Understanding ADS-B (Dec 2015) Alan Hepburn ICS #16828

Understanding ADS-B (Dec 2015) Alan Hepburn ICS #16828 Introduction Anybody who pays any attention to the aviation press cannot fail to be aware that a major change is heading for operators in the United States on January 1, 2020 the mandate to be ADS-B equipped above 10,000', and in the Class B/C airspace surrounding major terminals to much lower altitudes. The first part of this article will look at ADS-B in general in general. The article was written for a Canadian audience, and has been Americanized. However, it still makes sense to start off looking at how ADS-B is implemented internationally, then look at the extensions being implemented in the USA, since the international implementation is much simpler than the US version, albeit much less useful to the pilot. In the second part, we ll look at a review of the author s own approach to equipping for the ADS-B world, which entailed the installation of an L3 Lynx NGT9000+transponder. Surveillance Technologies In the beginning, there was primary radar. A rotating directional groundbased antenna sent out a pulse of RF energy, and looked for the small amount of energy reflected off targets when it arrived back at the antenna a few microseconds later. Azimuth was determined by the direction the antenna was pointing, and distance could be inferred from the round-trip time. To make sure they were talking to the right aircraft, controllers would instruct a target to turn on to a new heading, the watch the maggots on their screen to see which one altered course. That s the way they did things when the author learned to fly in Scotland in 1968. Now, let s look at Mode A/C transponders as they have existed since the 1960s, and which you probably have installed in your aircraft right now. SSR (secondary surveillance radar) ground stations transmit an interrogation to these transponders on 1030 MHz. This signal is again sent in the direction that the rotating radar antenna is pointed. Every airborne Mode A/C transponder that hears this interrogation transmits a response on 1090 MHz. The response, if you have an altitude encoder connected to the transponder, will include your pressure altitude. So, for years, we have been providing ATC with our vertical position. Since the interrogation is directional, only those transponders in line with the SSR antenna will respond. However, the potential exists for more than one transponder to respond at once. The guy who shouts loudest (which generally means the nearest aircraft) will be heard. The distance is determined based on the time it takes to receive the response. Since the return pulse signal is generated by a transmitter on the target aircraft, the range of SSR is greatly superior to that of primary radar, and since each aircraft can be assigned a separate code, target turning for identification is not required. Now, let s look at the Mode S transponder, which has been around since the 1980s. S stands for selective interrogation. To a Mode A/C ground station, a Mode S transponder looks just like a Mode C transponder. Indeed, one of the trickiest design requirements for the Mode S transponder was that it had to be interoperable with existing Mode A/C SSR ground stations. However, it is also able to respond to interrogations that are unique to mode S either All Call interrogations, that solicit a response from all Mode S listeners, or selective interrogations, to which only a specific aircraft will reply. Every airplane is assigned a unique, 24 bit identifier. On initially entering the area of coverage of a Mode S interrogator, the All Call interrogation will be used to determine the aircraft s identifier. Once identified, the aircraft will be unresponsive to All Call interrogations, and will only respond to interrogations containing its unique identifier. Responses will include a

squitter, or data field, containing this 24 bit address, to allow confirmation of the source of the response, and some other information, but not nearly as much data as is required to provide a full ADS-B response. Like Mode A/C, Mode S, as used for surveillance and TCAS, uses an interrogate/respond protocol. In these roles, the transponder will only respond when interrogated. If you are curious about your Mode S identifier, you can see it as a 6 character code which is listed for your aircraft in the US Civil Aircraft Register (http://registry.faa.gov/aircraftinquiry/nnum_inquiry.aspx). It doesn t matter if you have no transponder at all. You have still been assigned a 6-character hexadecimal (base 16) identifier. Prior to the advent of ADS-B, the only use of Mode S in North America was as a component of TCAS. Now let s look at ADS-B. The basic idea is pretty simple. Participating aircraft need to carry a Mode S transponder. The aircraft will respond to Mode A,C and S interrogations as just described, but periodically the Mode S transponder will also broadcast its GPS position, Mode S identifier, and other data, such as the tail number and/or flight number, etc. Thus the receiving station knows the position of the target in three dimensions. The outgoing transmission is referred to as ADS-B Out. Note that it is a broadcast, not a response to an external interrogation. Not just any Mode S transponder will do. You need a Mode S extended squitter (ES) unit. The extended squitter has room to accommodate the extra data. The outgoing data is segmented into 56 bit packets, and it will take several of these packets to convey the whole ADS-B Out message. The