Automatic Dependent Surveillance–Broadcast

ADS-B is an airspace surveillance system which could eventually replace secondary surveillance radar as the main surveillance method for controlling aircraft worldwide. In the United States ADS-B is an integral component of the NextGen national airspace strategy for upgrading and enhancing aviation infrastructure and operations. Other uses of ADS-B data include post-flight analysis, inexpensive flight tracking, planning, and dispatch. ADS-B consists of two distinct functions - "ADS-B Out" and "ADS-B In". Each "ADS-B Out" aircraft periodically broadcasts information about itself, such as identification, current position, altitude and velocity through an onboard transmitter. ADS-B Out provides air traffic controllers with real-time aircraft position information that is, in most cases, more accurate than the information available with current radar-based systems. With more accurate information, ATC can manage and separate aircraft with improved precision and timing. The ADS-B Out system relies on two avionics components aboard each aircraft: a high-integrity satellite navigation source (i.e. GPS or other certified GNSS receiver) and a datalink (the ADS-B unit). There are several types of certified ADS-B data links, but the most common ones operate at 1090 MHz, essentially a modified Mode S transponder, or at 978 MHz. To obtain ADS-B Out capability at 1090 MHz, user-operators can install a new transponder or modify an existing transponder if the manufacturer offers an ADS-B upgrade (plus install a certified GNSS position source if one is not already present). ==Benefits==

ADS-B provides many benefits to both pilots and air traffic control that improve both the safety and efficiency of flight. When using an ADS-B In system, a pilot is able to view traffic information about surrounding aircraft if those aircraft are equipped with ADS-B Out. This information includes altitude, heading, speed, and distance to the aircraft. In addition to receiving position reports from ADS-B Out participants, in the US, TIS-B can provide position reports on non-ADS-B Out-equipped aircraft if suitable ground equipment and ground radar exist. ADS-R re-transmits ADS-B position reports between UAT and 1090 MHz frequency bands. Aircraft equipped with universal access transceiver (UAT) ADS-B In technology will be able to receive weather reports, and in the US, weather radar through flight information service-broadcast (FIS-B), which also transmits readable flight information such as temporary flight restrictions (TFRs) and NOTAMs. ADS-B ground stations are significantly cheaper to install and operate compared to primary and secondary radar systems used by air traffic control for aircraft separation and control. Unlike some alternative in-flight weather services currently being offered commercially, there will be no subscription fees to use ADS-B services or its various benefits in the US. The aircraft owner will pay for the equipment and installation, while the Federal Aviation Administration (FAA) will pay for administering and broadcasting all the services related to the technology. Safety Situational awareness ADS-B makes flying significantly safer for the aviation community by providing pilots with improved situational awareness. Pilots in an ADS-B In equipped cockpit will have the ability to see, on their in-cockpit flight display, other traffic operating in the airspace and have access to clear and detailed weather information. They will also be able to receive pertinent updates ranging from temporary flight restrictions to runway closings. Improved visibility Even aircraft only equipped with ADS-B Out will benefit from air traffic controllers' ability to more accurately and reliably monitor their position. When using this system both pilots and controllers will see the same radar picture. Other fully equipped aircraft using the airspace around them will be able to more easily identify and avoid conflict with an aircraft equipped with ADS-B Out. With past systems such as the Traffic alert and Collision Avoidance System (TCAS) aircraft could only see other aircraft equipped with the same technology. With ADS-B, information is sent to aircraft using ADS-B In, which displays all aircraft in the area, provided those aircraft are equipped with ADS-B Out. ADS-B provides better surveillance in fringe areas of radar coverage. ADS-B does not have the siting limitations of radar. Its accuracy is consistent throughout the range. In both forms of ADS-B (1090ES & 978 MHz UAT), the position report is updated once per second. The 978 MHz UAT provides the information in a single, short-duration transmission. The 1090ES system transmits two different kinds of position reports (even/odd) randomly. To decode the position unambiguously, one position report of both kinds or a reference position nearby is needed. ADS-B enables improved safety by providing: • Radar-like IFR separation in non-radar airspace • Increased VFR flight following coverage • ATC final approach and runway occupancy, reducing runway incursions on the ground • More accurate search and rescue response: • Helps pilots to see and avoid other aircraft • Cockpit final approach and runway occupancy • Visual separation in VFR and MVFR conditions • VFR-like separation in all weather conditions • Real-time cockpit weather display • Real-time cockpit airspace display Efficiency Reduced environmental impact ADS-B technology provides a more accurate report of an aircraft's position. This allows controllers to guide aircraft into and out of crowded airspace with smaller separation standards than it was previously possible to do safely. This reduces the amount of time aircraft must spend waiting for clearances, being vectored for spacing and holding. Estimates show that this is already having a beneficial impact by reducing pollution and fuel consumption. Traffic capacity improvement ADS-B enables increased capacity and efficiency by supporting: • Better ATC traffic flow management • Merging and spacing • Self-separation or station keeping • Enhanced visual approaches; • Closely spaced parallel approaches; • Reduced spacing on final approach; • Reduced aircraft separations; • Enhanced operations in high altitude airspace for the incremental evolution of the "free flight" concept; • Surface operations in lower visibility conditions; • Near visual meteorological conditions (VMC) capacities throughout the airspace in most weather conditions; • Improved air traffic control services in non-radar airspace; • Trajectory-based operations provide a gently ascending and descending gradient with no step-downs or holding patterns needed. This will produce optimal trajectories with each aircraft becoming one node within a system-wide information management network connecting all equipped parties in the air and on the ground. With all parties equipped with NextGen equipment, benefits will include reduced gate-to-gate travel times, increased runway utilization capacity, and increased efficiency with carbon conservation. Other applications The ADS-B data link supports a number of airborne and ground applications. Cockpit display of traffic information A cockpit display of traffic information (CDTI) is a generic display that provides the flight crew with surveillance information about other aircraft, including their position. Traffic information for a CDTI may be obtained from one or multiple sources, including ADS-B, TCAS, and TIS-B. In addition to traffic based on ADS-B reports, a CDTI function might also display current weather conditions, terrain, airspace structure, obstructions, detailed airport maps, and other information relevant to the particular phase of flight. Airborne collision avoidance ADS-B is seen as a valuable technology to enhance airborne collision avoidance system (ACAS) operation. Eventually, the ACAS function may be provided based solely on ADS-B, without requiring active interrogations of other aircraft transponders. Other applications that may benefit from ADS-B include: • Lighting control automation and operation • Airport ground vehicle and aircraft rescue and firefighting vehicle operational needs • Altitude height keeping performance measurements • General aviation operations control • Conflict management • ATS conformance monitoring • Aircraft spotting where personal receivers can be used to produce a virtual radar picture • A number of websites use crowd-sourced distributed networks of ADS-B receivers to track air traffic. ==Security risk==

Aircraft with transponder only, or no transponder capability at all will not be shown. Pilots who become complacent or overconfident in this system are thus a safety problem, not only for themselves but for other transponder-only aircraft, and glider aircraft without ADS-B transponder. Glider aircraft often use the FLARM system for collision avoidance with other glider aircraft, but this system is not compatible with ADS-B. Aircraft with ADS-B but without FLARM are thus a safety risk for gliders with FLARM but without ADS-B and vice versa. Some aircraft, like those used for towing gliders, have both FLARM and ADS-B transponders for this reason. A security researcher claimed in 2012 that ADS-B has no defence against being interfered with via spoofed ADS-B messages because they were neither encrypted nor authenticated. The FAA responded to this criticism saying that they were aware of the issues and risks but were unable to disclose how they are mitigated