SubscribeGetting the most from your Wide Area Multilateration network with smart deployment
Wide Area Multilateration Introduction
Wide Area Multilateration (WAM) is an attractive new surveillance technique for Air Traffic Control. A network of distributed sensors receives transponder signals from a target of interest, and forwards the received signals with precise timing information to a Multilateration Server. There the time-difference-of-arrival (TDOA) technique is used for computing the position of the target.
Multilateration (MLT) has a number of compelling advantages. The actual sensors are compact, purely passive, and have minimal requirements for power and network connectivity. As an example, COMSOFT's Quadrant MLT and ADS-B sensor needs about 10W of electrical power, weighs approximately 20 kg and is the size of a laptop backpack. Once a site offers electricity and network connectivity, the installation of a ground station requires just a couple of hours with minimum preparation. The ground station provides self-adjustment to the RF environment and thus can be used operationally right after installation.
The sensor is designed from the ground up to be essentially maintenance-free. Due to their resistance to adverse weather conditions and their minimal impact on the environment, sensors can easily being mounted in most locations, making it possible to obtain an obstacle-free omnidirectional view. The multilateration controller is a commercial-off-the-shelf server running in the benevolent operating conditions of a control centre. Accurate time stamping at the ground stations ensures that network latency plays only a minor role for the integrity of the surveillance function.
Low cost for the initial investment and infrastructure combined with the minimal ongoing operating expenses make WAM networks appealing where the lifetime costs of a radar cannot be justified or afforded.
But not only the cost factor is compelling. Multilateration sensors take advantage of the squitter signals transmitted by aircraft transponders, as well as of Mode-S and Mark-X secondary radar replies. They typically offer an update rate well in excess of one per second, and with a suitable sensor configuration they offer a much higher precision than secondary radar technology.
Principles of Multilateration
The TDOA technique takes advantage of the different travel times of signals from a target to spatially separated sensor locations. Since electromagnetic signals propagate with the known speed of light c, the time difference of arrival of the signal at different sensors translates into range difference between the target and the sensors (cf. figure 1).
Figure 1: TDOA technique in wide area multilateration networks
A time difference Δt = t2−t1 between the reception of the signal at sensor 1 and sensor 2 thus constrains the position of a target to the set of points such that the distance from the target to sensor 2 is c∙Δt smaller than the distance from the target to sensor 1. As an example, if the signal arrives at both sensors at the same time, Δt = 0 and the target has to be located somewhere on a plane perpendicular to the line connecting the two sensors and equidistant from both (cf. figure 2).
In the general 2-dimensional case, the set of points compatible with a given time difference forms a particular mathematical curve called a hyperbola, shown in figure 2 in blue. Depending on which sensor is closer to the target, one or the other branch of the hyperbola applies. In the 3-dimensional case the set forms a curved surface called a hyperboloid. While in the 3-dimensional case, two sensors confine the target position only to such a surface, three sensor detections restrict it to the intersection of two hyperboloids (i.e. a single curve), and adding a fourth sensor allows the unambiguous determination of the target position.
Figure 2: Possible target geometry for different TDOA values
While theoretically any four distributed sensors are sufficient to pinpoint a target, in practice the resolution and accuracy of the measurements are finite. If this is taken into account, the geometry of the sensor configuration has a major effect in determining the actual accuracy of the multilateration solution. If all sensor locations are at similar directions from the target, even small uncertainties in the time measurement lead to large uncertainties in the position. Thus, sensors are typically distributed over much of the area of interest. However, if the distance between the sensors becomes too large, the risk that the more remote sensors completely miss a particular signal increases. In general, the distance between any two sensors that are expected to contribute to a given solution lies somewhere between 10 and 100 NM.
Most practically deployed sensor networks will use a minimum of five distributed sensors. This has two advantages. First, having 5-fold coverage introduces an element of redundancy. Even if one sensor becomes non-operational, the remaining network can be used to derive a full multilateration solution, thus providing much higher availability than possible with a non-redundant network. Secondly, having redundant coverage allows the central controller to select the subset of sensor detections with the most favourable geometry providing the most accurate position result.
A further advantage of WAM systems is the nearly unlimited scalability of the concept. Additional sensors can be added, either to overcome line-of-sight restrictions, to improve the geometry in certain areas, or to increase the overall surveillance volume.
The planning of a multilateration system deployment needs to be supported by careful analysis and modelling. Powerful tools allow the modelling of the coverage of individual sensors as well as the combined coverage of sensor networks. A suitable analysis also predicts which level of accuracy can be achieved in different volumes of interest (e.g. different altitudes) for a given sensor configuration.
Extending coverage with ADS-B
Multilateration determines the position of an aircraft based on the detection of its transmitted signals at multiple receivers. As described above, the quality of the solution depends on the geometry of the sensor network as well as on the position of the target with respect to it. In particular, positional uncertainty increases as targets move outside the borders of the sensor network.
A multilateration solution also requires signal reception at four or more sensors. However, since sensor coverage is restricted by the requirement for a clear line-of-sight, 4- or 5-fold coverage with a favourable geometry may be hard to achieve, especially in areas with difficult terrain or in the presence of obstacles.
Automatic Dependent Surveillance - Broadcast (ADS-B) is another attractive new technology to provide ATC surveillance. ADS-B equipped aircraft periodically broadcast their own position, determined via their navigation system and ultimately derived from GPS. The signal is collected by ADS-B receivers, decoded, and used directly to build an air situation picture, either for ATC on the ground, or to improve the situational awareness in the cockpit of other aircrafts.
