ADS-B Primer

ADS-B Primer
ADS-B Primer
A pilot’s guide to practical ADS-B information without the acronyms
Updated January 29, 2014
FlyQ EFB includes support for in-flight weather and traffic through ADS-B receivers. ADS-B is a complex
topic including a myriad of acronyms (such as “ADS-B” itself), frequencies, and technical concepts. This
document attempts to translate the important ADS-B concepts to English.
ADS-B (“Automatic Dependent Surveillance-Broadcast”) is a system that the FAA is in the process of
deploying across the country that provides weather and, in some cases, traffic information while inflight. There is no charge to use the system other than purchasing an ADS-B receiver (typically $600$1400).
ADS-B is broadcast from ground-based stations not satellites (like XM). Thus, reception is limited by
terrain and altitude. In addition, the national roll-out of ADS-B is not yet complete so coverage is better
on the East and West coasts than in the Midwest.
Although ADS-B is broadcast in a standard data format and there is a commonly accepted basic standard
for devices to communicate with each other (Garmin GDL-90 format), each device vendor takes a few
liberties and adds their own take on the offerings. Thus, unlike the case with GPS devices, iPad apps
need to be specifically programmed for each and every different device; there is no universal
The most popular devices for iPad use are the XGPS 170 from Dual (the makers of the red hockey-puck
GPS), Stratus from Appareo, Clarity from Sagetech, GDL-39 from Garmin, SkyRadar from SkyRadar
Corporation, and iLevil from Levil Technologies. Many of the units work only with one iPad application.
Thus, unlike with an external GPS, pilots cannot always use an existing device on a new app.
ADS-B provides for sending traffic information to aircraft but it is dangerous to rely upon. Because of
limitations that the FAA imposed regarding when a ground station may send traffic information to an
aircraft, it is likely that all traffic data will not be available to most aircraft using ADS-B receivers.
Specifically, traffic information is only broadcast to an aircraft when a certified, in-dash ADS-B OUT
transmitter is within 15 NM of the ground station. The FCC does not allow any portable ADS-B device to
transmit. Thus, users of iPad-connected portable ADS-B receivers only see traffic if another aircraft with
an installed ADS-B OUT system is nearby. The problem is that the ADS-B receiver can’t differentiate
between there really being no other planes in the area and lots of planes in the area but none with a
certified, in-dash ADS-B transmitter causing traffic information to flow. Thus, pilots must be very aware
of the risks of relying on ADS-B traffic.
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Coverage Area
The FAA planned to have ADS-B coverage available for the entire United States by the end of 2013 but
they didn’t quite make it.
The graphic below shows where the operational stations are located as of December 2013. Dots
represent each of the operational stations. Each station broadcasts over a fairly large distance so you
should visualize “clouds” of coverage around each dot. Therefore, the East Coast, West Coast, and the
South are well-covered now. Some areas of the Northern Rockies are the last to remain without
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Acronyms and Frequencies
ADS-B is filled with acronym after acronym. Some of the more commonly-used ones are TIS-B, FIS-B,
UAT, 978 and 1090ES. None of them are especially critical to understanding how to get weather in the
TIS-B refers to traffic information while FIS-B refers to weather.
UAT and 978 MHz refer to the frequency used to communicate between ground stations and aircraft
flying at normal GA altitudes. 1090 ES (or 1090 MHz) refers to the frequency used to transmit from
aircraft to aircraft. Currently, it’s primarily used for commercial airline data rather than GA information.
All ADS-B receivers work on the 978 MHz frequency to receive ground-based weather and traffic. Some
of the more expensive units also receive (but not transmit) on the 1090 MHz frequency. These units can
receive traffic from an effectively larger area because they can receive “relayed” information from other
aircraft. Weather is never sent over the 1090 band. In other words, if Plane A is equipped with a 1090
MHz receiver but is too far from a ground station to receive data, it might receive traffic (but not
weather) data from Plane B if Plane B is between Plane A and the ground station and Plane B has a 1090
transmitter (ADS-B Out).
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By far the most common use for ADS-B is to receive weather updates while flying. Obviously this
dramatically enhances in-flight safety, especially for IFR flights. The FAA’s ADS-B broadcast provides
several weather products but some weather products are broadcast more frequently than others.
The following table describes the maximum range of the coverage (that is, how far away is the weather
you see) and the best-case time between weather updates. It is very important to note that the actual
coverage area (especially for radar) may less and the transmission frequency is best-case. In real-world
use, the information may come quite a bit less often due to reception issues and such. There is also
considerable difference in the radio sensitivity of different models of ADS-B receivers so one receiver
may see weather data that another receiver model will miss.
