Implementing a neural network emulation of a satellite retrieval algorithm

As shown in Stogryn et al. (1994), Krasnopolsky et al. (1995), Thiria et al. (1993), Cornford et al. (2001), and many other studies summarized in Chapter 9, neural networks (NN) can be used to emulate the physically-based retrieval algorithms traditionally used to estimate geophysical parameters from satellite measurements. The tradeoff involved is a minor sacrifice in accuracy for a major gain in speed, an important factor in operational data analysis. This chapter will cover the design and development of such networks, illustrating the process by means of an extended example. The focus will be on the practical issues of network design and troubleshooting. Two topics in particular are of concern to the NN developer: computational complexity and performance shortfalls. This chapter will explore how to determine the computational complexity required for solving a particular problem, how to determine if the network design being validated supports that degree of complexity, and how to catch and correct problems in the network design and developmental data set. As discussed in Chapter 9, geophysical remote sensing satellites measure either radiances using passive radiometers or backscatter using a transmitter/receiver pair. The challenge is then to estimate the geophysical parameters of interest from these measured quantities. The physics-based forward problem (equation 9.4) captures the cause and effect relationship between the geophysical parameters and the satellite-measured quantities. Thus, the forward problem must be a single-valued function (i.e. have a single possible output value for each set of input values) if we have access to all of its input parameters. As a result, the forward problem is generally well-posed, i.e. variations in the input parameters are not grossly amplified in the output. One could, however, imagine some geophysical processes for which the forward problem was ill-posed for some parameter values as a result of a sudden transition from one regime of behavior to another (e.g. the onset of fog formation producing a sharp change in shortwave albedo in response to a minor change in air temperature).