A special-purpose, high-resolution, radar that maps the airport surface has proved a useful tool to monitor aircraft movements under conditions of poor visibility. Such radars are referred to as ASDE (Airport Surface Detection Equipment). The rationale for the design and critical parameter selection for the ASDE-3 is presented. Key features of the chosen design are a rotodome with variable focus antenna, frequency-agile TWT transmitter, and a digital scan converter. Each feature brought specific and significant improvement to the system performance and these improvements, are discussed in some depth.