The National Weather Service (NWS), Federal Aviation Administration (FAA), and Department of Defense (DOD) began installing the Automated Surface Observing Systems (ASOS) in the mid 1990s. At the completion of the ASOS program more than 900 sites, mostly at large and medium size airports, were installed nationwide. It was recognized that ASOS, due to limitations in sensor technology, was limited in its capability to report some of the so-called subjective elements, so provisions were made to permit augmentation of the ASOS observation by human observers. However, while ASOS has manual augmentation at most large airports, these account for only about 15% of the total. Thus an ongoing program in the NWS, called Product Improvement, has been tasked with investigating ways to improve ASOS in many areas, including its capability to observe subjective weather elements. This paper addresses the NWS efforts to solve a specific ASOS limitation, for which manual augmentation is required, the inability to identify drizzle. In 2000 the NWS procured a sensor that was to be tested for its capability to measure particle size with sufficient accuracy to distinguish drizzle from rain. The ability to quantitatively distinguish drizzle from rain was considered a necessary first step in understanding the nature of drizzle and developing a strategy for objectively identifying the occurrence of drizzle. The specific sensor procured for this test was not considered a candidate for use on ASOS due to its high cost. However, it was hoped that it would provide an objective technique for identifying drizzle and thus could be used as a reference sensor against which candidate replacement precipitation identification sensors could be tested. The result of this procurement was the purchase of what was designated the Meteorological Particle Sensor (MPS) from Droplet Measurement Technologies Inc. The results of three years of testing the MPS in the field at the NWS Sterling Test Facility are described in this paper.