Introduction
The purpose of the National Oceanic and Atmospheric Administration's (NOAA) Ground-Based GPS-IPW project is to:
- Evaluate the engineering and scientific bases of surface-based GPS meteorology,
- Demonstrate the feasibility and utility of using surface-based GPS observations for improved weather forecasting, climate monitoring, and satellite sensor calibration/validation,
- Transfer this observing system technology to operational use.
The Global Systems Division (GSD) Demonstration Branch (DB) established the world's first GPS network dedicated to atmospheric remote sensing in 1994. This project is a collabortaion between NOAA Research and several organizations and institutions including:
- NOAA's National Geodetic Survey (NGS) which manages the network of Continuously Operating Reference Stations (CORS); including some of the GSD/DB GPS-IPW network sites
- NOAA's National Data Buoy Center (NDBC)
- The U.S. Department of Transportation Federal Highway Administration (DOT)
- The U.S. Coast Guard (USCG)
- International GPS Service (IGS) which maintains global network of tracking sites; including some of the GSD/DB GPS-IPW network sites
- Scripps Institution of Oceanography (SIO)
- University of Hawaii at Manoa (UHhttp://lumahai.soest.hawaii.edu/Dept/meteorology/index.html)
- University NAVSTAR Consortium (UNAVCO)
- The Department of Energy Atmospheric Radiation Measurement (ARM) Program.
Why monitor atmospheric water vapor?
Water vapor is one of the most significant constituents of the atmosphere since it is the means by which moisture and latent heat are transported to cause "weather". Water vapor is also a greenhouse gas that plays a critical role in the global climate system. This role is not restricted to absorbing and radiating energy from the sun, but includes the effect it has on the formation of clouds and aerosols and the chemistry of the lower atmosphere. Despite its importance to atmospheric processes over a wide range of spatial and temporal scales, water vapor is one of the least understood and poorly described components of the Earth's atmosphere.
Water vapor measurement techniques
While an important goal in modern weather prediction is the improvement of short-term cloud and precipitation forecasts, our ability to do so is severely limited by the lack of timely and accurate water vapor data. Prior to the demonstration of GPS atmospheric remote sensing by the University NAVSTAR Consortium and North Carolina State University (from which key GPS-IPW researchers relocated to the Univ. of Hawaii at Manoa) in 1993, typical water vapor observing systems included radiosondes, surface-based radiometers, satellite-based radiometers, and some research aircraft. Each of these systems has advantages and limitations.
System |
Advantages |
Limitations |
| Radiosondes | Provide information about the vertical distribution of water vapor in the atmosphere | Labor intensive and (at present) are only launched twice-daily at upper-air sites. |
| Surface-based radiometers | Capable of high temporal resolution | Costly, require frequent calibration, and do not function well in all weather conditions, especially rain. |
| Research aircraft | Provide routine observations by using commercial aircraft (using the newly-developed Water Vapor Sounding System - WVSS) | Expensive and provide limited spatial and temporal coverage. These systems make observations only during ascent and descent to major airports. |
| Satellites using infrared sensors or microwave sensors | Provide very broad area (whole earth disk in the case of GOES) observations. | Estimates using IR sensors are reliable only in cloud free areas. Estimates using microwave sensors are valid in cloudy regions. However, they are available only over the oceans and are generally less accurate than the IR-based estimates. |
| Ground-based GPS-IPW | Provide unattended, autonomous, frequent, and accurate observations at very low cost that are unaffected by weather conditions or time-of-day. | Does not yet provide information about the vertical distribution of atmospheric water vapor, and since it only provides measurements of water vapor directly above the antenna, it has low spatial resolution. |
Although it has limitations, many researchers believed that ground-based GPS-IPW will be most useful as an element of a composite upper-air observing system. In this role, it will provide frequent water vapor observations to constrain numerical weather prediction models as well as independent calibration and validation of satellite measurements. In the latter case, ground-based GPS-IPW can provide data when satellites cannot obtain good measurements; mainly in cloudy regions. Where, from a forecasting perspective, the need to have accurate water vapor data is greatest.
How do ground-based GPS receivers measure water vapor?
GPS satellite radio signals are slowed as they pass through layers of Earth's atmosphere; the ionosphere and the neutral atmosphere. This slowing delays the arrival time of the transmitted signal from that expected if there were no intervening media. It is possible to correct for the ionospheric delay, which is frequency dependent, by using dual-frequency GPS receivers. The delays due to the neutral atmosphere, however, are not frequency dependent. They depend on the constituents of the atmosphere which are a mixture of dry gasses and water vapor.
In a ground-based measurement system, the signal delays from several (typically 6 or more) satellites in view are simultaneously mesured. These delays are mathematically adjusted (scaled) such that all satellites are seemingly directly overhead (at zenith) simultaneously using the function 1/sin (elevation angle of the satellite). The averaged vertically scaled signal delay introduced by the atmospheric constituents is called the Zenith Total (or Tropospheric) Delay (ZTD). ZTD can be separated into two terms called the zenith hydrostatic delay (ZHD) and the zenith wet delay (ZWD).
At sea level, ZTD has a magnitude of about 250cm to which the hydrostatic and wet components contribute about 97% and 3%: approximately proportional to the ratio of the total mass of dry air to water vapor in the atmosphere. ZTD can be estimated by constraining the positions of many widely-spaced GPS receivers and measuring the apparent error in position. When all system related errors are accounted for, the residual error is presumed to come from the neutral atmosphere.
The ZHD is calculated by measuring the surface pressure and applying a mapping function. The ZHD is then subtracted from the ZTD to give the ZWD. With the ZWD known, IPW directly above a GPS antenna through a factor that is proportional to the mean temperature of the atmosphere. The mean air temperature is currently estimated from a surface temperature measurement and its relationship to the climatological history of the temperature profile for that region based on radiosonde data.
Further information about Ground-Based GPS-IPW can be found in the publications listed in our bibliography.
GSD/DB demonstration network hardware and software
As of February 1999, the GSD/DB demonstration network contained 40 active sites. This network will be expanded to approximately 65 sites in 1999 and is it planned to have almost 200 sites by 2002.
There are three types of sites in the network; NOAA Wind Profilers Sites (NPN), Other NOAA Sites (ONS), and Other Agency Sites (OAS). The network currently consists of 23 NPN, 5 OAS, and 12 ONS sites. GSD/DB is responsible for all GPS-IPW equipment at the NPN and ONS sites, but only the surface meteorological equipment at OAS sites. For specific information about each site, see the Site Information page.
All sites are equipped with a surface meteorological instrumentation package and GPS receiver. These instruments are connected back to GSD/DB 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 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 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 benifit site climatology purposes.
GPS hardware
Receivers
Measurment 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 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. The GPS installation at Haskell, Oklahoma (HKLO) is illustrated below.
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The remaining network systems are installed at other NOAA facilities, U.S. Coast Guard Navigation Center (NAVCEN) sites, or U.S. Department of Transportation's Nationwide Differential GPS (NDGPS) sites. Typical installations at each of these three types of sites are shown below. Stennis MS is an other NOAA facility, Cape Canaveral FL is a USCG site, and Whitney NE is a DOT site.
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Antenna Positional Stability
Because the 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.
Software
The network is controlled by a software system developed by GSD/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 (illustrated below.) These values are then processed with Meteorology data from the sites to produce and Integrated Precipitable Water Values (IPW) 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 IPW for each half hour within the sliding window. As the 8 hour sliding window progresses throughout the day, more IPW 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 IPW are available, a first guess and median value. The first guess IPW is the first value for the half hour meeting the quality control criteria. The median IPW 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.