as that is classified. A possible mitigation is multilateration to verify that the claimed position is close to the position from which the message was broadcast. Here the timing of received messages is compared to establish distances from the antenna to the plane. The lack of any authentication within the standard makes it mandatory to validate any received data by use of the primary radar. Because the content of ADS-B messages is not encrypted, it may be read by anybody. dongle. These signals are not encrypted. Inexpensive hardware and free software can be used to display the speed, course, altitude, callsign and identification of an aircraft equipped with an ADS-B transponder. ==Theory of operation==

The ADS-B system has three main components: 1) ground infrastructure, 2) airborne component, and 3) operating procedures. The source of the state vector and other transmitted information as well as user applications are not considered to be part of the ADS-B system. Physical layer Two link solutions are used as the physical layer for relaying ADS-B position reports: universal access transceiver, and 1090 MHz extended squitter. Universal Access Transceiver (UAT) A universal access transceiver is a data link on the frequency 978 MHz. UAT is intended to serve the majority of the general aviation community in the United States. The data link is approved in the Federal Aviation Administration's "final rule" for use in all airspace except class A (above 18,000 ft. MSL). UAT is intended to support not only ADS-B, but also flight information service – broadcast (FIS-B), traffic information service – broadcast (TIS-B), and, if required in the future, supplementary ranging and positioning capabilities. Due to the set of standards required for this rule, it is seen as the most effective application for general aviation users. UAT will allow aircraft equipped with "out" broadcast capabilities to be seen by any other aircraft using ADS-B In technology as well as by FAA ground stations. Aircraft equipped with ADS-B In technology will be able to see detailed altitude and vector information from other ADS-B Out equipped aircraft as well as FIS-B and TIS-B broadcasts. The FIS-B broadcast will allow receiving aircraft to view weather and flight service information including AIRMETs, SIGMETs, METARs, SPECI, national NEXRAD, regional NEXRAD, D-NOTAMs, FDC-NOTAMs, PIREPs, special use airspace status, terminal area forecasts, amended terminal aerodrome forecasts (TAFs), and winds and temperatures aloft forecasts. These broadcasts serve to provide early adopters of the technology with benefits as an incentive for more pilots to use the technology before 2020. Aircraft receiving traffic information through the TIS-B service will see other aircraft in a manner that is similar to how all aircraft will be seen after they have been equipped by 2020. The availability of a non-subscription weather information service, FIS-B, provides general aviation users with a useful alternative to other monthly or annual fee-based services. The UAT system is specifically designed for ADS-B operation. UAT is also the first link to be certified for "radar-like" ATC services in the United States. Since 2001 it has been providing en-route separation (the same as mosaic radar but not of single-site sensors) in Alaska. UAT is the only ADS-B link standard that is truly bidirectional: UAT users have access to ground-based aeronautical data (FIS-B) and can receive reports from proximate traffic (TIS-B) through a multilink gateway service that provides ADS-B reports for 1090ES-equipped aircraft and non-ADS-B equipped radar traffic. UAT-equipped aircraft can also observe each other directly with high accuracy and minimal latency. Viable ADS-B UAT networks are being installed as part of the United States' NextGen air traffic system. 1090 MHz extended squitter In 2002 the Federal Aviation Administration (FAA) announced a dual-link decision using the 1090 MHz extended squitter (1090 ES) link for air carrier and private or commercial operators of high-performance aircraft, and universal access transceiver link for the typical general aviation user. The format of extended squitter messages has been codified by the ICAO.) and aircraft position and other status information is routed to the transponder. ATC ground stations and aircraft equipped with traffic collision avoidance system (TCAS) already have the necessary 1090 MHz (Mode S) receivers to receive these signals, and would only require enhancements to accept and process the additional extended squitter information. As per the FAA ADS-B link decision, and the technical link standards, 1090 ES does not support FIS-B service. (However, the signal strength of the signal received from