What is ADS-B

Aircraft · 7 min read · Jan 03, 2022
Automatic Dependent Surveillance-Broadcast

ADS-B is an acronym that stands for Automatic Dependent Surveillance-Broadcast. This technology helped aviation evolve from radars to satellites for aircraft location. This is why it has been taken as the primary technology to support the FAA’s Next Generation Air Transportation System known as NextGen.

How the ADS-B works can be easily inferred from the acronym meaning. As the FAA defines, ADS-B is automatic since it does not require any human interaction to transmit the information, dependent because the position and velocity vectors are derived from the Global Positioning System (GPS) or other suitable Navigation Systems (i.e., FMS), surveillance refers to providing a method to determine the 3-dimensional position and identify the aircraft, vehicles, or other assets.

Finally, the word broadcast at the end refers to the transmission of the information, which is made available to anyone with the corresponding ADS-B receivers. There are two forms of ADS-B, so let’s take a look at each of them here below.

ADS-B out system, ADS-B In equipped aircraft and ground stations

ADS-B Out is the form of ADS-B that actually replaces radar technology, as it can broadcast the required information regarding an aircraft’s GPS location, altitude, ground speed, and any other relevant data to ground stations and other aircraft. One of the most important aspects where ADS-B excels over radar technology is that the information is broadcast every second instead of every 5 to 12 seconds.

Another aspect to highlight is the way they broadcast the information. Radars depend on radio waves that are limited to line of sight. In other words, obstacles such as mountains and other solid objects pose a problem for this type of wave, and long distances are not possible. However, the whole ADS-B ecosystem improves this situation.

ADS-B uses the 1090MHz frequency by means of burst transmissions sent periodically by the Mode S transponder. The 1090MHz is the frequency approved for transmitting data around the world.

However, in some airspace, it is necessary to use another frequency called the Universal Access Transceiver (UAT) frequency, which is at 978MHz. This is especially useful within the United States because of the high volume of aircraft flying simultaneously, and the UAT is applied to airspace users flying under 18,000 ft MSL on properly equipped aircraft.

These transmissions are called squatters, and they are sent from the aircraft equipped with an ADS-B Out system. Aircraft operating in most controlled U.S. airspace must be equipped with ADS-B Out.

The data is then received by any aircraft equipped with ADS-B In either directly or relayed by ground stations, increasing the pilot’s situational awareness. Air traffic controllers also receive the information via ADS-B ground stations called MLAT stations.

A flight tracking radar showing some information that would be visible in an ADS-B In system.

ADS-B ground stations and Air traffic control

ADS-B ground stations provide great flexibility and versatility to the system because they are smaller and more adaptable than radar towers. Therefore, they can be placed in many locations that could not cover with radar technology. By covering the territory of the whole country, including areas that may be difficult to reach, the ADS-B technology enables a higher level of visibility no matter the ground formations or any other obstacles.

When the data is received by the ADS-B ground station, it is displayed for the controllers by means of the air traffic management system (ATM). Then the controllers use it to keep flying aircraft within safe separation distance over the corresponding airspace.

ADS-B In equipped aircraft

It can be very critical for a pilot to have its aircraft equipped with both ADS-B Out and ADS-B In. It is not only important to send the information so air traffic controllers can trace the flight at all times, make any adjustments to the route if necessary, and know where to send the help in case of emergency.

Also, it is important to monitor the information sent from controllers and other aircraft for the pilot to be even more aware of what is going on in the airspace regarding other flights, traffic information, and the weather. This way, the pilot can take any measures deemed necessary for the safety of the flight.

ADS-B In-equipped aircraft can gain access to the graphical weather displays in the cockpit as well as text-based advisories, including Notices to Airmen and significant weather activity. The FAA provides three forms of ADS-B In Services (Ref. AIM Chapter 4, Section 5 (PDF)).

Can I fly without ADS-B?

Equipping your aircraft with ADS-B systems requires flying in controlled airspace since it allows safe aircraft separation.

For example, in the continental United States, ADS-B Out has been required to aircraft owners since January 2, 2020, for flight in:

  • Class A, B, and C airspace;
  • Class E airspace at or above 10,000 feet MSL, excluding airspace at and below 2,500 feet AGL;
  • Within 30 nautical miles of a Class B primary airport (the Mode C veil);
  • Above the ceiling and within the lateral boundaries of Class B or Class C airspace up to 10,000 feet (note that ADS-B is not required below a Class B or Class C airspace shelf, if it is outside of a Mode C veil);
  • Class E airspace over the Gulf of Mexico, at and above 3,000 feet MSL, within 12 nm of the U.S. coast.

Also, according to the Civil Aviation Authority of New Zealand (CAA), you need to have an operational ADS-B OUT system to fly in controlled airspace above flight level 245 since 2019, and this will apply to flying at any altitude after the 31st of December 2022.

On the other hand, some solutions are offered to those with not equipped aircraft. When looking forward to flying in ADS-B rule-controlled airspace, the operators must apply for an ATC authorization, not more than 24 hours and not less than one hour before the flight. This is done using the FAA’s ADS-B Deviation Authorization Preflight Tool (ADAPT), which is only available to aircraft with functioning, altitude-encoding transponders.

