Why Did You Do It That Way: WSR-88D Design Choices

During the mid-1980's, engineers and meteorologists were designing a Doppler weather radar known at that time as the NEXRAD (Next Generation Weather Radar). Later, during the limited production phase of the radar it became known as the WSR-88D or the Weather Surveillance Radar - 1988 Doppler. The government had developed and written the NEXRAD Technical Requirements (NTR) document specifying the capabilities of the radar. In some cases the design was actually specified in some detail leaving the engineers with little leeway in choices. However, in other cases only the capability was specified allowing the contractor engineering staff considerable flexibility and ingenuity in the hardware and in some cases, the software design. For example, the beam width was specified as about 1o and the transmitter wavelength was also dictated with little ambiguity. However, the overall sensitivity of the system was specified but the contractor was left with determining the methodology to meet this requirement. Engineers would then do a series of requirements and trade-off studies arriving at the needed antenna gain, receiver sensitivity, transmitter power, pulse width, etc., to not only meet this requirement but at the same time do it in such a way as not to prohibit another NTR "shall". Another example was an NTR requirement to achieve a 50-dB clutter suppression. This was an arduous requirement for which a study of the known technology of the day was essential (as frequently occurred during system design). Another one of the difficult tasks was, in the face of the Doppler "dilemma"; the contractor was required to achieve velocity dealliasing of 50 m s-1 out to a range of 230 km. Further, range ambiguities had to be identified and corrected wherever possible. Could this be done in the signal processor and at the same time prohibiting any degradation in data quality? Or would these capabilities actually have to be allocated to another area of the system design.

Other similar questions had to be answered in the workstation and display product design when also considering such factors as human engineering and the potential for operator confusion when seconds counted and lives would literally be on the line. This paper will examine the "why" of some of these and other particular radar design characteristics in an attempt to answer some often asked questions.