Focusing on radar-based surveillance, this book has been written to provide a comprehensive introduction to the science, sensors and systems that form modern aviation weather surveillance systems.
Inspec keywords: lightning; meteorological radar; atmospheric turbulence; Doppler radar; radar polarimetry
Other keywords: Doppler weather radars; lightning; hazardous weather features; harmful atmospheric effects; atmospheric turbulence; aviation weather sensor; aviation weather surveillance; polarisation diversity radars
Subjects: Radar equipment, systems and applications; Atmospheric electricity; Instrumentation and techniques for geophysical, hydrospheric and lower atmosphere research; Convection, turbulence, and diffusion in the lower atmosphere; Atmospheric, ionospheric and magnetospheric techniques and equipment
This chapter discusses the following topics: (1) Aviation and electronics: a symbiotic relationship. (2) Phases in evolution of aircraft navigation and (3) Modern aviation weather surveillance.
This chapter discusses following aspects of aviation. (1) Goal of aviation systems. (2) Phases of aircraft flight. (3) Mechanics of aircraft flight. (4) Aircraft navigation systems. (5) Air traffic control and air traffic services. (6) Radars in aircraft navigation and air traffic control and (7) Aeronautical communication systems.
Aircraft fly within the atmosphere and are wholly dependent on it for the generation of the aerodynamic forces that sustain and regulate flight. Aeroplanes are also propelled by air-breathing engines that ingest air from the atmosphere to support combustion and generate thrust. Further, all navigational and communication signals, including visual, must penetrate a layer of the atmosphere before reaching the aircraft. The same is true for signals from aircraft to ground-based facilities and controllers. The aviation process is therefore strongly affected by the state of the atmosphere. Atmospheric processes are very diverse in terms of their origin, physical nature, spatial and temporal scales, and intensity. However, from the point of view of effects on aviation, they may be classified into five different groups: (i) phenomena involving physical motion of air; (ii) hydrometeorological phenomena; (iii) phenomena inducing and facilitating ice formation on aircraft surfaces; (iv) phenomena causing low visibility; and (v) phenomena involving atmospheric electricity. The following sections discuss different facets of the complex interaction between weather and aviation, the characteristics of different types of atmospheric phenomena, and their effects on aviation.
As mentioned before, aviation is much more sensitive to severe local phenomena and local variation of aviation-significant parameters than to large-scale or global climatic processes and trends which are of great significance in many branches of meteorology. The local phenomena may, of course, be driven by the overall climatic conditions. For example, the occurrence of a large low-pressure system over an area would give rise to myriads of thunderstorms. However, the type of instrumentation and modelling that provide information and forecasts about global and regional weather patterns do not provide details on a local scale that are adequate for aviation applications. This chapter concentrates on the types of local phenomena that serve as origins for many of the atmospheric effects on aviation discussed in the preceding chapter.
This chapter is devoted to evolving the attributes and characteristics desired of electronic weather surveillance systems designed in support of modern aviation. In addition to technical parameters, there are considerations of human factors such as the skill level and availability of human operators and data interpreters, environmental factors such as restrictions on electromagnetic spectrum and radiated power, and infra structural factors such as communication facilities, quality of electric power, and engineering support base, in the design and choice of surveillance equipment. Finally, the global and local regulatory and legal framework governing aviation activity, and the allocation of responsibility and liability in this field has a strong bearing on the development and deployment of weather surveillance systems in support of aviation.
Weather radars have many distinguishing features relative to those used in other roles, even within the field of aviation. This chapter focuses on the special features of weather radars, assuming a fair acquaintance with the principles, construction, and working of radars in general.
The optimisation of weather radar systems for aviation use is carried out in several ways. The first step is the choice of the basic radar parameters such as carrier frequency, pulse repetition frequency, transmitted power level, antenna gain and sidelobe levels, antenna scan rate, receiver characteristics, clutter-rejection characteristics, etc. to optimally fulfil the role envisaged for the radar under the inherent and imposed constraints. Thus weather radars designed to cover terminal areas would be significantly different from those intended for en route coverage in terms of their basic parameters. The next step is to design the data formats and communication formats and protocols to integrate the radars into the surveillance system. A third aspect of the optimisation process consists of proper choices of the locational and operating parameters of the radar. As already pointed out, a major advantage of the radar as a sensor for aviation weather is its capability for airport-centric observation, and the consequent matching of resolution requirements with aviation needs. This calls for appropriate placement of the radar relative to the surveillance domain. Among operational factors that require optimisation are the strategies for scanning, data collection and processing, and adaptive choice of variable parameters such as pulse-repetition frequency, pulse staggering, if any, transmitted pulsewidth, and processing and/or display modes for signals and data. Some of these aspects are considered in this chapter in relation to the major weather radar systems developed for aviation weather surveillance support.
There is the need for additional instrumentation which would be more widely distributed and be cost-effective for providing the basic aviation significant weather information, if necessary over limited areas. These can supplement Doppler radar data where their effective areas overlap with the radars, and can form an independent source of weather information about wind shear and shifts where Doppler radar support is not available. A number of such sensing instrument systems have been developed, and some of the important ones will be covered in this chapter.
