Per Jarlemark

Wet tropospheric effects on precise relative GPS height determination

SummaryConsiderable interest has been generated recently in the use of the Global Positioning System (GPS) for precise height determination. A major error source in these measurements is the propagation delay due to atmospheric water vapour. In order to achieve the high precisions required for such applications as absolute sea-level monitoring improvement of wet delay modelling is necessary. Results from a GPS campaign show a significant correlation (0.91) between the variability of the wet delay measured using a water vapour radiometer (WVR) at the Onsala site and the absolute value of the residual error in the height determination of a 134 km baseline from Onsala to Jönköping. This correlation indicates that the atmosphericvariability as inferred from the WVR data includes information on the quality of the GPS height estimate. During periods of high atmospheric activity, e.g., during the passage of a weather front, the use of a six-parameter gradient model reduces the spread for the vertical coordinate from 40 mm to 20 mm (with standard deviations of 17 mm and 9 mm respectively) over the 134 km baseline (less than 1 × 10−7) using 8 hour data spans on 11 different days over a six month period.

Ground-based microwave radiometry and long-term observations of atmospheric water vapor

Microwave radiometer data and radiosonde data from the time period 1981-1995 have been used to study long-term trends in the integrated precipitable water vapor (IPWV). The two instruments have operated 37 km apart on the Swedish west coast. Model parameters are estimated for the entire data sets as well as for subsets of the data. The IPWV model parameters are a mean value, a linear drift with time, and the amplitude and phase of an annual component. The radiosonde data, which are uniformly sampled in time, show an increase in the IPWV of 0.03 mm/yr with a statistical standard deviation of 0.01 mm. The microwave radiometer data, which are not at all uniformly sampled in time, show -0.02+/-0.01 mm/yr. We show that the disagreement is caused by the different sampling of the data for the two instruments. When the two data sets are reduced to include only data that are sampled simultaneously, we find an agreement between all estimated model parameters, given their statistical uncertainties. This suggests that if the microwave radiometer had also been operating continuously over the 15-year period, its data would have implied a linear trend similar to the result obtained from the radiosonde data. The general quality of the data, in terms of the short time scatter, has been improved over the time period. The root mean square (RMS) difference between the IPWV measured by the radiometer and by the radiosondes was 2.1 mm during the first 5 years and was reduced to 1.6 mm during the last 4 years. These values include the real difference in the IPWV between the two sites. The bias, radiometer-radiosonde, was 0.1 mm for the whole data set and varied between -0.2 and 0.9 mm for smaller data sets of a few years.

Time Transfer by Passive Listening Over a 10-Gb/s Optical Fiber

A technique for time and frequency transfer over an asynchronous fiber optical transmission control protocol (TCP)/IP network is being developed in Sweden by SP Measurement Technology together with STUPI. The technique is based on passive listening to existing data traffic at 10 Gb/s in the network. Since the network is asynchronous, intermediate supporting clocks will be located and compared at each router. We detect, with a specially designed high-speed optoelectronic device, a header recognizer, the frame alignment bytes of the synchronous optical network (SONET)/synchronous digital hierarchy (SDH) protocol, as a reference for the supporting clock comparison. The goal of the project is to establish a time transfer system with an accuracy on the nanosecond level. In this paper, we present the results of a time transfer over a distance of 5 km. We have compared two clocks: a cesium clock at the Swedish National Laboratory for time and frequency and a remote rubidium clock. The results of the time transfer with the fiber link have been simultaneously compared to measurements with a Global Positioning System (GPS) carrier phase link in terms of precision and stability. The root-mean-square (rms) difference between the time difference measured with the fiber link and the GPS link is approximately 300 ps. A large part of the difference is due to the heating of the GPS antenna cable, which introduces daily delay variations on the order of 1 ns from peak to peak. For one of the days with small day-to-day variations in temperature, the corresponding rms difference is 72 ps, and the Allan deviation is below 30 ps for averaging times longer than 5 min.

