Ins and Outs
ADS-B stands for Automatic Dependent Surveillance – Broadcast:
- Automatic because it periodically transmits information with no pilot or operator involvement required.
- Dependent because the position and velocity vectors are derived from the Global Positioning System (GPS) or other suitable Navigation Systems (i.e., FMS).
- Surveillance because it provides a method of determining 3 dimensional position and identification of aircraft, vehicles, or other assets.
- Broadcast because it transmits the information available to anyone with the appropriate receiving equipment.
ADS-B replaces radar technology with satellites, bringing major advantages. Radar relies on radio signals and antennas to determine an aircraft's location. ADS-B uses satellite signals to track aircraft movements.
ADS-B Out works by broadcasting information about an aircraft's GPS location, altitude, ground speed and other data to ground stations and other aircraft, once per second. Air traffic controllers and properly equipped aircraft can immediately receive this information. This offers more precise tracking of aircraft compared to radar technology, which sweeps for position information every 5 to 12 seconds.
Radio waves are limited to line of site meaning radar signals cannot travel long distances or penetrate mountains and other solid objects. ADS-B ground stations are smaller and more adaptable than radar towers and can be placed in locations not possible with radar. With ground stations in place throughout the country, even in hard to reach areas, ADS-B provides better visibility regardless of the terrain or other obstacles.
Aircraft operating in most controlled U.S. airspace must be equipped with ADS-B Out.
ADS-B In provides operators of properly equipped aircraft with weather and traffic position information delivered directly to the cockpit. ADS-B In-equipped aircraft have 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).
Traffic Information Services – Broadcast (TIS-B)
TIS-B is a client-based service that provides ADS-B Out/In equipped aircraft with surveillance information about aircraft that are not ADS-B equipped. To qualify as a TIS-B target, an aircraft must be equipped with a transponder, and be within radar coverage.
Automatic Dependent Surveillance ‐ Rebroadcast (ADS-R)
ADS-R is a client-based service that 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.
Flight Information Services – Broadcast (FIS-B)
FIS-B provides the meteorological and aeronautical data to the cockpit. This service is not a client-based service, but rather is always broadcast into the airspace on the UAT frequency. This information is not broadcast on the 1090MHz frequency.
Some of the currently available FIS-B products are:
- Airmen's Meteorological Information (AIRMET)
- Significant Meteorological Information (SIGMET)
- Convective SIGMET
- CONUS NEXRAD
- Regional NEXTRAD
- Special Use Airspace (SUA) Status
- Terminal Aerodrome Forecast (TAF)
- Winds & Temperatures Aloft
- TIS-B Service Status
How do I receive ADS-B In Traffic Services and what do I see today?
Aircraft equipped with an ADS-B In only receiver are capable of receiving weather and proximate traffic. The most reliable traffic information that would be received is air-to-air via ADS-B. If an aircraft is not ADS-B Out equipped, the ADS-B ground stations will not identify the aircraft as a TIS-B/ADS-R client. If flying near a "client" aircraft, an aircraft without ADS-B Out may be able to receive the ADS-B traffic information specifically generated for the "client" aircraft.
In order to be considered a TIS-B/ADS-R "client" by the ADS-B ground stations, an aircraft must meet the following criteria:
- Be in a region where ADS-R or TIS-B are offered
- Be ADS-B Out equipped and have produced valid position data within the last 30 seconds
- Be ADS-B In on only one link
However, as of January 4, 2016, the following additional criteria must be satisfied in order to be considered a TIS-B/ADS-R "client" by the ADS-B ground stations: Broadcast ADS-B with NACp>4, NACv>0, SDA>0, SIL>0, and NIC>4.
It is important to note that aircraft with dual link ADS-B-In capability (i.e. 1090MHz and 978MHz) are not provided with ADS-R services. This is because the aircraft is already capable of receiving ADS-B messages from both links directly via air-to-air. In this case, only TIS-B traffic services will be provided. Once the above criteria have been met, the ground system will identify the aircraft as a TIS-B/ADS-R "client" and using the client's ADS-B Out position information it creates a "hockey puck" around the aircraft. The "hockey puck" is 30 nm in diameter and +/- 3,500 ft.
Any "not-on-the-same ADS-B Link" aircraft that falls within the "hockey puck" will be transmitted via ADS-R on the appropriate ADS-B In link to the "client" aircraft. Similarly, any non-ADS-B Out Transponder equipped aircraft within the "client" aircrafts hockeypuck will be transmitted via TIS-B on the appropriate ADS-B In link.
The information that is received both air-to-air and ground-to-air by the ADS-B In receiver is for situational awareness use only.