08How flight trackers know where every plane is

The short version: Most airliners and many small planes broadcast their own GPS position, altitude, speed and identity about once a second, unencrypted, on 1090 MHz (and, for some small US aircraft, 978 MHz). Flight-tracking sites collect those broadcasts from tens of thousands of ground receivers, most of them run by volunteers, and fill gaps by timing older transponder replies across several receivers. Owners can ask the FAA to hide a plane from public sites, but that does not stop the radio signal. In the US, federal law allows anyone to listen to aeronautical radio; the rules differ by country, and the UK's are described as stricter.

What is ADS-B, and why can anyone receive it?

What you see: A flight-tracker app showing a plane over your house with its flight number, altitude and speed, updating every second or so.

ADS-B (Automatic Dependent Surveillance-Broadcast) is the aircraft announcing itself. "Dependent" means it depends on the aircraft's own navigation, and "broadcast" means it is sent to everyone rather than in answer to a question. Under the FAA's description, ADS-B Out carries the aircraft's position, both barometric and GPS altitude, horizontal and vertical velocity, its ICAO aircraft address (tied to the tail number for US aircraft), its Mode A squawk code and a Flight ID such as an N-number or airline call sign.¹²² It updates about once per second, and the FAA's specification calls for no encryption.¹²² The position comes from GPS or WAAS (see GPS), and the equipment must transmit at all times unless the FAA or ATC directs otherwise.¹²³

That openness is why flight tracking exists as a hobby and a business: a cheap receiver hears the same broadcast the FAA hears.

ADS-B In is the receiving side in the cockpit, and it is not required.¹²² Aircraft that have it can get TIS-B traffic information on either link and, on 978 MHz only, free FIS-B weather and flight information.¹²²

What is the difference between 1090 and 978 ADS-B?

What you see: A tracking site listing a plane's data source as "ADS-B" or "UAT," or a pilot shop selling two kinds of box.

The US runs two ADS-B links. 1090ES (1090 MHz Extended Squitter) is the worldwide standard, sent by a Mode S transponder. UAT (Universal Access Transceiver) on 978 MHz is a US system.¹²⁴ Flightradar24 describes UAT as "used primarily for smaller aircraft flying only below 18,000 feet in the United States."¹²⁵

Under 14 CFR 91.225, Class A airspace (above 18,000 ft, about 5,500 m) requires 1090ES, while in the other mandated airspace an aircraft may use either link.¹²⁶ A receiver that listens only on 1090 misses the small planes using 978.

Where is ADS-B Out required in the US?

What you see: Nothing in the sky - but this table is why nearly every plane near a big airport shows up on a tracker.

The rule took effect on January 1, 2020: "After January 1, 2020, unless otherwise authorized by ATC, no person may operate an aircraft" in the covered airspace without compliant equipment.¹²⁶

Source: eCFR, 14 CFR 91.225.¹²⁶ Aircraft certificated without an engine-driven electrical system, such as balloons and gliders, are excepted in some of this airspace.¹²⁶

As of October 1, 2026, the FAA counted 173,663 US civil aircraft equipped with ADS-B Out, 164,573 of them listed as good installs, and 7,200 equipped US air carrier aircraft.¹²⁷

What does a transponder do?

What you see: A plane on a tracker with an altitude but a vague or estimated position, or labeled "MLAT."

Before ADS-B there was secondary surveillance radar, and it is still in use. The FCC's aviation frequency table lists 1030 MHz and 1090 MHz, the latter described among other uses as "Vehicle Squitter."¹²⁴ In plain terms, a ground radar asks on 1030 MHz and the aircraft's transponder answers on 1090 MHz. A Mode S transponder carries a unique address for each aircraft, so the radar can talk to one plane at a time. That address is the ICAO address, and for US aircraft the FAA ties it to the tail number.¹²² The four-digit squawk code assigned by ATC rides along too.¹²²

ADS-B 1090ES reuses that same transponder to send unprompted "extended squitter" messages. Some older civil aircraft and many military aircraft carry only Mode S transponders, which send less data, so trackers have to work out where they are by other means.¹²⁵

In the FAA's air traffic control order (JO 7110.65), Mode 3/A is "designated as the common military/civil mode for air traffic control use," and a Mode C transponder replies with the aircraft's pressure altitude in 100-foot increments.¹²⁸ The same order covers the three emergency codes in the US: 7500 for hijack or unlawful interference, 7600 for loss of two-way radio communications, and 7700 for an emergency.¹²⁸

How does Flightradar24 know where planes are?

