T. Ragne Emardson

Three months of continuous monitoring of atmospheric water vapor with a network of Global Positioning System receivers

Three months of continuous data from the Global Positioning System (GPS) using 20 sites in Sweden and 5 sites in Finland have been used to estimate the integrated amount of atmospheric water vapor. The quality of the data has been assessed by comparisons with a microwave radiometer (water vapor radiometer (WVR)) at the Onsala Space Observatory and with data from four different radiosonde stations. We found the agreement in integrated water vapor (IWV) between the GPS estimates and the radiometer data to be 1–2 kg/m2 in terms of daily root-mean-square (rms) differences. A major part of these rms differences were caused by a bias between the data sets. This bias (WVR-GPS) varied from day to day between −1.0 and +2.5kg/m2 with a mean value of +1.3kg/m2. Comparisons with radiosonde data showed rms differences around or slightly above 2kg/m2 for each station using the entire 3 month data set. Also here the GPS estimates were, on the average, below the radiosonde results. We show that the radomes used to protect the GPS antennas are likely to cause a large part of the observed bias. Spatial structure functions were calculated by using the GPS and the radiosonde data. An overall consistency between the GPS-based and the radiosonde-based structure functions indicates that the spatial correlations between the GPS estimates are not affected by the estimation process used in the GPS data analysis.

A Comparison of Precipitable Water Vapor Estimates by an NWP Simulation and GPS Observations

Abstract Simulated time series of the total precipitable water (PW) vapor from a limited area numerical weather prediction model are compared to estimates derived from observations done with ground-based Global Positioning System (GPS) receivers. The model data examined are from the delayed-mode High Resolution Limited Area Model (HIRLAM) data assimilation (reanalysis) and the short-range forecasts on double nested grids. The observational data are derived from GPS measurements at 25 sites in Sweden and Finland over a 4-month period, August–November 1995. In general, the HIRLAM reanalysis system demonstrates considerable skill in reproducing the spatial and temporal evolution of the PW as depicted by the GPS estimations. Using a 0.2° horizontal resolution and 31 vertical levels, the HIRLAM reanalysis generates a PW time series that has, in comparison to that of the GPS estimates, an average offset of −0.1 mm and a root-mean-square difference of 2.4 mm. The average correlation between the PW time series fr...

Ground-Based GPS for Validation of Climate Models: The Impact of Satellite Antenna Phase Center Variations

The amount of water vapor in the atmosphere is an important indicator for climate change. Using the Global Positioning System (GPS), it is possible to estimate the integrated water vapor (IWV) above the ground-based GPS receiver. In order to optimally determine the IWV, a correct model of the received signal phase is essential. We have studied the effect of the satellite antenna phase center variations (PCVs) on the IWV estimates by simulating the effect and by studying the estimates of the IWV based on the observed GPS signals. During a period of five years, from 2003 to 2008, a new satellite type was introduced, and it steadily grew in numbers. The antenna PCVs for these satellites deviate from the earlier satellite types and contribute to excess IWV estimates. We find that ignoring satellite antenna phase variations for this time period can lead to an additional IWV trend of about 0.15 kg/m2/year for regular GPS processing.

Spatial interpolation of the atmospheric water vapor content between sites in a ground‐based GPS Network

We have studied three methods of spatially interpolating GPS-estimated wet delay time series. Five months of data from three sites in the Swedish continuously operating GPS network were used for interpolation to a fourth site. Different geometries were used, resulting in interpolation distances of 40–120 km. To assess the quality of the interpolations we compared the obtained time series with estimated wet delay from a GPS station and with radiosonde data. A method based on the theory of atmospheric turbulence showed the lowest errors when compared to both the GPS estimates and the radiosonde data. The rms errors using this method are typically 1 cm.