the aircraft at the ground station is still dependent on the range from the aircraft to the receiver, and interference, obstacles, or weather could degrade the integrity of the received signal enough to prevent the digital data from being decoded without errors. When the aircraft is farther away, the weaker received signal will tend to be more affected by the aforementioned adverse factors and is less likely to be received without errors. Error detection will allow errors to be recognized, so the system maintains full accuracy regardless of aircraft position when the signal can be received and decoded correctly. This advantage does not equate to a total indifference to the range of an aircraft from the ground station.) Today's air traffic control (ATC) systems do not rely on coverage by a single radar. Instead, a multi-radar picture is presented via the ATC system's display to the controller. This improves the quality of the reported position of the aircraft, provides a measure of redundancy, and makes it possible to verify the output of the different radars against others. Relationship to ADS-A/ADS-C There are two commonly recognized types of ADS for aircraft applications: • ADS-addressed (ADS-A), also known as ADS-Contract (ADS-C) • ADS-broadcast (ADS-B) ADS-A is based on a negotiated one-to-one peer relationship between an aircraft providing ADS information and a ground facility requiring receipt of ADS messages. For example, ADS-A reports are employed in the Future Air Navigation System (FANS) using the Aircraft Communications Addressing and Reporting System (ACARS) as the communication protocol. During a flight over areas without radar coverage, e.g., oceanic and polar, reports are periodically sent by an aircraft to the controlling air traffic region., Traffic information service – broadcast (TIS–B) Traffic information service – broadcast (TIS–B) supplements ADS-B's air-to-air services to provide complete situational awareness in the cockpit of all traffic known to the ATC system. TIS–B is an important service for an ADS-B link in airspace where not all aircraft are transmitting ADS-B information. The ground TIS–B station transmits surveillance target information on the ADS-B data link for unequipped targets or targets transmitting only on another ADS-B link. TIS–B uplinks are derived from the best available ground surveillance sources: • multilateration systems for targets on the airport surface • ADS-B systems for targets equipped with a different ADS-B link Another potential aircraft-based broadcast capability is to transmit aircraft measurements of meteorological data. ==Implementations by country==

Australia Airservices Australia operates more than 70 ADS-B ground receiver sites and Australia has full continental ADS-B  coverage above FL300 (30,000 feet). ADS-B equipment is mandatory for all aircraft flying at or above this altitude. ADS-B equipment is also mandatory for all IFR aircraft at any altitude. This applies to all Australian registered aircraft. Canada Nav Canada commissioned operational use of ADS-B in 2009 and is now using it to provide coverage of its northern airspace around Hudson Bay, which previously had almost no radar coverage. The service was then extended to cover some oceanic areas off the east coast of Canada including the Labrador Sea, Davis Strait, Baffin Bay, and part of the North Atlantic Tracks around southern Greenland. The service was later extended to cover the Canadian Arctic, and to the rest of Canada. proposing a mandate for Aireon-compatible ADS-B Out for all aircraft in Class A Airspace by 2021 and Class B Airspace, to be completed by 2022, requiring a transponder capable of delivering antenna diversity performance. In response to stakeholder feedback, Nav Canada later announced that such equipment will not be mandated according to that timeline, but rather that suitably equipped aircraft would be handled on a priority basis. The dates upon which equipment are required for operation in Canadian airspace were announced to be 10 August 2023 for Class A Airspace, 16 May 2024 for Class B Airspace, and no sooner than 2026 for Class C, D, and E Airspace. In May 2021, the not-for-profit Canadian In-Flight Information Broadcasting Association (CIFIB) announced its intention to build and operate a network broadcasting FIS-B (Flight Information Services - Broadcast)(weather) and TIS-B (Traffic Information Services - Broadcast)(traffic) information on 978 MHz UAT. By spring 2022, five ground stations were operating in Ontario, and more are currently being built, with the goal of the network covering areas with higher traffic. As of 2025, twelve stations operate in four provinces. The Canadian network is fully compatible with the United States