complete message is sent every 5 seconds or so. This broadcast function is where the B in ADS-B comes from. When it is not transmitting on 1090 MHz, the Mode S (ES) transponder listens on that frequency, as well as listening for interrogations on 1030 MHZ, so it can hear the ADS-B Out broadcasts of nearby similarly equipped aircraft, and determine where these aircraft are in three dimensions. Anywhere in the world, even well out of range of groundbased surveillance equipment, a Mode S ES equipped airplane will receive position reports from other Mode S ES equipped aircraft nearby. But all that a Mode S (ES) equipped airplane is going to see is other Mode S (ES) aircraft, not other aircraft who are not Mode S (ES) equipped. All that is needed to convey this traffic information to the pilot is a suitable display device, which typically is not part of the transponder itself. The display device may be a dedicated unit, but a number of GPS navigators, such as the Garmin panel-mounted products and multi-function displays (MFDs) can provide this function. So can portable ADS-B receivers, which then display the traffic on a portable electronic device (PED, e.g. an ipad running a suitable app.). A Mode S (ES) transponder will set you back $4,000. The GPS position must come from a certified WAAS GPS sensor, which until recently meant that only aircraft equipped with a WAAS navigator could participate. That came with a $15K price tag. As we ll see, some less expensive options have recently become available. On the ground, suitably equipped ADS-B towers relay your GPS position data to ATC. In the US, a network of around 800 ground stations provides fairly good coverage down to about 1,500 above ground. SSR stations also interrogate Mode A/C targets in the normal way, and the information presented to the controller is a merger of both ADS-B and conventional data. If all traffic was equipped with a Mode S (ES) transponders, there would no longer be a need for expensive, rotating antennas to interrogate Mode A and C targets. That s a major advantage from the surveillance server provider s point of view mega savings. Many countries already have deployed ADS-B in this way. Australia, for example, has fairly good ADS-B coverage above 10,000'. Canada has only implemented ADS-B over Hudson s Bay, where there was limited radar coverage. The Canadian deployment was aimed at allowing reduced separation of airline traffic operating above FL285 over Hudson s Bay, where there were understandably no ground-based SSR stations. However, with the two exceptions of Hudson s Bay and the Davis Strait between eastern Canada and

Greenland, the Canadian surveillance system continues to use only Mode A/C technology. SSR surveillance at low level in Canada only extends a couple of hundred miles north of the border. After that, there is no surveillance at low altitudes whatever. Just look at all that uncontrolled green territory on the Canadian Low Enroute charts. This may be hard for a US pilot to get his mind around, but it s reality. A significant disadvantage of ADS-B, as so deployed, is that an extensive network of ground stations is required, and there needs to be ground to build them on. A ground-based ADS-B network would be totally uneconomic in the vast undeveloped expanses of northern Canada, and in many other countries. Once you get out over the oceans, no ground, so no ground stations. Canada s plan, instead, is to put the listening stations aboard satellites in low earth orbit. The position reports from ADS-B Out equipped aircraft would be received by these satellites and relayed back down to ATC. NavCanada is partnered with Iridium, a satellite communications company, to develop this spacebased ADS-B system, which will be of interest to many countries who do not wish to deploy hundreds of ADS-B ground stations. The partnership operates under the name Aireon, and already has members from several other countries. The payoff for a space-based system is perhaps even bigger over the oceans. Currently, as the disappearance of flight MH270 demonstrated, we are still reliant on aircraft relaying their observed position via HF radio or satellite data link, and consequently the separation between oceanic traffic is very large. If aircraft could automatically relay their GPS positions via space-based ADS-B, the oceanic separation limits could be made similar to those pertaining over land, alleviating the present congestion of oceanic routes, and aircraft would be visible to ATC at all times. Ten space-based ADS-B relay satellites have already been launched. The system is scheduled to be up and running by 2018. In a recent conversation with an avionics inspector at the Rochester FSDO, he indicated that the US is also interested in space-based ADS-B, as many US radars are getting pretty long in the tooth. I suspect that the first target for implementation will be high level traffic, which will already have Mode S (ES) equipment, due to the 2020 mandate. If every aircraft was ADS-B equipped, and GPS were 100% reliable, ATC would no longer require SSR. They d just need an array of listening stations ground or space based to relay the broadcast position reports to ATC. A ground based ADS-B station comes at about 10% of the cost of an SSR station. Even if SSR is not completely eliminated, there are significant economies to be realized if its role can be partially assumed by ADS-B. Indeed, in Australia, they are already decommissioning some SSR facilities. No plans have been announced about a schedule mandating