The WVSS-II: A commercial aircraft sensor for water vapor information
Obtaining accurate information on atmospheric water vapor in the upper atmosphere has long been a well documented problem. Radiosonde sensors have had accuracy problems at the very low end of relative humidity values and at the high end near 100%. Relative humidity sensors on balloons have suffered accuracy losses and very slow response times as they ascend into the upper troposphere. Satellite information on water vapor suffers from lack of absolute accuracy and lack of vertical resolution. Adding to the complexity of obtaining water vapor information from commercial aircraft are the requirements for long-lived sensors and long intervals between required calibrations. This paper describes the key factors that have finally made a 12-year effort to find the right combination of technology and software to measure water vapor information from commercial aircraft (in a manner satisfactory to both air carriers and users of the data) a substantial success. A summary of the two components that have together contributed to the 2nd generation water vapor sensing system (WVSS-II) will be presented. The first technical achievement was the use of the diode laser at 1.37 microns to measure water vapor via Beer's law with sophisticated software. The extreme accuracy of the WVSS-II (research quality measurements in accuracy and precision) and the system's long lifetime (20 years) are due in part to the adaptation of the telecommunication industry diode lasers to the aviation measurement application. The second achievement was the invention of the flush mounted UCAR air sampler. A key factor in the FAA certification of the WVSS-II was the determination by the FAA that the air sampler profile and location were not subject to icing -- thus requiring no heater for the air sampler. Various test results from the use of the WVSS-II are described. Several results from research aircraft are presented. Another set of results from actual operation units on the United Parcel Service (UPS) B-757 aircraft are illustrated. These latter results are from radiosonde comparisons of opportunity (when the UPS aircraft made an ascent or descent near a radiosonde site at a time close to a balloon launch) and from a third party inter comparison study conducted at the UPS main hub in Louisville, KY this past June. Several comparisons of aircraft data and radiosonde data are presented which illustrate the extreme accuracy and consistency of the WVSS-II data. Providing the air carriers with sensor system attributes they desired was instrumental in achieving the WVSS-II as a commercial product. These elements are reviewed. The evolving market for the WVSS-II and the derivative products from it will be described. These include the application for major and regional jets, turboprop aircraft, certain military aircraft, and unmanned aerial vehicles (UAVs). There are also applications for mobile platforms used for Homeland Security and eventual applications for a select group of non-scheduled aircraft like business jets and other general aviation aircraft.