The purpose of this evaluation methodology was to determine whether the mean time between failure rate (MTBF) of the ASOS sensors was comparable with accepted standards. The reliability of the ASOS was evaluated by determining the percentage of time the ASOS system was operational and how often the individual sensors were non-operational.
The site selection criteria for sites utilized in this analysis methodology were:
Site was a commissioned ASOS site.
AOMC had no known interruptions in communications with the site.
Seven sites that met the selection criteria and each with a history of user comments and/or complaints were analyzed (see Table 3.6-1 ). Figure 3.6-1 shows the sites included in the ASOS Sensors Mean Time Between Failure Methodology.
The mean time between failure and the availability of the basic ASOS sensor group for selected sites was graphed for the period of February 1, 1997 through April 30. 1997. The objective of this process was to determine whether the mean time between failure and availability of the sensors was comparable with the levels produced in the ASOS Aviation Demonstration Final Report dated February 1996.
The following ASOS sensors were evaluated:
APRES ASOS Pressure Measuring Subsystem
APWX ASOS Present Weather Subsystem
ASKY ASOS Cloud Measuring Subsystem
ATDP ASOS Temperature & Dew Point Measuring Subsystem
AVIS ASOS Visibility Subsystem
AWIND ASOS Wind Subsystem
AZR ASOS Freezing Rain Subsystem
The following assumptions were made prior to the start of this comparison:
All sites are commissioned ASOS sites.
All recorded maintenance actions are documented at AOMC.
Sampling provided is representative of all commissioned sites.
The NWS ASOS Operations and Monitoring Center provided all data used in this element. Data was maintained at the ASOS Operations and Monitoring Center in tabled form and provided from the Average Mean Time Between Failure/Availability reports. The operational availability (Ao) was computed using the mathematical expression:
MTBF
Ao = ---------------------
MTBF + MOT
Where:
MTBF = Mean Time Between Failure of the system to report an acceptable form
MOT = Mean Outage Time to include Mean Time To Repair (MTTR), Mean Supply Response Time (MSRT), Mean Maintenance Delay Time (MMDT), and Mean Other Logistics Delay Time (MOLDT)
The Microsoft EXCEL Spreadsheet was used to graph the data and present the results of this analysis.
This assessment methodology utilized a sampling consisting of 90 days of data from seven sites. The 1995 ASOS Aviation Demonstration Report utilized 180 days of data for 26 sites.
Calculation of the sensor availability and sensor outages included end-to-end failures, hardware failures, as well as failures induced by power interruptions, maintenance activities, and other contributing causes. One hour is the minimum outage duration that could consistently be reported during the reassessment. To report a duration of less than one hour, a missing sensor failure must have coincided with a special.
Table 3.6-2 presents the statistical data for all failures for the sensors included in this analysis for 90 days from February 1 through April 30, 1997.
ASOS demonstrated a satisfactory level of availability, with overall sensor rates above 98%. Individual sensors exceeded the availability of the 1995 ASOS Aviation Demonstration with the exception of TDP and AVIS sensor, which decreased 0.0036 and 0.0102 percent respectively. The AZR sensor (not contained in the 1995 ASOS Aviation Demonstration), the APWX sensor and the APRES sensor all performed without any indicated failures during the study.
The mean sensor missing time indicated an overall decrease in sensor missing time throughout the sampling with an average decrease of outage time of 1.37 hours. This decrease included the exception of the ASKY, which indicated an increase of .7 hours. The size of the sampling emphasizes the impact of the ORH data that contained a 31 hour outage for the ASKY sensor. Figure 3.6-2 compares the sensor availability results obtained from the ASOS Re-assessment to those indicated in the ASOS Aviation Demonstration Report. Figure 3.6-3 compares the mean sensor missing times results obtained from the ASOS Re-assessment to those included in the ASOS Aviation Demonstration Report.