ascent: observations every 6 seconds (approximately 100-meter-verticalThese rates can be adjusted up or down via a real-time ACARS message.
resolution) until 90 seconds into the flight, then every 20 records
until an altitude of 20,000 feet
descent observations every 30 seconds from approximately 20,000 feet to
the surface
en route observations every 3 minutes
The original FAA requirement focused on ascent/descent information only and the response time specifications were modestly set to entice greater creativity in the approach to measurements of water vapor on commercial aircraft and to encourage several companies to bid in the competitive procurement process. These specifications were a response time of <6 s to 90% full-scale response for heights less than 5000 feet and <20 s for heights less than 20 000 feet. Results from early testing (Hills and Fleming, 1994) suggested mean values of 4 s and 12 s for corresponding altitudes for thin-film sensors. Typically radiosonde response times are about 1 s at 0°C to 10 s at -20°C to 100 s at -43°C (Salasmaa and Kostamo, 1975). In the upper troposphere, where temperatures are often as cold as -60°C, these response times must be much longer.
At flight level, a major advantage of the thin-film technology on the aircraft, as opposed to its use on the radiosonde, is the dynamically heated environment in which the measurement is made. As an example, using Eq. (2) and values of Mach number = 0.8, Ts = -60°C (213.15K) the temperature in the probe is 240.4K - greater than 27 degrees of warming. Thus, one can estimate response times to be in the range of 40 - 60 s and thus perhaps 4 - 6 times faster than for the radiosonde at this same cold static (ambient) temperature.
Two other characteristics of measurement systems that are useful to know are the sensitivity of the sensor (e.g., the lowest value of RH that can be measured) and the precision. The LMC WVSS sensor system is quite sensitive at the low RH range and is considered valid down to RH = 0.4%. The lowest RH value recorded in the probe (in an early sample of 12 000 reports) was 1.02% and the lowest value of RH static recorded (over all temperatures in the same sample) was 1.75%. [Note that this WVSS has a precision of 0.01% and this is why the RH values in this report are indicated to two digits beyond the decimal point - accuracy alone would not warrant this.]
The accuracy of the WVSS will depend on several factors. One of these is initial calibration and how well that calibration is maintained over time.
Initial calibration of the WVSS is performed by BFG. This is performed by standard laboratory procedures at RH values near 0% (using dry nitrogen) and 70% (using a chilled mirror reference traceable to NIST). The HMM30D provides a voltage from 0 to 5 volts that represents RH from 0 to 100%. The HMM30D signal conditioner is affected by ambient temperature. Therefore, BFG added circuitry to the electronics to compensate for output changes due to temperature. This typically reduced the error by a factor of five so the errors are less than +/- 0.5% RH over the temperature range likely to be encountered in the probe. Tests by LMC of the WVSS at BFG and at the National Center for Atmospheric Research (NCAR) indicated that the WVSS could be calibrated to within 1% for the range tested.