What is the difference between ADS-B Out and ADS-B In?

ADS-B Out is the transmit function: the aircraft broadcasts its position, altitude, velocity, identification, and other data. This is the component mandated by the FAA (14 CFR 91.225/91.227, effective January 1, 2020) for all aircraft operating in most controlled airspace. ADS-B In is the receive function: the aircraft receives ADS-B Out transmissions from nearby aircraft and displays them on a cockpit traffic display. ADS-B In also receives Traffic Information Service-Broadcast (TIS-B) and Flight Information Service-Broadcast (FIS-B) data from ground stations. ADS-B In is not mandated but provides significant situational awareness benefits. Together, they enable aircraft-to-aircraft surveillance without requiring ground-based radar.

Why are there two ADS-B frequencies?

1090 MHz Extended Squitter (1090ES) uses the existing Mode S transponder frequency and is the international standard (ICAO). All transport category aircraft, international operations, and aircraft operating above FL180 must use 1090ES. The message format is 112 bits transmitted at 1 Mbps using pulse position modulation. 978 MHz Universal Access Transceiver (UAT) was developed by the FAA as a lower-cost alternative for aircraft operating below 18,000 feet in US airspace only. UAT provides a longer message format (276 or 420 bits) at 1.041 Mbps, enabling FIS-B weather and NOTAM delivery. UAT is not recognized internationally. Ground stations (ADS-B ground-based transceivers) relay data between the two frequencies so that 1090ES and UAT aircraft can see each other.

What RF parameters does ADS-B require?

1090ES: Frequency 1090 MHz, modulation PPM (pulse position modulation), data rate 1 Mbps, transmit power 125 to 500 watts peak (depends on class), antenna gain approximately 0 dBi (omni), update rate 2 per second for airborne position, message length 112 bits (56 microseconds). Receiver sensitivity: minus 84 dBm minimum. Link budget for 150 NM range at FL350: transmit power 250 W (54 dBm), path loss approximately 138 dB at 1090 MHz, receive antenna gain 12 dBi (ground station), received power approximately minus 72 dBm, providing 12 dB margin. UAT: Frequency 978 MHz, modulation CPFSK, data rate 1.041 Mbps, transmit power 10 to 20 watts, antenna gain approximately 0 dBi.

Aviation Surveillance

ADS-B

/ay-dee-ess-bee/ — Automatic Dependent Surveillance-Broadcast

Aircraft broadcasts GPS-derived position, altitude, velocity, ID on 1090 MHz (Mode S Extended Squitter) or 978 MHz (UAT). ADS-B Out: FAA mandated (2020) for controlled airspace. ADS-B In: receives traffic/weather. 1090ES: international (ICAO), 1 Mbps PPM, 125-500 W peak. UAT: US-only below FL180, includes FIS-B weather. Range: 150+ NM. Update: 2/sec. Position: <0.05 NM accuracy.

Freq: 1090 / 978 MHz

Mandate: FAA 2020

Range: 150+ NM

Understanding ADS-B

ADS-B represents the most significant change in air traffic surveillance since radar was introduced in the 1950s. Instead of relying on ground-based radar interrogating aircraft transponders, ADS-B has the aircraft continuously broadcasting its own position derived from GPS. This enables surveillance in areas where radar coverage is impractical or impossible: oceanic airspace, remote regions, and low altitudes.

The RF system is deceptively simple: a transmitter on 1090 or 978 MHz broadcasting short data bursts. But the system's value lies in its ecosystem: ground stations relay data between frequencies, TIS-B fills gaps for non-ADS-B aircraft, and FIS-B delivers graphical weather. The entire NextGen airspace modernization program is built on ADS-B as its surveillance backbone.

ADS-B Link Comparison

Parameter	1090ES	978 UAT	Primary Radar	Secondary Radar	MLAT
Frequency	1090 MHz	978 MHz	2.7-2.9 GHz	1030/1090 MHz	1090 MHz
Range	150+ NM	100 NM	60-250 NM	200 NM	30-50 NM
Update	2/sec	1/sec	6-12 sec	4-12 sec	1 sec
Accuracy	<0.05 NM	<0.05 NM	0.1-0.25 NM	0.1 NM	10-50 m
Coverage	Global	US only	Limited	Moderate	Airport

Common Questions

Frequently Asked Questions

Out vs In?

ADS-B Out: transmit position/velocity/ID. FAA mandated (14 CFR 91.225/227, Jan 2020) in controlled airspace. ADS-B In: receive traffic displays, TIS-B, FIS-B weather. Not mandated but enhances situational awareness. Together: aircraft-to-aircraft surveillance without ground radar.

Two frequencies?

1090ES: Mode S transponder frequency, international (ICAO), required above FL180 and internationally. 112-bit message, 1 Mbps PPM. 978 UAT: FAA lower-cost alternative, US-only below FL180. Longer messages enable FIS-B weather/NOTAMs. Ground stations relay between frequencies for interoperability.

RF parameters?

1090ES: 125-500W peak, PPM, 1 Mbps, −84 dBm sensitivity. 150 NM link budget: 54 dBm TX, 138 dB FSPL, 12 dBi GS antenna = −72 dBm received (12 dB margin). UAT: 978 MHz, CPFSK, 1.041 Mbps, 10-20W, shorter range. Both use omni aircraft antennas (~0 dBi).

Related Terms

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