The TAT probe is located on the right side in the same relative position at three o'clock. Only three inches of the WVSS or TAT probes extend outward from the aircraft skin (far enough outside to avoid measurements being contaminated by surface-friction heating). The remaining 5-inch-deep "can" of the WVSS is located beneath the airplane skin and contains the signal-conditioning electronics for the HMM30D. The HMM30D has been reconfigured by LMC/BFG for this commercial aircraft application. Part of this reconfiguration included adding a temperature compensation circuit to eliminate the temperature dependence found in the original signal conditioning module of the HMM30D. This temperature-compensation circuit was successfully tested and validated at the National Center for Atmospheric Research and contributes to a more accurate result than would have been achieved from the original module.
A WVSS probe can be removed from outside the aircraft, disconnected and a new probe replaced in about 15 - 30 minutes. The WVSS probe is replaced at six-month intervals to maintain the desired accuracy. Beginning in 2000, the thin-film sensor will be re-calibrated every six months, and the sensor replaced and calibrated every twelve months. The recalibration and replacement of the thin-film sensor is accomplished by BFG personnel under a NOAA contract. The removal and replacement of the probe is accomplished by air carrier personnel. All expenses associated with the WVSS program are paid by the federal government (FAA and NOAA); these include the WVSS probes, their installation and maintenance, and their eventual removal or replacement with the Second-generation WVSS for operational use.
The WVSS probe is heated to conform to FAA regulations that all commercial
aircraft probes be heated for anti-icing purposes. The complete WVSS system,
hardware and software, has been certified by the FAA, and a Supplemental
Type Certification (STC) has been obtained for B-757 aircraft.