Have you ever wondered what does the channel numbers on the SBAS approach charts stand for?

When you select a PBN approach, we find WAAS approach type in United States, or EGNOS in Europe, amongst others. Below the approach procedure designator, i.e. RNAV (GPS) Y 04L in KJFK, a WAAS with CH 77519 appears underneath. Just below channel numbers there is a combination of letters and numbers.

If we remember, WAAS is system which augments GPS signal in other to improve GPS position. To do that, a geostationary satellite system broadcast a correction signal to aircraft. In U.S.A is WAAS. Then, it’s not necessary to place any ground station at airports to broadcast this correction signal to correct aircraft GPS position like in GBAS system. But why there is a channel on the approach chart though?

When the system was designed, the channel number was considered as an optional equipment capability which allowed to pilot to use a 5-digit number to select a specific approach instead of using the menu method as we usually do to choose from our navigation database. These numbers correspond to only one type of approach and they’re unique.

Below channel numbers, there is a combination of 4 letters and numbers. This is the approach identifier. In conventional radionavaids, when an ILS or VOR is selected, there is Morse code to aurally ident it. In this case, is visually identified to verify the selection and extraction of the correct approach chart from the navigation database. The first letter corresponds to augmentation system used: W for WAAS or E or EGNOS. The next two numbers are used for runway designator. When there are parallel runways, lest say LEFT, CENTER or RIGHT, letter A, B or C are added at the end. So, in KJFK, for the approach RNAV (GPS) Y 04L, our approach ident will be W04A.

The increase in density of air traffic in Europe, United States, and remote areas such as the North Atlantic Ocean, made the necessity to implement Single European Sky programs (SESAR) and NEXTGEN. To achieve this goal, technology plays a vital role. The ADS-B is one of them.

According to stablished critera by every civil aviation administration, as of 2.020, aircrafts must be equipped with ADS-B system. In Australia, pioneers in remote airspace management is already implemented since December 2.009 above FL300. Thanks to this technology they were able to reduce aircraft separations from 30 NM to only 5 NM, increasing airspace capacity significantly. But, What is it? And How does it work?

Working principles.

ADS-B (Automatic Dependent Surveillance – Broadcast), is a surveillance system which will replace information already obtained from radars.

This new system allows navigation systems onboard of an aircraft to obtain its position from GPS signals. Signals are joint together with other flight data gathered from other aircraft’s systems and broadcasted. Signal broadcasted is received by ground stations, inflight stations or satellite’s receivers and represented on a screen.

Nowadays, to be able to supply air traffic control with radar, air traffic controllers have one o more radar stations on ground to provide aircraft’s position. This information is obtained from radar echoes PSR (Primary Surveillance Radar), or exchanging information between aircraft and ground station, thanks to the transponder. This is known as SSR (Secondary Surveillance Radar). Between the two systems, SSR is the most accurate with MODE S transponder.

System and capacity.

The ADS-B has two basic capacities known as “OUT” and “IN”.

ADS – B “OUT” defines its capacity to broadcast ADS – B information. As an example of what kind of information broadcasts, the A330 with “OUT” capacity sends out the next information automatically and in a continuously manner:

• Latitude and longitude, Horizontal Integrity Limit (HIL), the difference between barometric altitude and geometric altitude and ground speed (GS). All obtained from GPS signal;
• Barometric altitude is obtained from ADIRS;
• Track and vertical speed given by IR’s;
• The ATC flight number introduced in the preflight check is given by FMS;
• Emergency status; and
• Selected altitude and heading, and barometric pressure (QNH/QNE) from the FCU.

This last function allows ATC, if they are equipped with proper system, to see on their screens their clearance and what pilot selected on the same radar tag. Very similar to what it happens with MODE S “enhanced” which uses “Down – link of Airborne Parameters” (DAP). This is what happens in airports such as London Heathrow. But this is another story…

On the other hand, ADS – B “IN”, defines its capacity to receive information from other ADS – B “OUT” stations which broadcast information.

Sounds pretty obvious that an aircraft equipped with both functions will be able to broadcast and to receive ADS – B information, to and from other ADS – B stations.

For an aircraft to able to be equipped with ADS – B technology, is necessary to have datalink equipment in VHF band. To do that, aircrafts use mainly two different equipments: 1.090ES and UAT978.

The UAT978 (Universal ADS Transceiver) is an equipment only used in United States below 18.000 feet. Created for general aviation, if it’s equipped with “IN” capacity, will be able to receive weather information free of charge. However, the rest of the world Will be using 1.090ES complying with ICAO requirements, what in fact has higher data transmission capacity.

But, What is 1.090ES all about? Basically, is a mode S transponder modification, currently on board of aircrafts. As the mode S transponder does, transmits on 1.090 Mhz., broadcasting information instead of waiting for the interrogation from a SSR station. This transponder has a group of extra capacities added to mode S, that the reason for its name: “Extended Squitter”.

