Possibility and application of all-weather lidar-radio sensing complexes

The article discusses the main problems of the development and applying of all-weather meteorological complexes of remote optical and radar atmospheric sensing. Discussed different areas of application depending on type of airports. The concept of a new mobile three-band true all-weather meteorological single platform complex for operational measurements in airports is proposed. Shown the results of multi-bands systems measurements in PPI mode and vertical profiler performed under various conditions.


Introduction
Aero navigation requires a comprehensive diagnosis of the meteorological situation for the real-time detection of dangerous weather phenomena in the take-off / landing zone, such as windshear in the boundary layer, wakevortex behind the aircraft, and also to determine horizontal and vertical visibility. The measurement of each parameter is carried out by separate systems, which does not provide allseason and all-weather measurements. Despite the complex market of remote detection systems, there is a certain tendency to develop all-weather meteorological systems for air navigation. So, for example, there are 3 independent groups working in this area abroad.
The Japanese company Mitsubishi electronics in 2011 released a concept project [1] on the integration of its own Doppler wind lidar with X and C band weather radar systems. In 2015, the company added another additional Ka-band radar range to the system.
In 2010, the American concern Lockheed Martin presented vision of an all-weather system for meteorological support for aero navigation. IR lidar Windtracer (Lockheed Martin) and X-radar METEOR 50DX (Selex), installed at the airports of Nice, Frankfurt and Munich [2]. This system showed high efficiency in various weather conditions, which proved the correctness of the merging approach of several measuring systems in one. It should also be noted that the Windtracer infrared lidar combined with the Weather Doppler Radar (TDWR) terminal is currently used at major airports in the world (Las Vegas, Munich, Frankfurt, Hong Kong).
Since 2010, the French company Leosphere (now Vaisala) has repeatedly tested its product (windcube -Doppler infrared lidar) with meteorological radars from Selex and Thales. The results each time showed that such integration of infrared systems and short-wave meteorological radars  [3]. All equipment is operated and serviced separately.

Multiband remote systems concept
Due to the complex network structure of the information of airport has required a specific software environment, often individual at each airport. Thus, in the short term there are no prospects for equipping meteorological facilities with a single all-weather hardware-software system, the advantage of which is mobility and a single infrastructure at the level of technical composition. In most cases, preference is given to manufacturers of serial equipment of a custom type, which solves a limited number of tasks in meteorology. The number of equipment types in this case exceeds several tens [4]. Nevertheless, the statement of the problem of creating all-weather single platform systems is extremely important and can be in demand in cases: • small and «far away» airports when a fully automated hardware-software environment with remote operator is required; • aircraft-carrying cruisers, since there is a limited take-off and landing sector, and there are no additional airfields [5,6]; • military aerodromes, where mobility and ease of use play an important role in deployment and operation. A comparative analysis of existing remote meteorological systems capable of measuring altitude profiles of atmospheric parameters allows us to conclude that, unlike radio sounding, remote atmospheric sounding devices based on the Doppler Effect have qualitatively better characteristics, but their potential maximum measurement range depends on weather conditions. In Russia Federation, the first attempt to combine the lidar and the radar on a single platform to ensure all-weather measurements was made by the company Laser Systems JSC together with specialists from the BSTU VOENMEH and the Military Space Academy named after A.F. Mozhaysky [7]. It was shown that the functioning of classical X-band radar systems requires specific weather conditions, when the reflectivity of atmospheric objects is not lower than a certain value. Therefore, measuring wind speed, atmospheric turbulence, detecting the windshear in clear atmosphere by radar is impossible.
At the same time, for optical radar (lidar) using a laser as a radiation source, the opposite situation occurs. Since the radiation wavelength a value of the order of 1 μm, the presence of an insignificant amount of aerosol is sufficient for its effective scattering in the atmosphere, which takes place even in clear atmosphere.
The optical part of the dual-band complex is based on the WINDEX-2000 pulsed heterodyne Doppler lidar with two mirror scanning module, which makes it possible to scan the entire upper hemisphere.
To verify the accuracy of measurements, comparative tests of remote wind measurement systems based on WINDEX-2000 and a radiosonde operating on the differential-ranging method of measuring wind speed were carried out. The measurement accuracy was estimated by calculating the correlation using the Pearson method and estimating the error of the correlation coefficient. According to the results of more than 800 independent measurements from three different WINDEX-2000 during 12 sessions of launching a GPS radiosonde, the correlation of the wind speed for the Vx component was 0.91 and for the Vy component 0.97. The average value of the correlation coefficient was 0.95 with a correlation coefficient error of 1.15%. The error of the wind speed along two coordinates does not exceed 1 m/s [8].
This result indicates a high accuracy of measurements and, in the case of remote measurements, an increased speed of updating information (up to 3 minutes, opposite to 30 minutes during aerosounding).

Result of multiband system measurements
Numerical modeling of the effects of electromagnetic waves of different nature showed that a system with an X-band radar (3 cm) and an infrared lidar (1.5 μm) cannot be called completely all-weather system for two reasons: • there is a range of atmospheric reflectivity (from 0 to 20 dBZ) when the signal from the IR channel begins to experience strong absorption, and for the X channel signal this type of reflectivity is almost transparent; • for X-band systems, the dead zone of measurements is about 3 km, when the dead zone of the pulsed lidar of the IR range is 100 meters (see Figure 1). These reasons make it difficult to integrate data, because in most cases, it turns out that these two ranges work independently and complement each other, but complete all-weather is not achieved, since there can be "blind" zone up to 3 kilometers. Screenshots of the lidar data processing software are presented in Figure 1 showing data gaps in range up to 3 km in PPI mode measurements with twoband X-radar and lidar systems. These gaps can be filled with Ka-band radar channel, which can operate in mists and moderate precipitation. The next stage in the development of all-weather systems at the company Laser Systems JSC was the development of a prototype lidar -radar complex, which included an additional Ka-band radar, humidity and temperature profiler and local sensors. The main characteristics of the complex are listed in Table 1. As a result of previous work [6,[8][9][10], the three-band (X, Ka and IR) all-weather meteorological single platform complex "Lira-3" is currently undergoing trial operation. The results of measuring the vertical profile of the wind speed obtained in April 2019 in St. Petersburg are presented in Figure 2. The graphs show in green the data obtained from Ka-radar, red -infrared lidar and blue -X-radar.  Figure 2. Vertical wind profiles from three-band meteorological system As it can be seen from picture, multiband systems can provide wind speed and direction information in reasonable quality.
A promising way to develop systems for remote meteorological analysis is not only remote measurements but also local measurements, ensuring the implementation of a full range of meteorological work. Thus, the existing mobile meteorological lidar -radar complex for remote analysis must be supplemented with the following equipment: • mast with local sensors (temperature, humidity, pressure, wind speed and direction); • automated rainfall system; • lightning detector; • meteorological temperature profiler; • equipment for receiving satellite and facsimile meteorological information; • set of radio stations.
As a result of such a combination, the complex will be able to: • short-term forecast of dangerous weather phenomena associated with clouds (thunderstorm, rain, hail); • wind profile measurement in the atmospheric boundary layer; • windshear detection; • obtaining current meteorological information; • automated processing and analysis of hydrometeorological information; • indication of measurement results on a remote display device; • forecasting hydrometeorological conditions; • archiving of measurement results.