studies with an instrumented platform, were performed..© Results indicated that the flow disturbances and turbulences that existed above the platform followed a particular geometry. In the peripheral area where the collectors were located, smoke studies indicated the occurrence of an ill-defined but orderly recirculatory flow system, moving upward in the windward section and downward in the leeward section. Since the vertical component of the recirculatory flow is greatest at the extreme upwind and downwind peripheral positions, minimum and maximum collections might be expected at these positions, respec- tively. Due to the circular platform geometry, it was also expected that the variation of collection would be symmetrical about the minimum-maximum collection axis. Ground Bias At present the relationship between the mean platform value and the equivalent ground value is empirical. It is assumed that for each bias ratio, with or without qualifications, there is a factor which empirically relates the mean platform value to the ground value. This ground factor, g, is defined as follows: equivalent ground value = gxmean platform value The relationship automatically takes into account the intrinsic efficiency of the platform collectors which, in this study, are limited to periphery-located OCC's and AOC)'s. SINGLE-WIND SYSTEM The most important bias characteristic of a s.w. system is the bias ratio because it not only describes the extent of relative bias but also determines the ground factor (assuming the information is available). With the ground factor known, the ultimate objective of most bias studies is met, i.e., the computation of the equivalent ground value. Thus there exists a need for fundamental experimental data to describe the variation of ground factor with bias ratio. Also needed is data to study the variation of the bias ratio with wind speed, particle size and density. In this system an unqualified and particular ground factor is associated with each bias ratio since the ratio is a specific measure of the resultant bias effects. In the case of uniform collection (bias ratio of unity) the values are ground values if collecting efficiency is 100 %. For practical reasons, a system with winds of approxi- mately equal speeds and of directions varying within a 30° sector are considered to be a s.w. systen.