Use of remote sensing to validate electro-optic prediction models

Software programs used to simulate electro-optic sensor performance and predict infrared signatures are extremely powerful tools. They are, for example, used to determine the required system specification for an infrared sensor to be successful in a certain application, or they are used to determine the susceptibility of a ship against a specific infrared surveillance or threat sensor. But to be of real value, such software must be validated for real-world scenarios, which requires significant resources and time, and a sound scientific methodology. Many modeling and simulation tools are developed and marketed as tested products, but lack sufficient validation.

The electro-optic/infrared technology group at the Institute of Maritime Technology (IMT) in South Africa is using remote sensing techniques to determine infrared sensor performance and measure the infrared signature of important targets (for example, ships, aircraft, and unmanned vehicles), specifically to validate some of these modeling and simulation codes. We have participated in a number of joint international experiments or efforts, the most noteworthy of which are the small targets trial (STT),¹ the Falsebay Atmospheric Experiment (FATMOSE),² and the ongoing First European South African Transmission Experiment (FESTER).³ FESTER is a joint international experiment we are conducting with the Netherlands defense research institute TNO and the German defense research institute Fraunhofer IOSB.

FESTER is a long-term experiment (>12 months' period) involving continuous study of atmospheric effects and the above- and below-water environment, with additional characterization of ship infrared signatures through measurement of cooling and heating rates of physical targets at thermal equilibrium and during transient thermal conditions (looking at cooling/heating rates) conducted during intense observation periods. Steady-state thermal signature measurements are indented to validate codes used to predict infrared signatures of targets under thermal equilibrium conditions.

One type of physical target used for infrared signature validation is different thickness metal plates, which are installed on our scientific vessel, Sea Lab I. The plates are coated with a standard naval gray ship paint, insulated on the inside, and fitted with a variety of instruments, including temperature sensors/loggers (iButtons), pyrogeometers, and pyranometers. These instruments measure plate temperatures and the sky and solar irradiance levels incident on the plates. A water spray cooling system was also installed to accelerate the plate cooling rate. A photo of the vessel and the starboard metal plates, showing the sensors fitted to the plates, appears in Figure 1.

Figure 1. A photograph of the scientific work boat, Sea Lab I (left), used for the First European South African Transmission Experiment (FESTER), with a close-up of the different thickness starboard metal plates (right) showing the different sensors fitted to the plates (e.g., iButtons, pyrogeometer, and pyranometer) and the water-spray cooling system.

To collect data on atmospheric behavior, scintillometers, multi-spectral transmissometers, and imaging electro-optic sensors are used to characterize atmospheric effects that influence electro-optic sensor performance, such as scintillation, refraction, atmospheric transmittance, and image distortion. In addition to standard weather sensors, other sensors (some on buoys) are used to measure underwater currents, conductivity, temperature (including vertical temperature profiles above and below water), and density, as well as sea surface and near-surface air temperatures to gain a more accurate characterization of the air-sea surface temperature difference.

Another important goal is to establish the degree of path homogeneity along the main propagation link path, which stretches over an 8km distance (at 12m above sea level) between our facility and the location of the electro-optic source equipment on the coast of St. James.

A planned expansion of FESTER is simultaneous characterization of propagation effects in the radio frequency part of the electromagnetic spectrum. This is intended to investigate the degree of correlation between the influence of certain atmospheric effects on both electro-optic and radio frequency propagation.

The only way to properly validate electro-optic sensor performance and infrared signature prediction codes is to compare model predictions to physical measurements made in experiments under real-world conditions. Most current ship infrared signature prediction models only cover thermal equilibrium or steady-state conditions. The next evolution will be codes that can also predict dynamic infrared signatures of ships. Characterizing dynamic infrared signatures is already one of the experiment's objectives, and we foresee FESTER evolving to cover any characterization gaps that may still be discovered.

Remote sensing activities in other technology programs at IMT are discussed elsewhere.³