network, and aircraft can use the same UAT (ADS-B In) receivers in both countries and the provided services operate seamlessly when crossing the border. China An American company ADS-B Technologies created one of the largest and most successful ADS-B systems in the world (an eight-station, 350+ aircraft network that spans more than 1,200 NM across Central China). This was also the first UAT installation outside the United States. As of March 2009, more than 1.2 million incident/failure-free flight hours had been flown with these ADS-B systems. In 2024, the Ministry of State Security accused that ADS-B receivers deployed by foreign organizations constitute a national security risk. Authorities had mounted clampdowns on enthusiasts who installed ADS-B receivers. Iceland As of 2010, Isavia is in the process of installing ADS-B across the North Atlantic Ocean. The system is made up by 18 ADS-B receiver stations in Iceland, Faroe Islands, and Greenland. India The Airports Authority of India (AAI), which manages the country's airspace, first commissioned German company Comsoft to install ADS-B ground stations at 14 airport sites nationwide in 2012. Comsoft finished installing seven new ADS-B ground stations under a second phase of deployment which India subsequently integrated into its ATC system in 2014, thus completing its ground network for automatic dependent surveillance-broadcast (ADS-B) tracking of aircraft. In line with the International Civil Aviation Organization's aviation system block upgrade plan, AAI has said that its ADS-B network will provide redundant, satellite-based surveillance where radar coverage exists, fill gaps in surveillance where radar coverage is not possible due to high terrain or remote airspace and enable it to share ADS-B data with neighboring countries. The network covers the Indian subcontinent, plus parts of the Bay of Bengal and the Arabian Sea. Sweden LFV Group in Sweden has implemented a nationwide ADS-B network with 12 ground stations. Installation commenced during the spring of 2006, and the network was fully (technically) operational in 2007. An ADS-B–supported system was opened in Spring 2009, and is currently operational in Kiruna, Sweden. Based on the VDL Mode 4 standards, the network of ground stations can support services for ADS-B, TIS-B, FIS-B, GNS-B (DGNSS augmentation) and point-to-point communication, allowing aircraft equipped with VDL 4-compliant transceivers to lower fuel consumption and reduce flight times. United Arab Emirates United Arab Emirates commissioned three operational redundant ADS-B ground stations in early 2009 and is now using ADS-B to provide enhanced coverage of its upper airspace in combination and integrated with conventional surveillance radars. United States To reduce congestion and cope with growing aircraft traffic, the Federal Aviation Administration has been developing the Next-Generation Air Transportation System (NextGen), including ADS-B. ADS-B equipment is built to meet one of two sets of industry standards, DO-260B and DO-282B. Aircraft operating in the United States in the airspace classes listed below are required to carry equipment that produces an ADS-B Out broadcast. ADS-B Out broadcasts information about an aircraft through an onboard transmitter to a ground receiver, moving air traffic control from a radar-based system to a satellite-derived aircraft location system. The FAA airspace requirements intentionally exclude some airspace that is frequently used by general aviation. The US Drug Enforcement Agency's aircraft and (other airborne US government surveillance) use the ADS-B method to identify in air while conducting operations. Operators can choose the 1090 megahertz extended squitter broadcast link, or the universal access transceiver broadcast link. FAA did not adopt higher performance standards that would enable all of the initial ADS-B In applications, but these can optionally be adopted. FAA forecasts an increase in the GA fleet from 224,172 aircraft in 2010 to 270,920 aircraft in 2031, growing an average of 0.9% per year. Funding resources In April 2011, US federal legislation via House Bill for FAA reauthorization permits an "equipping fund" that includes a portion for some general aviation aircraft. The fund would provide financing at competitive rates backed by loan guarantees. A public-private partnership has been formed as the NextGen Equipage Fund, LLC which is managed by NEXA Capital Partners, LLC. US implementation timetable The Federal Aviation Administration ADS-B implementation was broken into three segments each with a corresponding timeline. Ground segment implementation and deployment began in 2009 and was completed by 2013. Airborne equipment is user-driven and is expected