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

Assessment of Ground-Based Microwave Radiometry for Calibration of Atmospheric Variability in Spacecraft Tracking

In a suggested radio propagation experiment using a deep space antenna, accurate calibration of the propagation delay through the Earth’s atmosphere is essential. One or two microwave radiometers can be used for this purpose. Differences in precise locations of the radiometer(s) and antenna to be calibrated leave a residual wet path delay value. We computed the Allan Standard Deviation (ASD) of this residual, as well as the one resulting from different pointing positions in the plane of the sky, by simulations. Pointing offsets, e.g., to avoid solar radiation into the radiometer beam, lead in general to an increased ASD. However, for many observation geometries a deliberate pointing offset can compensate for the location differences. In the case studied we found a reduction of the ASD with up to 45% compared to the ASD obtained for a zero pointing offset. The size of the calculated ASD depends strongly on the model parameters used, e.g., the turbulence strength parameter C_n^2, which has a significant natural variation over a year

Analysis of Temporal and Spatial Variations in Atmospheric Water Vapor Using Microwave Radiometry

The amount of atmospheric water vapor is highly variable in both time and space. In this thesis some aspects of the variations in the atmospheric water vapor are treated. Short term variations of up to approximately a day in the radio propagation delay due to water vapor has an important role as one of the limiting factors in space geodesy and radio astronomical interferometry. Since the atmospheric water vapor is a greenhouse gas, its change over many years is of interest in climatology. A microwave radiometer at the Onsala Space Observatory was used for data acquisition. Its data are used for characterizations of the variations in the amount of water vapor. The accuracy of the amount of water vapor determined using microwave radiometry is evaluated, as well as methods that utilize the knowledge of short-term atmospheric variability in order to improve the accuracy of the water vapor estimates. The role of the water vapor in space geodetic techniques such as Very-Long-Baseline Interferometry (VLBI) and the Global Positioning System (GPS) is also discussed. In these techniques the radio wave propagation is used for precise positioning. It is shown that an increased statistical knowledge of the variations in the amount of water vapor can improve the estimation of the contribution of water vapor to the propagation path length. The result is better positioning, while at the same time the amount of water vapor can be derived from the geodetic data, which is of interest for the meteorological community

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

A GPS Carrier-Phase Aided Clock Transport for the Calibration of a Regional Distributed Time Scale

Clock transportation is a historically proven time transfer method for the calibration of time links and time scales. With the establishment of satellite-based time transfer methods, however, clock transportation has become less attractive especially on long baselines. In order to match for instance the GPS common view time transfer method with calibration uncertainties of a few nanoseconds, it is necessary to transport high quality, expensive clocks such as caesium beam frequency standards. The stability of the clock during transportation and the duration of the transport set the limit of the prediction uncertainty. Being able to measure the clock during transportation instead of predicting it would yield some major advantages: (a) the use of less expensive and small clocks such as rubidium or quartz oscillators for transportation, (b) no need for environmental conditioning of the transported clock, and (c) the duration of the transport is not critical as long as the clock can continuously be measured. One solution to the clock measurement problem during transport is the use of GPS carrier-phase observations as described and evaluated in this paper. It is shown that a calibration uncertainty of less than one nanosecond is potentially achievable

Ionospheric Effects on GNSS RTK

GNSS signals are influenced by free electrons as they propagate through the ionosphere. Studies have shown how the spatial variations of electron density in the ionosphere, affects measurements with network-Real Time Kinematic (NRTK) (Emardson et al 2011). This paper aims to predict what can be expected from measurements during the next solar maximum that is expected to occur around 2025 and discusses how it would affect RTK for automated transport applications. The ionospheric activity and its impact on positioning in the coming solar cycle maximum is discussed. This study focuses on data from Kiruna in northern Sweden (67.8N), mainly captured in January 2014 - in the middle of the most active time during the last cycle - which has been analyzed to predict the coming solar cycle. Based on the data, it was concluded that there is a risk of occasions with simultaneous signal slips on several satellites caused by the ionosphere which could cause temporary (minutes) loss of positioning ability for the RTK equipment. It is expected to occur a couple of times per month during the most active months of the solar cycle

Temporal correlations of atmospheric mapping function errors in GPS data analysis

The developments in global satellite navigation using GPS, GLONASS, and Galileo will yield more observations at various elevation angles. The inclusion of data acquired at low elevation angles allows for geometrically stronger solutions. The vertical coordinate estimate of a GPS site is one of the parameters affected by the elevation-dependent error sources, especially the atmospheric corrections, whose proper description becomes necessary. In this work, we derive time-series of normalized propagation delays in the neutral atmosphere using ray tracing of radiosonde data, and compare these to the widely used new mapping functions (NMF) and improved mapping functions (IMF). Performance analysis of mapping functions is carried out in terms of bias and uncertainty introduced in the vertical coordinate. Simulation runs show that time-correlated mapping errors introduce vertical coordinate RMS errors as large as 4 mm for an elevation cut-off angle of 5\ub0. When simulation results are compared with a geodetic GPS solution, the variations in the vertical coordinate due to mapping errors for an elevation cut-off of 5\ub0 are similar in magnitude to those caused by all error sources combined at 15\ub0 cut-off. This is significant for the calculation of the error budget in geodetic GPS applications. The results presented here are valid for a limited area in North Europe, but the technique is applicable to any region provided that radiosonde data are available. \ua9 Springer-Verlag 2006