Rüdiger Haas

The Odin satellite - I. Radiometer design and test

The Sub-millimetre and Millimetre Radiometer (SMR) is the main instrument on the Swedish, Canadian, Finnish and French spacecraft Odin. It consists of a 1.1 metre diameter telescope with four tuneable heterodyne receivers covering the ranges 486-504 GHz and 541-581 GHz, and one fixed at 118.75 GHz together with backends that provide spectral resolution from 150 kHz to 1 MHz. This Letter describes the Odin radiometer, its operation and performance with the data processing and calibration described in Paper II.

Impact of atmospheric turbulence on geodetic very long baseline interferometry

We assess the impact of atmospheric turbulence on geodetic very long baseline interferometry (VLBI) through simulations of atmospheric delays. VLBI observations are simulated for the two best existing VLBI data sets: The continuous VLBI campaigns CONT05 and CONT08. We test different methods to determine the magnitude of the turbulence above each VLBI station, i.e., the refractive index structure constant C-n(2). The results from the analysis of the simulated data and the actually observed VLBI data are compared. We find that atmospheric turbulence today is the largest error source for geodetic VLBI. Accurate modeling of atmospheric turbulence is necessary to reach the highest accuracy with geodetic VLBI.

Three months of local sea level derived from reflected GNSS signals

By receiving Global Navigation Satellite System (GNSS) signals that are reflected off the sea surface, together with directly received GNSS signals (using standard geodetic‐type receivers), it is possible to monitor the sea level using regular single difference geodetic processing. We show results from our analysis of three months of data from the GNSS‐based tide gauge at the Onsala Space Observatory (OSO) on the west coast of Sweden. The GNSS-derived time series of local sea level is compared with independent data from two stilling well gauges at Ringhals and Gothenburg about 18 km south and 33 km north of OSO, respectively. A high degree of agreement is found in the time domain, with correlation coefficients of up to 0.96. The root‐mean‐square differences between the GNSS‐derived sea level and the stilling well gauge observations are 5.9 cm and 5.5 cm, which is lower than for the stilling well gauges together (6.1 cm). A frequency domain comparison reveals high coherence of the data sets up to 6 cycles per day, which corresponds well to the propagation of gravity waves in the shallow waters at the Kattegat coast. Amplitudes and phases of some major tides were determined by a tidal harmonic analysis and compared to model predictions. From the GNSS‐based tide gauge results we find significant ocean tidal signals at fortnightly, diurnal, semi‐diurnal, and quarter‐diurnal periods. As an example, the amplitudes of the semi‐diurnal M2 and the diurnal O1 tide are determined with 1σ uncertainties of 11 mm and 12 mm, respectively. The comparison to model calculations shows that global ocean tide models have limited accuracy in the Kattegat area.

An inter-comparison study to estimate zenith wet delays using VLBI, GPS, and NWP models

Water vapour is a key variable in atmospheric processes and plays a crucial role in atmospheric motions on a wide range of scales in space and time. The water vapour content is approximately proportional to the zenith wet delay (ZWD) which, in turn, constitutes a crucial parameter in geodetic microwave space techniques (VLBI and GPS). Apart from being determined by measurement techniques, the ZWD can also be derived from numerical weather prediction (NWP) models such as the non-hydrostatic MM5 model and the hydrostatic HIRLAM model. At the station Robledo de Chavela (Madrid) ZWD values were derived from VLBI, GPS, MM5, and HIRLAM for the beginning of December 1996. The results of the different techniques agree to the sub-centimetre level with correlation values of 0.87 (GPS vs. MM5), 0.81 (GPS vs. HIRLAM), and 0.84 (MM5 vs. HIRLAM). The correlation VLBI vs. MM5 of 0.78 is based on a short VLBI time series and should be considered preliminary. Further studies with longer time series are necessary to confirm this value. The bias and RMS difference values are all contained in the margin provided by the internal errors.

Sea level measurements using multi-frequency GPS and GLONASS observations

Global Positioning System (GPS) tide gauges have been realized in different configurations, e.g., with one zenith-looking antenna, using the multipath interference pattern for signal-to-noise ratio (SNR) analysis, or with one zenith- and one nadir-looking antenna, analyzing the difference in phase delay, to estimate the sea level height. In this study, for the first time, we use a true Global Navigation Satellite System (GNSS) tide gauge, installed at the Onsala Space Observatory. This GNSS tide gauge is recording both GPS and Globalnaya Navigatsionnaya Sputnikovaya Sistema (GLONASS) signals and makes it possible to use both the one- and two-antenna analysis approach. Both the SNR analysis and the phase delay analysis were evaluated using dual-frequency GPS and GLONASS signals, i.e., frequencies in the L-band, during a 1-month-long campaign. The GNSS-derived sea level results were compared to independent sea level observations from a co-located pressure tide gauge and show a high correlation for both systems and frequency bands, with correlation coefficients of 0.86 to 0.97. The phase delay results show a better agreement with the tide gauge sea level than the SNR results, with root-mean-square differences of 3.5 cm (GPS L1 and L2) and 3.3/3.2 cm (GLONASS L1/L2 bands) compared to 4.0/9.0 cm (GPS L1/L2) and 4.7/8.9 cm (GLONASS L1/L2 bands). GPS and GLONASS show similar performance in the comparison, and the results prove that for the phase delay analysis, it is possible to use both frequencies, whereas for the SNR analysis, the L2 band should be avoided if other signals are available. Note that standard geodetic receivers using code-based tracking, i.e., tracking the un-encrypted C/A-code on L1 and using the manufacturers’ proprietary tracking method for L2, were used. Signals with the new C/A-code on L2, the so-called L2C, were not tracked. Using wind speed as an indicator for sea surface roughness, we find that the SNR analysis performs better in rough sea surface conditions than the phase delay analysis. The SNR analysis is possible even during the highest wind speed observed during this campaign (17.5 m/s), while the phase delay analysis becomes difficult for wind speeds above 6 m/s.

