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Sunday, September 1, 2013

Space-Based ADS-B

Automatic dependent surveillance-broadcast (ADS-B) is a cornerstone program of the NextGen airspace modernization initiative. It promises to allow for more efficient communications and operations between air traffic control facilities on the ground and pilots in the air.

ADS-B has been in use in various terrain-constrained parts of the country, namely Alaska, for many years. ADS-B Technologies, based in Anchorage, and its partner Globalstar have developed what they call a practical and cost effective space-based ADS-B system that is compatible with virtually any 978 Megahertz Universal Access Transceiver, or 1090 MHz Extended Squitter (ES) ADS-B avionics source. The companies discussed this partnership and their experiences with ADS-B implementation during an Avionics Magazine Webcast, “Real Space-Based ADS-B for NextGen.” A partial transcript of the Webcast follows.

Skip Nelson, president and CEO, ADS-B Technologies

We fly with these systems almost every day, and we’ve seen them save lives, and we’ve seen them save money. ADS-B really works.

ADS-B Link Augmentation System (ALAS) does not replace existing ADS-B avionics. It is simply a peripheral component that can take ADS-B global by dramatically increasing the range and capability of an already certified and proven technology.

Instead of communicating directly with the ground station, ALAS takes the ADS-B payload and sends it to the Globalstar constellation via an L-Band link. The satellite then turns the signal and transmits it to the Globalstar gateway on C-Band. At the gateway, the process is reversed, and the payload is converted by a local GBT into an ASTERIX Cat 21 or Cat 33 message, and then sent via secure circuit to an ATC automation system. Alternatively, the payload could be sent downstream in a raw format to be processed by an existing ADS-B network. For ADS-B In, the process is simply reversed, and S-Band is used.

So the ALAS system architecture is deceptively simple and straightforward. We begin with the equipment that one would find in any terrestrial ADS-B network; that is, the installed aircraft ADS-B avionics and the ground component, which is almost always a ground-based transceiver (GBT). Then we add the ALAS components. In the aircraft, we simply couple to the RF output of the installed avionics and convert the ADS-B message into an L-Band transmission, which is then sent to the satellite and immediately retransmitted to the ground on C-Band, where the process is reversed and the message becomes available as an RF input to the GBT.

We have done this in countless tests over many flight hours, and I promise you that it works and it works very well. It works with Globalstar’s first-generation 9.6 kilobits-per-second speed, and it will work even better with their second-generation 256 kilobits-per-second speed.

So why did we choose Globalstar for the backbone of our system? … When we fully considered latency, reliability and flexibility, in terms of growth and future modifications, a simple, high-speed, bent-pipe LEO seemed the obvious choice.

We also like Globalstar because although they had had some technical problems in the mid-part of the last decade, they were now coming back with a vengeance, successfully launching a new generation of satellites and almost doubling their capacity every few months.

To us, Globalstar is fast, less than 200 milliseconds aircraft to ground, extremely reliable; and for us, reliability means the best safety case with the fewest potential points of failure. It’s also efficient. When it comes to air traffic control, a bent pipe with a regional gateway ground station gets the ADS-B message from the aircraft to the controller’s glass as quickly and as directly as possible.

We also see no problem with the concept of many ground stations rather than many satellites. Satellites can cost $50 million each, but a Globalstar gateway ground station may cost less than $10 million, and it provides a wide range of additional services, such as satcom, asset tracking and machine-to-machine interfaces, in addition to air traffic control.

Many ground stations also promote global harmonization. We are convinced that most nations will want their surveillance data landed as quickly and as directly as possible, either within their own borders or at least within their own region. Globalstar is ideal because additional gateway ground stations can be built quickly and economically.

And finally, with virtually no buy-in for the ANSPs and a very modest equipage cost for the aircraft owners and operators, we are confident that the ALAS Globalstar concept will be the least-expensive and most-efficient solution for all concerned, now and in the future.


Mike Melum, chief scientist and engineering director, ADS-B Technologies

ALAS can fill in coverage gaps, or it can operate stand-alone in remote areas, where there is no ground-system coverage. It can provide many additional enhancements and features using Globalstar’s additional available bandwidth, and with the second-gen system, up to 256 kilobits per second.

ADS-B originally was designed to allow a statistical maximum of about 500 or more aircraft in the area of a single ground station, to utilize the same frequency to broadcast their data bursts and to receive broadcast data from the ground station.

ALAS is inserted in the RF coaxial line between the existing ADS-B transceiver and the aircraft 1090- or 978-Megahertz antenna with no effect on the existing ADS-B operation.

Each second, the aircraft’s ADS-B transceiver sends a burst of critical data. Each second, ALAS couples off a sample, or a snapshot, of that transmission without degrading the data in any fashion. That sample is then reformatted for transmission over the Globalstar satcom link. On earth, it is transformed exactly back to its original form, as if it were received directly via the existing terrestrial system for insertion into air traffic control automation.

This approach was taken so that the system can be added to an existing ADS-B installation, preserving the customer’s investment in ADS-B equipment, no change to air-to-air functionality, and without impacting the progress towards equipage and satisfying the 2020 mandate.

As many of you are aware, there are other ongoing efforts to establish space-based ADS-B. ALAS is highly reliable. Since it uses proven Globalstar full-duplex link with forward error correction and handshaking to ensure that each second, ADS-B updates occur without error.

