Sensors in the natural world include those which equip us with our five senses: sight, hearing, smell, taste, and touch. These convert the various and diverse inputs to electrochemical signals that can be used to inform or control the living organism. In a similar way, in man-made devices, sensors are also used to measure various stimuli. However, because of the broad range of potential inputs and outputs, the accepted definition of a sensor is refined. In this definition, all devices that convert input energy into output energy are referred to as transducers, and sensors form a small subset of the group as defined below: 'A sensor is a transducer that receives an input signal or stimulus and responds with an electrical signal bearing a known relationship to the input' (Fraden 2003). Systems of sensors and transducers are constructed for a variety of applications, including surveillance, imaging, mapping, and target tracking. In some cases, the sensors provide their own source of illumination and are referred to as active sensors. Passive sensors, on the other hand, do not provide illumination and depend on variations of natural conditions for detection.

Radiometers are instruments for detecting or measuring radiant energy, and although the term can be applied to sensors operating over any band in the electromagnetic spectrum, it is most often applied to devices used to measure infrared (IR) radiation. The radiation observed by such a sensor is either emitted by the object being observed or reflected from it. In this chapter, these relationships will be quantified in terms of measurable characteristics of the object.

The basic principles of active noncontact range finding are similar for electromagnetic (radar, laser, etc.) and active acoustic sensing. A signal is radiated toward an object or target of interest and the reflected or scattered signal is detected by a receiver and used to determine the range. As shown in Figure 5.1, a source of radiation is modulated and fed to a transmit antenna, or aperture, which is usually matched to the impedance of the transmission medium to maximize power transfer. This can take the form of a horn for acoustic or radar sensors, or an appropriately coated lens for a laser. The antenna also operates to concentrate the radiated power into a narrow beam so as to maximize the operational range and to minimize the angular ambiguity of the measurement. When the transmitted beam strikes the target, a portion of the signal is reflected or scattered because the target has a different impedance, or refractive index, than the medium through which the signal is propagating. A small percentage of the reflected power travels back to the receiver (which is often collocated with the transmitter), where it is captured by the receiver antenna and converted to an electrical signal that can be filtered to remove extraneous noise before being amplified and detected. Distance measurement methods can be classified into three categories: interferometry, time of flight, and triangulation. The method used by a particular sensor usually depends on the maximum range and the measurement accuracy required. For example, interferometric methods can be extremely accurate, but are prone to range ambiguity, while time-of-flight methods operate at longer ranges with poorer accuracy.

The environment in which a sensor is expected to operate exerts a strong influence on its performance. The effects include interactions of the acoustic or electromagnetic radiation with the target and its surrounds (the background), and particularly with the atmosphere through which the beam must travel between the target and the sensor. These interactions include attenuation by the atmosphere and attenuation and scattering by hydrometeors and other suspended particulates. For electromagnetic sensors, clear-air attenuation is mostly caused by molecular absorption and scattering by oxygen and water.

Noise is the unwanted energy that interferes with the ability of the receiver to detect the wanted signal. It may enter through the antenna along with the desired signal or it may be generated within the receiver itself. In underwater sonar systems, external acoustic noise is generated by waves and wind on the water surface, by biological agents (fish, prawns, etc.), and by man-made sources such as engine noise. In radar and lidar sensors the external electromagnetic noise is generated by various natural mechanisms such as the sun and lightning, among others. Man-made sources of electromagnetic noise are myriad, from car ignition systems and fluorescent lights to other broadcast signals. The deliberate transmission of noise in an attempt to mask a target echo or otherwise deceive a sensor is known as jamming.

The range resolution of a sensor is defined as the minimum separation (in range) of two targets of equal cross section that can be resolved as separate targets. It is determined by the bandwidth of the transmitted signal, Δf (Hz), which is generated by widening the transmitter bandwidth using one of the following modulation forms: amplitude modulation, frequency modulation, phase modulation.

The analysis in this book, so far, has concentrated on using sensors to detect the presence of targets in noise and clutter. However, sensors are capable of much more than that. Consecutive measurements of a target's position (and sometimes velocity) can be used to calculate the target state (position, velocity, and acceleration), from which a good estimate of the future position can be made. This process of estimation is generally referred to as tracking. For most time-of-flight sensors that operate in polar space, this involves following the target independently in both range and angles to obtain good estimates of its position in three dimensions. This chapter is concerned with the mechanics of the tracking process.

Transponders were originally electronic circuits that were attached to some item whose position or presence was to be determined. The transponder operated by responding to a request received from an interrogator, either by returning some data from the transponder, such as an identity code, or returning the original properties of the signal received from the interrogator with a minimum time delay. Because the interrogator signal is much stronger than the returned signal, the former would swamp the latter unless some characteristic of the response were different. This difference can usually be achieved by separating the interrogation and response temporally or by changing the frequency. It is also possible to encode the response with some form of spread-spectrum modulation that can be decoded by the receiver.