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What is ADS-B?

‍Automatic Dependent Surveillance-Broadcast ADSB-out is essentially a 1090MHz Mode-S transponder that is modified to transmit GPS coordinates to show an aircraft's position to other locations equipped with ADSB-in. ADSB-out transmits automatically without the requirement to be prompted by a secondary surveillance RADAR source.

What does ADS-B transmit?

Using "ADS-B out", each aircraft periodically broadcasts information about itself, such as identification (ICAO code), current position, altitude and velocity. In most cases, the information provided is more accurate than the information available with current radar-based systems. With more accurate information, ATC will be able to position and separate aircraft with improved precision and timing.

The limitations of ADS-B.

ADSB-out is transmitted on 1090MHz using an inefficient modulation technique called Pulse Position Modulation. The direct impact of ADS-B is an increase in the frequency use of 1090MHz. The 1030/1090 MHz surveillance infrastructure sees its performance degraded when transponders are interrogated beyond their designed capability and performance.  
This happens when there are too many RF interrogations on 1030 MHz triggering too many replies from aircraft transponders on 1090MHz.  This happens when the density of airframes using 1090MHz in a given area exceeds the supporting capacity of the frequency.

A high level of activity on the 1030/1090 MHz infrastructure can result in detection losses of aircraft surveillance, generating delays and reducing the network capacity. This has, in the past, resulted in Air Traffic Controllers requesting GA pilots to limit the use of 1090Mhz transmissions. ie turn them off. The area around Frankfurt am Main in Germany, London and the Los Angeles Basin airspace is the USA is the most congested in the world.
The concerns about frequency congestion have resulted in the FAA adopting a second frequency (978MHz UAT) and also rejected the use of 1090MHz for UAS/UAV's stating that it "does not have the capacity for significant growth".  If you would like to understand more, please refer to the following article that explains why we and certain authorities are not pursuing ADSB-out on 1090MHz as the sole frequency for Electronic Conspicuity in future.

How PilotAware detects and processes ADSB.

PilotAware receives the ADSB-out packets directly using an integrated 1090 receiver and immediately decodes the aircraft's position. Because ADSB is a 3D transmission, PilotAware can calculate the aircraft's position in time and space. We call this a 3D target.

Here is an example of what an ADSB target looks like on in-flight applications.

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