Karol Dawidowicz

Accuracy of single receiver static GNSS measurements under conditions of limited satellite availability

Abstract At least two simultaneously operating receivers are required for differential global navigation satellite system (GNSS) positioning. In this mode, the systematic errors between stations can be estimated or reduced in order to achieve much higher accuracy. Precise point positioning (PPP) is a rather new category. PPP is a combination of the original absolute positioning concept and differential positioning techniques. In PPP we use observation data of a single receiver and additional information on individual GNSS errors derived from a GNSS network, usually from ground based augmentation systems (GBAS). GBAS systems can be divided by the area of operation into global, continental, national or regional ground support systems (e.g. ASG-EUPOS, CORS, SAPOS, SWEPOS). GBAS systems allow users with a single receiver to position in differential mode based on observations from the reference stations. This paper presents an analysis of the position determination accuracy using single receiver GNSS measurements conducted under conditions of limited satellite availability and processed using various types of GNSS services. The PPP-CSRS service was chosen as an example of a PPP service. For differential processing mode, the ASG-EUPOS service was selected. The analysis was based on four days of data from three GNSS stations. The PPP-CSRS results show that horizontal accuracies of ∼5 cm and vertical accuracies of 10 cm are achievable provided 0·5 h of open sky and low multipath dual frequency GNSS data. Accuracy clearly decreases for points measured under conditions of limited satellite availability. Analogous ASG-EUPOS service accuracies are noticeably better.

Impact of different GNSS antenna calibration models on height determination in the ASG- EUPOS network: a case study

Abstract Before 6 November 2006, the International global navigation satellite system (GNSS) Service used relative phase centre models for GNSS antenna receivers. When absolute calibration models were introduced, it resulted in significant differences in the scale of GNSS networks compared to the very long baseline interferometry and side-looking radar measurements. The differences were due to the lack of the GNSS satellite antenna calibration models. This problem was sufficiently resolved and the International GNSS Service decided to switch from relative to absolute models for both satellites and receivers. This decision caused some variations in the results of the GNSS network solutions – especially in the vertical component. To date, the problem of switching from relative to absolute antenna phase centre variations has been mainly considered for global or continental networks using relatively long observation sessions. The aim of this paper was to study the height differences caused by using different calibration models in GNSS observation processing done in the national GBAS network (ASG-EUPOS). The analysis was done using 3 days of GNSS data, collected with four different receivers and antennas, divided by 1 h observation sessions. The results of the calculations show that switching from relative to absolute phase centre variation models may have a significant effect on height determination in the ASG-EUPOS network, particularly in high accuracy applications.

Analysis of Current Position Determination Accuracy in Natural Resources Canada Precise Point Positioning Service

Abstract Precise Point Positioning (PPP) is a technique used to determine highprecision position with a single GNSS receiver. Unlike DGPS or RTK, satellite observations conducted by the PPP technique are not differentiated, therefore they require that parameter models should be used in data processing, such as satellite clock and orbit corrections. Apart from explaining the theory of the PPP technique, this paper describes the available web-based online services used in the post-processing of observation results. The results obtained in the post-processing of satellite observations at three points, with different characteristics of environment conditions, using the CSRS-PPP service, will be presented as the results of the experiment. This study examines the effect of the duration of the measurement session on the results and compares the results obtained by working out observations made by the GPS system and the combined observations from GPS and GLONASS. It also presents the analysis of the position determination accuracy using one and two measurement frequencies

Periodic signals in a pseudo-kinematic GPS coordinate time series depending on the antenna phase centre model – TRM55971.00 TZGD antenna case study

This study investigates the differences between position estimates obtained using individual and type-mean (igs08.atx) antenna calibration models in GPS pseudo-kinematic processing. We used 15-min observation windows to study the short-period oscillations. Continuous GPS observations from eight selected Polish EUREF Permanent Network stations with the same TRM55971.00 TZGD antenna were used. The results showed that the differences in the calibration models, directly propagate into the position domain, affecting sub-daily results and influencing periodic variations. The sub-daily variations have periods close to half a sidereal day with amplitudes of up to 10 mm in position components. It could also be demonstrated that the mean position offsets, resulting from the use of individual calibrations instead of type-mean igs08.atx calibrations, can reach up to 5 mm in the up component, while the offsets in the horizontal components generally remain below 1 mm.

