GPS/MET Preliminary Report

1. Introduction

"GPS Meteorology" is a name given to the body of science and technology which makes use of the Global Positioning System (GPS) for active remote sensing of the Earth atmosphere. At the University Corporation for Atmospheric Research (UCAR), scientists have already demonstrated one GPS Meteorology technique using ground based GPS receivers.[1] "Ground based GPS Meteorology is capable of accurate, continuous, integrated precipitable water vapor measurements at fixed sites. In the near future, a UCAR lead team intends to demonstrate an active limb sounding technique using radio occultation observations taken by an orbiting GPS receiver. We refer to this program and the technology under development collectively as "GPS/MET".

[Figure 1]

Figure 1 GPS/MET Measurement Geometry

As illustrated in Figure 1 above, high resolution atmospheric soundings can be retrieved from these observations when the radio path between a Low Earth Orbit (LEO) GPS receiver and one of the 24 GPS satellite traverses the Earth's atmosphere.[2] When the path of the GPS signal begins to transect the mesopause at about 85 km altitude, the signal is sufficiently retarded such that a detectable (1 mm) delay in the dual-frequency carrier phase observations is obtained by the LEO GPS receiver. As the signal path descends through successively denser layers of the atmosphere, the delay increases to approximately 1 km at the Earth's surface. Thus, the atmosphere creates a unique signal with approximately 6 orders of magnitude in dynamic range. A single LEO GPS receiver could observe more than 500 such occultations per day, with roughly uniform global coverage. The GPS/MET Phase I Program described herein is aimed to demonstrate the feasibility and utility of this "space based" GPS Meteorology.

The primary GPS/MET Program objectives are: (1) to construct a system and use it to collect GPS occultation data, (2) to develop and demonstrate algorithms for the recovery of accurate refractivity profiles and derivative products, such as pressure, temperature and moisture profiles, (3) to evaluate the net impact of GPS/MET occultation data on weather forecasts and global change research, and (4) to publish GPS/MET data in forms useful to other scientists investigating meteorological and related applications.

Note that GPS/MET promises to produce high quality data for two of the key global change variables identified by the Committee on Earth and Environmental Sciences: atmospheric temperature and moisture distribution. Moreover, the goals for GPS/MET address specific objectives identified in NOAA's 1992 Strategic Plan for Upper- Air Observations. If successful, this proof- of- concept demonstration could pave the way for a new, low cost operational sounding system capable of thousands of globally distributed soundings per day.

To achieve these objectives quickly and inexpensively, a modified commercial GPS receiver will be flown on a host satellite as a secondary payload. Launch of this satellite, the MicroLab-1, is scheduled for Fall, 1994. GPS occultation data, ancillary data taken from ground based observations, and derived information products will be published on the Internet beginning early 1995. To establish the predicted performance of the system, the GPS/MET team is conducting various tests and a detailed error analysis on the end- to- end system. Gridded data derived from operational observing systems will be used to validate GPS/MET data. Four Dimensional Data Assimilation (4DDA) software tools are under development for use in the assessment of the technique's net impact on weather forecasts.

The GPS/MET Program is sponsored by the National Science Foundation (NSF), the Federal Aviation Administration (FAA), and the National Oceanic and Atmospheric Administration (NOAA). Additional support is being provided by the National Aeronautics and Space Administration (NASA) and two industrial corporations: Orbital Sciences Corporation (OSC) and Allen Osborne Associates, Inc. (AOA). The GPS/MET team includes researchers from UCAR's UNAVCO and NCAR/MMM divisions, and from the University of Arizona (U of AZ) and CalTech's Jet Propulsion Laboratory (JPL).

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