What you see: A map full of tiny yellow planes, each one tappable, including over the open ocean.

Flightradar24 (FR24) says its primary source is ADS-B, with multilateration for aircraft that have only Mode S, satellite ADS-B over the oceans (more in satellites), and further inputs from North American and Australian radar data, FLARM and the Open Glider Network, and UAT.¹²⁵ It says it has "more than 50,000 receivers on seven continents and in space" and a database of more than 1.5 million aircraft.¹²⁵ In a blog post from April 2025, updated October 2025, FR24 said it had "recently surpassed 50,000 connected ground stations globally," of which about 7,000 are FR24-supplied kits and the rest are user-built.¹²⁹

FlightAware works the same basic way. In October 2020 it reported 30,000 ADS-B receivers in 193 countries on all seven continents and called it "the largest terrestrial ADS-B network in the world."¹³⁰ Volunteers host the receivers, and the company may give out a free FlightFeeder where it needs coverage.¹³⁰ The "About FlightAware" note on the same page gives no exact count, saying only "tens of thousands" of receivers.¹³⁰

The ground receivers only listen; nothing is sent to the aircraft.

What is MLAT, and why can't I see a plane on Flightradar24?

What you see: A plane you can hear overhead that is missing from the app, or one that appears only once it climbs.

MLAT (multilateration) finds a transmitter by comparing when the same signal reached several receivers. FR24 says it uses Time Difference of Arrival and that "Four FR24-receivers or more, receiving signals from the same aircraft, are needed to make MLAT work."¹³¹ It works only from roughly 3,000-10,000 ft (about 900-3,000 m) and above, because lower down too few receivers can hear the plane.¹³¹

So a plane can be missing for several ordinary reasons:

• It has no ADS-B and is too low, or in too thin a receiver area, for MLAT.¹³¹
• It uses 978 MHz UAT and no nearby receiver listens there.
• Its owner has asked for it to be blocked from public display (see LADD below).¹³²
• It is over the ocean, beyond the reach of ground receivers, where FR24 relies on satellite ADS-B, and it has no ADS-B to hear.¹²⁵

Coverage follows the volunteers, so it thins out wherever few of them live.

What is ADS-B Exchange?

What you see: A tracking site showing military jets and business aircraft that the bigger apps leave off.

ADS-B Exchange was "founded in 2016 ... as a hobby project."¹³³ It describes itself as offering "a transparent view of the skies without censorship or filtering," including military traffic, and like the others it uses MLAT and community-hosted receivers.¹³³ The site's footer links to JETNET's terms.¹³³

The difference is policy, not radio. FR24 and FlightAware honor the FAA's LADD blocking list; ADS-B Exchange describes its view as unfiltered.¹³³

Can owners hide a plane from flight trackers?

What you see: A private jet that shows up as a blank or generic entry, or not at all, on a mainstream tracker.

Two FAA programs exist, and neither switches off the radio.

LADD makes FAA-fed vendors hide a registration from public display. It grew out of BARR, a blocking system that NBAA, the FAA and data vendors began developing in 1997, and was later renamed LADD; NBAA welcomed new FAA privacy measures in a November 4, 2019 press release.¹³⁴ Vendors must block LADD-listed registrations, call signs and flight numbers and show no history for them.¹³⁴

LADD now operates under Section 803 of the 2024 FAA Reauthorization Act, filters data from the FAA's SWIM feed and participating websites, and offers "FAA Source" (FAA use only), "Subscriber Level" and "Unblocking" options, with changes taking effect on the first Thursday of each month.¹³² The FAA is direct about the limit: "The LADD program does not impact the ADS-B broadcast data," which "may be received by off-the-shelf receivers."¹³⁵

PIA (Privacy ICAO Address) assigns a temporary ICAO address that is not linked to the owner in the Civil Aircraft Registry.¹³⁵ To qualify, an aircraft must be US-registered, use 1090 MHz ADS-B, use a third-party call sign and fly in US-managed airspace; dual 1090/UAT aircraft were not eligible.¹³⁵ It costs nothing in the first phase and can be re-requested after 20 days, per the FAA page as updated March 22, 2025.¹³⁵