Weather Product
Maximum Coverage Area (radius)
Regional Nexrad Radar
US Nexrad Radar
250 NM
100 NM
100 NM
100 NM at airport surface, 500 NM
1000 NM
100 NM
500 NM
500 NM
Winds/temps Aloft
TFRs and NOTAM (D)
Approximate transmission
2.5 minutes
15 minutes (usually less often)
5 minutes
10 minutes
5 minutes
10 minutes
10 minutes
10 minutes
10 minutes
Do not confuse frequent transmission with more accurate information. In particular, NOAA only
updates TAFs and Winds Aloft every 6 hours. Thus, although you’ll see data come in every 10 minutes,
the data is usually exactly the same as the last broadcast.
It is important to realize that METARs and TAFs flow into the system slowly; you do not get the entire
country with each data burst. Thus, especially when the ADS-B system has been running for less than 30
minutes, you may not see METARs or TAFs from your nearest airports. When you select weather for a
particular airport in FlyQ EFB, the app will find the nearest current METAR or TAF from the selected
airport; this may not be the selected airport itself. Thus, it is possible, especially when the system is
starting up, that you’ll see METARs or TAFs from airports other than the selected one. This is clearly
marked on the FlyQ EFB display but pilots should be careful to notice the airport from where the METAR
or TAF is coming.
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ADS-B’s original purpose was to provide better traffic information to pilots and to ground-based
controllers to allow more efficient use of the national airspace (NAS). Weather data was added later, as
something of a “sweetener,” to encourage adoption of the system.
The basic concept was to mandate that all aircraft have what was essentially a more sophisticated
transponder that sends more information to the controllers. This is now the law and all aircraft are
required to have such a system by 2020 (unless Congress later delays the deadline, of course). This
system, called ADS-B “Out,” transmits information from the aircraft to ground stations. Upon receipt of
this data, the ground station replies by sending a stream of traffic information to the transmitting
aircraft and, because it’s broadcast not point-to-point, to all other aircraft in the area.
Crucially, no traffic information is broadcast from a ground station (978 frequency) unless there is an
aircraft nearby (within a 15 NM radius and within 3,500 ft AGL) that is equipped with an ADS-B Out
ADS-B Out requires a transmitter. However, the FCC and FAA only allow transmitters on certified
systems. The FAA does not allow any portable system (like one connected to an iPad) to be certified.
Thus, it is impossible to purchase a portable ADS-B Out system; you must purchase a much more
expensive certified system and have it installed in your plane to get ADS-B Out.
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In other words, although the ground stations have traffic information, they will only broadcast it when
an aircraft with a certified ADS-B Out system is within 15 NM of the ground station; there can be several
ADS-B In receivers in an area and the ground station will not transmit traffic unless there is also an ADSB Out system in the area.
The problem this creates cannot be understated. An ADS-B receiver that is not receiving traffic
information may not be receiving it either A) Because there is no other plane in the area or B) Because
there is simply no ADS-B Out plane in the area. The receiver cannot tell the difference. Thus, a pilot
looking at an iPad screen with ADS-B traffic can be misled into believing there is no traffic when, in fact,
there is considerable traffic but none with ADS-B Out. We believe this is dangerous and, therefore,
strongly advise that pilots rely primarily on their eyes, not their iPad, when looking for traffic.
In addition, in our testing, we see far more reliable traffic from dual-band (978 and 1090) receivers so if
traffic is important to you, we suggest you purchase a dual-band receiver such as the Clarity. This is
because these receivers get the same limited traffic from 978 as single-band receivers but also pick-up
traffic data from airliners flying overhead on 1090. As no weather is sent via 1090, weather is reliant on
ground stations and 978 and not directly affected by the use of a single versus dual –band receiver. That
said, we find the radio sensitivity varies dramatically between different brands so some ADS-B receivers
receive weather and traffic better than others. Thus, we suggest that pilots investigate each ADS-B unit
carefully; unlike GPS units, there really is a significant difference between models.
Bluetooth vs. Wi-Fi
Most ADS-B systems use Wi-Fi to communicate with the iPad. This is because Apple tightly controls
access to Bluetooth devices that want to connect to the iPad. Most manufacturers have not gone
through the time and expense to get this Apple certification. Wi-Fi, on the other hand, is completely
accessible to all iPads so it’s commonly used to avoid Bluetooth certification.
Bluetooth uses less power than Wi-Fi thus helping to preserve the iPad’s battery life. More importantly,
an iPad may only connect to one Wi-Fi device at a time while it may connect to multiple Bluetooth
devices (plus a Wi-Fi device). Thus, if the ADS-B receiver is using the Wi-Fi connection, the iPad cannot
connect to the Internet and cannot connect to other devices that also use Wi-Fi.
A good example of this problem is trying to use an ADS-B receiver along with a separate AHRS device.
AHRS units provide yaw, pitch, and roll information that is crucial to accurate Synthetic Vision, such as
the one included with FlyQ EFB. The most common standalone AHRS unit is made by Levil and uses WiFi. If a pilot chooses an ADS-B system that also uses Wi-Fi, he cannot connect to both devices at the
same time. If, on the other hand, the pilot chooses a Bluetooth ADS-B receiver, he may also add the
Levil unit. Of course, this specific problem goes away if the pilot chooses an ADS-B receiver that also
includes an integrated AHRS. Clarity, Levil, Appareo, and Garmin (the GDL 39 3D) make such a device
but they cost more and, if you already have a Levil AHRS, it means not using a device you already own.
