As always, you need to know a little bit about the early days of aircraft tracking, so let’s rewind to round about the middle of the last century, the 1950s. The development of radar during the second world war started a revolution for Air Traffic Control (ATC). Prior to radar, aircraft would make radio calls telling ATC where they were, what altitude they were flying at, and at what time they estimated to be at the next waypoint (and what that waypoint was of course). ATC were keeping track of the location of aircraft in analog ways: maps, boards, pen and paper. Radar revolutionised this by letting ATC see the position of aircraft in real-time.
Early radars however only were able to paint a radar target (or ‘blip’) on a screen showing the aircraft’s direction and range from the radar station. There was no additional information shown such as altitude, speed, or callsign, and it wasn’t evident which blip belonged to which aircraft, the controller had to keep that in their mental picture. To address this shortcoming, a device called a transponder (abbreviate XPDR in aircraft terms) was developed and deployed in all large passenger aircraft and later across all aircraft wanting to fly in controlled airspace. Using transponders, ATC radar systems could now listen to the response when a secondary radar beam swept by the target and associate every ‘blip’ on the controller’s screens with a code that was assigned by ATC and set by the pilots, known as the squawk. The squawk code – which is used to this day - is a four number octal code, meaning it is using only numbers 0-7. This first version of transponders implemented transponder Mode A. This mode only transmitted the pilot set squawk in response to an interrogator signal.
This twin-signal system came to be known as the Air Traffic Control Radar Beacon System (ATCRBS), and it consists of a primary radar beam that ‘paints’ any radar returns on the controller’s screen and a second signal called the ‘interrogator beacon’. As the ATC’s radar beam scans the horizon, it also transmits an interrogator beacon on a secondary antenna that is very slightly trailing the main antenna. The full complement of this system is referred to as a ‘secondary surveillance radar’ (SSR) because it provides this secondary information derived from the transponder response. The transponder in the aircraft listens to the frequency of the interrogator beacon at 1030 MHz, and as soon as a signal at 1030 MHz is received, it responds by transmitting its transponder code on a frequency of 1090 MHz.
There still was essential information missing though, most importantly the aircraft’s altitude. Thus Mode C was developed which on top of still sending the code set by the pilots, responds with the current pressure altitude of the aircraft. The pressure altitude is often referred to as the ‘flight level’, which is the altitude the aircraft altimeter is showing when the pressure on the altimeter is set to the standard atmospheric pressure of 29.92 inHg, or 1013 hPa. This added significant safety as it allows ATC to see when an aircraft is deviating from its cleared altitude or if it is on a potential collision course with another aircraft flying on a converging course at the same altitude.
But there’s an additional benefit to the airplane’s transponder: It also allows the pilots to indicate their status to ATC (a one-way communication) by setting special reserved transponder codes, such as 7700 for an in-flight emergency, 7600 if the communication radios are not working, or 7500 in case of a hi-jacking situation.
As the network of ground-based ATC radar stations grew, neighbouring radars were triggering transponder responses from aircraft much further away, making it increasingly difficult to differentiate which radar beam had triggered the transponder response. The solution to this was to develop a new transponder technology that uniquely identifies each aircraft, and thus Mode-S (‘S’ for ‘select’) was developed. Mode-S provides ground-air-ground integral data link capability, uniquely identifying each target using the fixed “hex address” also seen in modern day ADS-B messages. Mode-S thus in essence is a data packet protocol operating on the existing transponder frequency of 1090 MHz. For encoding and decoding of the data being transmitted, Mode-S is using differential phase shift key (DPSK) modulation for the ground to air uplink, and pulse position modulation (PPM) for the air to ground downlink. Using two different modulation methods that can coexist on the same carrier signal prevents interference between the different interrogation modes and responses.
With Mode-S it was now possible to add up to 8 additional parameters to the air-ground messages. As a minimum, they now included flight identification, aircraft velocity, and vertical speed telling whether the aircraft was flying level, climbing, or descending. This was an opportunity for new applications, such as using the transponder signal to help with collision avoidance directly between aircraft, without involving ATC. To provide this functionality, transponders now also started interrogating other transponders nearby to find out if any traffic was coming too close. Despite lacking actual position information in the Mode-S data, some clever signal strength algorithms were implemented to determine how far away (approximately) a nearby aircraft was located and along with the vertical speed and velocity data it could calculate whether the flight paths might be intersecting and on a collision course.
