Along the signal path from the atmosphere, through the sensors and the data logger to the final archive, the signal quality may be irreversibly comprised. These faults include aliasing caused by poor sampling practice and quantization in an analog to- digital converter. Aliasing and quantization will be defined in this chapter. Drift in some of the system parameters, such as temperature sensitivity, is generally preventable but is not always reversible. Sampling of a signal occurs in the time domain and, frequently, in the space domain with one, two, or three dimensions. In the time domain, the time interval between successive points is called the sampling interval and the data logger controls this interval. When two or more sensors are distributed, vertically, along a mast then the system is sampling both in the time domain and in the space domain. When multiple measurements are arrayed along the surface of the earth, the sampling is occurring in time and in two or three space dimensions. Most meteorological systems are undersampled both in time and space. Space undersampling is an economic necessity. The consequence of undersampling is that frequencies above a certain limit, called the Nyquist frequency, will appear at lower frequencies and this is an irreversible effect. Quantization occurs when the signal is converted from analog to digital in the analog-to-digital converter. Since the range of the converter is expressed in a finite number of digital states, signal amplitudes smaller than this quantity will be lost. This is another irreversible effect. These are not the only irreversible effects. For example, drift is caused by physical changes in a sensor or other component of the measurement system. Drift may have a causal component, such as undocumented temperature sensitivity, and a random component such as wearing of an anemometer bearing. The former is theoretically preventable and reversible, whereas the latter is irreversible. Each element of the system may include some signal averaging, and each element may add bias and gain. As noted in earlier chapters, a sensor is a transducer, a device that changes energy from one form to another.
