The visible light spectrometer on board the Solar Mesosphere Explorer spacecraft measures stratospheric NO3 in the 20--40 km altitude region and provides accurate daytime NO3 density profiles with nearly complete latitudinal coverage over an extended period of time. The instrument and data analysis are discussed in detail, and NO3 results for winter/spring 1982 are presented and compared to current theoretical models. Agreement with other measurements is good, and comparison with NOx models indicates that although the overall agreement is acceptable, improvements in the models are required before good agreement is reached at all latitudes. The data indicate that NO2 has a strong memory of the physical conditions present in the stratosphere over a time period of several days.