June 22, 2018, ©. Leeham News: Last week we started a series of Corners that deal with the largest navigation change since VOR and Radar was introduced after the Second World War.

It’s about leaving radars and transponders to keep track of where aircraft are, letting an ADS-B transmitter/receiver in the aircraft take over this role.

Figure 1. The ADS-B is mandatory in most US airspaces. Source: FAA

ADS-B, the future navigation cornerstone

Most people think of the ADS-B system (Automatic Dependent Surveillance-Broadcast) as an extension of the aircraft’s transponder.

Principally, it’s not. It’s operating principle is totally different. A transponder answers to someone interrogating it. The ADS-B doesn’t. It just shouts out its message every second, whether someone is interested or not.

Where it has links with advanced transponders (the type S) are, all ADS-B solutions outside of the US and most in the US use the mode S transponder’s 1090Mhz datalink transmitter for sending the message. The message format is an extended version of the type S transponders datalink message called Extended Squitter (ES) message.

In the US, an alternative ADS-B datalink is allowed to lessen fears of congestion for the 1090Mhz ES link. It’s a data link transceiver at 978Mhz, called the UAT (Universal Access Transceiver), which is allowed below 18,000ft altitude. The idea is general aviation aircraft which don’t fly above 18,000ft could use the 978Mhz link and by it reduce congestion for the 1090Mhz link.

The message these datalinks transmit is the one we described in the last Corner:

• Who the aircraft is;
• Where the aircraft is;
• Where it’s heading, at what altitude and at what speedl
• If it’s climbing or descending; and
• If it’s in distress.

Where is ADS-B mandatory?

In the US, ADS-B must be on aircraft flying over 18,000ft (Class A airspace) or in Class B or C airspace below 18,000ft (Figure 1) by 1 Jan. 2020. For the rest (Class E), it must be present if you fly above 10,000ft.

Small aircraft types, like the Super Cub, used for small VFR hops in rural areas, won’t need it. If these want to visit airports with control towers, they need ADS-B as well.

In Europe, all IFR traffic aircraft and VFR traffic aircraft over 5,700kg/12,566lb gross weight or faster than 250kts need ADS-B by 7 June 2020. For other countries, it varies, but many require it from 2020 for transport type aircraft.

This means that by 2020, aircraft flying between airports of some size will broadcast their presence and intentions.

When this is in use by all controlled aircraft, air traffic controllers in control centres or towers only need an ADS-B receiver to know where everyone is and where they are going. They also need a software program to process all the broadcast from the aircraft in their reception sphere and to present the air traffic picture to him.

But such programs will be standard items and eventually readily available. An ADS-B receiver is so much simpler to install and maintain than a radar with a transponder interrogator.

The picture of the air traffic will be more accurate and complete then controllers ever had before. The GPS receiver feeding the ADS-B with the situational data is the one today used for GPS based LPV airport approaches. These approaches are flown to the same minima as ILS class 1 approaches.

It’s an SBAS (Satellite Based Augmentation System) corrected GPS receiver with high precision. There are different SBAS systems in use around the world, WAAS for the US, EGNOS for Europe, MSAS for Japan and GAGAN for India. Several more are under construction (China, Russia…).

Now we have discussed the mandatory ADS-B-out requirement and its basic use. But there is also the optional ADS-B-in (listening) capability, which opens many possibilities.

In the next Corner, we will dig deeper and explore what the benefits are when aircraft receive ADS-B messages.