the expanded use of ADS-B in Canada, but studies are apparently under way to examine the feasibility of requiring all aircraft in airspace where a Mode C transponder is currently required to carry Mode S (ES) in future. If and when the relay stations are on satellites, the antenna should ideally be on top of the airplane. But for years, we will be looking at SSR interrogations from the ground and, assuming space based ADS-B goes ahead in some form, broadcast of ADS-B Out to satellites, so you d ideally have antennas both top and bottom. This is termed antenna diversity. Mode S ES transponders are available that support antenna diversity, but at a cost. The good news is that, for aircraft with small fuselages, it looks like enough signal will make its way from a belly-mounted antenna for the satellites to see it. Traffic Detection Systems A further advantage of ADS-B is that each ADS-B Out equipped aircraft in the system knows the location of similarly equipped traffic without requiring ATC to issue an advisory. While this ability is a benefit, it is of limited use as long as every aircraft in the system is not ADS-B equipped. What you d really like to be able to do is see every nearby aircraft, including those who are not ADS-B equipped, are too low to be seen by radar, or are simply not transponder equipped at all. Let s first dispense with the target aircraft with no transponder. There is no technology, short of military target

acquisition radar, that will let you see them. For transponder-equipped aircraft, however, the problem can be solved by a so-called active traffic system. These systems have been around since long before ADS-B was developed. In 1993, TCAS (Traffic Collision Avoidance System) was introduced, a system that not only locates traffic, but computes a collision avoidance strategy and presents it to the pilot Climb! Climb! TCAS is mandated for aircraft with more than a certain number of passengers. TCAS is good, but uneconomic for GA operators. While TCAS can detect Mode A/C traffic, both aircraft have to be equipped with TCAS, including a Mode S transponder for the collision avoidance negotiation to work. We will now look at the much more economical, though only slightly less capable Traffic Advisory Systems (TAS) which are arguably affordable for general aviation aircraft. They use similar technology to TCAS, and may issue verbal warnings, but they do not compute a collision avoidance strategy, and hence do not require a Mode S transponder. The airborne TAS interrogates nearby Mode A, C or S targets on 1030 MHz just as if it were a ground station. It does not, however, use a rotating antenna. Rather, it broadcasts the interrogation in all directions and uses a directional antenna to determine where the responses are coming from. It knows the pressure altitude of the responder if he is transmitting Mode C data, and computes the distance from the round trip time to receive a response. This allows the three dimensional position of nearby aircraft to be viewed on a suitable display device. Dedicated displays, MFDs, GPS navigators or PEDs again provided this display function. The main contenders in the TAS market are Garmin with their GTS 800 product line, Avidyne with the TAS 600, and L3 with its Skywatch products. At my local airfield, we are too far from the SSR station to be seen below 4,000'. Thus, some local pilots figure there s no point in even turning on their transponders on local, low level flights. But this makes them invisible to TCAS/TAS aircraft, so please keep the transponder squawking at all times when in the air. Typically, display devices can only be configured to talk to one traffic detection system at a time, so this would mean that you could see the ADS-B Out threats, or the targets detected by your TAS, but not both. However, in an installation with both sensors (TAS and ADS-B), the TAS receives the ADS-B information and merges it with the active traffic before passing it to the display. The ADS-B targets will have identification and heading information available. US Extensions to ADS-B When the preliminary design of ADS-B was carried out in the US, it was determined that if everybody simply replaced their Mode C transponder with a Mode S (ES) unit, the density of traffic in the US is such that the system would become overloaded. The solution to this was to introduce a second ADS-B Out frequency, 968 MHz, which would be used only by aircraft flying below 18,000'. Mode S (ES) transponders are usable at all altitudes, because, after all, even high flyers have to penetrate airspace below 18,000' to get where they are going. Users of this new frequency will retain their existing Mode C transponders, but will have to add a separate 968 MHz box to broadcast their GPS position on that frequency. Like a Mode S ES transponder, the 968 box listens on its own frequency for nearby 968 MHz equipped traffic. Since the 968 MHz device is both a transmitter and receiver, it is referred to as a Universal Access Transceiver or UAT. The UAT is not itself a transponder. It does not respond to interrogation. It is simply an alternative data link designed to broadcast the ADS-B Out data periodically, hence relieving the load on 1090 MHz. As such, its operation is quite a bit simpler than that of a Mode S ES transponder, and it is correspondingly cheaper. We ll see other uses for the receive capability of the UAT shortly. Aircraft equipped with either a Mode S ES transponder, or a UAT that are sending ADS-B Out data to ATC are referred to as ADS-B clients.

