A long term field trial called FESTER (First European South African Transmission Experiment) has been conducted by an international collaboration of research organizations during the course of almost one year at False Bay, South Africa. Main objectives of the experiment are a better insight into atmospherical effects on propagation of optical radiation, a deeper understanding of the effects of (marine) aerosols on transmission, and the connection of the mentioned effects to the general meteorological and oceanographic conditions/parameters. Modelling of wakes and possible infrared-radar synergy effects are further points of interest. The duration of one year ensures the coverage of most of the relevant meteorological conditions during the different seasons. While some measurements have been performed by permanent installations, others have been performed during intensive observation periods (IOP). These IOPs took place every two to three months to ensure seasonal changes. The IOPs lasted two weeks. We will give an overview of the general layout of the experiment and report on first results. An outlook on the planned analysis of the acquired data, which includes linkage to the Weather Research and Forecasting model (WRF), will be given.
An overview is given of the First European – South African Transmission ExpeRiment (FESTER), which took place in South Africa, over the False Bay area, centered around Simon’s Town. The experiment lasted from April 2015 through February 2016 and involved continuous observations as well as periodic observations that took place during four Intensive Observation Periods (IOPs) of 2 weeks each, which were spread over the year. The continuous observations aimed at a characterization of the electro-optical propagation environment, and included standard meteorology, aerosol, refraction and turbulence measurements. The periodic observations aimed at assessing the performance of electro-optical sensors in VIS / SWIR / MWIR and LWIR wavebands by following a boat sailing outbound and inbound tracks. In addition, dynamic aspects of electro-optical signatures, i.e., the changes induced by variations in the environment and/or target orientation, were studied. The present paper provides an overview of the trial, and presents a few first results.
The member nations of AC/323 SET-RTG056/RTG32 on Integration of Radar and Infrared for Ship Self Defence have performed the Validation Measurements for Propagation in the Infrared and Radar (VAMPIRA). The objective was to get insight into the radar and infrared synergy concentrated on propagation in a coastal environment including horizontal inhomogeneity and to validate radar and infrared propagation models. The trial was held in the period 25 March-5 April 2004 near Surendorf Germany. As part of the trial TNO made RF 1-way transmission measurements, 24 hours/day during the whole trial period. The transmission path over the Eckernforder Bucht in Northern Germany had a length of 8.2 km. The transmitted signal was a sweep consisting of 6 frequencies i.e. 3.36, 5.32, 8.015, 9.7, 13.45, and 15.71 GHz. The transmitter height was 11.5 m, the receiver height 6.4 m above 'normal null'. At each end of the path a meteorological station was installed measuring every 30s the air temperature, relative humidity, air pressure, wind speed and wind direction. About halfway the path the TNO meteo buoy was anchored measuring air temperature and relative humidity at 5 heights between 0.65 and 5.15m above the sea surface. Also the sea water temperature was measured by the buoy on a depth of 1m below the sea surface. The effects of evaporation ducting at the propagation at the various frequencies were clearly demonstrated. Some times very deep fadings were present at 13.45 and 15.71 GHz where at the same time almost no effect at 3.36 and 5.32 GHz was observed. The measured propagation at 15.71 GHz was more enhanced than at 13.45 GHz due to the ducting conditions and the elevation angle of the transmitter and receiver antenna. In several sample cases the 1-way propagation factors are computed for every 5 minutes using the propagation model TERPEM (Signal Science) and the vertical refractivity profiles computed by the TNO model TARMOS. The 1-way computed propagation factors compared very well to the measured data at all frequencies, although the computed fadings were not always as deep as the measured ones. A first promising result has been obtained computing the observed height of the RF source under various atmospheric conditions using the transmission phases computed by TERPEM.
A study is carried out to classify possible combinations of refractivity conditions for RF and IR over a wide range of meteorological conditions using different micrometeorological bulk models. The calculated refractivity profiles are analyzed for evaporation duct height (EDH), mainly relevant for RF propagation, and for gradients of the modified refractivity at different heights, relevant for both RF and IR propagation. These refractivity gradients are a direct indicator for the occurrence of sub- or super refraction at the height of interest. The present study reveals that under humid and unstable conditions evaporation ducts are found at approximately 3±2 m above cold (5°C) waters and at approximately 8±5 m over warm waters (25°C). Under dry conditions, these duct heights are approximately 9±5 m and 20±10 m, respectively. Duct heights decrease with increasing wind speed. Under humid and near-neutral conditions, duct heights range from 1 to 25 m, and decrease with increasing air temperature and/or wind speed. On the other hand, for dry and near-neutral conditions, and also for neutral conditions, the duct height is not well defined. Values between 1 m and 100 m are found, with an irregular dependence on air temperature and wind speed. Reliable modeling of duct height under these conditions remains questionable due to a lack of vertical mixing in the surface layer. The paper also shows that all four combinations of RF and IR sub- and super-refraction can occur in the atmosphere. The occurrence of a specific combination depends predominantly on temperature and humidity, and to a relatively minor part on wind speed. The magnitude of refraction effects in the two spectral bands is not necessarily coupled but varies with environmental conditions and height. Sub-sub refraction is generally weak and occurs under neutral conditions or at large heights. Super-super refraction occurs under warm and dry conditions and can reach medium strengths. RF-super refraction in combination with IR-sub refraction occurs under strong unstable conditions (e.g., surface temperature higher than air temperature) and can reach medium strengths. RF-sub refraction in combination with IR-super refraction occurs under stable and warm conditions. The magnitude of refraction can be very large, especially at low altitude.