FlightAware's PIA page gives a 60-day interval for the "initial phase," which differs from the FAA's current figure.¹³⁶ PIA is valid only in US and US-managed airspace, and the aircraft must revert to its registered address when leaving it.¹³⁵

Small planes have one more option. A UAT aircraft below 18,000 ft squawking 1200 (flying under visual rules) can select an "anonymous mode" that broadcasts a randomized address, at the cost of ATC services.¹²² The FAA keeps its own ADS-B surveillance data for at least six months regardless.¹²²

The two sides, as sourced: the business-aviation group NBAA publicly commended the FAA's 2019 privacy measures,¹³⁴ while ADS-B Exchange presents an unfiltered view as transparency.¹³³

What is ACARS?

What you see: Short bursts of digital noise on a scanner around 129-131 MHz, or a website listing text messages from airliners.

ACARS is text messaging between aircraft and their airlines, and increasingly air traffic control. ARINC developed it, and it has carried routine pilot-to-airline messages since 1978.¹³⁷ VHF ACARS runs mainly on 129.125 and 131.550 MHz with roughly 200 nm (370 km) line-of-sight range; HF datalink covers 2-22 MHz, and satellite links use Inmarsat around 1.5 GHz and Iridium around 1.6 GHz.¹³⁷ The FCC table lists 131.450, 131.550, 131.725, 131.825 and 136.850 MHz for datalink.¹²⁴ Oceanic controller messaging (CPDLC) and position reporting (ADS-C) ride over ACARS in remote airspace.¹³⁷

The faster successor is VDL Mode 2, which uses twelve 25 kHz channels from 136.700 to 136.975 MHz, with 136.975 MHz as the common signaling channel. It runs at 31.5 kbit/s raw, against 2.4 kb/s for plain VHF ACARS.¹³⁸ Europe's Single European Sky rule (January 2009) required new aircraft flying in Europe after January 1, 2014 to carry CPDLC (Controller-Pilot Data Link Communications), with VDL Mode 2 as the primary link.¹³⁸

US law does not name ACARS; the relevant text is the federal wiretap law's exemption for "any marine or aeronautical communications system," covered below.¹³⁹ Elsewhere, check the national regulator's rules.

What frequency is air traffic control?

What you see: A scanner or a streaming site playing clipped, fast voices: "Delta 1234, contact departure."

Pilots and controllers talk in plain voice on the VHF airband. The US frequency table assigns air traffic control and related services in 25 kHz steps across these ranges:¹²⁴

Sources: 47 CFR 87.173, Airframes.io and Wikipedia's VHF Data Link article.^124137138

Airband voice uses AM rather than FM. AM does not suffer from FM's "capture effect," so when two stations transmit at once, listeners hear a garbled mix of both instead of only the stronger signal.¹⁴⁰ Europe has also split the standard 25 kHz channels into three, giving 8.33 kHz spacing.¹⁴⁰ EUROCONTROL says Europe has "a known shortage of voice communication frequencies," and under EU Implementing Regulation 1079/2012 aircraft without 8.33 kHz-capable radios may not be able to operate in EU member states' controlled airspace from January 1, 2018.¹⁴¹

Is it legal to listen to air traffic control?

What you see: LiveATC-style streams of tower audio, or a scanner tuned to the local approach frequency.

In the US, yes. The federal wiretap statute says it is not unlawful to intercept radio communications from stations "for the use of the general public," those relating to "ships, aircraft, vehicles, or persons in distress," and those "transmitted by any marine or aeronautical communications system."¹³⁹ That is the legal footing for scanner listeners and for sites that stream tower audio. State scanner rules and police radio are separate questions, covered in scanners.

