Dr. Stephan Schulz, COMSOFT GmbH

Why and how ADS-B and Multilateration technology will become the leader in future surveillance

Introduction

Automatic Dependent Surveillance - Broadcast (ADS-B) is a compelling technology to provide unified surveillance to both airborne and ground parties. An ADS-B-OUT equipped aircraft periodically broadcasts its own position, determined via its navigation system and ultimately derived using a Global Navigation Satellite System (GNSS), typically GPS. The signal is received by ADS-B ground stations, decoded, and used to build an air situation picture.

ADS-B can be used over several different link layers, including UAT, VDL Mode 4, and 1090MHz Mode-S. However, transmission in the form of 1090MHz Mode-S extended squitter messages has gained significant support and seems to be most likely to become the future standard at least for large commercial aircraft, which typically already carry Mode-S transponders and can easily accommodate this new technology.

Using this technology, aircraft broadcast their position twice per second in the form of 112 bit Mode-S messages. Such a message contains the unique 24 bit Mode-S address of the aircraft, an encoding of the aircraft position, and, typically, the barometric altitude. Additional messages can carry further information, including call sign, speed, heading, and geometric altitude.

One major advantage of ADS-B over conventional radar surveillance is that the ADS-B signal can be received by ADS-B-IN equipped aircraft and displayed on a Cockpit Display of Traffic Information (CDTI), providing the pilot with additional awareness about nearby aircraft. However, even in the pure air-to-ground form, ADS-B is extremely attractive for a number of reasons.

• The ADS-B position is derived by the aircraft and transmitted digitally. Hence the accuracy of the position is typically high. It also is independent of the range between sensor and aircraft, simplifying integration of several sensors into a tracked air situation picture.
•  The position is updated twice per second, independent of constraints by the ground station as those imposed by e.g. a rotation antenna.
• The position is broadcast automatically, without the need for interrogation. Receivers can use simple omni-directional or wide-angle antennas to receive all messages from a large, defined part of the air space. A comparatively small, fixed antenna is sufficient to receive reports from up to 200NM away, as long as a clear line of sight can be established.
•  Since transmission is spontaneous, the number of messages is independent of the number of receivers. This results in a much more efficient use of the limited bandwidth of the 1090MHz frequency band than with conventional secondary or even Mode-S radar surveillance.

These advantages are offset to a certain degree by disadvantages.

• As with any secondary surveillance technology, successful surveillance requires the cooperation of the targets. However, pure ADS-B not only relies on a functional transponder, but also on the integrity of the aircraft navigation system. If this fails, the aircraft will not be able to broadcast its position, or worse, it may broadcast invalid positions.
• Similarly, it is relatively easy to broadcast fake ADS-B messages simulating non-existent aircraft. Both of these cases are broader, but not substantially different in risk from a classical secondary radar transponder reporting a wrong Mode-C altitude, however.
• For complete coverage, all potential targets have to be equipped with ADS-B capable transponders. As the technology is introduced gradually, there is a transition period in which the full benefit of pure ADS-B cannot be realized.
• Since ADS-B messages are broadcast, they are available to everybody with the right equipment. Except for regulatory action, there is no way to restrict the availability of aircraft positions.

Australia has already decided to introduce mandatory ADS-B over 1090MHz, and will complete the move by 2009. Other important air-spaces, e.g. the United States of America and the EUROCONTROL zone, are also moving in this direction. However, even now flexible deployment of the latest generation ADS-B sensors can mitigate or even negate many of the problems listed.

State-of-the-art ADS-B Sensor Technology

The new generation of ultra-compact ADS-B sensors from COMSOFT is designed from the ground up for its intended purpose. As a result of this design-for-purpose, they combine low energy consumption, attractive deployment options, and impressive capabilities. ADS-B sensors are available as commercial off-the-shelf (COTS) components.

These sensors are available in both outdoor and indoor variants. Outdoor versions can be easily mounted close to existing communication infrastructure, and offer a complete surveillance solution even under extreme environmental conditions and without access to a sheltered environment. Indoor versions are 2U 19" rack units and allow the convenient deployment within existing communication or processing facilities.

Actual capability and energy consumption are similar or identical for rack-mounted and out door in ASTERIX Category 021, without the need for further processing. Of course, the plot data generated by the sensor can contribute to the processing of a suitable tracker, e.g. the EUROCONTROL ARTAS, for integration with other data sources and a more stable, tracked air situation picture.

In addition to direct ASTERIX output, sensors may also offer access to the raw Mode-S messages received from the aircraft. Time synchronization, whether via GPS or other means, allow the sensor to forward these messages with a high-resolution, high-precision time stamp that is sufficient for Time Difference of Arrival (TDOA) applications like Multilateration.

Finally, the new sensor generation supports remote monitoring and control, typically via SNMP. Continuous self-monitoring and testing enables the timely detection of problems with the sensor, and can quickly provoke corresponding reactions by users of the sensor data.

Stand-alone ADS-B

The simplest possible deployment method for ADS-B is as a stand-alone system. As stated above, state-of-the art ADS-B sensors can generate a complete air situation picture without the need for subsequent post-processing. While the picture is not tracked, the high update rate and high fidelity of the ADS-B position reports generate a high-quality picture that is an excellent situational awareness tool.