Differences in GPS coordinate time series resulting from the use of individual instead of type-mean antenna phase center calibration model

It is well-known that the phase center of a Global Navigation Satellite System (GNSS) antenna is not a stable point coinciding with a mechanical reference. The phase center position depends on the direction of the received signal, and is antenna-and signaldependent. Phase center corrections (PCC) models of GNSS antennas have been available for several years. The first method to create antenna PCC models was the relative field calibration procedure. Currently only absolute calibration models are generally recommended for use. In this study we investigate the differences between position estimates obtained using individual and type-mean absolute antenna calibrations in order to better understand how receiver antenna calibration models contribute to the Global Positioning System (GPS) positioning error budget. The station positions were estimated with two absolute calibration models: the igs08.atx model, which contains typemean calibration results, and individual antenna calibration models. Continuous GPS observations from selected Polish European Permanent Network (EPN) stations were used for these studies. The position time series were derived from the precise point positioning (PPP) technique using the NAPEOS scientific GNSS software package. The results show that the differences in the calibrations models propagate directly into the position domain, affecting daily as well sub-daily results. In daily solutions, the position offsets, resulting from the use of individual calibrations instead of type-mean igs08.atx calibrations, can reach up to 5 mm in the Up component, while in the horizontal one they generally stay below 1 mm. It was found that increasing the frequency of sub-daily coordinate solutions amplifies the effects of type-mean vs individual PCC-dependent differences, and also gives visible periodic variations in time series of GPS position differences.

Determining normal heights with the use of Precise Point Positioning

In this paper, the authors analysed the Precise Point Positioning (PPP) performance in determination of normal heights using observations from a test network consisting of ten sites. One-week observation interval was processed using standard PPP approach and Multi-Station PPP (MS-PPP) with ambiguity resolution using the NAvigation Package for Earth Observation Satellites v. 3.3.1 software. In post-processing corrections like the Earth rotation parameters, ocean loadings, antenna phase centre offsets and variations etc. needed to obtain the most accurate position were utilised. By processing the various lengths observing sessions (1 hour, 30 minutes, 15 minutes) the authors examined the degradation of the position determination precision with shortening the observation time. In the study both high-end receivers used most often at CORS, as well as commercial two-frequency receivers were utilised. Normal heights were obtained using PL–geoid–2011 model. The results of the research show that PPP is a viable alternative for Relative GNSS Positioning in the case of GNSS levelling.

GNSS Antenna Caused Near-Field Interference Effect in Precise Point Positioning Results

Abstract Results of long-term static GNSS observation processing adjustment prove that the often assumed “averaging multipath effect due to extended observation periods” does not actually apply. It is instead visible a bias that falsifies the coordinate estimation. The comparisons between the height difference measured with a geometrical precise leveling and the height difference provided by GNSS clearly verify the impact of the near-field multipath effect. The aim of this paper is analysis the near-field interference effect with respect to the coordinate domain. We demonstrate that the way of antennas mounting during observation campaign (distance from nearest antennas) can cause visible changes in pseudo-kinematic precise point positioning results. GNSS measured height differences comparison revealed that bias of up to 3 mm can be noticed in Up component when some object (additional GNSS antenna) was placed in radiating near-field region of measuring antenna. Additionally, for both processing scenario (GPS and GPS/GLONASS) the scattering of results clearly increased when additional antenna crosses radiating near-field region of measuring antenna. It is especially true for big choke ring antennas. In short session (15, 30 min.) the standard deviation was about twice bigger in comparison to scenario without additional antenna. When we used typical surveying antennas (short near-field region radius) the effect is almost invisible. In this case it can be observed the standard deviation increase of about 20%. On the other hand we found that surveying antennas are generally characterized by lower accuracy than choke ring antennas. The standard deviation obtained on point with this type of antenna was bigger in all processing scenarios (in comparison to standard deviation obtained on point with choke ring antenna).

