from a given object as a bright blip, and older ones as somewhat dimmer blips, as they faded. One might expect three or four such blips on the display, depending on the scanning speed of the antenna. The operator could easily determine the direction of travel, from the dimmest to brightest blip. Blip/scan ratios One key characteristic in all radars is the pulse repetition frequency (PRF), which determines the maximum effective range. The time between pulses must be long enough so that a single pulse can reach the maximum range of the system and then return before the next pulse begins. For instance, a radar designed to have a range of needs to wait 2 milliseconds for a pulse to travel the maximum range and back at the speed of light (300,000 km/s). This means that such a radar can send out, at most, 500 pulses per second (PRF). If the radar sent out 1000 pulses per second, it would be impossible to determine whether a particular reflection came from an object at 150 km from the pulse just sent out, or an object at 300 km reflecting the previous pulse. On the other hand, a 150 km radar requires only 1 millisecond; this makes a PRF of 1000 possible. Intertwined with the PRF is the length of the pulse, or duty cycle. This determines the minimum range of the system. Longer pulses mean that more energy has the potential to be reflected by the object. However, the radar system cannot detect reflections while it is sending a pulse. To have a 30 km minimum range, for instance, a radar can have pulses no longer than 0.1 ms in duration. For an early warning radar the minimum range is generally not important, so longer pulses are used to maximize the returns, but the duty cycle was nonetheless an important design consideration. Those two factors combine to determine the strength of the signal returned from a distant object. Using a shorter duty cycle allows for better minimum range, but also means that less radio energy is being sent into space over a given time, reducing the strength of the return signal. Likewise, reducing the PRF in order to improve range has the effect of causing the system to spend more time listening, also reducing the total amount of energy broadcast. This means it is inherently difficult to produce a radar system