ADS-B complements multilateration in a number of ways. Since the position is determined and encoded by the aircraft, the accuracy is the same regardless of the concrete location of the aircraft in the coverage area. Moreover, since only a single sensor is needed, it is much easier and cheaper to achieve full coverage over large or complexly structured regions.
The Quadrant MLT and ADS-B sensor has the ability not only to detect telegrams in the 1090MHz Mode-S downlink format and transmit them with a precise time stamp to a multilateration server; the sensor also decodes the embedded ADS-B messages and translates them into an ASTERIX category 21 data stream describing the air situation picture. With this capability, even single-sensor coverage is sufficient to determine the position and attributes of ADS-B equipped aircraft. Thus, the same sensor network is able to deliver two independent surveillance streams, one based on independent measurements, the second based on the reported position of the aircraft.
High-Integrity ADS-B
The concept of ADS-B is impressive due to its simplicity. Instead of maintaining a complex and expensive ground infrastructure to accurately determine the position of an aircraft, the aircraft downlinks its position, together with unambiguous identifier, current course, speed and vertical movement. Since the information is directly obtained from the avionics, ATC and pilot share and use the same information. There is only one noteworthy drawback: If the GPS based positioning differs significantly from the actual position, there is no means to detect this by an independent system.
To ensure a higher level of integrity, ADS-B can be combined with the TDOA technique. As described above, it allows two sensors to restrict the position of an aircraft to a hyperboloid, i.e. a very limited range of positions. An erroneous ADS-B position is very unlikely to fall onto this hyperboloid, and hence can usually immediately be flagged as implausible. Even if a single ADS-B plot is accidentally compatible with the TDOA derived restrictions, each new position report will be tested against a different hyperboloid. Thus, the ADS-B position can be verified to a high degree of integrity using only two sensors - a requirement much easier to meet than the 4-fold coverage needed for full multilateration.
This approach combines the simplicity and accuracy of ADS-B with an independent confirmation of the announced position, resulting in a highly trustworthy surveillance system with less strict sensor placement constraints than required for a full multilateration network.
In figure 3 the coverage results at flight level FL55 for a network of six sensors is visualized. The aim was to cover the whole country of Slovenia with a minimum number of sensors positioned at specified sites. The challenge is posed by the inhomogeneous terrain, with valleys at 1500ft elevation surrounded by mountains reaching 5000ft, and occasional peaks rising up to 7000ft. Within the geographical confines of the network, full multilateration coverage can be achieved. Sensor coverage is even redundant in most of the central region. However, by additionally employing ADS-B, a much larger area to be covered. Even high-integrity ADS-B already offers a valuable extension. While at flight level FL100 and up, nominal multilateration coverage is achieved for a much larger region, the ADS-B coverage at FL55 is remarkable. Moreover, while the quality of multilateration solutions degrades with distance from the sensor network, ADS-B reports have uniform high quality, regardless of the position of the aircraft.
Figure 3: Calculated MLT and ADS-B coverage at FL55 for a network of 6 sensors
Optimized Deployment Strategies
The flexibility of combined MLT and ADS-B sensors enables the exact tailoring of a surveillance solution to individual needs. At a first level, single ADS-B sensors can be installed to complement existing radar solutions. They can serve as fall-back systems and gap-fillers, by providing additional surveillance coverage in situations where radar is restricted by line-of-sight.
Alternatively, current multilateration systems can be used as superior and more cost-effective drop-in replacements for existing or planned secondary radar installations. This will result in a much improved cost/performance ratio even without considering the additional capabilities of the system.
Such a network will offer significant side benefits. The ADS-B and high-integrity ADS-B reports will increase the surveillance area, by providing high-quality surveillance data in cases where radar and MLT are restricted by line-of-sight, or where the large distance from the target introduces inaccuracy for systems based on direct measurements. As long as ADS-B is not mandatory, this information is especially useful for providing enhanced situational awareness and as a planning tool.
Several large airspaces are on the road to a full ADS-B mandate. Moreover, national and international organizations are working on regulatory standards to support the use of ADS-B as a replacement of radar. This will immediately increase the value of existing and future WAM/ADS-B installations. In this case, radar-like separation and similar services can be offered in the area covered by high-integrity ADS-B, with single-coverage ADS-B serving as a high-quality fallback. Full multilateration will allow additional surveillance options for the core area of interest, and the even wider coverage of pure ADS-B can support long-term traffic flow planning, safe and convenient transfer of aircraft to and from neighbouring ATC regions, and a host of informational services.
Wide Area Multilateration Conclusion
Multilateration and ADS-B are two compelling new technologies for cost-effective and future-proof surveillance solutions. The current generation of combined ADS-B and multilateration sensors supports flexible deployment and enables attractive applications even now. Since the same hardware is used to implement several different surveillance solutions, systems can be scaled and extended as newly installed systems gradually take over more and more responsibilities from conventional radars - starting with initial installations for increased situational awareness, moving to gap-filling applications and wide-area multilateration as backups for secondary radar systems, and finally to full radar replacement.
Article Co-author - Dr Susanne Och
Susanne Och studied Physics at the University of Erlangen-Nuremberg, Germany. She received her PhD in 1997 after spending also two years at a European research facility in Munich. With experience in Radar remote sensing and software engineering she joined Comsoft's department for Surveillance Applications in 2007. Dr. Och works in the field of ADS-B product development with focus on site analysis and implementation.