19. The ACE-HF "SNR Summary Chart" provides perhaps the best overview of circuit quality, because it presents a simultaneous summary of predicted SNR at all frequencies and all times-of-day for the specified circuit, service grade and Required Reliability. In this chart, each grid point is colored green, yellow or red in accordance with the Required SNR value, and a legend is given at the top of the chart. You can move the mouse pointer over the chart to position the cross-hairs over the current time-of-day to more accurately read the chart. The chart may be changed to plot a summary of the Reliability computations. This chart is best used when a 90% Required Reliability is specified, where the green area is then bounded by the 90% contour and represents the Operational Working Area. The upper-frequency bound is then the FOT for the Required SNR and grade of service, and the lower-frequency bound is the Lowest Usable Frequency (LUF). Assured contacts will most likely result when the frequencies and times-of day of the green area are used as a guide.