Retrograde iliac endarterectomy - modified technique combining conventional surgical and interventional procedures

Background. Retrograde ring-stripper endarterectomy of iliac arteries through an inguinal approach modified by combining interventional procedures. Material and methods. 52 iliac arteries underwent modified retrograde ring-stripper endarterectomy. Passage of the lesions with a guide wire permits endarterectomy over the wire as a central splint under fluoroscopic control. Residual lesions are corrected by balloon or stent angioplasty. Results. Conversion to a conventional operation was required in 8 limbs. Initial technical success was achieved in 44 limbs. Cumulative secondary patience was 95.8% at 24 months (S.E. 4.7%). Conclusion. Modified ring-stripper endarterectomy with angioplasty is a safe and effective procedure. Long-term results are required

Comparison concept and quality metrics for GNSS antenna calibrations: Cause and effect on regional GNSS networks

Precise values for absolute receiver antenna phase centre corrections (PCC) are one prerequisite for high-quality GNSS applications. Currently, antenna calibrations are performed by different institutes using a robot in the field or in an anechoic chamber. The differences between the antenna patterns are significant and require a sound comparison concept and a detailed study to quantify the impact on geodetic parameters, such as station coordinates, zenith wet delays (ZWDs) or receiver clock estimates. Furthermore, a discussion on acceptable pattern uncertainties is needed. Therefore, a comparison strategy for receiver antenna calibration values is presented using a set of individually and absolutely calibrated Leica AR25 antennas from the European Permanent Network (EPN), both from the robot (Geo++ company) and from the chamber approach (University of Bonn). Newly developed scalar metrics and their benefits are highlighted and discussed in relation to further structural analysis. With our metrics, properties of 25 patterns pairs (robot/chamber) could be used to successfully assign seven individual groups. The impact of PCC on the estimated parameters depends on the PCC structure, its sampling by the satellite distribution and the applied processing parameters. A regional sub-network of the EPN is analysed using the double difference (DD) and the precise point positioning (PPP) methods. For DD, depending on the antenna category differences in the estimated parameters between 1 and 12 mm are identified also affecting the ZWDs. For PPP, the consistency of the observables, i.e. potential differences in the reference point of carrier phase and code observations, additionally affects the distribution among the different parameters and residuals

On the Role of GNSS Receivers for Antenna Patterns and Parameter Estimations

The precise processing of data derived by several global navigation satellite systems (GNSS) for global and regional networks relies on high-quality and calibrated equipment. Currently, an intensively discussed question in the IGS antenna working group is the best practice for publishing and distributing calibration values for receiver antennas for different systems and frequencies. There is the question of frequency band specific output of calibration values or system specific output, the magnitude of their differences and their impact the estimation parameters that are not yet assessed. We will address these points in our contribution. Several studies performed and evaluated at our calibration facility demonstrate a systematic impact of the receiver and the implemented signal tracking concept. The expected magnitudes in GNSS processing lead to differences on the coordinate domain of a few millimetres on a short and well-controlled baseline for original observations or frequencies. These effects are superimposed and amplified when forming linear combinations of independent signals and frequencies, which, however, are essential for global GNSS processing tasks such as ionosphere-free linear combination in global GNSS networks. These amplifications are critical as apparent biases in the coordinate and troposphere estimates are introduced with different magnitudes. For this reason, we present a quality assessment for different antenna-receiver combinations and provide an in-depth analysis and comparison for the majority of available and existing systems, signals, frequencies and linear combinations. The data were recorded under well-controlled conditions and include GNSS data of more than one week for each of the analysed number of four geodetic and reference station grade antennas. The analysis of the different combinations of antenna-receiver configurations provides metrics for assessing the impact of the receivers on the multi-system GNSS processing and the determination of the geodetic estimates. Consequently, validation with theoretical and expected metrics derived through multiple linear combinations is investigated, with additional focus on coordinate and troposphere estimates. The analysis uses the concepts of relative (baseline processing) and absolute (precise point positioning, PPP) GNSS processing

Applying phase center corrections also to code observables? – A PPP case study

Consistent and reliable receiver antenna calibration sets are an essential prerequisite for precise geodetic reference frames. In the context of the igs3-reprocessing campaign and the realisation of a new international terrestrial reference frame (ITRF20) using multi-GNSS, receiver antenna phase centre correction (PCC) sets play a key role. However, the consistency between different PCC realisations and the exchange of PCCs are today still a challenging task. In this contribution, we present results of our developed strategy for the characterisation of receiver antenna calibration sets. We emphasise the need of consistency between individual PCCs resulting from different calibration strategies and various realisations (igs14, igs20 and igsR3_2077). We discuss the need of defining the reception point for code and carrier phase observation, i.e. should PCC also be applied to code? Based on precise point positioning (PPP) we will show that the consistency of modelling both observation types in PPP is required and the differences are not only related to the residuals but also to the receiver clock estimates. We analyse this challenge at selected EUREF Permanent Network (EPN) sites, examining the receiver antenna pattern and the geographical location, as this also has a crucial role. The consistency of the reference point between code and carrier phase observations has additional effects on the parameters, as they are linked by the ionosphere free linear-combination. Depending on the type of receiver antenna, effects on the receiver clock estimates and ambiguities of up to 8 mm and systematic effects on PPP code post-fit residuals have been found

