su = constant curves and is equal to IT between two consecutive Sk = k/W' curves 2 Information Flows at the DPL Level Fig. 2.8 Relevant for the sampling of the field; the radiating line is drawn in violet in the center of the figure; the observation line is drawn in green; the curves at su = Sk are drawn in blue; the curves y = Vn are drawn in red; the sampling points on the observation curve B are given by the intersection points between the blue curves and the green curve (green points) passes over a hyperbola. It is useful to note that the time delay difference evaluated by considering any other pair of points along the segment connecting Sa and Sb changes more slowly in the space and hence would give a slower phase variation in the space. Now, let us consider a continuous source. In Fig. 2.8 a linear source D (violet thick line) radiates in free space. The field is observed on the observation curve S (plotted as thick green curve). This geometry has been analyzed in [22, 72]. The curves y = constant are ellipses whose foci are the two extreme points of the source. They are drawn as red curves in the figure. The curves = sk = W', for k = 1, SNDF are hyperbolas having foci at the extreme points of the source and intersecting the source at a distance of 2/2 from each other [22, 72]. This gives an intuitive explanation of the spatial sampling procedure. As first step, the average phase associated with the average delay of the field radiated at the extreme points is subtracted at each point of the observation curve. As second step, we collect a sam- ple each time the observation curve intersects a Sk curve (green points). These curves are related to the time delay difference evaluated by extreme points of the source and therefore give the fastest phase variation in the space associated with the time delay. This process "filters out" the artifacts due to the geometry and gives us a simple picture of how information flows from the DPL to the PL. For example, at instant tk the PL receives the request to send the stri