Jan M Johansson | Chalmers University of Technology (original) (raw)

Papers by Jan M Johansson

this booklet is produced within the Gfg 2 project, a three year coordination action, funded by th... more this booklet is produced within the Gfg 2 project, a three year coordination action, funded by the 7th Framework Program of the European Commission under the Environment theme. Its mission is to better assess the value of Global Navigational Satellite Systems (GNSS) for Global Environmental Earth Observation (GEEO) and GEOSS.

Geophysical Research Letters, 2002

Proceedings of the 10th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1997), Sep 19, 1997

Ionospheric Effects on Network-RTK A part of the atmosphere is ionized by the UV radiation from t... more Ionospheric Effects on Network-RTK A part of the atmosphere is ionized by the UV radiation from the Sun. This part is often referred to as the ionosphere. The resulting free electrons influence the GNSS signals as they propagate through the ionosphere. We have studied how the spatial variations of electron density in the ionosphere affect measurements with network-RTK. The aim is to predict what we can expect from measurements during the next solar maximum that is expected to occur around 2012. In order to perform a spatial characterization of the ionosphere, we have used archived GPS data from SWEPOS from a five year period, 1999-2004, around the previous solar maximum. We find that the effect of the ionospheric spatial variability on network-RTK measurements is greater during night time than during day time. It is also clear that the effect is larger for northern Sweden than for the southern part. This is especially true during night time. The effect is also largest in the months October and November and smallest in June and July. Also the number of cycle slips is larger in northern Sweden than in southern Sweden. We find that when monitoring the ionosphere and its influence on network-RTK performance it is desirable to have several different geographical regions under observation. The effects in northern Sweden may, for example not be that relevant for a user in southern Sweden. In this report we define the ionospheric delay errors as the standard deviation of the difference between the ionospheric delay at L1 at one location and the estimated value of this based on the three surrounding reference stations with 70 km separation. Using GNSS equipment that is state-of-the art around 2010, we find that when conditions are such that the ionospheric delay error is below 10 mm, which occurs some 70% of the time, a rover is able to fix the ambiguities more than 90% of the time. This ability decreases with increasing ionospheric variability and when the ionospheric delay error is larger than 25 mm, which occurs some 10% of the time, the rover ability to fix is less than 50%. When measuring with network-RTK during the next solar maximum, approximately, 80% of the time, we have conditions such that a rover has at least 75% chance of fixing the solutions. Overall the probability to find a correct fix solution when performing RTK measurements during the next solar maximum is approximately 85% and the mean time to fix is 55 seconds.

Proceedings of the ... IEEE International Frequency Control Symposium, May 1, 2007

SP Technical Research Institute of Sweden and STUPI have performed a time transfer experiment ove... more SP Technical Research Institute of Sweden and STUPI have performed a time transfer experiment over a 500-km-long baseline between Borås and Stockholm. The time transfer technique passively utilizes the data bit stream generated in an optical fiber computer network based on the packet over SONET/SDH technique. A small fraction of the optical signal is monitored both at the transmitter and at the receiver. When an occurrence of a unique bit sequence of the SDH frames is detected, an electrical pulse is generated and compared with a resolution of 100 ps to a local clock. With data from all four positions of an optical bidirectional link, two-way time transfer can be achieved and any symmetrical variations in delay can potentially be cancelled. The results presented here have been obtained over OptoSUNET, the new Swedish University Network. In the experiment, 10 Gbit/s traffic from SP over OptoSUNET is extended in Stockholm to STUPI, a clock laboratory which is the second node in this setup. This reconnection enables that a communication channel is established between two nodes, with no intermediate jump. The time-transfer experiment includes more than 500 km of fiber transmission, of which several km is via airlines. By comparing the results from a GPS carrier-phase link, a precision better than ± 1 ns is achieved over several months of measurements between two hydrogen-masers.

The EGU General Assembly, Apr 1, 2016

For about two decades, SWEPOS (the Swedish Permanent GNSS network) pillar stations have been used... more For about two decades, SWEPOS (the Swedish Permanent GNSS network) pillar stations have been used indifferent geodetic and geodynamic studies. To keep continuous measurements of these long lived pi ...

Real Time Kinematic (RTK) is a system that utilises Global Navigation Satellite Systems (GNSS) to... more Real Time Kinematic (RTK) is a system that utilises Global Navigation Satellite Systems (GNSS) to provide accurate positioning in real time. The contribution of the troposphere, the ionosphere and local effects, such as receiver noise and multipath are the most significant error sources affecting network RTK measurements We show how measurements with network RTK are affected by these different error sources under varying circumstances such as time of year or time of the day, network infrastructure, satellite systems and processing techniques We find that, for Scandinavian conditions, the effect of the ionospheric spatial variability on network RTK measurements is greater during nighttime than during daytime. The effect is also largest in the months October and November and smallest in the months of June and July. A densification of the reference network from 70 km to 35 km between the reference stations results in improved measurements. The error in the measured vertical position coordinate is reduced from 26 mm to 17 mm. The access to new GNSS reduces error in the measured vertical position coordinate from 26 to 21 mm. By using the L3-combination, the contribution from the ionosphere is reduced to virtually zero. However, this has been at the expense of the local errors

TOUGH (Targeting Optimal Use of GPS Humidity Measurements in Meteorology) is a research project s... more TOUGH (Targeting Optimal Use of GPS Humidity Measurements in Meteorology) is a research project supported by the EU 5th Framework Programme. The main purpose of this project is to develop and refine methods enabling the use of GPS data from existing European GPS stations in numerical weather prediction models, and to assess the impact of such data upon the skill of weather forecasts. The contributions from the Chalmers are: • GPS data analysis in near real time • Assessment and characterization of correlated estimation errors (spatial as well as temporal) • GPS system research, assessing the stability and accuracy of time series of the estimated atmospheric propagation delays.

