Bjorn’s Corner: Largest navigation change since radar, Part 6

July 20, 2018, ©. Leeham News: Last week we discussed how a TCAS (Traffic Collision Avoidance System) works and how ADS-B will improve the system.

Now we discuss other areas of aircraft navigation and separation which are enabled when all players have ADS-B and transmit their 3D position and where they are going.

Figure 1. Air traffic is getting denser and denser. Source: flightradar24.

New navigation applications based on ADS-B

When all aircraft in controlled airspace have ADS-B it changes the way Air Traffic Control (ATC) separates and directs aircraft.

Air traffic is increasing by 6%-7% year over year. Figure 1 shows the intensity of airliners flying in different parts of the world.

With separation and control tools like Radar and Secondary Radar (by means of transponder responses), there is a limit how many aircraft can share an airspace.

With the increased position accuracy of ADS-B, the separation between aircraft can be reduced. This allows a higher traffic volume for the same airspace.

The first area where ADS-B has been used to reduce separation requirements is in areas with no radar coverage, such as Oceanic airspace. Normally aircraft must operate with large separations as so-called “procedural separation” is used.  It builds on pilots reporting the aircraft’s position over HF radio (the normal VHF radio hasn’t the range) or datalink (FANS-1/A via satellite) at regular intervals. Air Traffic Control then establishes separation by queuing aircraft in long trains at near optimal altitudes and at the same airspeed.

The drawback is the aircraft must fly after one another on common tracks, at the same speed and stay on the same flight level as long as any conflict risk with other aircraft exists. With low positioning knowledge by ATC, the clearance to climb to a better flight level to fly more efficiently (as the aircraft gets lighter) or overtake a slower aircraft is often difficult to get, It often takes time until a secure separation is achieved and the climb can be allowed. The ATC is simply not sure where all aircraft are and cannot allow any fast movements in the air-space. The result is long trains of aircraft with many flying in a non-optimal way.

With ADS-B in every aircraft, one can install an aircraft to aircraft separation application which allows the pilot requesting the climb to include the nearest aircraft and the observed separation to these in the climb request. The controller knows the aircraft has the equipment to self-separate and allows the pilot to change altitude.

ADS-B also enables a simplified and cheaper TCAS system to be installed on general aviation aircraft. Only a software application is needed in the avionics system which takes the ADS-B reports from surrounding aircraft and calculates collision risks.

It then gives Traffic Awareness messages to the pilot through audio (“aircraft 2 o’çlock same altitude five miles”) and on the flight displays.

In the next Corner, we will discuss how radio traffic will gradually be replaced with Chat between the pilot and the air traffic controller.

9 Comments on “Bjorn’s Corner: Largest navigation change since radar, Part 6

  1. In ADS-B controlled airspace are the aircraft flying to a pressure altitude, determined by the individual aircraft’s air-data system, or are they flying to a GPS absolute altitude?

    I ask because there’s variation in the pressure altitude computations of different aircraft – if the data the ADS-B system is from the air data system, then it’s only at the altitude the aircraft thinks its at. That could be wrong – there was a collision in Africa involving a business jet for this reason a few years ago. Aircraft with inaccurate air-data systems can be spotted by air traffic control using radars, and are.

    If the ADS-B system is using an absolute position relative to the centre of the earth, then that’s fine, but the downside would be that all aircraft would have to fly to this definition of altitude, you couldn’t mix ADS-B and non ADS-B equipped aircraft in the same airspace.

    Or am I wrong?

    Also, are aircraft flying with ADS-B automatically compliant with Revised Vertical Separation Minima (RVSM) requirements (e.g. the accuracy and precision of their altitude measuring systems?

    • Hi Chris,

      for the reasons you list all aircraft are flying on pressure altitude as measured by the aircraft’s air data system (pressure altimeter for small aircraft). For flying at Revised Vertical Separation Minima (RVSM) above 29,000ft/FL290 you need an air data system which can operate to the higher accuracy so 1000ft vertical separation still is a safe distance.

      In the ADS-B out data packet, there is both the aircrafts 3D position relative to the earth and its velocity (its GPS state vector) and the pressure altitude from the air data system. What counts for separation and TCAS functions is the pressure altitude. With the state vector, the ATC applications can do a faster and more predictable prediction where your aircraft will be at a future point in time.

      • CL: good question, I would have guessed absolute, interesting.

        I would not have thought to ask.

  2. The thing on that map that I find amazing is just how empty of air traffic Africa is. Once you get south of the med. coastline there are only around a half dozen flights in the air.

    • It’s a matter of the population’s income. The large growth in Asian travel has come from a growing middle class getting the economy where travel can be accommodated. Africa will get there, let’s see how long it takes.

      • And bizarrely enough, they had a collision off Africa as there was no control and no one thought it could happen.

  3. Wonder when this “snapshot” was taken. Apparently only ONE a/c on its way over the South Atlantic from Europe to South America at the time. Really?

    • This is because there are no ADS-B ground receivers there. FR24 which has made the picture relies on private ground-based ADS-B 1090ES receivers which they send to enthusiasts prepared to put the antenna on the roof/balcony and then route the receiver’s output to his Internet via his computer or a dedicated compute box (Raspberry). ADS-B based ground surveillance over the Oceans will have to function via Satellite-based ADS-B receivers which then downlink the data to ATC.

  4. I would insert into some of Bjorns position comments, exact vs unknown.

    Position is roughly known, but not exact so you have to have a much larger safe block.

    Back in the late 80s I had a flight from Seattle to Anchyoae. Tailiwn was 300 knots or some such.

    To stay in our time block we were doing S turns over South Eastern Alaska (very large area)

    For me it was a real treat as I had lived in 3 places there and had only seen it from low levels. That time I got to see it from overhead and angles (route is off the outer Islands so its just a long distance slant look from the West)

    Best was seeing Lituya Bay and the Tidal Wave line as we flew inside of that an the Sun Angle was perfect to show it. (one of the highest waves ever rerecorded at 1700 feet)

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