This gave birth to the first iteration of the traffic collision avoidance system (TCAS, or ACAS with ‘A’ standing for ‘airborne’). For this system to work, the transponders need to be sending out their messages regardless of whether a ground-based radar beacon (or SSR signal) triggered them. Therefore, these transponders started sending out data at regular intervals even when they didn’t detect any interrogation signal, and they also started responding to other aircraft, not just ground radar beacons. This way, aircraft could avoid each other even in the middle of an ocean where no ATC radar coverage is available.
With further density increases in traffic and with global navigation satellite systems (GNSS) becoming ubiquitously available in every aircraft, a further refinement of the Mode-S transponder technology was developed, known as Mode-S ES (extended squitter), leading to an extensive overhaul of the tracking methodologies in use thus far and the development of the ADS-B standard.
Automatic Dependent Surveillance - Broadcast (ADS-B) is an aircraft tracking technology developed by the Radio Technical Commission for Aeronautics (RTCA). It is published in the document RTCA DO260-B: Minimum Operational Performance Standards (MOPS) for 1090 MHz Extended Squitter Automatic Dependent Surveillance – Broadcast (ADS-B) and Traffic Information Services – Broadcast (TIS-B). From the introduction in that document:
Since 2017 - give or take a few years depending on jurisdiction – ADS-B has been made mandatory for all aircraft wanting to fly under instrument flight rules (IFR), in controlled airspace, or both.“ADS-B is automatic because no external stimulus is required; it is dependent because it relies on on-board navigation sources and on-board broadcast Transmitting Subsystems to provide surveillance information to other users.”
The ADS-B information is transmitted on top of the aircraft transponder signal, and it stands alongside several similar technologies, each with their own purpose:
• ADS-A, with ‘A’ standing for ‘addressed’, is a point-to-point data exchange using ACARS data links.
• ADS-C, with ‘C’ standing for ‘contract’, also implements point-to-point data exchange but the underlying communication technology usually involves satellite data links.
ADS-B in (contrast to A or C) always broadcasts the aircraft state vectors to anyone who’s listening.
ADS-B messages are sent on 1090 MHz as Mode-S ES (“extended squitter”) messages, allowing up to 48 parameters to be transmitted in one “squit”. Mode-S ES and ADS-B are often used interchangeably these days, however, the subsystem broadcasting ADS-B messages from an aircraft may or may not be equipped with a Mode-S based transponder. A further important commonality between Mode-S and ADS-B consists in the ability for ATC to ‘select’ certain information to be sent by the aircraft.
As of 2017, the ADS-B standard provides for the broadcast of all real-time state vector information of any aviation operation relevant vehicle. This includes aircraft, rotorcraft, balloons, drones, gliders, airport service vehicles, tugs, airport fire brigade vehicles, ground obstacles, there’s even a transmitter category for parachutists.
So how is all this background information of any use to you in reference to RealTraffic?
You will now see that in order for a system like RealTraffic to function, it needs to be backed by a global network of receivers that decode the ADS-B data and forward it to central servers.
The transponder frequency of 1090 MHz is a relatively high frequency in terms of radio propagation characteristics and in essence requires line-of-sight visibility. It suffices for a building to be located between the receiver and the aircraft for the signal to be blocked. This explains why at some airports, landing aircraft can disappear at low altitudes shortly before landing, and then reappear when another receiver with a slightly different viewing angle to the airport has the transponder in sight again. If no receivers are near the airport, ground traffic unfortunately is often not visible at all because 1090 MHz doesn’t transmit well through buildings, trees, and hills.
Operational rules at airports differ as well: Standard procedure at some airports is to switch the transponder to standby after landing. This unfortunately will make the aircraft disappear. But increasingly, airports are changing the rules, especially under poor weather conditions, to keep transponders running so as to allow better traffic visibility by ATC and between aircraft and ground vehicles.
In the next part I'll focus on the many different data fields ADS-B can provide, and what information can be gleaned from that.