Here’s a visualization of the test with .2 and .5 m with the mote worn in front. The circles are in the position relative to the Purell, which was mounted on the left edge, center of the graph The radius of the circle is proportional to signal strength. Width of the edge represents 2 standard deviations. Data from the Purell experiment below.
We expected to have the inner arc be large circles, indicating strong signal strength close to the dispenser and the outer arc be small circles representing smaller signal strength farther from the dispenser. That’s not clearly the case, although there is a trend. Note however, that not all experiments have turned out this way. Some experiments have shown differences that are more clear.
There was some concern initially that the variation of the signals would be the problem. That does not seem to be the case as the measured standard deviations are rather small compared to the size of the circles. However, our experimentation in the lab suggests that when the motes are held in a fixed position the signal strength is constant, but moving them around over an angle of about +- 30 degrees or putting objects between them easily changes the RSSI by about +-10 points.
Perhaps we should be thinking of a single sampling position as a biased estimator and think about moving the motes around a bit at each experimental position in order to get an unbiased estimator with a larger standard deviation.
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