Fredrick Solheim

GPS Sounding of the Atmosphere from Low Earth Orbit: Preliminary Results

Abstract This paper provides an overview of the methodology of and describes preliminary results from an experiment called GPS/MET (Global Positioning System/Meteorology), in which temperature soundings are obtained from a low Earth-orbiting satellite using the radio occultation technique. Launched into a circular orbit of about 750-km altitude and 70° inclination on 3 April 1995, a small research satellite, MicroLab 1, carried a laptop-sized radio receiver. Each time this receiver rises and sets relative to the 24 operational GPS satellites, the GPS radio waves transect successive layers of the atmosphere and are bent (refracted) by the atmosphere before they reach the receiver, causing a delay in the dual-frequency carrier phase observations sensed by the receiver. During this occultation, GPS limb sounding measurements are obtained from which vertical profiles of atmospheric refractivity can be computed. The refractivity is a function of pressure, temperature, and water vapor and thus provides informat...

Sensing atmospheric water vapor with the global positioning system

Global Positioning System (GPS) receivers, water vapor radiometers (WVRs), and surface meteorological equip- ment were operated at both ends of a 50-kin baseline in Colorado to measure the precipitable water vapor (PWV) and wet delay in the line-of-sight to GPS satellites. Using high pre- cision orbits, WVR-measured and GPS-inferred PWV differences between the two sites usually agreed to better than 1 min. Using less precise on-line broadcast orbits increased the discrepancy by 30%. Data simulations show that GPS mea- surements can provide ram-level separate PWV estimates for the two sites, as opposed to just their difference, if baselines exceed 500 km and the highest accuracy GPS orbits are used.

The Promise of GPS in Atmospheric Monitoring

Abstract This paper provides an overview of applications of the Global Positioning System (GPS) for active measurement of the Earths atmosphere. Microwave radio signals transmitted by GPS satellites are delayed (refracted) by the atmosphere as they propagate to Earth-based GPS receivers or GPS receivers carried on low Earth orbit satellites. The delay in GPS signals reaching Earth-based receivers due to the presence of water vapor is nearly proportional to the quantity of water vapor integrated along the signal path. Measurement of atmospheric water vapor by Earth-based GPS receivers was demonstrated during the GPS/STORM field project to be comparable and in some respects superior to measurements by ground-based water vapor radiometers. Increased spatial and temporal resolution of the water vapor distribution provided by the GPS/STORM network proved useful in monitoring the moisture-flux convergence along a dryline and the decrease in integrated water vapor associated with the passage of a midtropospheri...

Sensing integrated water vapor along GPS ray paths

We demonstrate sensing of integrated slant-path water vapor (SWV) along ray paths between Global Positioning System (GPS) satellites and receivers. We use double differencing to remove GPS receiver and satellite clock errors and 85-cm diameter choke ring antennas to reduce ground-reflected multipath. We compare more than 17,000 GPS and pointed radiometer double difference observations above 20° elevation and find 1.3 mm rms agreement. Potential applications for SWV data include local and regional weather modeling and prediction, correction for slant wet delay effects in GPS surveying and orbit determination, and synthetic aperture radar (SAR) imaging. The method is viable during all weather conditions.

Propagation delays induced in GPS signals by dry air, water vapor, hydrometeors, and other particulates

Dry air, water vapor, hydrometeors, and other particulates (sand, dust, aerosols, and volcanic ash) in the atmosphere introduce microwave propagation delays. These delays must be properly characterized to achieve the highest accuracy in surveying and atmospheric sensing using Global Positioning System (GPS) signals. In this paper we review the theory of microwave propagation delays induced by the above atmospheric constituents and estimate their maximum delays. Because the structure of atmospheric refractivity can be highly complex and difficult to model, and because measurement tools are unavailable for characterizing most of the refractive components, we use simplified examples to illustrate its effects. Our results show that propagation delays due to water vapor, cloud liquid, rain, and sandstorms can be significant in high-accuracy GPS applications. For instance, propagation through 1 km of heavy rain can induce 15-mm delays in L1, and because delays due to scattering are dispersive and alias as ionospheric delays in L3 processing, L3 range errors are magnified to 20 mm. Depending upon the distribution of precipitation relative to the configuration of GPS satellites, such unmodeled delays can induce horizontal and vertical errors of several centimeters.

GPS surveying with 1 mm precision using corrections for atmospheric slant path delay

Multipath and atmospheric effects can limit GPS surveying precision. We surveyed a 43 km baseline using large diameter choke ring antennas to reduce multipath and pointed radiometer and barometric data to correct for atmospheric slant delay. Based on 11 daily solutions, atmospheric slant delay corrections improved vertical precision to 1.2 mm rms and horizontal precision to sub-mm. Applications for high precision GPS surveying include deformation monitoring associated with earthquake and volcanic processes, subsidence, isostasy, and sea level measurements; monitoring of atmospheric water vapor for climate and global change research, and to improve the resolution of synthetic aperture radar; calibration of satellite altimeters; and precise satellite orbit determination.

Pointed water vapor radiometer corrections for accurate global positioning system surveying

Delay of the Global Positioning System (GPS) signal due to atmospheric water vapor is a major source of error in GPS surveying. Improved vertical accuracy is impor- tant for sea level and polar isostasy measurements, geodesy, normal fault motion, subsidence, earthquake studies, air and ground-based gravimetry, ice dynamics, and volcanology. We conducted a GPS survey using water vapor radiometers (WVRs) pointed toward GPS satellites to correct for azi- muthal variations in water vapor. We report 2.6 mm vertical precision on a 50-km baseline for 19 solution days. Kalman filter or least-square corrections to the same data do not account for azimuthal distribution of water vapor and are degraded by 70%.

Microwave profiling of atmospheric temperature, humidity, and cloud liquid water

We describe a passive microwave radiometer that provides continuous unattended atmospheric temperature and humidity profiles up to 10 km in height, and low resolution liquid water profiles. Profile accuracies in cloudy and clear conditions up to 7 km height are better than 2.5 K (temperature) and 1.1 g/m3 (humidity) as determined by statistical comparison with radiosondes. The microwave profiler observes 12 channels in the 20 to 60 GHz range, 1 infrared channel, and surface temperature, pressure and humidity. Retrieval coefficients for specific locations are derived using local radiosonde data and a neural network or regression analysis. The commercial microwave radiometer design has logged more than a million hours in locations ranging form the arctic to the tropics. We provide examples of microwave profiler soundings in various locations and weather conditions, and comparisons with radiosondes. We discuss recent high resolution water vapor field analysis results based on simulated slant GPS and microwave profiler data.

Improved Retrieval of Integrated Water Vapor from Water Vapor Radiometer Measurements Using Numerical Weather Prediction Models

Abstract Water vapor radiometer (WVR) retrieval algorithms require a priori information on atmospheric conditions along the line of sight of the radiometer in order to derive opacities from observed brightness temperatures. This papers focus is the mean radiating temperature of the atmosphere (Tmr), which is utilized in these algorithms to relate WVR measurements to integrated water vapor. Current methods for specifying Tmr rely on the climatology of the WVR site-for example, a seasonal average-or information from nearby soundings to specify Tmr. However, values of Tmr, calculated from radiosonde data, not only vary according to site and season but also exhibit large fluctuations in response to local weather conditions. By utilizing output from numerical weather prediction (NWP) models, Tmr can be accurately prescribed for an arbitrary WVR site at a specific time. Temporal variations in local weather conditions can he resolved by NWP models on timescales shorter than standard radiosonde soundings. Curren...