The system described so far has two drawbacks: 1. Aircraft equipped with Mode S ES transponders broadcasting and listening on 1090 MHz are not be able to see UAT traffic broadcasting on 968 MHz, and vice versa; and 2. There was no incentive to opt for the 968 MHz box, so operators might all opt for the Mode S solution, particularly as their Mode C transponders became unreliable with age. The solution to the first drawback is to equip the SSR ground stations in the US with the ability to receive both 968 and 1090 MHz data. The ground stations know that, if a particular client is sending his position reports on 1090 MHz, he will be unaware of any client using 968 MHz and vice versa. So, the ground station will rebroadcast data on any 968 MHz clients to the 1090 MHz clients and vice versa. Since both clients listen on their own frequency, they will hear this rebroadcast traffic. Any information received direct from other aircraft on 1090 MHz or 968 MHz is referred to as ADS-B in data from the receiving aircraft s point of view, and the data re-broadcast by ground stations is termed ADS-R. As an additional improvement, the ground stations will also transmit information on any Mode A/C targets that they may be tracking on both 1090 and 968 MHz, so that clients will be aware of this traffic also. Collectively, the traffic information sent out by the ground stations is termed Traffic Information System or TIS-B data. Thus, in the US, an expensive TAS may become less relevant, though it will still provide a display of traffic really close to the ground, such as circuit traffic at airports not covered by SSR. One important feature has to be understood. Only traffic in the vicinity of known ADS-B clients, and only traffic that they will not already be receiving on their own ADS-B Out frequency will be transmitted. The vicinity is, in fact a hockey puck shaped piece of airspace with a radius of 30 nm, and extending vertically ± 3,500' for ADS-R messages and ±5,000' for TIS-B from the reported altitude of the client. Portable receivers are available that can eavesdrop on ADS-R and TIS-B traffic messages sent to client aircraft, but the traffic provided by the ground station will only be that in the vicinity of known clients. If there are no participating aircraft, nothing will be sent at all. So, the traffic displayed by these portable receivers may have little relevance to the aircraft in which the portable receiver is being used. If you want to receive relevant traffic data, you d better be a client i.e. be transmitting ADS-B Out data using either a Mode S (ES) transponder or a UAT. Once you cross leave the US, there will be no TIS-B or ADS-R messages to listen to, so the only traffic you will see are those aircraft equipped with the same ADS-B technology (Mode S ES or UAT) as your own. None of the locals will be using UATs. If you want a traffic system that works reliably outside the US, the only solution is to splurge on a TAS. To make ADS-B compliance, and specifically use of UATs even more attractive, the FAA decided to broadcast flight information messages as well. This information, which includes graphic weather radar imagery, METARs, TAFs, NOTAMs (including TFRs), winds aloft etc., is collectively known as FIS-B. Until recently, this FIS-B data was broadcast to anyone who wanted to listen in, but as of January 2016, these messages will only be sent if there are ADS-B clients in the area. Users with portable ADS-B receivers will still be able to eavesdrop on the FIS-B data, but if there are no clients in the area, the FIS-B transmissions will cease. The message is loud and clear. The FAA wants as many aircraft as possible to become ADS-B clients. FIS-B messages are tailored to be relevant to the area serviced by the ground station. METARs, for example, cover a 100 nm radius. Also, since it is only sent on 968 MHz, you need a UAT or a portable 968 MHz receiver to get it. Clients with only a Mode S ES transponder are out of luck!

