.g. COM-3 or COM-4). (For Windows the Microchip-CDC-driver is necessary.)
The figure below shows the schematic of the decoder.
The supply voltage comes from the USB-connector. L1 and C3 remove noise from the 5V supply voltage. C2 is part of the 3,3V USB-voltage regulator. Q1, C4 and C5 are the clock source for the microcontroller.
D1 & C6 generate 4,5V to pin3 of the miniadsb-connector. This is the receiver power supply voltage. Pin 1 of the miniadsb-connector is ground. At pin 2 the decoder receives the analog signal from the miniadsb-receiver.
Through the safety-resistor R2 the analog signal is fed into the comparator. R2 can be replaced by a wire, if the miniadsb-output contains a safety resistor.
The digitized signal leaves the comparator at pin 6 and is immediately fed back into the PIC at pin 11.
Pin 11 is the digital ADS-B input of the micrcontroler. If somebody likes to use a separate comparator, then the connection between pin 6 and pin 11 has to be removed, and the digital signal from the external comparator would have to be fed into pin 11.
The LED3 shows the digitized signal. some seconds after the decoder was switched on together with the receiver it should only smoulder a little bit.
The LED1 is flashing up, it the start of a ADS-B-frame was detected.
If the whole frame could be received, then LED2 lights up until this frame was read out by the PC
Up to 6 switches can be connected to the decoder to control its function. Only 4 switches are contained in my layouts, they can control all important functions. But even they are not really necessary, because the decoder can be controlled via USB (RS232) by my software.adsbScope. If you plan to use only adsbScope, then connect RB0 (pin 21) of the PIC permanently to Vss (ground). If you plan to test Planeplotter as well, then install the 4 default switches of my PCB-layouts.
If the switch at RB0 is closed (remote) then the switches at RB2 and RB4 have no function. The decoder is remotely controlled.
If the remote-switch is in open position then the data flow to the PC is controlled by the switches. For details read the handbook.
|The figure at the left shows an
SMD-layout with a size of 4cm x 4cm. But the size can be reduced, if
necessary. By the application of the following simplifications the
layout (single sided) can be shrinked down to 2.5cm x 3.5 cm without
possible simplifications and savings
If adsbScope is used, then switches can be removed and pin 21 will be permanently connected to ground.
L1 is not really necessary.
R3,4,5 and the 3 LEDs are not necessary for the decoder function.
R2 can be removed, if the receiver contains a safety resistor. If you are brave then you don't need any safety resistor as well, and you will get e better analog signal.
Jumper JP1 is not necessary, its function is done by adsbScope.
|Not all aeronautic enthusiasts
are electronic hobbyists too.
There are some industrial products on the market. I just test out the GNS5890-USB-stick. The GNS5890 is smaller then a matchbox and contains the ADS-B-receiver and the decoder. An antenna with 1 meter long antenna cable is included. This set is ideal for mobile use.
The decoder is fully compatible to my adsbPIC-decoder, and will be delivered with the (relabeled) firmware 8. To use it with adsbScope the CDC-driver has to be installed. You can use the driver-CD of the GNS5890 or my ZIP-file (since 16.12.2011). After driver installation the GNS5890 is supported by Planeplotter too.
The GNS5890 is a very strait design without any frills. It has no DIP-switches, RS232-interface or input for external comparator. This made it possible to shrink the size. The result is a capable and reliable device.
During the first tests if outperformed my receive chain (a stocked triple dipole antenna, miniadsb-receiver and adsbPIC-decoder). The frame rate was 20% up to 100% higher, and more aircraft have been tracked.
I pimped my miniadsb-receiver a little bit, and now both systems are comparable.
Global Navigation Systems – GNS GmbH
And again: not all aeronautic enthusiasts are electronic hobbyists too.
The fastest and cheapest way to get an own ADS-B-Receiver is the use of an DVB-T-Stick and the free software. This software can convert some DVB-T-sticks into ADS-B-receivers that decode the ADS-B-data and stream this data via network.
AdsbScope can receive this data from the network and visualize it.
Compared to a specialized ADS-B receiver this device will receive less data, but it is a nice cheap way to get an operational ADS-B-receiver.
Two different softwares are available. Both support DVB-T-Sticks with RTL2832U-interface chip and E4000 or R820T tuner-Chip.
If you use RTL1090, then MLAT has to be deactivated. Check the menu point "Config - MLAT clock". The correct setting is "do not use MLAT".
|The outlines and names of all
stated of the world can be overlayed. Some of the necessary files are
contained in the ZIP-file, the remaining files can be downloaded and
installed automatically by adsbScope.
Runways can be shown as well, if the necessary files are downloaded from jetvision.de.
If then adsbPIC-decoder is used, then the following parameters can be modified/red out: