Of course, pilot-controller communication is compromised when the crew is listening to other frequencies or engaged in other communications, such as talking to their company. If these communications could be accomplished silently and digitally, voice communications with ATC would improve. The Aircraft Communications Addressing and Reporting System (ACARS) is a commercial system that enables the crew to communicate with company personnel on the ground. It is often used to exchange routine flight status messages, weather information, and can serve as a non-voice communication channel in the event of an emergency. Many of the messages are sent and received automatically, such as the time the flight leaves the gate (triggered by the release of the parking brake), takeoff and touchdown times (triggered by landing gear switches), and arrival time (triggered when a cabin door is opened). Other information may include flight plans, significant meteorological information (SIGMETs), crew lists, cargo manifests, automatic terminal information service (ATIS) reports, en route and destination weather, clearances, and fuel reports. Some ACARS units can interface with onboard engine and performance-monitoring systems to inform company ground personnel of maintenance or operations related issues. [Figure 6-9]

Significant valuable meteorological data can be obtained by collecting data from aircraft fitted with appropriate software packages. To date, the predominant sources of automated aviation data have been from aircraft equipped with aircraft to satellite data relay (ASDAR) and ACARS, which routes data back via general purpose information processing and transmitting systems now fitted to many commercial aircraft. These systems offer the potential for a vast increase in the provision of aircraft observations of wind and temperature. Making an increasingly important contribution to the observational database, it is envisioned that ACARS data will inevitably supersede manual pilot reports (PIREPS).

Another use of ACARS is in conjunction with Digital ATIS (D-ATIS), which provides an automated process for the assembly and transmission of ATIS messages. ACARS enables audio messages to be displayed in text form in the flight decks of aircraft equipped with ACARS. A printout is also provided if the aircraft is equipped with an on-board printer. D-ATIS is operational at over 57 airports that now have pre-departure clearance (PDC) capability.


Unlike TCAS and terrain awareness and warning systems (TAWS), which have been used in airline and military aircraft for at least a decade, ADS-B is a relatively new air traffic technology. It is an onboard system that uses Mode S transponder technology to periodically broadcast an aircraft’s position, along with some supporting information like aircraft identification and short-term intent. By picking up broadcast position information on the ground instead of using ground radar stations, ADS-B represents a significant advancement over the existing ATC system by providing increased accuracy and safety. This is possible because ADS-B addresses the major deficiency of TCAS - accuracy. In the TCAS system, aircraft positions are only accurate to a few degrees; thus, the accuracy of TCAS decreases with distance. Moreover, the reliance on transmission timing for range data in TCAS is errorprone. The method used by ADS-B avoids this problem.

In addition to the broadcast of position to the ground, ADS-B can be used to enable a new collection of aircraft- based applications. Unlike conventional radar, ADS-B works at low altitudes and on the ground. It is effective in remote areas or in mountainous terrain where there is no radar coverage, or where radar coverage is limited. One of the greatest benefits of ADS-B isits ability to provide the same real-time information to pilots in the aircraft cockpit and to ground controllers, so that for the first time, both can view the same data.

ADS-B will also enable aircraft to send messages to each other to provide surveillance and collision avoidance through data link. Other aircraft in the immediate vicinity can pick up position information broadcasts from equipped aircraft. This enables equipped aircraft to formulate a display of nearby aircraft for the pilot; the pilot’s awareness of the current situation is enhanced. Combined with databases of current maps and charts, the onboard displays can show terrain as well as proximate aircraft. This is a powerful inducement for change. The heightened situational awareness offered by satellite navigation in conjunction with modern database applications and map displays, combined with the position of proximate aircraft, builds a picture in the cockpit equivalent to that on the ground used by the controller. This is particularly important in places like Alaska where aviation is vital, NAS infrastructure is minimal (because of the harsh conditions), and weather changes quickly and in unpredictable fashions.

Eventually, as the fleets equip, it may be possible to save money by retiring expensive long-range radars. Identified by the FAA as the future model for ATC, ADS-B is a major step in the direction of free flight. While ADS-B shows great promise for both air-to-air and air-to-ground surveillance, current aircraft transponders will continue to support surveillance operations in the NAS for the foreseeable future. If enough users equip with ADS-B avionics, the FAA will install a compatible ADS ground system to provide more accurate surveillance information to ATC compared to radar-based surveillance.

In the United States, two different data links have been adopted for use with ADS-B: 1090 MHz Extended Squitter (1090 ES) and the Universal Access Transceiver (UAT). The 1090 ES link is intended for aircraft that primarily operate at FL180 and above, whereas the UAT link is intended for use by aircraft that primarily operate at 18,000 feet and below. From a pilot's standpoint, the two links operate similarly and both support ADS-B and TIS-B. The UAT link additionally supports Flight Information Service-Broadcast (FIS-B) at any altitude when within ground based transmitter (GBT) coverage. FIS-B is the weather information component, and provides displays of graphical and textual weather information. Areas of approved use for the UAT include the United States (including oceanic airspace where air traffic services are provided), Guam, Puerto Rico, American Samoa, and the U.S. Virgin Islands. The UAT is approved for both air and airport surface use. ADS-B broadcast over the 1090 MHz data link has been approved for global use.