ADS-B ground stations receive and transmit aircraft position information.

ADS-B Compliance Should Reduce Potential for Midair Collisions

When it comes to collision avoidance, “see and avoid” has long been the primary principle when flying VMC. The current regulation, 14 CFR 91.111, requires that “No person may operate an aircraft so close to another aircraft as to create a collision hazard,” and “When weather conditions permit, regardless of whether an operation is conducted under instrument flight rules or visual flight rules, vigilance shall be maintained by each person operating an aircraft so as to see and avoid other aircraft.” However, see and avoid does not always work due to atmospheric and light conditions, the limitation of the human eye, contamination on the windshield, intercept angles, traffic density, blind spots in the aircraft, and pilot (or CFI) workload associated with normal cockpit duties. Over the last 36 years there have been an average of 30 midair collisions per year resulting in 75 deaths per year. A total of 2,700 people have died, mostly due to failure of the see and avoid system. As we shall see, ADS-B has the potential to significantly reduce accidents resulting from the limitations of see and avoid.

ADS-B is a component of the FAA NextGen modernization program to move from radar and other ground-based services to satellite-based services. The abbreviation can be decomposed as follows:

Automatic: Messages are sent out automatically without interrogation or pilot action.

Dependent: Position and velocity are derived from accurate GPS.

Surveillance: Primary purpose is for ATC location of aircraft, vehicles, or other assets.

Broadcast: Position data is broadcast by an aircraft to anyone (ATC and pilots) with appropriate receiving equipment.


In principle ADS-B is simple, but the implementation in the U.S. is actually quite complex and confusing. The basics are that every aircraft (in affected airspace) must have an accurate position source (generally provided by a WAAS GPS receiver), and a transmitter that will send this position to ATC and to other nearby aircraft at the rate of once per second. This is far superior accuracy and timeliness compared to conventional surveillance radar.

The typical ASR-9 radar system only provides a position about once every five seconds, and some other radars are even slower. At significant distances from the radar station, the position may be accurate to only about one mile, versus ADS-B accuracy of a few feet. In general, ATC cannot use radar to see aircraft on or near the ground, and radar provides no direct aircraft-to-aircraft position information. With ADS-B, ATC receives these position transmissions via a network of ground stations that have a very small physical footprint and low cost compared to radar. At all times, aircraft with the appropriate ADS-B receiving equipment can determine the position of all other ADS-B-equipped airplanes, including at non-towered airports and on the ground.

The initial concept was to use an enhanced transponder (ES or Extended Squitter) to transmit this position on the current transponder frequency of 1090 Mhz. This is the International Standard for ADS-B as implemented in other countries. However, due to frequency congestion concerns and the desire to uplink significant amounts of data such as weather information, an alternate frequency band has been allocated in the U.S. Transmitters that operate on this 978 Mhz frequency band are called UAT, or Universal Access Transceivers. The process of transmitting position data by an aircraft on one of these two acceptable frequencies is called ADS-B Out, whereas the receipt of data by an aircraft is called ADS-B In.

To ensure that all aircraft will be transmitting their positions, 14 CFR 91.225 requires that aircraft be equipped with a certified position source and comply with ADS-B Out requirements in specified airspace by January 1, 2020. The affected airspace is essentially all airspace in which a transponder is currently required, plus the Gulf of Mexico and the coastline of the U.S. out to 12 miles.

To provide adequate time for equipment to be developed, certified, and installed, a 13-year period was established for the phase-in of the system.  The 1090ES system must be used at and above FL180, and either 1090ES or 978UAT can be used below FL180. The regulation is explicit about the 19 data fields that must be transmitted; they include position, altitude (both geometric and pressure altitude), velocity, an indication of ADS-B In capability, transponder codes and status, and a number of fields concerning navigation accuracy. One key item to note is that while ADS-B Out compliance is mandatory by 2020 in the specified airspace, there is no mandatory requirement for ADS-B In.