In the UK the picture is stricter, though the sources are less direct. A 2012 Parliament petition, citing the Wireless Telegraphy Apparatus (Receivers) (Exemption) Regulations 1989, stated that "It is an offence to listen to any other radio services unless you are authorised by a designated person to do so."¹⁴² That is a petitioner's statement, not an official ruling; the petition drew 96 signatures and no government response.¹⁴² Ofcom's 2014 license-exemption notice states the general offense "to establish, install or use equipment for wireless telegraphy without holding a licence" but says nothing specific about receive-only scanning.¹⁴³

Section 48 of the UK's Wireless Telegraphy Act 2006 makes it an offense, "without lawful authority," to use wireless telegraphy apparatus "with intent to obtain information as to the contents, sender or addressee" of a message of which neither the listener nor anyone they act for is an intended recipient, or to disclose such information.¹²⁰ The penalty on summary conviction is a fine up to level 5 on the standard scale.¹²⁰ The section does not mention scanners by name.

Elsewhere in the world

What you see: The same flight-tracker apps working in Europe, but with no 978 MHz traffic.

Outside the US, ADS-B means 1090ES; UAT is a US system.¹²⁵ Europe's rule is EU Regulation 1207/2011, as amended, and it was carried into UK law after Brexit; legislation.gov.uk shows the retained text, with the airspace wording in paragraph 5 substituted from December 31, 2020.¹⁴⁴

Sources disagree on the date. OPSGROUP, writing in November 2019, listed "June 7, 2020 - European ADS-B Out Mandate" for IFR aircraft above 5,700 kg or 250 kt, requiring a GPS source coupled to a 1090ES transponder.¹⁴⁵ The current text of Article 5 says that by December 7, 2020 operators must have serviceable transponders, with Parts A and B applying to aircraft over 5,700 kg (about 12,600 lb) maximum takeoff mass or 250 kt (about 460 km/h) cruise speed first certificated on or after June 7, 1995, and fixed-wing aircraft also meeting Part C.¹⁴⁴

Operators with a retrofit program in place before that date had until June 7, 2023, and exemptions cover aircraft flying for maintenance or export or retiring by October 31, 2025, and up to 3 consecutive days with an inoperative transponder.¹⁴⁴

The main difference from the US: Europe's rule keys on aircraft weight and speed, while the US rule keys on airspace.

In Boston: why do planes fly low over certain neighborhoods?

What you see: A steady line of jets climbing out over Cambridge or Somerville, or arriving low over the harbor, on some days and not others.

Logan International, run by Massport, sits on Boston Harbor 2.5 miles (4 km) northeast of Back Bay on 2,384 acres (965 ha).¹⁴⁶ It has six runways:¹⁴⁶

Planes are low near an airport because they are lined up on final approach or have taken off along a fixed procedure, and which runway is in use mostly follows the wind. Runway 14/32 shows the wind's role plainly: it opened November 23, 2006, after being proposed in 1973, runs one way only (landings on 32, takeoffs on 14), is court-limited over land, and operates only in a strong northwest wind; residents want at least 11.5 kt and Massport favors 10 kt.¹⁴⁶ Chelsea, East Boston, Winthrop and Revere residents opposed it.¹⁴⁶

The bigger change came from RNAV, area navigation along precise GPS-defined paths. The FAA's Environmental Assessment for the runway 33L RNAV departure came out in May 2013.¹⁴⁷ Cambridge Day reported that RNAV began in June 2013, while the City of Cambridge says the procedures were implemented in 2014.¹⁴⁸¹⁴⁷

RNAV was introduced "to make flights more efficient and reduce fuel burn," and it produced "a higher concentration of flights in a smaller cluster of neighborhoods," in Cambridge Day's words.¹⁴⁸ Departures from 33L became "more concentrated over parts of north and west Cambridge," according to the city.¹⁴⁷ People under that narrow track hear more flights even if the total number has not changed.

Citing Massport data, Cambridge Day reported complaints from Medford rising from 0 in 2013 to 7,861 in 2017, Somerville from 162 to 3,840, Arlington from 6 to 2,225, and Cambridge from 263 in 2013 into the thousands by 2016.¹⁴⁸

The FAA, Massport and MIT then studied ways to spread the noise more evenly.¹⁴⁷ A 2021 MIT proposal added a departure leg over central Somerville and Porter Square, and Cambridge and most communities on the Massport Community Advisory Committee, a body created by the state legislature, voted against it.¹⁴⁷ Each proposed fix moves some of the noise somewhere else, which helps explain why proposals have drawn opponents.

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