The Way Forward So, what is the GA supposed to do? There is no indication that UAT technology will be adopted in any country outside of the US, so if you foresee any need to access ADS-B services internationally, then a Mode S (ES) transponder is your only option. If your Mode C transponder has a problem, consider replacing it with a Mode S (ES) unit, since that is probably the cheapest solution, and will ensure future international compatibility. Most manufacturers have a Mode S ES transponder that takes up the same rack space as their earlier Mode C product. With only a Mode S (ES) transponder, you will not have access to ADS-B weather. But then, that information again stops at the border. While you are able to call up US weather north of the border until you get out of range of US ADS-B towers, they have no weather information on non-us airports. The solution to that problem is to continue to subscribe to XM weather in the territory covered by that service. After 2020 ADS-B will be a requirement in the US after January 1, 2020. Does this mean that all aircraft will be banned from US airspace if they want to continue with just a Mode C transponder? By no means. The ruling will apply only to Class A, B and C airspace, and to Class E airspace above 10,000'. So the light VFR guy will simply have to do some careful route planning to avoid the class B and C airspace around major terminals, and stay below 10,000'. This B and C airspace is outlined with a solid blue or purple line on the sectional charts. A quick glance at one of these charts will show that avoiding such airspace is not too difficult. Many US operators will no doubt adopt this approach. You will not be able to simply get flight following and blast right through the Class B/C airspace on an RNAV direct routing as you often can today, though. The days of using major airports as a customs stop will be over, since places like Syracuse, Rochester, Buffalo and Cleveland (using a southern Ontario departure as an example) will be off limits to anybody who is not ADS-B equipped. Instead, you ll have to use places without Class C airspace, like Massena, Watertown, Oswego County, Niagara Falls, Erie and Sandusky. IFR operators without ADS-B Out will have to avoid the same airspace, and this may result in them being assigned some fairly devious routings. Their exclusion from airspace above 10,000' may be more significant to IFR operators than to VFR. No more going up there to get out of icing conditions or turbulence, or to take advantage of tailwinds. If you don t feel comfortable living with these restrictions, are there some inexpensive options for making an airplane ADS-B compliant? Until recently, the need for a certified GPS source meant an IFR certified GPS navigator, such as a Garmin 430W, but those who waited to see how technology would respond to this mandate have been rewarded. You can now buy a Mode S transponder with a built-in a certified GPS source for around $4,000 not much different from the cost of a Mode S transponder alone. That will make you compliant with the 2020 mandate. Currently, there are few international requirements for ADS-B carriage by the little guys. If you just want to comply with the 2020 mandate, a standalone UAT may well suffice. Conclusion You will still be able to fly in the US after 2019 without being ADS-B compliant, but your operations will be somewhat restricted. There are some fairly inexpensive options to meet the ADS-B mandate, but you will not enjoy the benefits of ADS-B in as implemented in the US. The equipment required to become an ADS-B client will do little to enhance your capabilities outside the US (unless you fly overseas to countries requiring ADS-B equipment), but may be useful when Canada gets space-based ADS-B working.

If you want to receive FIS-B messages, and have adopted the Mode S ES method of compliance, a portable dual channel receiver and PED will fill your needs. If, on the other hand, you have gone the UAT route, then you can send FIS data to any suitable display, including a PED. If you can afford it, you are probably better off with XM Weather, which covers both the US and Canada, and has higher resolution weather radar information in areas where this service is available. Let s just hope that the availability of free ADS-B weather to UAT users south of the border does not change the business case for XM Weather. The ADS-B services just described are already up and running in the US, and many operators are starting to take the warnings about not waiting until the last minute seriously, and are becoming ADS-B equipped. Low flyers tend to opt for a UAT, given its access to free FIS data. If you are not ADS-B equipped, you may already get the feeling that you are a second class citizen when flying down there, and this will only increase as 2020 approaches. In the concluding part of this article, I ll describe my personal approach to becoming ADS-B compliant, using a product that came on the market in 2015 the L3 Lynx transponder.