Several features of modern ADS-B sensors support such a stand-alone use. This includes the ability to independently send surveillance data to various clients. The use of geographical, altitude, and address filters allows focusing on relevant parts of the traffic.

Stand-alone ADS-B surveillance as a complete surveillance solution is particularly attractive for smaller airports that so far have no independent surveillance system, and that cannot afford a radar-based solution. In Europe, increased competition by low-fare airlines has led to significantly increased traffic to such airports.

Even in cases where other surveillance systems already exist, ADS-B can be usefully employed. One such opportunity is the filling of gaps in conventional surveillance. Outdoor ADS-B sensors have only minimal requirements with regard to placement and environment. As purely passive devices, there is no problem with electromagnetic emissions and interference. Hence the sensors can be deployed nearly wherever power supply and data network connectivity can be established. This allows positioning on or behind line-of-sight obstacles partially blinding e.g. a secondary radar system. In this way, ADS-B allows more complete surveillance coverage at low level or in rugged terrain.

In addition to such spatial gap-filling, ADS-B can be a cost-effective temporal gap-filler, i.e. it can be used as a redundant backup system for a more conventional surveillance system, either as a warm standby or even as a replacement system for scheduled downtime.

High-integrity ADS-B

As stated above, one of the disadvantages of ADS-B is the reliance on the aircraft navigation system. Using pure ADS-B, there is no way to verify the position or even presence of an aircraft. However, modern ADS-B receivers often include a highly stable system clock, kept synchronized via GPS. Thus, they are able to generate highly precise time stamps. If at least two spatially separated ADS-B sensors receive a given extended squitter message and can deliver it with a sufficiently precise time stamp to a central high integrity ADS-B controller, the central controller can use simple time difference of arrival (TDOA) techniques to perform a plausibility check on the announced position.

For this purpose it computes the distances from the announced ADS-B position to both sensors. Since the Mode-S signal is travelling at the known speed of light, any difference in the distances between the target and the two sensors should result in a corresponding difference in the time of arrival of the signal at the involved sensors. The high-integrity ADS-B controller verifies if this delay is observed within the limits of accuracy. While for the case of two sensors, all the points on a known hyperboloid yield the same TDOA, the chance that the announced position is on this hyperboloid by accident is negligible. Moreover, if observations for a moving target remain consistent over time, a very high confidence can be placed in the correctness of the announced position.

A high-integrity ADS-B solution has a number of advantages. While still using a small number of sensors and rather simple central equipment, it can distinguish verified, unverified, and known invalid ADS-B plots. Moreover, if a sensor fails, the system gracefully degrades to a normal ADS-B solution. It still requires targets with ADS-B-OUT capability, though.

Multilateration and ADS-B

If a population of mixed ADS-B and non-ADS-B targets has to be controlled, the surveillance solution cannot rely on an announced position. However, if multiple sensors receive a given signal, the TDOA technique can be extended to full Multilateration. This approach does not rely on the contents of the received messages, but only on the ability of a central Multilateration controller to identify messages received at different sensors and compare their time of arrival.

As mentioned above, the TDOA between two sensors constrains the position of the emitter to a hyperboloid. If more than two sensors receive the signal, any pair generates such a solution hyperboloid and independently constrains the position of the target. Three sensors are sufficient to pinpoint a target on a two-dimensional plane, while four sensors will determine a unique three-dimensional position.

Since the technique does not depend on the message contents at all, it can be driven by non-ADS-B transponder messages. In particular, the TCAS DF11 acquisition squitter will allow tracking of all TCAS-equipped aircraft. Furthermore, TCAS replies and other transponder messages can increase the frequency of updates. Since TCAS and Mode-S are mandated in several air-spaces, this solution currently has even wider applicability than pure ADS-B.

To further improve the coverage and update frequency, a simple interrogator can be added to the system to request specific information from aircraft transponders.

While Multilateration requires a much higher sensor density than either ADS-B solution, it offers the advantage of completely independent position derivation, and also can handle non-ADS-B targets. For ADS-B equipped targets, on the other hand, it offers two independent surveillance data streams from a single sensor network.

Conclusion

Even without full equipage of air-fleets, modern ADS-B sensors can significantly increase the availability and quality of air traffic surveillance. The low cost and high flexibility of these sensors make them attractive as back-up and gap-filling solutions, or even as primary surveillance solutions in low-traffic/low-risk situations where radar is not cost-effective. A redundant pair of ADS-B sensors, directly connected to a suitable display, provides air traffic control with a high-quality air situation picture.

Higher integrity can be ensured via TDOA analysis at multiple sensors. Even with just two spatially separated sensors, high integrity of the ADS-B data stream can be ensured. Using a complete sensor net, the system can independently derive the position even for non-ADS-B aircraft, and thus be a complete replacement for secondary radar.

Since the various applications use the same sensor technology, the entry barrier to these new technologies is low. A single ADS-B test installation can be gradually extended to whatever level of coverage and reliability is ultimately required.