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Common ADS-B Receivers
There are no portable ADS-B transmitters. The following are some of the more common portable ADS-B
receivers. Prices are not listed as they change quickly. However, the general range is between $600 and
$1,400 depending on the brand and the specific features of the device.
Dual XGPS 170
This unit is made by the same company (Dual) that makes what is probably the most popular aviation
GPS for the iPad, the Dual XGPS 150 (the red hockey puck). Unlike most other ADS-B units, Dual has
gone through the Apple Bluetooth certification process so this unit uses Bluetooth rather than Wi-Fi to
connect to the iPad. This is a major benefit as it frees the iPad’s Wi-Fi system to talk to the Internet or
emerging in-cockpit systems such as the Aspen Connected Panel. It also means the iPad requires less
power to communicate with the device.
FlyQ EFB works with the Dual XGPS 170. FlyQ EFB automatically detects the presence of a Dual ADS-B
receiver and immediately begins receiving weather and traffic data from the device. No manual
configuration is necessary once the one-time Bluetooth pairing between the iPad and the Dual unit is
The unit itself is small, well-built, and battery powered. It includes a WAAS-enabled GPS but no AHRS. It
works with AOPA FlyQ EFB and several other iPad apps so it’s a safe investment.
The XGPS 170 is a single-channel (978 MHz) receiver so it receives data from ground stations but not
from aircraft transmitting on the 1090 MHz frequency. As a single-channel receiver, its traffic is more
limited than a dual-band receiver.
Clarity by Sagetech
Sagetech makes very precise, military-grade systems. Their new Clarity line of ADS-B receivers have
exceptional technical specs, giving them superb reception and reliability characteristics.
There are two models, both of which include an internal 6-8 hour battery and use Wi-Fi to communicate
with the iPad. Both units are physically small and very well-built.
The first model, simply called Clarity, is a WAAS-enabled GPS with a dual-channel (978 MHz and 1090
MHz) ADS-B receiver that receives data from both ground-based and in-flight systems. This effectively
increases the range of coverage. The second unit, called Clarity SV (for Synthetic Vision), adds an AHRS
unit so one device can also provide yaw, pitch, and roll data to systems such as AOPA FlyQ EFB that
include Synthetic Vision (3D). Clarity works with a number of different iPad apps including FlyQ EFB.
Stratus by Appareo
There are two Stratus units. Both are battery-powered ADS-B / WAAS GPS systems that are sold
exclusively by Sporty’s and work exclusively with ForeFlight. They both use Wi-Fi rather than Bluetooth
and have eight hour battery lives.
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The original model is a single-channel receiver that works on the 978 MHz frequency only and does not
receive data from other aircraft on the 1090 MHz frequency.
The newer model is smaller and includes an AHRS. It receives on the 1090 MHz frequency as well as the
978 MHz frequency.
SkyRadar by Radenna Systems
The original SkyRadar ADS-B was the first widely-used ADS-B system for the iPad. The original unit has
been supplanted by two newer units: SkyRadar L and SkyRadar-D2. Both include a WAAS-enabled GPS
and use Wi-Fi to communicate with the iPad. Neither unit is battery-powered so must be plugged into
ship’s power.
The SkyRadar-L is smaller, less-expensive, more modern-looking device. It’s a single-channel 978 MHz
unit. The larger D2 model adds reception on the 1090 MHz frequency for air-to-air data broadcasts.
As single-channel receivers, their traffic is more limited than dual-band receivers.
Garmin GDL-39
The Garmin GDL-39 is a Bluetooth GPS / ADS-B receiver that works exclusively with the Garmin Pilot iPad
The unit includes both 978 MHz and 1090 MHz support for both ground-to-ship and ship-to-ship
reception. It requires a corded connection to ship’s power or an optional battery pack. The base model
does not have an AHRS but the new GDL-39 3D includes an AHRS.
iLevil from Levil Aviation
Levil pioneered the portable AHRS market with their AHRS-G device (which is supported by FlyQ EFB).
There are two iLevil units. The iLevil SW is a new and completely redesigned system that begins with the
AHRS and WAAS GPS support in the older device and adds ADS-B capabilities. Like the older unit, it uses
Wi-Fi to communicate with the iPad and runs for up to 3 hours on the internal battery or can be
powered by a USB connection to a power source. It also includes photovoltaic solar cells to help charge
the device but these solar cells do not provide enough power to replace USB charging.
The second iLevil unit is the iLevil AW and includes everything in the SW version (except the solar panel)
and also includes a port for receiving pitot-static data from experimental aircraft to measure pressure.
The iLevil AW is physically very different than the SW unit.
The iLevil units are supported by several iPad apps including AOPA FlyQ EFB.
Both iLevil units are single-channel (978 MHz) units so have limited traffic visibility.
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