to be completed both voluntarily based on perceived benefits and through regulatory actions (Rulemaking) by the FAA. The cost to equip with ADS-B Out capability is relatively small and would benefit the airspace with surveillance in areas not currently served by radar. The FAA intends to provide similar service within the NAS to what radar is currently providing (5 nmi en route and 3 nmi terminal radar standards) as a first step to implementation. However, ADS-B In capability is viewed as the most likely way to improve NAS throughput and enhance capacity. In December 2008, Acting FAA Administrator Robert A. Sturgell gave the go-ahead for ADS-B to go live in southern Florida. The south Florida installation, which consists of 11 ground stations and supporting equipment, is the first commissioned in the United States, although developmental systems have been online in Alaska, Arizona, and along the East Coast since 2004. The completed system will consist of 794 ground station transceivers. The December 2008 action is in compliance with a late-term executive order from George W. Bush which mandated accelerated approval of NextGen. FAA segment 1 (2006–09) ADS-B deployment and voluntary equipment, along with rule-making activities. Pockets of development will exploit equipment deployment in the areas that will provide proof of concept for integration to ATC automation systems deployed in the NAS. It is being developed at the FAA's William J. Hughes Technical Center near Egg Harbor City, New Jersey. • Oceanic In-trail – ADS-B may provide enhanced situational awareness and safety for Oceanic In-trail maneuvers as additional aircraft become equipped. • Gulf of Mexico – in the Gulf of Mexico, where ATC radar coverage is incomplete, the FAA is locating ADS-B (1090 MHz) receivers on oil rigs to relay information received from aircraft equipped with ADS-B extended squitters back to the Houston Center to expand and improve surveillance coverage. On 14 June 2012, FreeFlight Systems and Chevron received STC for the first rule-compliant ADS-B installation in GOMEX helicopters that was awarded by the FAA. Early adopters ; Cargo Airline Association: Cargo carriers, notably UPS. With the addition of the Garmin G1000 flight instrument system to their fleet in 2006, ERAU became the first fleet to combine a glass cockpit with ADS-B. ; University of North Dakota: UND has received an FAA grant to test ADS-B and has begun to outfit their Piper Warrior fleet with an ADS-B package. Privacy The FAA in America has come up with two systems to address privacy concerns • Limiting Aircraft Data Displayed (LADD): Aircraft owners or designated representatives may request limiting aircraft data displayed (formally referred to as blocking) or unblocking flight tracking data. Flight tracking services that draw the data from FAA agree to block the information, though as noted elsewhere in this article, the data are unencrypted and may be available from non-FAA sources. • Privacy ICAO Aircraft (PIA): Program to improve the privacy of eligible aircraft by enabling aircraft owners to request an alternate, temporary ICAO aircraft address, which will not be assigned to the owner in the Civil Aviation Registry (CAR). These are used in conjunction with a third party call sign: DCM (Garmin/Fltplan.com), FFL (ForeFlight), FWR (FlightAware), XAA (ARINCDirect/Collins Aerospace). ==System design considerations==

A concern for any ADS-B protocol is the capacity for carrying ADS-B messages from aircraft, as well as allowing the radio channel to continue to support any legacy services. For 1090 ES, each ADS-B message is composed of a pair of data packets. The greater the number of packets transmitted from one aircraft, the fewer aircraft that can participate in the system, due to the fixed and limited channel data bandwidth. System capacity is defined by establishing a criterion for what the worst environment is likely to be, then making that a minimum requirement for system capacity. For 1090 ES, both TCAS and ATCRBS/MSSR are existing users of the channel. 