Air traffic control (ATC) and radar were first used together in 1943 when Air Force air traffic controllers began using Ground Controlled Approach (GCA) equipment to help military pilots land safely in poor visibility.  Civil aviation first used GCA equipment at La Guardia Airport in 1945 where it was instrumental in tripling the landing rate to 15 planes per hour.  GCA matured and was widely accepted through the mid-1950s at which time the Instrument Landing System (ILS) began replacing GCA.  Though ILS uses similar course guidance principles to GCA, ILS instrumentation on-board aircraft displayed course deviation directly to the pilot.

As air traffic increased, not only did coverage area and scan rates of rotating radars need to increase, but long range radars were deployed along primary air routes through the 1960s.  Also added during this time were automatic aircraft tracking and weather tracking.  Modern radars use Doppler effect to discriminate between moving and stationary targets and to measure storm velocities.

Another innovation to emerge during World War II was the Identification Friend or Foe (IFF) system.  IFF depended on receiver/transmitter units (aka transponders) in ‘friendly’ aircraft that responded to coded radar ‘interrogations’.  In the early 1960s, the FAA published a standard for transponders and ATC interrogators for civil aviation and by the end of the decade 200 ground-based interrogators were deployed in the US and transponders became mandatory equipment in aircraft operating in “positive controlled airspace at high altitudes and near major airports” (1).

In the 1970s, the Air Traffic Control Radar Beacon System (ATCRBS), also known as Secondary Surveillance Radar (SSR), was upgraded to improve surveillance performance in dense airspace and transmissions between ground and aircraft.  With each aircraft equipped with a transponder, interrogations from ground radar returned unique data so controllers could identify the blips on their screens.  In the 1980s, the airborne collision avoidance system (ACAS) was developed.  It uses air-to-air surveillance and is required on all commercial aircraft operating in the US and Europe.  In the 1990s, radar surveillance of runways and taxiways was added.  This, coupled with transponders, a technique called ‘multilateration’, make surveillance accuracy at the surface good enough to feed into the automatic safety warning devices.

Next in the evolution of surveillance technology was the Traffic Collision Avoidance System (TCAS).  With TCAS, one aircraft interrogates another’s transponder, thereby detecting potential airspace conflicts.  The resulting information can be graphically displayed depicting another aircraft’s range and bearing.  TCAS does not require ground infrastructure but does require a TCAS system on one aircraft and at least a Mode C transponder on the other.  Note: A Mode C transponder is currently required in all Class A, B, and C airspace, within 30 miles of primary Class B airspace airports, and anytime you’re above 10,000 feet (FAR Part 91 Section 215 – ATC transponder and altitude reporting equipment and use).

And most recently, Automatic Dependent Surveillance – Broadcast (ADS-B) was developed to transmit aircraft position and velocity in 3 dimensions to other aircraft via an air-to-air datalink and to ground stations via an air-to-ground datalink (ADS-B Out).  In turn, other aircraft can use this state vector information along with their own state vector to calculate and display relative range and bearing (ADS-B In).  In addition, data gathered from ADS-B Out equipped aircraft are fed to ATC.  Like TCAS, ADS-B requires ALL aircraft be equipped in order to receive full benefit from the system.  If one aircraft lacks a Mode C transponder, that aircraft is invisible to the TCAS equipped aircraft.  Likewise with ADS-B, if an aircraft is not broadcasting its state vector, the aircraft with ADS-B won’t ‘see’ them.  Therefore, SSR installations must continue to be operated in the airspace until ALL aircraft have ADS-B  Out capabilities.  Hence the justification for legal mandates to ‘motivate’ aviation industry and consumers to equip.

TCAS II Mandate – All commercial turbine-powered transport aircraft with more than 30 passenger seats (or MTOW above 33,000 lb)

Mode C Transponder Mandate – All aircraft. In Class A, Class B, and Class C airspace areas; and in all airspace within 30 nautical miles of an airport listed in appendix D, section 1 of this part from the surface upward to 10,000 feet MSL;

ADS-B (Out) Mandate – 14 Part 91 Automatic Dependent Surveillance—Broadcast (ADS–B) Out Performance Requirements To Support Air Traffic Control (ATC) Service; Final Rule

However, even with an FAA mandate and incentives like free weather, aircraft owners have to pay for ADS-B equipment, and if the benefits of equipping aren’t obvious, some may delay their acquisition until they get closer to the compliance date of January 1, 2020.

ADS-B itself has a bit of history.  In the mid-1990s, the FAA’s ‘Surveillance Vision Plan – Rev 2’ was published.  The SVP described a “transition from ground-based radar surveillance to a joint satellite-based and ground-based surveillance system” in 5-year segments through 2015.  From an historical perspective, the SVP is interesting reading.  However, the system has evolved from lessons learned from prototypes starting in early 2000s.  One of the most notable programs was the Capstone Project in Alaska (2).  From Capstone came the Universal Access Transceiver (UAT), a device that can communicate with the ground and with nearby aircraft to exchange the ADS-B position data.  The beauty of UAT is that it uses readily available components to keep costs down and can be incorporated into radios in smaller and lower-flying aircraft (3).

Next in this sequence of events was the publication of FAA’s Notice of Proposed Rulemaking (NPRM) for ADS-B Out on October 5, 2007.  Simultaneously, the FAA chartered the ADS-B Aviation Rulemaking Committee (ARC) to provide the aviation community with a forum for ‘discussion and review’ of the NPRM.  On September 26, 2008, the ARC published it results in “Recommendations on Federal Aviation Administration Notice No. 7–15, Automatic Dependent Surveillance—Broadcast (ADS–B) Out Performance Requirements to Support Air Traffic Control (ATC) Service; Notice of Proposed Rulemaking”.  Following the FAA’s consideration of these results, the “Automatic Dependent Surveillance – Broadcast (ADS–B) Out Performance Requirements To Support Air Traffic Control (ATC) Service; Final Rule” was published May 28, 2010 wherein the FAA added to existing regulations “equipage requirements and performance standards for ADS–B) Out avionics on aircraft operating in Classes A, B, and C airspace, as well as certain other specified classes of airspace within the U.S.”

On September 30, 2011, the ADS-B In ARC provided recommendations that clearly define how the community should proceed with ADS–B In while ensuring compatibility with ADS–B Out avionics standards defined in §§ 91.225 and 91.227 of Title 14, Code of Federal Regulations.  This was in response to the FAA’s request for the ARC to provide a forum for the U.S. aviation community to define a strategy for incorporating ADS–B In technologies into the NAS.  From this report, it easy to misconstrue the ARC’s message in the cover letter signed by Brown and Hendricks stating “…there is not a NAS user community business case for near-term ADS–B In equipage. Therefore, at this time, the ARC does not support an equipage mandate.”  For clarity, the meaning behind this statement is a sentiment by airline representatives on the ARC that they are very reluctant to equip with ADS-B In because of its significant cost and yet uncertain benefits… Nonetheless, they (the airlines and the ARC) all estimate the potential benefits to be massive – they just have not yet been proven.

So this brings us current with significant progress being made in infrastructure build-out with “nationwide ADS-B ground infrastructure …  expected to be completed in 2013” (4, page 11).  Now we get to write the rest of the story with our response to the challenge of equipping our aircraft to become part of the ADS-B ‘network in the sky’.  But before you do, you should probably read this

[to be continued…]