The dual frequency implementation plan introduces some complications.  Because it is desirable for an aircraft transmitting on one of these frequencies to receive position data from aircraft that are using the other frequency, a secondary function of the ground stations is to retransmit to airborne aircraft the position of nearby aircraft operating on the other frequency. This is called ADS-R (retransmit), and when a ground station receives a report from any compliant aircraft that indicates that it is ADS-B In capable, thereafter for 30 seconds the ground station will transmit the position of all other aircraft on the other frequency that are within 15 miles laterally and 5,000 feet vertically from the reference aircraft. Some available ADS-B In systems listen on both 1090ES and 978UAT, and these systems can directly receive position reports from all other ADS-B Out aircraft without the requirement to be in range of a ground station.

Another ground station function is called TIS-B. This is the transmission to ADS-B Out aircraft (that are ADS-B In capable) of the position of legacy aircraft that are transponder equipped and have been detected by ATC radar systems but that are not ADS-B Out capable. TIS-B transmits, for 30 seconds, position information for aircraft within 15 miles laterally and 3,500 feet vertically from the reference aircraft.

To benefit from ADS-B traffic information, either directly from other aircraft or from the ground stations, the aircraft must have ADS-B In, and must have some device on which the traffic can be displayed. This is typically a panel-mounted GPS receiver, a multifunction display or, potentially, a portable tablet device.

The ground stations have been deployed under contract for the FAA by ITT, now Exelis. The FAA has reported that ground station deployment per the original planned coverage has been completed “on-time and within budget.”   There are 634 ground stations, 3 major data processing distribution centers, and 2 network operations centers; also, service delivery point equipment has been installed at 271 ATC facilities. Integration of ADS-B position data into current ATC procedures using radar is on-going.

The range of a ground station is generally 150 miles, but the system is line of sight so coverage improves with altitude. The following graphic provided by the FAA shows ADS-B ground stations and coverage at 5,100 feet MSL as of September 14, 2014.

Some coverage gaps exist at lower altitudes, and it is possible that additional stations may be added in the future to improve low-altitude coverage.


The FAA says that ADS-B ground station deployment has been completed “on time and on budget.”


Another uplink service provided by the ground stations is the transmission of weather information called FIS-B (Flight Information Service, Broadcast).  The information is similar to that provided through XM, but it is available to ADS-B In aircraft without charge. However, because of frequency congestion, this is only transmitted on the 978 frequency. Unlike XM, FIS-B cannot generally be received by an airplane on the ground.

XM is available in three subscription levels, Aviator LT, Aviator, and Aviator Pro; the reports available under FIS-B are similar to Aviator LT. While FIS-B data includes NEXRAD images, METARs, and TAFs (and many other products), it does not include reports found in the more advanced XM subscriptions, such as individual lightning strikes.

To meet the mandate, and to receive the benefit of ADS-B In information, equipment must be installed in the aircraft. Since Twin Commanders are certified aircraft, this equipment must be certified as compliant and must be installed under an approved STC, or more rarely under Field Approval.  Since Twin Commanders generally fly at and above FL180, the ADS-B Out system must be based on a 1090ES transponder. Several avionics manufactures have secured approval for their ADS-B Out equipment including Trig, FreeFlight, Garmin, NavWorx, Avidyne, and BendixKing.  Other vendors have products that are currently in the certification process.

Some of the products such as FreeFlight, NavWorx, and Garmin have an integrated WAAS GPS receiver, so they can be installed with the required position source without an approved WAAS GPS in the panel. Most Twin Commanders have Garmin navigators, and Garmin ADS-B Out 1090ES products for certified aircraft include a panel-mounted transponder (GTX 330 w/ES), and a remote-mounted transponder (GTX 33 w/ES). These need to be paired with an approved position source such as the GTN 7xx/6xx or the GNS 5xxW/4xxW; the GNS480 is expected to be added as an approved source by the end of 2014. Note that the older 530/430 non-WAAS units are not approved sources.

