Avionics. Aircraft Antennas and Sensors

In the early years of aircraft flight, technological innovation was directed at improving flight performance through rapid design improvements in aircraft propulsion and airframes. Secondary development energies went to areas such as navigation, communication, munitions delivery, and target detection. The secondary functionality of aircraft evolved into the field of avionics. Avionics now provides greater overall performance and accounts for a greater share of aircraft lifecycle costs than either propulsion or airframe components.

Landau (1) defines avionics [avi(ation) + (electr)onics] as the branch of electronics dealing with the development and use of electronic equipment in aviation and astronautics. The field of avionics has evolved rapidly as electronics has improved all aspects of aircraft flight. New advances in these disciplines require avionics to control flight stability, which was traditionally the pilot’s role.

Aircraft Antennas. An important aspect of avionics is receiving and transmitting electromagnetic signals. Antennas are devices for transmitting and receiving radio-frequency (RF) energy from other aircraft, space applications, or ground applications. Perry and Geppert (4) illustrate the aircraft electromagnetic spectrum, influenced by the placement and usage of numerous antennas on a commercial aircraft. Golden (5) illustrates simple antenna characteristics of dipole, horn, cavity-backed spiral, parabola, parabolic cylinder, and Cassegrain antennas.

Radiation pattern characteristics include elevation and azimuth. The typical antenna specifications are polarization, beam width, gain, bandwidth, and frequency limit.

Computers are becoming increasingly important for the new generation of antennas, which include phased-array antennas and smart-skin antennas. For phased-array antennas, computers are needed to configure the array elements to provide direction and range requirements between the radar pulses. Smart-skin antennas comprise the entire aircraft’s exterior fuselage surface and wings. Computers are used to configure the portion of the aircraft surface needed for some sensor function. The computer also handles sensor function prioritization and deinterleaving of conflicting transmissions.

Aircraft Sensors. Sensors, the eyes and ears of an aircraft, are electronic devices for measuring external and internal environmental conditions. Sensors on aircraft include devices for sending and receiving RF energy. These types of sensors include radar, radio, and warning receivers. Another group of sensors are the infrared (IR) sensors, which include lasers and heat-sensitive sensors. Sensors are also used to measure direct analog inputs; altimeters and airspeed indicators are examples.

Many of the sensors used on aircraft have their own built-in computers for serving their own functional requirements such as data preprocessing, filtering, and analysis. Sensors can also be part of a computer interface suite that provides key aircraft computers with the direct environmental inputs they need to function.

Aircraft Radar. Radar (radio detection and ranging) is a sensor that transmits RF energy to detect air and ground objects and determines parameters such as the range, velocity, and direction of these objects. The aircraft radar serves as its primary sensor. Several services are provided by modern aircraft radar, including tracking, mapping, scanning, and identification. Golden (5) states that radar is tasked either to detect the presence of a target or to determine its location. Depending on the function emphasized, a radar system might be classified as a search or tracking radar.

Stimson (6) describes the decibel (named after Alexander Graham Bell) as one of the most widely used terms in the design and description of radar systems. The decibel (dB) is a logarithmic unit originally devised to express power ratios, but also used to express a variety of other ratios. The power ratio in dBis expressed as 10 logi₀ P₂/Pi,where P₂ and Pi are the power levels being compared. Expressed in terms of voltage, the gain is (V₂/V₁)² dB provided the input voltage V₁ and output voltage V₂ are across equal resistances.

Stimson (6) also explains the concept of the pulse repetition frequency (PRF), which is the rate at which a radar system’s pulses are transmitted: the number of pulses per second. The interpulse period T of a radar is given by T = 1/PRF. For a PRF of 100 Hz, the interpulse period would be 0.0i s.

The Doppler Effect, as described by Stimson (6), is a shift in the frequency of a radiated wave, reflected or received by an object in motion. By sensing Doppler frequencies, radar not only can measure range rates, but can also separate target echoes from clutter, or can produce high-resolution ground maps. Computers are required by an aircraft radar to make numerous and timely calculations with the received radar data, and to configure the radar to meet the aircrew’s needs.

Aircraft Data Fusion. Data fusion is a method for integrating data from multiple sources in order to give a comprehensive solution to a problem (multiple inputs, single output). For aircraft computers, data fusion specifically deals with integrating data from multiple sensors such as radar and infrared sensors.

For example, in ground mapping, radar gives good surface parameters, whereas the infrared sensor provides the height and size of items in the surface area being investigated. The aircraft computer takes the best inputs from each sensor, provides a common reference frame to integrate these inputs, and returns a more comprehensive solution than either single sensor could have given.

Data fusion is becoming increasingly important as aircrafts’ evolving functionality depends on off-board data (information) sources. New information such as weather, flight path re-routing, potential threats, target assignment, and enroute fuel availability are communicated to the aircraft from its command and control environment. The aircraft computer can now expand its own solution with these off-board sources.