Codephase center corrections for multi GNSS signals and the impact of misoriented antennas

In absolute positioning approaches, e.g. Precise Point Positioning (PPP), antenna phase center corrections (PCC) have to be taking into account. Beside PCC for carrier phase measurements, also codephase center corrections (CPC) exist, which are antenna dependent delays of the code. The CPC can be split into a codephase center offset (PCO) and codephase center variations (CPV). These corrections can be applied in a Single Point Positioning (SPP) approach, to improve the accuracy in the positioning domain. The CPC vary with azimuth and elevation and are related to an antenna, which is oriented towards north. If the antenna is wrongly oriented, the effect cannot be compensated and wrong corrections will be added to the observations. The Institut für Erdmessung (IfE) established a concept to determine CPC for multi GNSS signals, where a robot tilts and rotates an antenna under test precisely around a specific point. Afterwards time differenced single differences are calculated, which are the input to estimate the CPC by using spherical harmonics (8,8). First studies in our working group showed, that an improvement of the position in a SPP are possible, if antenna pattern for the codephase are considering and correctly applied. In this contribution, we present the improvement of a SPP and PPP approach by considering CPC for different low cost antennas with multi GNSS signals. Beside the positioning domain, an analysis of the CPC in observation domain, by evaluating the deviations of single differences from zero mean, is performed. Furthermore, we quantify the impact of a disoriented antenna, e.g. oriented in east direction, in the positioning and observation domain by using north oriented CPC. We show, that this impact can be compensating in a post-processing by rotating the antenna pattern. Finally, we present some results of different calibrations, where the antennas are disoriented on the robot and compared to the estimated CPC pattern with the post-processing approach and discussed their impact on the positioning

Multi-GNSS Receiver Antenna Calibration

Global Navigation Satellite Systems (GNSS) are not only widely used for precise positioning, navigation and timing but also for establishing of terrestrial reference frames for geospatial applications, such as land and water management. The quality of GNSS carrier phase measurements depends on the knowledge about the location of the exact electrical reception point of the GNSS receiver antenna, also known as phase center. Because the location of this receiving point varies with the direction of the incoming satellite signal, phase center corrections (PCC), including a phase center offset (PCO) and phase center variations (PCV), have to be taken into account. These corrections are determined by a calibration of the antennas either in an anechoic chamber using artificially generated signals or in the field by use of a robot and real GNSS signals. The frequency dependent PCC are published in the IGS Antenna Exchange format (ANTEX). In order to take the benefits from the higher quality of the newer frequencies (like GPS L5) and satellite systems (e.g. Galileo or Beidou) so that multi-GNSS measurements can be processed, PCC have to be provided also for these signals. In this contribution, the calibration procedure developed at the Institut für Erdmessung (IfE) is presented. The robot model as well as the data acquisition and analysis is shown. Furthermore, the estimation process of the PCC using spherical harmonics is explained in details. We show, that an absolute GNSS receiver antenna calibration using a robot and real signals can successfully be carried out at the Institut für Erdmessung (IfE). The results underline an overall good repeatability with an RMS for the difference patterns of different calibrations smaller than two millimeters. It is shown that the L5 patterns significantly vary from L2, so that specific calibration values are needed. In addition, the concept of a joint estimation approach of same frequencies (like GPS L1 and Galileo L1) and its difference to the "classical" approach of frequency and system dependent pattern is presented. It can be seen, that differences up to 5.5 mm are present, if the joint estimated PCC are compared to the "classical" EL1X PCC. This underlines the demand of not only frequency but also GNSS specific PCC

Multi-frequency multi-GNSS receiver antenna calibration at IfE: Concept - calibration results - validation

