4. The most important feature of ACE-HF is the ability to generate animated area coverage maps. "ACE" stands for "Animated Communications Effectiveness", the key feature used to create area coverage maps that may be shown sequentially as a function of time-of-day, frequency, or sunspot number. ACE-HF area coverage maps are based on similar displays developed for US Navy communications. In each map, the area covered is shown as a clear area. Areas beyond are colored red to show that receivers beyond the curve are "disconnected" at the specified level of circuit integrity. Terminator and day/night zones are clearly shown. The example shows the limits of coverage at a specified SNR level and 50% reliability. The display may be animated slowly or very quickly, and may be paused at any frame. The great advantage of ACE-HF is that the effects -- sometimes astonishing effects -- of the day's passage on HF coverage may be easily understood. With ACE-HF maps, you can see at a glance when the HF bands will be open in different parts of the world. In ACE-HF, the effect of time is always emphasized.