ADS-B and Next-Gen Avionics

What is ADS-B?

ADS-B is a replacement for (or supplement to) traditional radar based surveillance of aircraft. ADS-B is a major change in surveillance philosophy – instead of using ground based radar to interrogate aircraft and determine their positions, each aircraft will use GPS to find its own position and then automatically report it.

Why would we want ADS-B?

There are three benefits driving the transition to ADS-B. Firstly, the GPS positions that are reported by ADS-B are more accurate than the current radar positions and are more consistent. This means that in the IFR environment closer aircraft spacing can be used than at present, and this provides much-needed capacity improvements in congested airspace. Secondly, ADS-B surveillance is easier and less expensive to deploy than ground radar. This means that airspace which previously had no radar and only procedural separation services can now have the benefits of ATC services. And finally, because ADS-B is a broadcast service that can be received by other aircraft as well as ATC on the ground, ADS-B offers the option for an aircraft to have accurate and inexpensive traffic awareness of other nearby aircraft.

Will I need ADS-B?

Almost certainly. The benefits of ADS-B only become available if substantially all the aircraft participate. Closer spacing is only available if all the aircraft have improved position reporting. If radar is not deployed, ATC can only see ADS-B equipped aircraft. Without an ADS-B output, an aircraft would be invisible to the traffic receiver on another aircraft. For airspace where ADS-B has been deployed as the primary separation mechanism it is likely that having ADS-B equipment will be an entry requirement.

A very important point is that for all these benefits to work, an aircraft only needs ADS-B “Out”. That is, the aircraft must report position information to ATC and to other aircraft. There is no requirement for ADS-B “In” – that will always be an optional feature.

When will I need ADS-B?

That depends on the airspace you want to fly in. Widespread mandates for ADS-B are forecast between now and 2020. The biggest is already in place – the FAA has mandated ADS-B Out in all US airspace where transponders are currently required, with a deadline of 2020, although the majority of the US ADS-B infrastructure is already fully operational.

Australia and Fiji have active ADS-B programs, as does Europe, although the current European requirements primarily affect Air Transport, rather than GA.  Over the next few years other countries are expected to confirm their ADS-B plans.  Because the expected lifetime of the avionics being installed today extends into the ADS-B deployment period, it is worth taking into account future capabilities when buying equipment now.

In the US particularly, the forecast demand for ADS-B installations is well in excess of the number of qualified facilities, and it would be sensible to plan your ADS-B requirements now and not wait until the deadline.

What equipment do I need?

To support ADS-B “Out”, the aircraft must have a GPS receiver as the position source, and a datalink transmitter to actually send the ADS-B data.

The datalink transmitter that most aircraft will use is a Mode S transponder, using a feature called “Extended Squitter”. This is often referred to as 1090 ES, because the Extended Squitter (ES) transmissions are transmitted on the 1090 MHz frequency.  The Mode S transponder with Extended Squitter is the international standard for ADS-B output. Specific to US airspace – and not approved elsewhere – is the UAT datalink transmitter as an alternative to the Mode S transponder. UAT transmitters may only be used on GA aircraft flying at lower altitudes in the USA.

The GPS receiver used must be an IFR certified receiver. Although that GPS is not required to be WAAS capable, that may be a moot point. Many legacy GPS receivers that were designed before ADS-B was planned do not include the necessary calculation of integrity and accuracy that ADS-B needs to operate. It is unlikely that these older devices can be upgraded, and therefore a new GPS receiver would be required. This does not necessarily mean discarding an existing GPS navigator - a secondary receiver can be used to provide ADS-B data without disrupting the existing installation - there is no rule that requires a single common position source.

Should I use UAT or 1090 ES?

If you are flying outside the USA, there is no choice – the only approved solution is 1090 ES. That is also true for large aircraft and high altitude aircraft in the USA – you must use 1090 ES. If however you are flying a GA aircraft in the US, you may instead elect to use a UAT solution.