Multi-frequency and multi-constellation GNSS have the potential to boost the overall performance of GNSS-based positioning, navigation and timing. This has an impact on the realisation of global reference frames, geophysical monitoring applications as well as enabling new applications. To this end, all error sources should be adequately corrected for. However, currently multi-frequency multi-GNSS receiver antenna calibration values are still missing. In this paper, the newly developed multi-frequency multi-GNSS calibration process at Institut für Erdmessung (IfE), Leibniz University Hannover, is presented. The basic concept and the assumptions for the antenna calibration are described. Resulting phase centre corrections (PCC) for GPS and Galileo for typical antennas are presented. We show that the repeatability of the estimated patterns are almost better than 2 mm in terms of maximum deviation and that the used tracking strategies by the receivers have marginal impact on the patterns, at maximum 1.2 mm for the studied receiver-antenna combinations. Furthermore, applying phase centre corrections for multi-frequency multi-GNSS carrier phase observations reduces significantly (up to 37%) the antenna related biases as validated on short baselines. Moreover, a validation in the coordinate domain shows that with IfE PCC a short baseline can be computed with high accuracy: the topocentric coordinate differences to the known baseline are in most cases smaller than 1 mm for the horizontal components and smaller than 2.2 mm in vertical

Impact of Different Phase Center Correction Values on Geodetic Parameters: A Standardized Simulation Approach

For highly precise and accurate positioning and navigation solutions with GNSS, it is mandatory to take all error sources – including phase center corrections (PCC) – adequately into account. These corrections are provided by different calibration facilities and are published in the official IGS antenna exchange format (ANTEX) file for several geodetic antennas. Currently, the IGS antenna working group (AWG) is discussing which metrics should be used as a basis for accepting new calibration facilities as an official IGS calibration facility. To this end, requirements have to be set for comparing different sets of PCC for the same type of antenna. Mostly, characteristic values of difference patterns (dPCC) are analysed, e.g. maximum deviations, RMS of dPCC, or percentage of dPCC values that are smaller than 1 mm. For users and station providers, however, it is most interesting to investigate the impact of dPCC on geodetic parameters, e.g. topocentric coordinate deviations and troposphere estimates. Since the impact is not only depending on the antenna in use and the station’s location but also on the applied processing strategies, a standardized comparison strategy is needed. In this contribution, we present the impact of different PCC values on geodetic parameters using a standardized simulation approach. We show results for several globally distributed stations using different processing strategies and their respective impact on the geodetic parameters. This includes the application of different elevation cut-off angles, observation weightings w.r.t satellite coverages and elevation angles as well as use of different frequencies and linear combinations. The obtained results are analysed in detail, repeated behaviours are grouped and compared to widely used characteristic values of dPCC. Thus, an overall conclusion of the similarity of different PCC models can not only be drawn on the pattern level, but also their impact on geodetic parameters can be assessed

Potential of GPS Common Clock Single-differences for Deformation Monitoring

Global satellite navigation systems (GNSS) are a standard measurement device for deformation monitoring. In many applications, double-differences are used to reduce distance dependent systematic effects, as well as to eliminate the receiver and satellites clock errors. However, due to the navigation principle of one way ranging used in GPS, the geometry of the subsequent adjustment is weakened. As a result, the height component is generally determined three times less precisely than the horizontal coordinates. In addition, large correlations between the height and elevation dependent effects exist such as tropospheric refraction, mismodelled phase center variations, or multipath which restricts the attainable accuracy. However, for a kinematic analysis, i. e. for estimating high rate coordinate time series, the situation can be significantly improved if a common clock is connected to different GNSS receivers in a network or on a baseline. Consequently, between-station single-differences are sufficient to solve for the baseline coordinates. The positioning geometry is significantly improved which is reflected by a reduction of the standard deviation of kinematic heights by about a factor 3 underlining the benefits of this new approach. Real data from baselines at the Physikalisch-Technische Bundesanstalt campus at Braunschweig where receivers are connected over 290 m via an optical fiber link to a common clock was analysed. © 2016 Walter de Gruyter GmbH, Berlin/Munich/Boston

Estimation and validation of receiver antenna codephase variations for multi GNSS signals

Besides antenna phase center corrections (PCC) for carrier phase measurements, which have to be considered for precise GNSS application, also codephase variations (CPV) exist. These are antenna dependent delays of the code which vary with azimuth and elevation. Such variations are not provided operationally in the antenna exchange format (ANTEX) at the moment. Previous studies in our working group show, that CPV should be taken into account when using code-carrier combination. Depending on the antenna type they can amount up to some dm. At Institut für Erdmessung (IfE), a concept to determine the CPV has been established. This procedure uses a robot that rotates and tilts the antenna under test precisely in the field. Real world modulated signals from the satellites are used, which is challenging in anechoic chamber procedures. Time differenced single differences are used to estimate PCC and CPV as spherical harmonics (8,8) in a post-processing approach. In this contribution we present the concept CPV of Galileo signals for several kinds of receiving antennas (mass market and high grade). In addition, we discuss the repeatability and stability of CPV for those antenna. Typical values of the CPV reaches up to 500 mm. The RMS of patterns resulting from multiple calibrations are 80 mm for Galileo C1X and 48 mm for GPS C1C