TITLE: GPS Integrated Precipitable Water (30-min) Data [NOAA/FSL]

	Seth I. Gutman
	325 Broadway
	Boulder, C0 80305-3328 USA
	Voice: 303 497-7031
	Email: Seth.I.Gutman@noaa.gov


	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 


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 

	  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 
	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: 


	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. 


4.1 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
	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.]