UAV boom

The technology race to integrate drones into airspace has begun all over the world.
OCTOBER, 16 / 2019 / 5 MINUTES
Фото вышки в Örnsköldsvik
Various solutions are being tested globally to make drones visible to ATM services and other air traffic participants. Existing technologies can be classified as follows:
1. Secondary surveillance radar (SSR) and MLAT/WAM systems.
2. Automatic dependent surveillance—broadcast systems.
3. Cellular networks 4G / 5G.
4. Visual observation systems.
5. Other non-aeronautical solutions.
In this review, we are not going to analyze the last two points, but we will focus on the first three as the most relevant.
Secondary surveillance radars (SSR) and MLAT/WAM systems
SSR can potentially be used for monitoring unmanned aerial vehicles (UAVs). However, it can immediately be noted that this solution has a number of critical shortcomings and is unlikely to be applied.
Wide area multilateration a multitude of antennas instead of radar dishes /
The modern development of this direction is multi-position monitoring systems (MLAT — Multilateration / WAM — Wide Area Multilateration), operating on the multilateration principle basis. MLAT/WAM is a approved surveillance technology and includes a single network of mobile stations that allow the precise location of aircraft equipped with airborne transponders. MLAT/WAM are widely used at various airports in the world, and there is also a number of projects where similar systems cover entire regions and even countries. The solutions based on multilateration are intended to replace SSR in the future, when the SSR service life comes to the end.

MLAT/WAM stations are the ADS-B 1090ES receivers too, therefore, with the introduction of MLAT/WAM, automatic dependent surveillance-broadcasting (ADS-B) is launched. This is one of the advantages of this technology.

Another important advantage of the MLAT/WAM is that the system does not use satellite data and is more independent and protected, unlike ADS-B, which is based on the satellites information. The global cybersecurity problem in aviation is being solved precisely through the use of either secondary radars or multi-position surveillance systems.
Perhaps the only disadvantage of the MLAT/WAM is that, there is the necessary requirement to determine the exact aircraft’s coordinates: it has to be inside the network of at least three stations.
MLAT/WAM is being implemented in Europe, the USA, Canada, Russia, Great Britain, Australia, Malaysia, South Korea, Namibia, India and other countries. MLAT/WAM is a good candidate for making UAVs visible to ATC. To do this, it is enough to equip the drone with a mobile transponder, and the system will «see» it the same way as it «sees» other manned aircraft.
Automatic Depended Surveillance-Broadcast Systems
Automatic Dependent Surveillance-Broadcasting (ADS-B), like MLAT/WAM, is a global standard and is used in aviation for surveillance purposes. However, unlike the MPSN, the ADS-B relies on satellite data and allows aircraft equipped with transponders to broadcast their location and other information to everyone around them over the broadcast range.
UAVs should be able to detect nearby manned aircraft, and this is easy if the drone is equipped with an ADS-B receiver. Since all the new civilian aircraft have equipment compatible with the ADS-B 1090ES, the UAV will be able to «see» the aircraft surrounding it.
Starting from 2020, the world's leading drone manufacturer DJI will equip all UAVs weighing more than 250 g with ADS-B 1090ES In mobile receivers (for the USA — UAT). Sky review has already written about DJI technology, and has also interviewed Adam Lisberg, Corporate Communications Director of DJI, North America.