While it is true that the regulations require ADS-B only to fly in controlled airspace, it is also true that you may benefit from having your aircraft equipped with ADS-B systems.

What is the main benefit of Automatic Dependent Surveillance-Broadcast (ADS-B)?

There is not one main benefit since ADS-B provides a series of benefits. However, some are more relevant than others, so we will focus on those relevant ones.

From having more efficient spacing and optimal routing in non-radar environments, such as the busy airspace in the Gulf of Mexico, mountainous regions of Colorado, and the lower altitudes of Alaska, to improving the awareness and problem-solving possibilities of ATC, here are the most relevant benefits of ADS-B.

Higher level of safety

The better surveillance coverage provided by ADS-B allows for data to be transmitted more timely and accurately, thus improving the global level of safety.

Pilots getting access to Traffic Information Service–Broadcast (TIS-B) via ADS-B In can have a better outlook of the airspace they are flying in. This includes having information about altitude, ground track, speed, and distance of aircraft flying in radar contact with controllers, and within a 15-nautical mile radius, up to 3,500 feet above or below the receiving aircraft’s position.

Moreover, the versatility of ADS-B ground stations allows covering a greater range to obtain and transmit more accurate information on an aircraft’s position in an emergency, to a much lower altitude and with 45 percent more coverage than currently possible with secondary radar. This means a great improvement of safety even in the most difficult situation.

Enhanced awareness

Knowing the aircraft’s location with or without ADS-B equipped is an essential piece of information. Accurate weather and traffic information allow for the pilots and controllers to share the awareness of the specific situation, making it possible to make timely decisions with the vital capability of seeing things beforehand and avoiding them.

In addition, the most recent technologies allow pilots to have a clear view of the terrain via terrain maps in their cockpits, so they can fly over any obstacle without any problem, even in the lowest visibility situations.

Rescue activities in case of accidents

There have been a few cases of accidents where finding the aircraft for rescue purposes has been difficult. However, ADS-B’s highly accurate positioning data via GPS-based surveillance reduces the need for search in rescue activities because the last reported positions are precise. This way, controllers tracking aircraft can dispatch the rescue teams quickly to the place where they are needed.

It is important to highlight the importance of ground stations when these situations occur in difficult areas to reach. Because these ADS-B ground stations are smaller, they are usually placed to cover those hard-to-reach areas without any problem.

Many plane tracks visible in a clear blue evening sky over a city.

Traffic Information Services – Broadcast (TIS-B)

TIS-B is a client-based service that provides properly equipped aircraft with surveillance information about aircraft that are not ADS-B equipped. This service is great for improving situational awareness even outside controlled airspace where most aircraft without ADS-B fly. Not knowing there are aircraft flying nearby just because they do not have ADS-B could be highly dangerous.

The only possible drawback is that any aircraft must be equipped with a transponder and be within radar coverage to qualify as a TIS-B target. Fortunately, it is very rare to see any aircraft flying without a transponder. Most of the time, aircraft not ADS-B equipped fly within the range of ground-based radar to ensure the necessary communications.

Automatic Dependent Surveillance ‐ Rebroadcast (ADS-R)

ADS-R is considered an air-to-air transmission service. It relays ADS-B information transmitted by an aircraft broadcasting on one link to aircraft equipped with ADS-B In on the other link.

For example, the information for an aircraft equipped with a 1090MHz ADS-B Out system will be re-broadcasted to an aircraft equipped with ADS-B In on the UAT (i.e., 978MHz) frequency, and vice versa.

This service provides another layer of situational awareness for the pilots since it allows them to visualize one another on their cockpit display.

Flight Information Services – Broadcast (FIS-B)

FIS-B provides meteorological and aeronautical data to the cockpit. This service is not a client-based service such as the ADS-R. Instead, it is always broadcast into the airspace on the UAT frequency but never on the 1090MHz frequency.

Currently available FIS-B products include but are not limited to:

  • Airmen’s Meteorological Information (AIRMET)
  • Significant Meteorological Information (SIGMET)
  • Convective SIGMET
  • Regional NEXTRAD
  • Special Use Airspace (SUA) Status
  • Terminal Aerodrome Forecast (TAF)
  • Winds & Temperatures Aloft
  • TIS-B Service Status

What if my ADS-B Out fails?

It could be worrisome to be in the middle of your flight and see your ADS-B system fail. However, the whole ecosystem is created with contingencies to cover almost all possible situations, which is one of them.

When an aircraft’s ADS-B fails in flight, it is commanded to continue to its destination. The corresponding ATC will coordinate with any subsequent ATC facilities along the remaining flight route to keep track and provide the information needed to complete the flight safely. When failure happens on the ground and access to ADS-B rule airspace is needed, requesting authorization through ADAPT is necessary.

An Emirates aircraft flying and leaving a clear white trail across a blue sky.

Final considerations about ADS-B

As you see, ADS-B is a very important acronym for aviation activities to be carried out smoothly and safely, especially those involving controlled airspace. Both weather data and position information represent a critical aspect for flying safely, so transmitting and receiving such information in a timely and accurate manner is essential.

Fortunately, the aviation industry keeps evolving, and so evolve the technologies and regulations applied by the authorities that rule airspace. Therefore, you can rest assured that flying becomes even safer every year and even every day.

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Jet pilot @NASA

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