TITLE: GPS Integrated Precipitable Water (30-min) Data [NOAA/FSL] CONTACTS: Seth I. Gutman OAR FSL R/FS3 325 Broadway Boulder, C0 80305-3328 USA Voice: 303 497-7031 Email: Seth.I.Gutman@noaa.gov 1.0 DATA SET OVERVIEW This dataset contains 30 minute GPS precipitable water vapor data from the NOAA Forecast Systems Laboratory Ground-Based GPS Meteorology Division. This data was taken as part of the Ground Based GPS-IPW project from stations throughout the IHOP region. The GROUND-based GPS-IPW provides accurate water vapor observations that are unaffected by the weather conditions or the time-of-day. 2.0 INSTRUMENT DESCRIPTION 2.1 Instrumentation All sites are equipped with a surface meteorological instrumentation package and GPS receiver. These instruments are connected back to FSL/DD via either dedicated FTS2000 telephone lines or the Internet. Also, as a backup means, meteorological data from NPN sites can be obtained via a GOES-DOMSAT downlink. Surface meteorology sensors Two types of surface meteorology (MET) packages (or payloads) are used by the demonstration network. At the NPN sites, payloads are known as PSOS and they are a customized version of the MARS package. At ONS and OAS sites, payloads are known as GSOS. GSOS payloads were custom designed and are fabricated for the demonstration network by NOAA's National Data Buoy Center (NDBC). Both packages measure only minimal meteorological parameters; pressure, temperature, and relative humidity. Because they are not used as part of the water vapor measurement calculations, other common meteorological parameters such as wind speed and direction are not gathered, And, if fact, the relative humidity data is also not used during water vapor measurement calculations. It is recorded for the benefit site climatology purposes. GPS hardware Receivers Measurement of water vapor relies on the extreamly precise determination of a site's position. Therefore, dual frequency (L1 and L2) GPS receivers must be used. Antenna Installations Approximately one-half of the demonstration network systems are located at NPN sites. The common method for installing a GPS antenna at NPN sites is to place it on one of the corner fence posts away from the equipment shelter that houses the GPS receiver, radar electronics, and communications equipment. Antenna Positional Stability Because the demonstration network antennas are not formally monumented, questions have been raised concerning the stability of these sites and their suitability for geodetic surveying. In 1996, NGS provided daily positions for nine sites in the network. These positions were used to calculated the stability of these sites. Site stabilities ranged from 4.1 mm - 5.3 mm (d_NORTH); 6.3 mm-8.7 mm (d_EAST); and 10.1 mm-14.3 mm (d_UP). The overall network average was determined to be: d_NORTH = 4.4 mm; d_EAST = 7.6 mm; d_UP = 12.7 mm. 2.2 Station Locations Station locations and site information for the stations included in this dataset can be found on the NOAA/FSL website: http://gpsmet.fsl.noaa.gov/jsp/net_now/net_now.jsp?region=ihop 3.0 DATA COLLECTION AND PROCESSING The demonstration network is controlled by a software system developed by FSL/DD in conjunction with SIO and UH. GPS precise predicted orbits from SOPAC and data from individual sites (GPS and MET) are continuously downloaded by a server. At the beginning of a processing cycle, several processing nodes download the GPS data and predicted orbits for selected sites from the server. The GAMIT (GPS at MIT) software package provided by MIT processes this data and produces Tropospheric Signal Delay (ZTD) values. These values are then processed with Meteorology data from the sites to produce and Integrated Precipitable Water Values (IPWV) which are then distributed to interested parties and made available online. The current processing system incorporates an 8 hour sliding window technique, with two processing cycles for each hour starting at 02 and 32 minutes after the hour. Each cycle produces 16 individual IPWV for each half hour within the sliding window. As the 8 hour sliding window progresses throughout the day, more IPWV are produced for each half hour until that particular half hour is no longer within the sliding window. This produces a possibility of 16 values for each half hour. Within each half hour, two IPWV are available, a first guess and median value. The first guess IPWV is the first value for the half hour meeting the quality control criteria. The median IPWV is the median of all results for that particular half hour. This result can be the median of 1 to 16 values depending upon quality control criteria. 4.0 DATA FORMAT AND FILE NAMING 4.1 Data Format These data are in columnar ASCII format. Column name Units Description ----------- ---- ----------- SITE Site abreviation YEAR YYYY Four-digit year of observation Date JJJ.dddd Julian date of observation Time HH:MM:SS Time of observation IPW cm Integrated Precipitable Water Values PRESS mb Barametric Pressure TEMP C Temperature RH % Relative Humidity TD m WD m HD m TM K PI cm FERR QCFLAGS Quality Control Flags 4.2 File Naming Conventions The files for this dataset are named in the format ssssYYJJJ.pwv
Where: ssss is the station ID/Abbreviation. YY is the two-digit year JJJ is the Julian date pwv stands for Precipitable Water Vapor For example: dqua02176.pwv 5.0 DATA REMARKS For further information on the Integrated Precipitable Water Values contained in this dataset, refer to NOAA/FSL Ground-Based GPS Meteorology website: http://gpsmet.fsl.noaa.gov/jsp/index.jsp. A series of Power Point Presentations that may be of interest are also located from this website: http://gpsmet.fsl.noaa.gov/jsp/downloads/downloads.jsp 6.0 REFERENCES NOAA/FSL, cited 2002: Ground-Based GPS Meteorology Demonstration Network. [Available on-line from http://gpsmet.fsl.noaa.gov/jsp/index.jsp.]