It is also worth noting the global trend of the ADS-B introduction. It is mandatory to equip all the commercial aircraft with 1090ES Out equipment from 2020, due to the part of the air navigation systems development in the United States (NextGen) and Europe (SESAR).
As part of the ADS-B line in Russia, there are several points of view on which ADS-B type is better to use for drones. In addition to the 1090ES, the Swedish development of VDL-4 is offered. It is worth noting here that the 1090ES is already in use on commercial aircraft, and UAVs equipped with such transponders can be compatible. In case of ADS-B VDL-4, it is not entirely clear how exactly this type of ADS-B will make drones visible in practice, because neither Russian nor foreign aircraft are equipped with ADS-B VDL-4 equipment. In the 2000s the largest international aircraft manufacturers criticized the VDL-4 («Airbus work on VDL Mode 4», «Boeing Position Paper on VDLM4») and refused to equip their aircraft with this type of ADS-B. Among the main reasons were: low data transfer rate and bandwidth, the use of the VHF band and the identified interference. Obviously, in the case of VDL-4, the installation of additional equipment for universal compatibility will be required, which will steadily lead to inappropriate financial costs.
If we do not consider the features of different types of the link, and identify the key disadvantages of ADS-B, we can highlight the fact that the technology is tied to satellite data. The interference or the total blocking of the satellite signal can create certain difficulties.
4G / 5G Cellular Networks
The 4G and 5G technologies are also considered for urban flights (UAM — Urban Air Mobility). The 4G or 5G technology readiness level is relatively high, but not as high as for ADS-B or MLAT/WAM. It will take some time to adapt the technology to aviation targets.
The fastest 4G mobile networks currently offer an average speed of around 45 Mbps. The chip's manufacture Qualcomm believes that 5G can reach speeds about 10−20 times faster. For comparison, the data transfer rate for ADS-B 1090ES and UAT is 1 Mbps, for ADS-B VDL-4 this indicator is even lower — only 19 Kbps. The potential data transfer rate of 5G networks reaches incredible 10 Gbps, and the speed of 1 Gbps has already been recorded with Verizon clients.
Experts also note that navigation systems based on satellite signals in cities may not be accurate enough, and buildings create additional difficulties in receiving signals. In this context, 5G is assessed as a promising solution for urban flights.
4G / 5G can be used to track UAVs in two main ways: by receiving messages about the UAV’s location by a dedicated server (according to on-board navigation systems); determining the location of the 4G / 5G terminal (coinciding with the UAV’s location) using the means of a mobile operator (the so-called LBS — Location Based Service). The first method is more accurate, but the second does not depend on the GPS signals availability and the UAV navigation system operation. The collected information about the UAV location can be broadcast to manned aircraft equipped with ADS-B In data receiver using the TIS-B service at 1090 frequency.
Uber and AT&T are already testing a 5G network for Uber Elevate Air Taxi. AT&T's 5G is currently operating in 19 US cities, and it is planned to cover at least another 30 cities. AT&T plans to deploy a nationwide 5G network by mid-2020.

In Singapore it is expected to create the first «skyport» for the Volocopter urban taxi, which, along with CityAirbus and Uber Elevate, is undergoing active testing and is one of the fastest growing projects in the field of urban taxi.
The disadvantages of cellular networks include potential interruptions in data transmission and possible difficulties when switching between networks of various cellular operators. It is also worth noting that due to technical features there is a limit on the flight’s altitude. That is why 5G networks are supposed to be used for low-level urban flights.
Summarize and the key point highlighting
It is important to note that none of the technologies considered were originally created for drones. Experts periodically talk about the «bottlenecks» of aviation technology, for example, about the problem that the address space may not be enough for all drones, or that the systems simply cannot work when there are thousands of UAVs in the coverage area. Nevertheless, each of the presented technologies can be applied in its part, and there are practical examples.
The ADS-B 1090ES equipment is the most obvious and effective step to increase the UAV flight safety here and now. Thus, drones will be able to become visible to manned aircraft and controllers in the nearest future, and they will not need to additionally re-equip the entire fleet. DJI, the world’s largest drone manufacturer, has already taken this path.
In terms of ground infrastructure, MLAT/WAM is seen as the most promising solution. Multilateration is a reliable and accurate technology that allows for cybersecurity even in the conditions of weak or incorrect ADS-B satellite data. Three advantages of this approach can be distinguished:
— the stations included in the multi-position systems are the ADS-B 1090ES receivers — and this means that, by deploying the MLAT/WAM, the ADS-B 1090ES implementation takes place at no additional cost;
— due to the fact that the MLAT/WAM uses the multilateration principle to determine the aircraft and UAVs coordinates, the system is able to provide a high level of security and flight safety even in cases where satellite coordinates are incorrect or the signal is completely blocked;
— MLAT/WAM are the development of the direction of secondary radars, and their implementation is correlated with plans for the transition to an updated air navigation system of the country.
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