Wet path delay and delay gradients inferred from microwave radiometer, GPS and VLBI observations

Very Long Baseline Interferometry (VLBI) is collocated with a permanent Global Positioning System (GPS) receiver and a Water Vapor Radiometer (WVR) at the Onsala Space Observatory in Sweden. Both space geodetic techniques are affected by the propagation delay of radio waves in the atmosphere, while the remote sensing technique is sensitive to the atmospheric emission close to the center of the 22 GHz water vapor emission line. We present a comparison of estimated equivalent zenith wet delay and linear horizontal delay gradients from an independent analysis of simultaneous VLBI, GPS, and WVR observations. Using different constraints for the variability of the delay and the horizontal gradient in the analysis of the VLBI and the GPS data did not have a large influence on the agreement with the WVR estimates. We found that the weighted rms differences between wet delay estimates from the geodetic techniques and the WVR estimates generally increased for an increased variability in the atmosphere.

Ultra-rapid UT1 measurement by e-VLBI

The latency of UT1 measurement with Very Long Baseline Interferometry (VLBI) has been greatly reduced by using e-VLBI technology. VLBI observations on the baseline formed by the Kashima 34-m and the Onsala 20-m radio telescopes achieved ultra-rapid UT1 measurements, where the UT1 result was obtained within 30 min after the end of the observing session. A high speed network and a UDP-based data transfer protocol ‘Tsunami’ assisted the high data rate and long-distance data transfer from Onsala to Kashima. The accuracy of the UT1 value obtained from the 1-h single baseline e-VLBI experiment has been confirmed to be as the same level with the rapid combined solution of Bulletin-A. The newly developed technology is going to be transferred to the regular intensive VLBI sessions, and it is expected to contribute to the improved latency and accuracy of UT1 data.

Crustal motion results derived from observations in the European geodetic VLBI network

Geodetic VLBI observations have been performed with the European geodetic VLBI network since early 1990 on a regular basis. The purpose of these observations is to determine crustal motion in Europe and to establish a stable reference frame for other space geodetic techniques. Over the years the size of the network and the number of participating stations has steadily increased. Today, the network extends from the island of Sicily in the south to the island of Spitsbergen/Svalbard in the north and from the Iberian peninsula in the west to the Crimean peninsula in the east. The area covered by the network is affected by two main geodynamic processes which are post-glacial rebound effects in the northern part, and the evolution of the Alps-Apennines orogenic systems in the southern part. With nearly 10 years of VLBI observations the determination of crustal motion in Europe is carried out with high accuracy. Baseline measurements are achieved with an accuracy of a few parts per billion. We compare the evolution of baseline lengths and topocentric station displacements with geophysical models. Strain rates in Europe on a large scale are determined from the results of the VLBI analysis.

Recent Progress in the VLBI2010 Development

From October 2003 to September 2005, the International VLBI Service for Geodesy and Astrometry (IVS) examined current and future requirements for geodetic VLBI, including all components from antennas to analysis. IVS Working Group 3 “VLBI 2010”, which was tasked with this effort, concluded with recommendations for a new generation of VLBI systems. These recommendations were based on the goals of achieving 1 mm measurement accuracy on global baselines, performing continuous measurements for time series of station positions and Earth orientation parameters, and reaching a turnaround time from measurement to initial geodetic results of less than 24 h. To realize these recommendations and goals, along with the need for low cost of construction and operation, requires a complete examination of all aspects of geodetic VLBI including equipment, processes, and observational strategies. Hence, in October 2005, the IVS VLBI2010 Committee (V2C) commenced work on defining the VLBI2010 system specifications. In this paper we give a summary of the recent progress of the VLBI2010 project

MM5 derived ZWDs compared to observational results from VLBI, GPS and WVR

Abstract Modelled values of zenith wet delay (ZWD) from the non-hydrostatic numerical weather prediction (NWP) model MM5 are compared to estimated values retrieved from observations by geodetic very long baseline interferometry (VLBI), global positioning system (GPS) receivers, and water vapour radiometers (WVRs). In addition, sparse radiosonde (RS) data are used to augment the available data sets. The comparison is done for three stations of the European geodetic VLBI network for six observing sessions during the year 1999. The stations (Madrid, Onsala, and Wettzell) were primarily chosen to have the maximum number of collocated measuring techniques. In general, the time series for the different techniques show a good agreement. The correlation values between the techniques amount to 75–95%. The RMS differences of MM5 with respect to the other techniques obtain values of ±1.3–1.6 cm. The bias between MM5 and VLBI lies at about 1.0 cm, the bias between MM5 and GPS varies in the range of 0.0–0.6 cm and appears to be station dependent.