Measurements and Error Sources in Time Transfer Using Asynchronous Fiber Network

We have performed time transfer experiments based on passive listening in fiber optical networks using Packet over synchronous optical networking (SONET)/synchronous digital hierarchy (SDH). The experiments have been performed with different complexity and over different distances. For assessment of the results, we have used a GPS link based on carrier-phase observations. On a 560-km link, precision that is relative to the GPS link of <; 1 ns has been obtained over several months. In this paper, we describe and quantify the different error sources influencing the fiber time transfer measurements. We show that the temperature dependence of the optical fiber is the major contribution to the error budget, and, thus, reducing this effect should be the best way of improving the results.

The atmospheric influence on the results from the Swedish GPS network

Abstract The SWEPOS network currently consists of 21 continuously operating GPS stations. The GPS receiver at the Onsala site is collocated with a microwave radiometer measuring the integrated amount of water vapour along the line of sight. We have studied the correlation between GPS estimates of position and variations in the atmosphere using data from the autumns of 1995 and 1996, and found only small correlations between the estimated positions and the atmospheric quantities studied. We are, however, aware of other systematic errors, e.g. caused by radomes above the GPS antennas which may presently dominate the error budget

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.

Time and frequency transfer in an asynchronous TCP/IP over SDH-network utilizing passive listening

A technique for time and frequency transfer over an asynchronous TCP/IP network is being developed by SP, Swedish National Testing and Research Institute together with STUPI. When implemented, users will be able to compare their clocks by connecting to the system. The technique is based on passive listening to existing data traffic in the network. Since the network is asynchronous, intermediate clocks are located and compared at each router. We use the frame alignment bytes of the SONET/SDH protocol as references in order to compare these clocks. As a test bed for the experiment, we will use the Swedish University Computer Network (SUNET). A preliminary assessment of the technique in a lab environment will be performed late 2005

Thermal influence on the receiver chain of GPS carrier phase equipment for time and frequency transfer

In this study a temperature controlled environment is used in order to quantify the thermal influence on all major parts of state of the art geodetic GPS receiving equipment. Temperature variations, effective as time delay variations, were identified as a dominating error source that degrades the capabilities of carrier phase GPS based time and frequency transfer considerably. For purely code-based measurements with uncertainties in the ns range is temperature rarely an issue. In contrast carrier phase observations offer potentially a two orders of magnitude better accuracy and are therefore suitable for exploiting the characteristics of maser quality clocks. However, the stability of the environment around the receiver equipment defines the achievable accuracy. Four distinct parts of the receiver chain were subject to systematic measurements of the temperature-delay dependency: antenna preamplifier, antenna and clock cables, power distribution devices and geodetic receivers. A temperature controllable climate chamber was deployed with the respective component to follow a long time-constant temperature stepping. Signal through devices were mainly tested in a vector-voltmeter approach. Zero base line GPS processing was used to test receivers. With individual component temperature dependence being far above the expected accuracy of carrier phase based time and frequency transfer it underlines the necessity to include temperature as an important parameter into time/frequency solutions.

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.

A fiber based frequency distribution system with enchanced output phase stability

Experimental results on the stability of the output phase of a frequency distribution system from several days of measurement is presented, in addition to a discussion regarding the influence of control loop parameters. The setup handles the issue that the output phase stability of a system depends on perturbations along the transmission length. This is especially critical if the signal is transmitted through optical fiber, at lengths of a few 100 m. An experimental evaluation using a laser based transmitter at a wavelength of 850 nm, and 625 m of multimode fiber where 575 m where placed outdoor, a temperature dependence of 100 ps/°C was detected. To compensate for these slow variations in real time, a setup using two-way transmission, in conjunction with an adjustable optical delay, was constructed. This device is adjusted to induce a delay variation of equal magnitude but opposite direction, in comparison to the delay change of the fiber. Calculating the modified Allan deviation of the transmitted signal, it is apparent that without active compensation, the deviation at τ below 1000 s is comparable to the values from the measurement system without transmission. At longer integration times, however, the slow variations in the fiber transmission will deteriorate the modified ADEV substantially. When activating the dynamic adjustment of pre-delay in the system, the deviation at shorter times will increase with a few dB, however, the modified ADEV decreases continuously with τ, eventually below the values for the uncompensated system. In conclusion, activating a dynamically controlled pre-delay in a fiber based frequency transmission system will induce a small penalty on fast variations of the output phase, however giving a remarkable improvement on slower variations. The usefulness of this added functionality must therefore be determined by the application of the signal.