Besides, because it’s working on the same frequency, it’s able to comply with airspaces where SSR radar service is provided and with ADS at the same time.

Advantages.

Regarding all conventional radar information which air traffic controller receives, ADS – B is more reliable. All data is sent directly from the aircraft’s navigation equipment.

There is other factor which affects service which is the transmission speed. Nowadays, secondary surveillance radars use interrogation/response of onboard transponders to obtain data information from aircrafts. Let’s say for a moment and aircraft flying under radar coverage equipped with transponder. The SSR antenna begins with an interrogation on 1.030 Mhz. and aircraft through its transponder will response on 1.090 Mhz. with information asked. Once this information is received on ground, is presented to air traffic controller’s screen. To the contrary, ADS – B broadcast twice per second automatically without the necessity to be interrogated by any other equipment. An ADS – B “IN” antenna receiver is only needed.

With ADS – B, all radar antennas could be easily replaced by ADS – B receivers, simpler to install, easy to maintain, more energy efficient, and in the end, cheaper.

Applications.

If I explained it correctly until now, maybe you were able to guess other advantages or capacities this technology has.

If we add an ADS – B receiver antenna to an aircraft, we would give it ADS – B “IN” capacity. We only would need a way to represent this information in the cockpit: CDTI (Cockpit Display of Traffic Information).

All this will be translated into being represented on places we’re all familiar with, such as TCAS screen, on a MFDU (Multifunction Display Unit), or on a ND (Navigation Display). We would be able to see on one of these screens onboard of our aircraft the same information as the air traffic controllers on their radar screens. There is no doubt how this increases pilot’s situational awareness in highly congested airspaces.

As we mentioned before, he appearance of ADS – B has brought about some new applications: TIS – B (Traffic Information Service) and FIS – B (Flight Information Service).

TIS – B allows information regarding aircrafts with transponder but not ADS – B equipped, flying under radar coverage, to be broadcasted by ADS – B “OUT” stations. This information is received by ADS – B “IN” equipped aircrafts being able to see on their screens onboard other aircrafts around them which are not equipped with ADS – B.

FIS – B allows aircrafts equipped with ADS – B “IN” to receive weather information, ATIS, or NOTAM from ADS – B “OUT” ground stations. This type of service is well known as FIS – B.

New Procedures stablished on NAT – HLA oceanic airspace.

Of course, ADS – B is a very substantial improvement in air traffic control. The ITP (In Trail Procedure), allows aircraft to choose optimum flight levels without being “blocked” by other aircraft flying at a distance with no radar further than “ITP Distance”.

This is, if an airplane wish to climb or descend crossing other airplane’s flight level, and both are ADS – B (“IN” & “OUT”) equipped, sending the request via CPDLC to ATC, it will show  distance from other aircraft, their flight level and callsigns before being sent to ATC on CPDLC’s screen.

Thanks to ADS – B, ATC will receive more accurate information and will have a total picture of airplanes instantly with no ADS – B equipped and ADS – B airplanes. So, it will be easier for ATC to see if they have separation enough to give the clearance for level change request.

Near and future developments.

On the other hand, over remote areas and over oceans, receiver antennas on ground is not possible or feasible. Because of that, for some time companies have been working on a low altitude satellite constellation able to receive ADS – B signals from aircrafts.

This constellation called Iridium is made of 66 active nanosatellites and 9 as spare. They are in an orbit just at 785 km from earth’s surface, being able to receive ADS – B signal and send it to ATS centres. It’s expected to be fully operational in the end of 2.018.

Curiously, and to mention flight MH370 disappearance, the company FlightAware has signed an agreement with Aireon (Owner of Iridium Constellation) to supply airlines with a fleet management and tracking capabilities based on this system. This is the solution to ICAO’s system called GADS (Global Aeronautical Safety System) to continuously monitor aircraft’s position.

In Europe, comparing to United States, there are no immediate plans of integrating ADS – B position to ATC’s system in all regions, and to provide ATC service. That’s the reason why Iridium is fastest solution. In Italy, for example, will be implemented.

The implementation of ADS – B is, as you have read, a very remarkable change in what we already knew about surveillance. There is no doubt the reduction on aircraft’s separation in remote areas will be very valuable, especially in HLA airspace in the North Atlantic Ocean, as it happened before in Australia with no effect on safety at all.  

Besides, being able to handle information rapidly and accurately, crews and air traffic controllers will be able to increase their situational awareness which will lead to take decisions easily.

Finally, implementing ADS – B procedures like ITP, will allow an optimization of airspace. Aircrafts will be able to fly closer to their optimum flight level, reducing fuel burn and CO2 emissions.