The demand for accuracy in the above applications has been hard to satisfy by our existing radiosonde and satellite remote sensing systems. While a number of measurement principles for measuring water vapor with balloon-borne radiosondes have been attempted, only a few give reliable results - even these have had trouble with accuracy at the extremes of very wet (100% relative humidity) and very dry conditions. Problems of the carbon hygristor have been documented at both the high and low ends of the relative humidity (RH) range (Wade, 1993, 1994). Problems of thin-film capacitors to properly account for RH at very cold temperatures have also been noted (Miloshevich, et. al., 1998). The satellites provide good horizontal resolution of relative amounts of water vapor in the upper atmosphere (and especially of the gradient of water vapor on a global scale); however, such measurements suffer from the lack of absolute accuracy and have poor vertical resolution where it is really required - in the lower troposphere. The satellites' remote sensing systems may never have the capability to provide (by themselves) the vertical resolution information required for all atmospheric science applications.
Clearly, a composite observing system is required for water vapor, and one of the new methods of contributing to a composite system will be measurement from commercial aircraft. These aircraft are capable of providing high vertical resolution measurements of water vapor information upon ascent and descent, and excellent spatial resolution at flight level. They are also capable of providing calibration information across multiple satellite fields-of-view from long trans-oceanic flights.
The first real-time water vapor measurement system was provided via a competitive government contract won by the Lockheed Martin Corporation (LMC) and its major subcontractor B. F. Goodrich (BFG) Aircraft Sensors Division. This system is referred to as the Water vapor Sensing System (WVSS). Other subcontractors were the AlliedSignal Corporation, who incorporated WVSS software into their avionics equipment used on B-757 aircraft, and United Parcel Service (UPS) who helped certify the WVSS on their B-757 aircraft and are now flying the WVSS on six (as of this writing) such aircraft. The funding for this contract was provided by the Federal Aviation Administration (FAA) and the National Oceanic and Atmospheric Administration (NOAA).
The fundamental measurement concept used for this first-generation WVSS is the thin-film capacitor applied for the direct measurement of relative humidity. This is the same concept as found in some radiosondes (in fact, both sensors are provided by the same company, Vaisala), but there are two major differences that make the WVSS more accurate and more reliable than the water vapor information from randomly chosen radiosondes. The aircraft sensor is a more rugged, semi-permanent fixture, made more accurately than the original product by the removal of the temperature dependence. This is contrasted by the harsh reality of radiosonde production where they are designed to be disposable, mass-produced at low cost, and individual calibration is required for each sensor. A second major difference is the Mach number effect (the dynamic heating associated with the aircraft's rapid speed) that allows for a significantly faster sensor response time in the cold regions of the upper troposphere. On the aircraft and in radiosondes, the output voltage of the capacitive signal conditioner is a linear function of the RH. However, for radiosondes, Vaisala's software internal to their radiosonde processing system uses a "temperature correction factor" to boost RH values at low temperatures - this to compensate for the very slow response time and for nonlinear effects at cold temperatures. Such a temperature correction has not been required for the WVSS - see Appendix B.
This first real-time water vapor measurement system on commercial aircraft
is working in an experimental mode and is available on the Internet in
real-time via the
address:
The WVSS program was the first attempt to obtain real-time water vapor information from modern commercial aircraft. Although there were numerous ground tests of the WVSS before the first flight, it has been a rather daring decision to expose the WVSS immediately on the Internet; however, this was viewed as the best way for all involved to quickly access the new measurement system. It also opened the door for constructive criticism - allowing a more rapid understanding of the nuances of the system not discernible until placed in actual flight conditions. We do not regret that decision.
As of this writing, over 1200 reports per day are being received. As users are becoming familiar with this data, they need to be aware how the sensor system works - its calibration and its error characteristics. Providing this information is the purpose of this report.
Section 2 will outline the relevant equations associated with measuring water vapor from modern high-speed commercial aircraft. Section 3 of this report will address some of the technology details of the WVSS. Section 4 will describe the calibration procedure used for the current aircraft measurement technology. Section 5 provides a sample of early results and the expected error structure. Section 6 indicates the intended use of this first-generation WVSS.