A UAT solution will often be more expensive than a 1090 ES based solution, because the 1090 ES ADS-B capability is built into many existing ATC transponders, whereas the UAT solution is a separate datalink radio. Although there is some hot debate on the subject, you will still need a transponder if you install UAT, which is why the total cost is higher. To be fair, if you already have a transponder and it has plenty of life left in it you can dodge that part of the cost, but in general the cost favours the 1090 ES solution.

That raises the obvious question – why would anyone use UAT?

The key difference between the two solutions is that UAT has spare uplink bandwidth, whereas 1090 ES only has the capacity for ADS-B position reporting. That means that a UAT radio can receive additional data streams, in addition to the traffic information common to both datalinks. The FAA is providing a weather reporting function using the spare datalink bandwidth of the UAT radio, and the FAA is hoping that this “added value” feature will encourage GA operators to install ADS-B equipment sooner that they otherwise might.

Weather reporting is nothing to do with ADS-B Out though, it is an ADS-B In application.

What about ADS-B In?

An aircraft with ADS-B In can hear position reports from all the other nearby aircraft – independently of ATC. Such a facility usually drives what is called a “Cockpit Display of Traffic Information”, or CDTI. In practice this kind of display is often integrated with a Multi-Function Display or moving map GPS display.  In the US, a UAT receiver can also receive weather data.

To support ADS-B In obviously requires a datalink receiver, in addition to the datalink transmitter that is providing the ADS-B Out function. Most UAT based ADS-B Out solutions will include a datalink receiver as well as a transmitter – as already mentioned the key advantage of the UAT system is the ability to uplink other information, so there’s a limited opportunity for a transmit-only UAT system. 1090 ES transponder based solutions today do NOT include the datalink receiver. Instead, the 1090 ES based ADS-B receivers are generally packaged as a separate system. At the high end, these are usually integrated with TCAS systems. For GA aircraft a separate ADS-B receiver is used.

It is worth pointing out that in the US the FAA infrastructure will rebroadcast information between 1090 ES and UAT systems so that the traffic information is available identically on either data link. With the weather data available on UAT, the UAT data link therefore contains all the information you might need.  There is no requirement for your ADS-B In receiver to use the same technology as your ADS-B Out system, and in the US there is therefore a good argument for using a 1090 ES ADS-B Out solution together with UAT for ADS-B In.

For GA aircraft there are many portable ADS-B receivers that support UAT or dual datalinks, and these can be used with multi-function displays, moving map GPS receivers and tablet computers.  Note however that the ADS-B ground network will only uplink traffic data to your receiver if your aircraft has reported that it is ADS-B capable.  It does that with special data flags in the ADS-B Out transmissions which makes an important point - you can only get traffic on your ADS-B receiver if your aircraft is ADS-B Out equipped.

What is antenna diversity?

A key benefit of ADS-B is that an aircraft with an ADS-B receiver can detect other nearby aircraft, and that needs to work for aircraft both above and below, and in any relative position. Large transport aircraft with TCAS already use more than one antenna for their TCAS and transponder systems, in order to ensure that there are no radio blind spots caused by the wings or fuselage. Having more than one antenna is called diversity, and the principle of antenna diversity can be applied to ADS-B installations.

In small GA aircraft the transmission pattern of a typical transponder antenna, although far from uniform, shows significant radiation above the aircraft as well as below, even when the antenna is on the aircraft belly. On GA aircraft there is therefore no regulatory requirement for diversity, although on an aircraft with ADS-B In, adding a second receive antenna may give a better all-round traffic picture.

What does it cost?

All the current Trig Mode S transponders – TT21, TT22 and TT31 – are also ADS-B “Out” certified. There is NO extra cost to the Trig transponder for the ADS-B capability.

The problem is the GPS receiver. Since the GPS needs to be an IFR certified receiver, it is often the most expensive part of the solution. Fortunately, as mentioned earlier, it is not necessary to replace or upgrade an existing navigator to get a compliant solution. Trig has a stand-alone WAAS GPS receiver, the TN70, that is compatible with all Trig transponders. Including the WAAS GPS antenna and installation kit, the TN70 list price is $3,500.