1090 ES ADS-B must not reduce the capacity of these existing systems. The FAA national program office and other international aviation regulators are addressing concerns about ADS-B non-secure nature of ADS-B transmissions. ADS-B messages can be used to know the location of an aircraft, and there is no means to guarantee that this information is not used inappropriately. Additionally, there are some concerns about the integrity of ADS-B transmissions. ADS-B messages can be produced, with simple, low-cost measures, which spoof the locations of multiple phantom aircraft to disrupt safe air travel. There is no foolproof means to guarantee integrity, but there are means to monitor this type of activity. This problem is however similar to the usage of ATCRBS/MSSR where false signals also are potentially dangerous (uncorrelated secondary tracks). There are some concerns about ADS-B dependence on satellite navigation systems to generate state vector information. There are some general aviation concerns that ADS-B removes anonymity of the VFR aircraft operations. In order for the ADS-B system to function to the fullest extent, equipment for all aircraft in the airspace is required. This demands that transponder technology be scalable from the smallest aircraft to the largest aircraft to allow for 100% equipage for any given airspace. Current transponder technology is capable of equipping larger, traditional aircraft but a new type of transponder is required for equipping aircraft that are smaller and lighter or don't have electrical systems like the large traditionally transponder-equipped aircraft. The requirements for these smaller and lighter aircraft are mainly size, weight, and power (SWAP) and transponder technology must allow for equipage of these types of aircraft to enable saturation of ADS-B for total visibility in any given airspace. On 7 June 2002 the FAA published a historical overview of its decision on the ADS-B link architecture for use in the National Airspace System (NAS). ==Technical and regulatory documents==

• Minimum Aviation System Performance Standards (MASPS) • Minimum Operational Performance Standards (MOPS) • RTCA DO-242A — ADS-B MASPS. Describes system-wide operational use of ADS-B. ==Satellite (space-based) ADS-B collection==

A significant step forward for ADS-B is the reception by artificial satellites of the ADS-B signal. It was tested for the first time in 2013 on ESA's PROBA-V and it is being deployed by companies like Spire Global using low-cost nanosatellites. Aireon is also working on space-based ADS-B with the Iridium satellite network, a LEO (Low Earth Orbit) satellite network that was originally created to deliver phone and data service anywhere on the planet. By capturing ADS-B position data from aircraft flying below the satellite, the network will give the following capabilities: • Air traffic control using surveillance-based separation standards will be possible over water, in areas that radar does not currently cover. Currently, air traffic control uses the larger procedural separation standard in oceanic and remote areas. • As is currently possible in radar-covered areas, a position history will be available for lost aircraft, as in the case of Malaysia Airlines Flight 370. The system only receives ADS-B on aircraft broadcasting on the 1090 MHz frequency. This limits the system generally to airliners and business aircraft, despite the fact that small aircraft are frequently off the radar due to mountains blocking the signal at low altitudes. The system could be compromised by smaller, private aircraft with exclusive belly-mounted ADS-B antennas, due to the aircraft fuselage blocking the signal. The rationale for using the Iridium satellite network for this new capability was due to: • The Iridium satellites fly very low and thus can receive the ADS-B out signals more reliably (transponders and ADS-B were designed for ground reception). • Iridium satellites are replaced relatively frequently due to the increased air friction at their lower altitude, and thus lower lifespan. Thus the system would be deployed on iridium faster. • Iridium provides worldwide coverage, including the poles. In September 2016, Aireon and FlightAware announced a partnership to provide this global space-based ADS-B data to airlines for flight tracking of their fleets and, in response to Malaysia Airlines Flight 370, for compliance with the ICAO Global Aeronautical Distress and Safety System (GADSS) requirement for airlines to track their fleets. In December 2016, Flightradar24 entered an agreement with Gomspace for space-based tracking in 2016. SpaceX subsequently placed 66 operational and 9 spare Iridium satellites in orbit over the course of 8 launches between 14 January 2017 and 11 January 2019. Another 6 spare satellites remain on the ground. The ICAO describes space-based ADS-B as a technology equalizer, offering developing nations an airspace surveillance capability. By 2020, 34 nations will deploy the system, including the 17 members of the Asecna in Africa, and the Cocesna air navigation services agency in Central America. More frequent updates in the North Atlantic Tracks allowed reducing longitudinal separation from and lateral separations from . The FAA plans an evaluation in the Caribbean airspace from March 2020 until 2021, to complement the unreliable Grand Turk Island radar which allows reducing separation from . ==See also==