Detta hafte har producerats av Gfg2-projektet, en trearig koordineringsaktivitet finansierad av E... more Detta hafte har producerats av Gfg2-projektet, en trearig koordineringsaktivitet finansierad av Europeiska kommissionens sjunde ramprogram inom dess miljotema. Projektets uppgift ar att belysa vardet av jordobservationer med hjalp av globala navigationssystem (GNSS = Global Navigational Satellite Systems) for Global Environmental Earth Observation (GEEO) och Global Earth Observation System of Systems (GEOSS).

The Onsala Space Observatory (OSO) on the Swedish west coast is the fundamental geodetic station ... more The Onsala Space Observatory (OSO) on the Swedish west coast is the fundamental geodetic station of Sweden and operates several co-located geodetic and geophysical infrastructures that contribute to the GGOS. Currently, work is ongoing to establish a twin telescope to be part of the VGOS network.

Journal of Geodynamics, 2021

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This page was generated automatically upon download from the ETH Zurich Research Collection. For ... more This page was generated automatically upon download from the ETH Zurich Research Collection. For more information, please consult the Terms of use.

Astronomy & Astrophysics, 2015

Aims. We report about a 230 GHz very long baseline interferometry (VLBI) fringe finder observatio... more Aims. We report about a 230 GHz very long baseline interferometry (VLBI) fringe finder observation of blazar 3C 279 with the APEX telescope in Chile, the phased submillimeter array (SMA), and the SMT of the Arizona Radio Observatory (ARO). Methods. We installed VLBI equipment and measured the APEX station position to 1 cm accuracy (1σ). We then observed 3C 279 on 2012 May 7 in a 5 h 230 GHz VLBI track with baseline lengths of 2800 Mλ to 7200 Mλ and a finest fringe spacing of 28.6 µas. Results. Fringes were detected on all baselines with signal-to-noise ratios of 12 to 55 in 420 s. The correlated flux density on the longest baseline was ∼0.3 Jy beam −1 , out of a total flux density of 19.8 Jy. Visibility data suggest an emission region 38 µas in size, and at least two components, possibly polarized. We find a lower limit of the brightness temperature of the inner jet region of about 10 10 K. Lastly, we find an upper limit of 20% on the linear polarization fraction at a fringe spacing of ∼ 38 µas. Conclusions. With APEX the angular resolution of 230 GHz VLBI improves to 28.6 µas. This allows one to resolve the last-photon ring around the Galactic Center black hole event horizon, expected to be 40 µas in diameter, and probe radio jet launching at unprecedented resolution, down to a few gravitational radii in galaxies like M 87. To probe the structure in the inner parsecs of 3C 279 in detail, follow-up observations with APEX and five other mm-VLBI stations have been conducted (March 2013) and are being analyzed.

The BIFROST (Baseline Inferences for Fennoscandian Rebound Observations Sea Level and Tectonics) ... more The BIFROST (Baseline Inferences for Fennoscandian Rebound Observations Sea Level and Tectonics) project was started in 1993. The first primary goal was to establish a new and useful three-dimensional (3D) measurement of the movements in the Earth's crust based on Global Navigation Satellite System (GNSS) observations, able to constrain models of the GIA (glacial isostatic adjustment) process in Fennoscandia. We will present GNSS 3D velocity solutions derived from a complete reprocessing applying state-of-the art processing strategies and using the GIPSY/OASIS as well as the GAMIT/GLOBK software packages, and we compare these to recent GIA model predictions. The step where velocities are derived from daily position results also calls for some attention. Here we apply a time-series analysis, where we identify outliers and shifts in the time series, and we also evaluate characteristics of the random noise. In addition, we compare our results to velocity estimates from within the GLOBK (where the complete network is considered compared to the time-series analysis which is done station by station). Of special interests in activities like BIFROST are issues regarding reference frames, which are especially important while searching for true vertical velocities and while comparing to GIA models, as well as long term stability in the observation system including new generation of satellites and changes in the ground segment. These issues will also be discussed in the poster.

Agriculture-GNSS techniques are widely used for agriculture. Related to positioning are machinery... more Agriculture-GNSS techniques are widely used for agriculture. Related to positioning are machinery guidance, mapping, and livestock monitoring. Reflected GNSS signals can be used for soil moisture estimation, land classification, crop development monitoring, and biomass monitoring. A soil moisture map, generated by using GNSS reflectometry measurements (red: dry; blue: wet) (© A. Egido, Starlab). Energy-Many GNSS applications are related to energy, e.g. tracking of goods, time synchronization in smart grids, space weather, subsidence in mining, wind scatterometry, and lake level monitoring. GNSS-R installation at the Onsala Space Observatory, for monitoring of the sea level. Health-Health organizations use GNSS positioning services to manage people and equipment, monitor disease propagation, and direct search and rescue operations.

Proceedings of the 2005 IEEE International Frequency Control Symposium and Exposition, 2005.

A technique for time and frequency transfer over an asynchronous TCP/IP network is being develope... more 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,

2006 Optical Fiber Communication Conference and the National Fiber Optic Engineers Conference, 2006

A nationwide system for accurate time distribution is being developed, utilizing synchronization ... more A nationwide system for accurate time distribution is being developed, utilizing synchronization in an SDH-network. The first experimental results based on this technique are presented, performed on, but not limited to, STM-64.