Software Defined Radio
Note: My images aren't working properly. I will get them fixed as soon as I can figure out what is wrong.
This set of VIs will allow you to detect aircraft in the surrouning airspace and decode their ID and locations by searching for ADS-B messages transmitted at 1090 MHz.
Software Requirements:
NI-LabVIEW 2011
NI USRP Drivers
Hardware Requirements:
NI USRP 292x
Antenna tuned to 1090 MHz.
Antenna
There are three requirements for the antenna.
- It must be tuned for 1090MHz.
- It must be vertically polarized.
- It must be omnidirectional.
You can make a simple antenna with one piece of wire that is ~6.8 cm long (1/4 wavelength at 1090 MHz), with 4 more 6.8 cm wires radiating downward at a 45 degree angle to form a ground plane (see Figure 1).
Figure 1: Example Antenna
Detecting and Demodulating the Signal
The messages we are trying to decode follow the ADS-B protocol. They are transmitted at a 1090MHz carrier and encoded using Pulse Position Modulation (PPM) at a rate of 1 Mb/s. Each 112 bit (112 microsecond) message is preceded by an 8 microsecond preamble which identifies the starting point of the message (See Figure 2). The decoder scans until it finds a valid preamble, then proceeds to decode the rest of the message.
Figure 2: ADS-B Message Format
The PPM Demodulator works as follows:
- The message is sampled at 4 MegaSamples/second (MS/s)
- The message is decimated by a factor of 2.
- The samples are compared to a threshold to convert them to a '1' or '0'.
- Each bit now consists of two samples, with '10' corresponding to a binary '1' and '01' corresponding to a binary '0'
Reading the Message's Data
The message data is encoded following the ADS-B Protocol as outlined in the document RTCA DO-282B.
The first 5 bits of the message contain the Downlink Format (DF), which specifies the structure of the rest of the message. Both the DF = 17 and DF = 18 formats are used for ADS-B messages, while DF = 19 is reserved for military applications, but can in some cases also contain ADS-B messages. Bits 6-8 contain either the CA (DF = 17), CF (DF = 18), or AF (DF = 19). For our purposes, this field is only important in DF = 18 and DF = 19 messages, since it serves to distinguish between ADS-B messages and other messages transmitted in those formats. All DF = 17 transmissions contain ADS-B messages.
The following 24 bits contain the aircraft address information as registered with the International Civil Aviation Organization (ICAO). This unique identifier is what allows the tracking of a single aircraft across multiple messages. Bits 33-88 contain the message itself.
The first 8 bits of the message contain the Type Code and Subtype Code, which state what information the following bits will contain. For this project, the type codes of interest are 0, 9-18, and 20-22, which contain Airborne Position Messages. In this message, bits 55-88 contain the latitude and longitude, encoded using Compact Position Reporting. The final 24 bits are a parity check, which is not used in this decoder.
Acknowledgements
The state machine that forms the framework of the decoder and the PPM demodulator were designed by Erik Luther.