To receive ADS-B In services, Garmin’s GDL 88 has dual-band 978 and 1090 receivers. (It is also capable of ADS-B Out on 978UAT, but this would normally be disabled in an aircraft that is operating with ADS-B Out on 1090ES). Garmin also offers a GTS 8xx series of active traffic systems that incorporate a 1090ES ADS-B In receiver and correlate ADS-B In traffic into the overall traffic picture. A Twin Commander owner could receive ADS-B In with the GDL 88 or with a GTS 8xx active traffic system.

A number of portable products are also available that support ADS-B In traffic and weather. Note that no portable product will be certified for the required ADS-B Out function! The Appareo Stratus II product links to an iPad or iPhone via WiFi, and integrates traffic and weather into the ForeFlight application. The Garmin GDL 39 links to a tablet via Bluetooth, and integrates traffic and weather into the Garmin Pilot application on either IOS or Android platforms. Garmin has announced the Flight Stream product that will link panel-mounted avionics including the GTN and GNS series navigators and the GDL 88 ADS-B receiver to the Garmin Pilot application running on a tablet. A number of other receivers are available that display on other tablet software.


Many pilots have not yet moved forward with the acquisition of ADS-B equipment. A recent survey of 26 Twin Commanders listed for sale shows one aircraft that is fully ADS-B Out/In compliant. I found 19 percent had GTN or GNS WAAS GPS units and GTX 330 transponders. To become compliant, these aircraft need only upgrade the 330 to the 330 w/ES, a modification available from Garmin for about $1,200. I found a substantial portion of the fleet (31 percent) that still have the Collins TDR-90 transponder, and this would need to be replaced. Many of these aircraft would also need an upgrade to the compliant GPS position source.

A notable 2012 midair collision near Washington D.C. involved an NTSB employee accompanied by a CFI doing a Flight Review and an FAA employee preparing to fly an instrument approach. Both pilots were highly experienced, and the occurrence was in VMC with one of the pilots in contact with ATC. No traffic system was available on either aircraft.

Because of the employment of the pilots, the NTSB asked the Transportation Safety Board of Canada to do the investigation. The report was very thorough and concluded that “neither pilot saw the other aircraft in time to avert a midair collision likely due to the inherent limitations of the see-and-avoid principle. In the absence of a means of alerting visual flight rules (VFR) pilots to potential conflicts to enhance the current see-and-avoid technique, the risk for midair collisions remains.” ADS-B provides pilots with information about potentially conflicting traffic, and does so without the need for assistance from ATC. For the majority of GA aircraft that do not already have some form of installed traffic system, it should be a significant tool in the reduction of future midair collisions.

It appears that more than 50 percent of Twin Commanders have no active traffic system. For these aircraft, an ADS-B In system can offer traffic information and alerts in the cockpit that may allow the pilot to know where to look, reducing the limitations of see and avoid. For Twin Commanders without current inflight weather information, the ADS-B system can provide weather information in the cockpit to facilitate critical inflight decisions.

Pilots ask, “When should I do the required upgrade?” The FAA forecast is that 150,000 GA aircraft will need ADS-B Out in the U.S. and the FAA reports just under 5,000 now compliant. Therefore, between now and the end of 2019, 29,000 systems must be installed each year. The Aircraft Electronics Associate reports 680 avionics shops in the U.S., so the math is that each shop must complete about one installation per week to meet the mandate. This is unlikely to happen.

Author Howard Wolvington

Author Howard Wolvington

As we approach the deadline, free market supply and demand is likely to result in premium installation costs, and significant wait times for installation. Some aircraft will not be updated by the 2020 mandate date, and will be limited to flight below 10,000 feet and outside of B and C airspace and the Class B 30-mile veil until installation can be completed.  Thus, the prudent Twin Commander owner should get started on this upgrade as soon as practical.

Howard Wolvington is an independent CFI in the Seattle, Washington, area who specializes in Technically Advanced Aircraft. He holds the ATP and Gold Seal CFI-CFII-MEI certificates and has been designated as the 2014 National CFI of the Year. He may be contacted at