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.

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.

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.

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.

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.

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.

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.