An Automatic Meteorological Data Collection System That Is Installed at Global Positioning System Monitoring Stations

Abstract

The Demonstration Division of NOAA’s Forecast Systems Laboratory is conducting a long-term experiment to test the effectiveness of using the precise geodetic position measurements made by a network of Global Positioning System monitoring stations to determine the total amount of water vapor contained in the sectional volume of the atmosphere above each station. By knowing the exact position of the GPS satellites along their orbits and the precise location of the GPS monitoring receivers on the ground, an interpretation of the location error (actual location versus receiver-derived apparent location) yields a good indication of the amount of water vapor in the atmosphere. This result occurs because the monitoring station’s apparent location error is partially caused by the water vapor. Many factors influence the propagation of the electromagnetic waves as they travel through the Earth’s atmosphere from the constellation of GPS satellites (distributed along their orbits) to the GPS monitoring receivers (distributed throughout a ground surface network). One of these factors is the total amount of atmospheric water vapor. It is the quantity of this water vapor that the Demonstration Division is measuring. Other factors that affect the local speed of propagation of the electromagnetic waves transmitted from the GPS satellites, as the waves travel toward the GPS ground receivers, are the degree of ionization of the Ionosphere and the mass density distribution of the air in the Atmosphere. By subtracting the effects of the ionization and of the mass density distribution from the monitoring station’s total position error, the fraction of the total error caused by atmospheric water vapor can be isolated. With this value, the quantity of water vapor in the atmosphere can be calculated. The effect of the mass-density distribution of the atmosphere can be more precisely determined if its pressure, temperature, and relative humidity are accurately measured at the GPS monitoring stations. For this purpose, special meteorological data-collection systems have been installed at the same sites where GPS monitoring receivers are providing position-error data. These automatic systems were designed and built by the National Data Buoy Center. In them a microcontroller provides two-way data communication with a digital barometer and with a digital temperature/humidity sensor. The microcontroller also manages the digital interrogations and replies necessary for the transmission of the meteorological data to a central collection station at the Forecast Systems Laboratory in Boulder, Colorado. This data communication is via existing digital circuits used by the U.S. Coast Guard and by the National Geodetic Survey for monitoring and controlling the Differential Global Positioning System Aids-to-Navigation network. The meteorological data collection electronic packages and supporting hardware are called GPS Surface Observing Systems (GSOS). They can be quickly and easily installed at existing GPS monitoring sites. Besides a barometer, a temperature/humidity sensor, and a microcontroller, a GSOS assembly has auxiliary subsystems, such as a remote DC power supply, a solar radiation shield, a barometric pressure port, a lightening surge suppressor, a repeater module for the data communication cable, an airtight and watertight equipment enclosure, a solid-state data recording memory (one-week capacity), and numerous other components. Total electrical power consumption of the system is low. The operating temperature range of a GSOS package has been demonstrated to be from –60 °C to +50 °C. Several field installations have also survived severe hurricanes.