ADS-B Out Basics

Introduction

ADS-B Automatic Dependent Surveillance Broadcast (Ref.1) is an avionics system that allows planes to transmit their navigational status on a periodic basis without interrogation. It is an outgrowth of the ATCRBS Air Traffic Control Remote Beacon System – Mode A, Mode C and Mode S (Ref.2). It is a particular case of Mode S (Ref.3). Before Mode A/C/S, the only way to detect aircraft was by using PSR Primary Surveillance Radar. This just gave a slant distance and azimuth. ADS-B “Out” allows planes to transmit GNSS and navigational sensor information, giving accurate information to ground stations and other local aircraft. ADS-B “In” permits an aircraft to receive TIS-B (Traffic Data)/FIS-B (Weather Data) information and direct aircraft communications. Ref.4 is an excellent overview of ADSB.

Fig.1 ADS-B Block Diagram

Figure 1 shows an ADS-B Block Diagram. Older Mode A/C SSR Secondary Surveillance Radars interrogate planes on 1030MHz and receive replies on 1090MHz. Planes equipped with ADS-B periodically send out information on 1090MHz and if equipped, can also receive information on 1090MHz.

Packet Structure

Fig.2 ADS-B_1090ES_OUT 112 bit Packet Structure

Figure 2 shows a typical ADS-B packet structure. The packet starts with a particular 8usec Preamble with 4 x 0.5usec pulses located at 0, 1, 3.5 and 4.5usecs. The data rate is 1Mbps. Each bit period is 1usec. A “1” is encoded with a pulse in the 1st 0.5usec and no pulse in the second 0.5usec. A “0” is encoded with a no pulse in the 1st 0.5usec and a pulse in second 0.5usec (This is referred to as PPM in this context).

Modulation Method

Fig.3 ADS-B_OUT Scicos Simulation
Fig.4 Scope CH1 Packet Display CH2 ASK RF Output
Fig.5 Scope CH1 Packet Display Zoom of Preamble
Fig.6 Spectrum Display of ASK RF Output
Fig.7 ASK Spectrum of Carrier = 1KHz Modulated by Data = 100bps

Figure 3 shows the Scicos model used to simulate ADS-B_1090ES. The baseband packet is first constructed in Scicoslab and read in as a structure V. This directly multiplies the RF Carrier at 1090MHz to generate ASK. Figure 4 shows the baseband packet structure and the ASK modulation. Figure 5 shows a closeup of the Packet Preamble. Figure 6 shows the ASK spectrum. Due to differences in the carrier frequency of 1090MHz and the effective pulse rate of 2Mbps (0.5usec pulses), it is hard to display the data nulls +/- 2MHz from the carrier. Figure 7 is a clearer picture showing ASK modulation with a carrier at 1KHz and data at 100bps. The data nulls at +/- 100Hz from the carrier are clearly shown.

Decoding with RTL-SDR

Fig.8 GNU Radio Companion Model for ADS-B_1090ES Decoding with RTL-SDR
Fig.9 ADS-B_1090ES 120usec 112 bit Packet
Fig.10 ADS-B 8usec Preamble

Figure 8 shows the GNU Radio Companion (Ref.5) RTL-SDR model to decode the ADS-B packets. Since the ADSB effective pulse width is 0.5usec, we need an Fs of at least >= 2 x 2Msps >= 4Msps to cover at least up to the first null. The RTL is setup for a max sample rate of 3.2Msps, so the decoded pulses will not be perfect rectangles. Figure 9 shows a decode of a full 112bit packet. Figure 10 shows an expanded view of the ADSB 8usec preamble.

Fig.11 YouTube Video ADS-B Out

Please send your comments, questions and suggestions to:
jclark@clarktelecommunications.com

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References

#1. – “ADS-B Automatic Dependent Surveillance Broadcast”
https://en.wikipedia.org/wiki/Automatic_Dependent_Surveillance-Broadcast

#2. “The Story of Mode S”, Emily Chang etal
http://web.mit.edu/6.933/www/Fall2000/mode-s/

#3. – “ADS-B for Dummies 1090MHz Extended Squitter”
https://www.sigidwiki.com/images/1/15/ADS-B_for_Dummies.pdf

#4. “The 1090MHz Riddle”, Junzi Sun
https://mode-s.org/decode/book-the_1090mhz_riddle-junzi_sun.pdf

#5. “GNU Radio Companion”
https://www.gnuradio.org/

By Jeremy Clark

Jeremy Clark is a Senior Telecommunications Engineer and Advanced Amateur Radio Operator VE3PKC. He is the author of E-Books on Telecommunications, Navigation & Electronics.