Quadrant: From Mode S to Multilateration

Author: Dr Michael Leeson, Development Teamleader ATE (Air Traffic Equipment), COMSOFT GmbH

COMSOFT, the German ATC supplier, continues to develop its suite of surveillance products to provide a complete end-to-end solution. After establishing itself as a competent and reliable supplier of equipment for processing, tracking, analysing and displaying many types of airspace information they then turned their attention to the collection of surveillance information; the result was Quadrant.

Quadrant has developed from a single ADS-B sensor into a comprehensive and flexible surveillance solution. Quadrant now incorporates multiple ADS-B sensors with a central processor to enable multilateration; it provides local transmitters for site monitoring, reference transponders and ground-based vehicle transponders; it includes a distributed interrogation capability to extract information from Mode A, Mode C and Mode S aircraft.

Quadrant has developed to meet the changing requirements of established and emerging air navigation service providers (ANSP). The flexibility of the system makes it attractive for many users whether they are looking at a token ADS-B capability, a method of enhancing and augmenting an existing capability or a complete operational surveillance system. The development path has adapted to meet the changing needs and emerging markets.

ADS-B

The development of Quadrant emerged from an existing multilateration system and began with the creation of a robust and flexible ADS-B sensor. Initially ADS-B was seen as the key market as it provided an expandable surveillance solution with the addition of more sensors. The Quadrant ADS-B sensor is specifically designed for deployment in remote areas where regular access for maintenance is not convenient. It is small enough to be carried as hand-baggage and consumes less than 10W. It can obtain its power from an existing supply, via solar panels or over the Ethernet feed that provides its connectivity. The sensor is intended to be installed once and does not require any routine on-site maintenance as there are no moving parts and no fans.

Standards change, improvements can be made and occasional defects need fixing. All software and firmware on a Quadrant ADS-B sensor can be installed, updated, monitored and controlled remotely via the Ethernet connection. The Quadrant Control and Monitoring System (QCMS) provides a comprehensive monitoring capability and enables loading, coverage and numerous performance statistics to be reported.

The compiled ADS-B information is provided via ASTERIX CAT021 messages. All recent versions are supported and individual clients may select the version they wish to receive; new versions are easily incorporated through the remote upgrade process. Up to 20 clients can connect to each sensor to receive an independent stream each with a specific version and a bespoke UAP. However, most users prefer to use the often limited bandwidth between remote sites to transfer data to a single point from where it can be integrated and distributed in a highly flexible way using the Quadrant Integrator.

Multilateration

A geographically distributed network of Quadrant ADS-B sensors provides the basis of a multilateration system. The multilateration capability is an integral part of the ADS-B sensor and no modification or upgrade is necessary.
Quadrant began as a multilateration system and this is where its strengths still lie. The ADS-B sensor and Quadrant Central Processor share the processing necessary to perform multilateration and this minimises the bandwidth demands. The detection of a transmitted signal, its decoding into recognisable formats and the timing of its arrival are all performed within the sensor. There is no need to transfer huge amounts of sampled data: the intelligence to recognise the message resides in the sensor. New RF messages and improved detection algorithms can all be uploaded to the sensor remotely. The sensor is truly installed once and left alone.

Detection and timing of signals is only part of the multilateration solution, the remaining intelligence lies within the Quadrant Central Processor. Synchronisation of the clocks in each sensor is a major issue with any multilateration system and the Quadrant approach makes use of the distributed intelligence of the system. The accurate clock within each sensor is aligned with the clocks in other sensors by processing timing events from reference transponders and GPS. The Quadrant Central Processor uses all of the available information to maintain an accurate and consistent time frame. This provides Quadrant with sufficient positional accuracy to support ground movement, local area and wide area surveillance.

The parallel development of multilateration and an ADS-B capability has meant that the obvious method for identifying a target is via its 24 bit ICAO address (more commonly known as the Mode S address). The availability of this address in both long and short squitter messages, as well as many other downlink formats, makes it easy to associate multiple messages and multilateration plots with the same aircraft. Quadrant is able to create target tracks, based upon this address, and collect a rich data set used to produce both ADS-B and multilateration outputs.

Multilateration surveillance data is provided as ASTERIX CAT020 format messages. In common with the ADS-B output, all recent versions are available and each client may select the preferred version. New versions are incorporated by updating the software in the Quadrant Central Processor.

Other aircraft transmissions can also be used to generate multilateration plots including Mode A and Mode C replies. Quadrant can optionally include other “plot association” algorithms, based upon timing and geographical proximity, to enable additional plots to contribute to a target track. Making use of the content of these messages is harder to achieve; indeed it is not possible to differentiate between Mode A or Mode C without knowledge of the related interrogation. But there is much useful information that can be obtained from interrogation and hence Quadrant provides a complete interrogation capability.

Interrogation

In keeping with the Quadrant philosophy, the interrogation system is provided by multiple remote transmitters, which contain some of the required intelligence, and a Quadrant Interrogation Controller. Multiple transmitters enable a wide airspace to be covered with low-power transmissions which are targeted at a small area. This reduces the impact of Quadrant on other surveillance systems by reducing the amount of False Replies Uncorrelated In Time (FRUIT).

The Quadrant Interrogation Controller was developed to integrate completely with the Quadrant Central Processor in order to meet the demand for multilateration of aircraft without ADS-B capability. The Quadrant Interrogation Controller determines the information required from each aircraft, identifies the most suitable transmitter and schedules the necessary uplink formats. The transmitter network operates to the same time frame as the Quadrant Central Processor and this enables interrogations and replies to be correlated.

Transmitters have the ability to vary transmission power and can use omnidirectional and sectorised antennas to restrict the transmission to the necessary area. The choice of antenna pattern, the optimal power and indeed the location of transmitters is crucial to achieving good performance at an acceptable price. The location of multilateration sensors is also crucial to the achievable accuracy of the system. As the sensors and transmitters are designed to be installed once and left, and as the installation geography is the most costly component to change, Quadrant uses numerous modelling and analysis tools to determine a suitable configuration.

Monitoring, Analysis and Expandability

The Quadrant coverage tool provides a model of system performance based upon terrain data, antenna patterns and the performance of sensors and transmitters. The Quadrant ADS-B sensor is able to detect aircraft beyond 250NM if there is line-of-site. In general, it is terrain which determines coverage and multilateration accuracy that drives the selection of sites. Location of transmitters and sensors, and the expected performance can be visualised.

A model is only as good as its data and no chances are taken with the Quadrant coverage tool. Real sensors can be deployed on a temporary basis to any potential site and the actual coverage measured using available ADS-B reports. Coverage, loading and distribution of downlink formats are just some of the numerous sensor statistics available

ADS-B and multilateration are new concepts for many customers and are considered a risky alternative to radar. The advantages over radar are attractive: higher accuracy, lower cost, greater redundancy, but radar is a well known technology and potential users must be convinced and assured that Quadrant is a reliable and viable alternative. A phased introduction is the recommended approach and is beneficial in many ways: a single sensor provides ADS-B coverage but also enables a huge amount of data to be collected and analysed. Additional sensors increase the coverage area incrementally but also enable multilateration to be demonstrated. ADS-B and multilateration is completely passive and can work alongside existing SSRs and will make use of the responses to their interrogations. When the amount of parasitic interrogations is insufficient, or when an SSR is replaced, the active components of Quadrant can fill in the gaps.

Every Quadrant installation is different and brings new challenges. The robust technology, the intelligent design and the reassuring analysis provided by Quadrant ensure that each challenge is met fully, safely and reliably.