Analyzing the Impact of Different Pcv Calibration Models on Height Determination Using Gps/Glonass Observations from Asg-Eupos Network

ABSTRACT The integration of GPS with GLONASS is very important in satellite-based positioning because it can clearly improve reliability and availability. However, unlike GPS, GLONASS satellites transmit signals at different frequencies. This results in significant difficulties in modeling and ambiguity resolution for integrated GNSS positioning. There are also some difficulties related to the antenna Phase Center Variations (PCV) problem because, as is well known, the PCV is dependent on the received signal frequency dependent. Thus, processing simultaneous observations from different positioning systems, e.g. GPS and GLONASS, we can expect complications resulting from the different structure of signals and differences in satellite constellations. The ASG-EUPOS multifunctional system for precise satellite positioning is a part of the EUPOS project involving countries of Central and Eastern Europe. The number of its users is increasing rapidly. Currently 31 of 101 reference stations are equipped with GPS/GLONASS receivers and the number is still increasing. The aim of this paper is to study the height solution differences caused by using different PCV calibration models in integrated GPS/GLONASS observation processing. Studies were conducted based on the datasets from the ASG-EUPOS network. Since the study was intended to evaluate the impact on height determination from the users’ point of view, a so-called “commercial” software was chosen for post-processing. The analysis was done in a baseline mode: 3 days of GNSS data collected with three different receivers and antennas were used. For the purposes of research the daily observations were divided into different sessions with a session length of one hour. The results show that switching between relative and absolute PCV models may cause an obvious effect on height determination. This issue is particularly important when mixed GPS/GLONASS observations are post-processed.

URBAN AREA GPS POSITIONING ACCURACY USING ASG-EUPOS POZGEO SERVICE AS A FUNCTION OF SESSION DURATION

ABSTRACT GNSS observations carried out in a network of Continuously Operating Reference Station (CORS) are a complex systems which offer post-processing as well as corrections sent in realtime. In Poland, such a system has been in operation since June 2008, known as the Polish Active Geodetic Network (ASG-EUPOS). Usually the measurements performed in real time characterized lower accuracy than static measurements. For users who demand the highest precision results the post-processing services are provided. The paper presents an analysis of the position determination accuracy using ASG-EUPOS POZGEO service. It is well known that the final accuracy is e.g. the measuring conditions, time of observations or number of measured frequencies dependent. We processed 4 consecutive days of GPS data to determine how the accuracy of derived positional coordinates depends on the length of the observing session, the characteristics of horizon visibility on points and the used in post-processing observations (L1 or L1+L2). The POZGEO results show that horizontal accuracies of about 1-2 cm and vertical accuracies of 4 cm are achievable provided 0.5 hours dual frequency GPS data. The accuracy clearly decreases for point measured under conditions of strongly limited satellite availability

ANTENNA CALIBRATION MODELS IN HEIGHT DETERMINATIONS IN ASG-EUPOS' POZGEO-D SERVICE - A CASE STUDY

ABSTRACT GNSS observations in a network of permanent stations are a complex systems which offer both post-processing and corrections sent in real-time. In Poland such a system, known as the Polish Active Geodetic Network (ASG-EUPOS), has been in operation since June 2008. The GNSS development forces also continuous modernization of ASG-EUPOS (e.g.: GPS/GLONASS receivers mounting, ASG+ project) which aims to improve the accuracy of position determination. One of the factors limiting the accuracy (especially the vertical component) is antenna phase center variations (PCV) problem. PCV problem is resolved using the antenna calibration process. As a result, antenna phase center corrections models (PCC) are created. So far three methods have been developed to determine GNSS antenna PCV. For this reason and because of some problems in introducing of absolute models at present we can speak of three models of receiver antennas PCV (so called: relative, absolute converted and absolute). The aim of this paper was to study the height differences caused by using different calibration models in GNSS observation processing done in the ASG-EUPOS POZGEO-D service. The analysis was done using 3 days of GNSS data, collected with four different receivers and antennas, divided by one hour observation sessions. The results of the calculations show that switching between PCV models may have a visible effect on height determination, particularly in high accuracy applications.