Wet delay variability calculated from radiometric measurements and its role in space geodetic parameter estimation

The “wet delay,” the excess radio path length due to atmospheric water vapor, has been derived from 71 days of microwave radiometer measurements at the Onsala Space Observatory, Onsala, Sweden. The temporal and spatial variability in the wet delay was analyzed. When we estimated daily “variance rates,” the parameter characterizing a random walk process, values in the range 3.1×10−9 to 1.1×10−7 m2/s were found for the timescales 10–20 min. We estimated horizontal gradients in the wet delay and found that the temporal variability of the gradient components changed significantly from day to day. The variations in the gradients were also found to be significantly larger if data acquired only at relatively high elevation angles were used in the calculation. The effect of the wet delay variations on Global Positioning System (GPS) geodetic estimates was analyzed by performing Monte Carlo simulations. We used a Kalman filter with parameters for geodetic GPS data processing, first modeling the atmosphere as a horizontally homogeneous random walk process in time. In this case the estimated wet delay was found to be more sensitive to a detuning of the Kalman filter than the vertical component estimates. The RMS errors in the wet delay estimates increased from 2.2 to 3.6 mm when the atmospheric variance rate changed from 1.0×10−8 to 1.0×10−7 m2/s and when the filter parameter was set to 1.0×10−8 m2/s. When simulated wet delay gradients were added to the data, it was seen that if gradients are not estimated by the Kalman filter on days with large gradient variability, the scatter introduced by the gradients can dominate the other modeled error sources.

Time transfer using an asynchronous computer network: Results from three weeks of measurements

We have performed a time transfer experiment between two atomic clocks, over a distance of approximately 75 km using an 10 Gbit/s asynchronous fiber-optic computer network. The time transfer was accomplished through passive listening on existing data traffic and a pilot sequence in the SDH bit stream. In order to assess the fiber-link clock comparison, we simultaneously compared the clocks using a GPS carrier phase link. The standard deviation of the difference between the two time transfer links over the three-week time period was 243 ps.

Time transfer using an asynchronous computer network: An analysis of error sources

We have performed a time transfer over a distance of approximately 75 km using an asynchronous computer network based on optical fibers. In order to validate the results from this fiber-link, we have compared the results with a GPS-link, which consists of carrier phase observations. All electronic cabinets were equipped with temperature and humidity sensors. Here we present experiments where the temperature and humidity of the delay in the electrical components were investigated. All components showed some temperature dependence, but no significant humidity dependence was found. By using the derived temperature coefficient for the components the standard deviation of the difference between the fiber link and GPS link decreased from 243 ps to 184 ps.

High-rate GNSS techniques for the detection of large seismic displacements

Measurement of co-seismic strong-motion displacements at sub-second temporal resolution is of great importance for earthquake studies. We have investigated the usage of highrate sampled Global Navigation Satellite System (GNSS) data to measure seismic motion by implementing an industrial robot simulating the displacements close to an earthquake epicenter. The robot arm is tracked by GNSS signals. Two baselines-400 m and 60 km-from the robot to reference stations are used to process the observed GPS data. Both methods give similar (within 0.5 mm) Root Mean Square (RMS) differences between the estimated and the commanded coordinates. The RMS differences are 3.5 mm in the east component, 5.6 mm in the north component, and 8.1 mm in the vertical component.

Utilizing an Active Fiber Optic Communication Network for Accurate Time Distribution

To achieve a robust and accurate time scale at an arbitrary location in Sweden, we propose and demonstrate a system where an existing back-bone communication network carries the timing reference. Furthermore, when using the proposed technique the timing is extracted from the data packages already transmitted in the system, thus not taking up additional bandwidth. This is a unique feature of this time transfer technique, enabling an implementation through add-on modules to the existing network, and it is independent of the manufacturer of the router equipment. The feasibility and accuracy of the distributed time is experimentally verified in the Swedish University Network, SUNET, which connects university cities in Sweden. The long-haul transmission in this network is based on SDH STM-64, which is a synchronous transmission at 9.953 Gbit/s over optical fibers. However, the proposed time distribution technique is applicable in any packet based digital transmission.

Assessment of GPS derived speed for verification of speed measuring devices

Speed information from GPS is increasingly used and provides an alternative to conventional methods such as wheel speed sensors. We investigate the possibility to use GPS derived speed as a reference when verifying laser and radar-based speed measuring devices used in traffic enforcement. We have set up a realistic test scenario where a GPS equipped vehicle was driven at three different speeds (40, 90 and 130 km/h) through a pre-defined measurement zone. An independent and traceable reference speed was calculated by accurately measuring the length of the measurement zone (approximately 15 metres), and the time it took to pass through it. The reference speed was compared to the average GPS speed for each passage. This comparisons show that the standard uncertainty of such GPS speed measurements is less than 0.05 km/h. Hence, GPS derived speed meets the accuracy requirements for verification of laser and radar based speed measuring devices.