Aviation, as a spatially distributed activity, extends over more area than is covered by any one sensor and also requires more kinds of data products than can be generated by any one instrument. Combining data from as many diverse sources as possible would not only enhance spatial coverage but also provide a more comprehensive product set for aviation support. This has led to a systems approach to the aviation weather surveillance problem. Such systems gather and fuse data from many sources and provide high-level products specifically tailored for aiding aviation activities. With parallel development of sensors, aviation weather data are available from a number of sources with different characteristics and wide spatial distribution. In the modern information age with powerful hardware and software available for voluminous data handling in many fields of activity, designing systems for collecting and processing weather data from many sources would seem to be a straightforward affair, but the problem is rendered complex by the widely different types of information sources. First, there are a host of generically diverse sensing instruments with differing coverage, types of observed parameters, update rates, and data formats. These include a variety of radars, in situ sensors and sensor clusters, weather satellites, and others, with their primary data in digital, graphical or even photographic forms. Then there is the problem of multiple and overlapping coverage by similar instruments in certain areas, while other areas may not be covered at all. Further, these devices operate asynchronously, without any centralised or master control to coordinate their operation. Finally, and most importantly, there is a large amount of very valuable information available from noninstrument sources such as pilot reports, meteorologists' impressions and interpretations, and model-based computations. Combining data from all these sources to provide reliable data products involves a high level of ingenuity. Further complexity is added to comprehensive aviation weather data systems by their ambitious specifications driven by the demands of aviation support. To facilitate the utilisation of their output data by operating personnel of diverse backgrounds and even by automated systems, the modern aviation weather data systems are expected to perform highly sophisticated processing which mimics or replaces human capabilities in some ways, especially in cognitive and inferential aspects. Thus the multi source data are not only fused and collated, but are used to recognise the nature of the hazardous phenomena, locate their spatial boundaries, estimate their intensity, and evaluate their hazard potential. In the end the data products are to be delivered in a final form that can be used directly and immediately for aviation-related decisions. In addition to these sophisticated processing functions, the high-level weather data systems also interface automatically with other air traffic management systems to share the data and make them available in a timely fashion at locations where they are required. Yet another requirement of modern aviation weather surveillance systems is predictive ability. It is not enough to know accurately just the current weather scenario. To operate the aviation system smoothly and to minimise the disrupting effect of sudden decisions on individual flights as well as support facilities, it is necessary to know of hazardous weather developments in advance. Even a few or several minutes of warning in the case of fast-evolving phenomena such as microbursts and gust-frontal wind shifts is beneficial. Longer lead times are both desirable and possible in predicting changes in less agile parameters such as visibility, snowfall intensity and environmental winds. Predictions may be made from computational forecast models run by specialised agencies, by extrapolating observed phenomena and parameters, and/or by sensing and recognising precursor phenomena. The inclusion of all these possibilities imparts a great degree of sophistication to modern high level aviation weather data processing systems, and makes their design a challenging task. It is only in the 1990s that such comprehensive data systems are being attempted.
This chapter deals with the basic features of the process of automating the detection and recognition of some of the weather phenomena that are of direct concern to aviation. However, before starting such a discussion it must be pointed out that this is an area of immense breadth, scope and diversity for the following reasons: (i) diverse nature of weather phenomena; (ii) multiplicity of types and specifications of sensors; (iii) options on different levels of automation desired; (iv) diverse nature and format of the data products; and (v) varying capacities and sophistication of the host computers used for implementing the automation. Further variations in the automation approach are provided by the background and algorithmic preferences of the developers. Finally, numerous attempts have been made independently by many research and industry groups and individuals around the world to develop automation algorithms. The result is an exceptionally wide array of computer programs for automatic detection and recognition of aviation significant weather features and phenomena, with considerable parallelism, duplication and overlap among some of the programs intended for similar applications. Many of these automation programs were or are only concept proving attempts, but a number of programs have matured, been field-tested and implemented in real systems.
The role of turbulence as a frequent and serious atmospheric hazard to aviation was addressed in Section 3.3. It was pointed out in Chapter 4 that thunderstorms generate some of the strongest turbulence found naturally in the atmosphere. However, atmospheric turbulence is of widespread occurrence, and 'clear air turbulence' is a commonly experienced source of aviation hazard and discomfort. The basic capability of coherent weather radars to sense and measure turbulence through an estimate of the Doppler spectral width was discussed in Chapter 6. In view of the paramount importance of turbulence in the aviation context, this chapter is devoted to a closer and deeper study of some of the fundamental aspects of atmospheric turbulence, and its relationship with aviation on the one hand and radar measured variables on the other.
Atmospheric electricity manifests itself most visibly and violently as lightning. As aircraft fly through air spaces with actual or potential lightning activity, a good understanding of the interaction between lightning and aircraft is imperative. The statistical fact that lightning has not caused too many fatal accidents has often led to the assumption that lightning may not be a serious hazard for aviation. However, lightning can and does cause significant functional impairment of modern aircraft. The increasing use of digital devices in navigational and control systems of contemporary aircraft is likely to make the aircraft more susceptible to direct and indirect lightning effects. Several systematic experimental studies conducted in recent years have provided quantitative information about the impact of lightning on aircraft and served to dispel some preconceived notions regarding the susceptibility of aircraft to lightning strikes. Such knowledge is likely to be of great value in designing aircraft and their flight paths, and in mitigating the effects of lightning on aircraft.
It is clear from the discussion hitherto in this book that the primary sensor of the modern aviation weather surveillance system is the Doppler weather radar in its different forms. It not only estimates the various reflectivity-derived parameters such as the intensities of rainfall and hailfall, but also detects and provides qualitative and quantitative pictures of wind velocity and turbulence fields in the atmosphere. Modern aviation weather surveillance systems, by virtue of their ability to measure these parameters, and automatically derive higher-level weather products and perform their interpretation from the aviation management point of view, represent a quantum jump over the state of the art existing till recently. Yet there is scope for further improvement, and some courses of action for effecting these are visible and being pursued already. One of the major directions of possible improvement is the development and deployment of polarisation diversity radars.