This is in contrast to other approaches, where the actual ADS-B signal from the aircraft would be received in space and subjected to a wide variety of propagation anomalies and potentially tremendous interference from other aircraft and ground systems. In other words, with that system approach, you might receive a usable packet within 15 seconds, or so. And this also assumes that the aircraft is above a relatively high latitude where there’s overlapping satellite 1090 spot beam coverage.

The full-duplex connectivity of Globalstar adds great utility to the ALAS approach. ALAS conveys not only the ADS-B Out data from the aircraft; it can provide ADS-B In and many other functions. ALAS is designed to utilize the full-duplex secure 256-kilobit-per-second Globalstar connection to the aircraft to provide two-way voice and data very economically between aircraft and ground facilities. Weather camera photos, flight data recorder backhaul, text messages and much more are just a few possibilities that can flow over the available bandwidth.

ALAS requires minimal equipage, but the benefits are significant. And by minimal equipage, we mean a single, small, lightweight ALAS box with an interface to aircraft power and installation of a small-patch antenna on the top of the aircraft. The ALAS box is coupled to the RF coax between the ADS-B transceiver and its antenna. There can be other interfaces depending on which auxiliary capabilities are chosen to utilize the available full-duplex bandwidth of the Globalstar network. Also with ALAS, there’s no possibility of packet collisions or interference, as with traditional ADS-B, since each aircraft has its own unique connection to the satcom link.

Voice transmissions on the average are assumed to be about 30 seconds in duration, about every 10 minutes, so voice requires more bandwidth but for short duty cycles. Simultaneously, ADS-B In data can flow into the aircraft while flight data recorder-type information can flow in the opposite direction, all at a very consistent rate.

This can all happen at today’s 9,600-bit-per-second rate. Once second-gen is in place, much, much more capability will be available, and all of the information over this link can be prioritized.


Tony Navarra, president of Global Operations, Globalstar

Since 1999, Globalstar has operated a low-earth-orbiting satellite constellation, utilizing a unique bent-pipe architecture that offers several key advantages over other satellite systems. This architecture was chosen to provide low-latency voice and high-speed data services to mobile subscribers. These advantages are extremely important for any aviation-related services, such as a space-based ADS-B service.

The altitudes of the satellites, along with the distributed locations of the gateways over the earth ensure a redundant signal path to and from multiple satellites, as subscriber terminals are connected to the public-switch networks, the internet, or virtual private networks, VPNs.

Our bent-pipe architecture allows the satellites to act as mirrors in the sky, that merely retransmit the signals they receive from subscriber terminals to the gateways, and vice versa, and then on to other subscribers, the internet, or the VPNs.

The intelligence or control and management of the various services and features are maintained at the gateways and not in the satellites. In other words, the brains of the Globalstar system are on the ground, rather than in the satellites, which permits easy and efficient upgrades to the network, as well as the ability to add new services.

This capability is very important for systems such as ALAS. The aviation industry will not be stuck for years with whatever systems happen to be installed or attached to a satellite. Rather, with Globalstar’s solution, enhancements and upgrades can be added dynamically over the 15-year design life of our satellite constellation.

The Globalstar satellites operate in eight planes in a 52-degree inclined orbit to the equator, 45 degrees apart. This configuration creates a mesh of overlapping satellite beams as they pass over the earth’s surface. The Globalstar’s constellation uses code division multiple access, or CDMA, technology, to assure the highest quality of signals, low latency, high speeds, low probability of signal intercept, and redundant overlapping signals, as subscribers units receive and transmit their data to multiple satellites, combining signal strength and assured reception.

The Globalstar system is now in its second generation of operations. Services began in 1999. The 24 second-generation satellites have all been launched, and 22 are now in service, with the remaining two satellites going into service by August of this year.

The satellite operations and ground operating control centers, called SOCCs and GOCCs, control and manage the connection of phone calls worldwide, providing mobile business-to-business data applications to rural subscribers and ensuring security position location information to both individual subscribers as well as companies for their employees and their asset locations. We have two SOCCs and two GOCCs that are fully redundant that maintain and assure our 24-hour by 7-days-a-week operations.

The C-Band frequency is used to provide wide-band signalling between the gateways and the satellites, and the L- and S-Band frequencies are transmitted and received via the 16 transmit and 16 receive beams on the Globalstar satellites. These frequencies carry the voice and data between the satellites and the user terminals, which are already deployed in many different products and in subscribers’ hands today.

ADS-B Technologies used a specific data modem, our model 1720, to successfully conduct its airborne field trials, as previously described. In real-time, the aircraft ADS-B waveform is transmitted by ALAS and received at the satellites and gateway and is then forwarded over the Globalstar data network to the public network, infrastructure, internet or private VPN links, as requested by the airline, fleet operators and aviation regulatory agencies.

We used our extensive traffic-modeling capability, which has been in place for more than 15 years here at Globalstar, to analyze Globalstar’s ability to carry the ALAS signalling. Our initial results indicate that we can serve 8,000 aircraft over North America; and with the upcoming upgrade to our second-generation ground infrastructure, we can continue to meet ADS-B’s planned link-augmentation system and the growth of civil and commercial aviation use well into the future.

To listen to the full version of this Webcast, visit www.avationtoday.com/webinars.

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