G21A-0092
Scientific Applications in Geodesy and Geodynamics - Innovations Offered by the new GNSS Signals
Over the past 12 years the International GNSS Service (IGS) has demonstrated innovative solutions to maximize the benefits of GPS/GNSS signals in space. Today the IGS provides a large set of high quality products for a huge number of applications e.g. in geodynamics, surveying or atmosphere monitoring. A key objective of the IGS is to provide users anywhere in the world access to highest level GNSS data, products and resources for scientific applications, through an "open data policy". This is naturally dependent upon the availability and performance of the various satellite systems. Recognizing the importance of the upcoming new European satellite navigation system (GALILEO) and the modernization programs for GPS and GLONASS the IGS decided to set up a GNSS-Working Group. One of the major goals of this WG is to prepare a consolidated feedback to GNSS system engineering based on relevant IGS experience of providing highest accuracy products for the existing systems. Concerning the work of IGS Analysis Centres as well as other IGS Working Groups the opportunities offered by the various GNSS modernization programs should be reflected. The set of new signals provided by GNSS ongoing modernization programs will allow for improved ambiguity resolution techniques over longer baselines (TCAR, MCAR). Linear combinations of 3 frequencies provided by Galileo and GPS allow to mitigate second-order ionospheric effects and might subsequently improve the determination of the remaining tropospheric refraction.The optimal signal linear combination in terms of wavelength and noise level depends on the baseline length. Nevertheless in case of controlled intersystem biases the new processing models will improve the positioning accuracy of reference points and subsequently contribute to the stability of the reference frame and the determination of Earth Rotation Parameters. This presentation will give a summary of the findings of the IGS GNSS-WG concerning the determination of geodetic parameters from GNSS data and touch upon the strategies of the International GPS Service for optimizing the future use of multiple integrated GNSS.
G21A-0093
Preparing for Galileo - New Results from Anechoic Chamber Absolute Antenna Calibrations
The antenna phase center variations are crucial corrections for high precision GNSS applications. They are applied according to IGS conventions as absolute antenna phase center corrections which are dependent on the carrier frequency. With GLONASS As being upgraded and first Galileo signals available, this frequency dependence is getting more important and has to be clarified. Up to now extrapolation from the GPS frequencies or mean values for all GLONASS frequencies are used. Anechoic chamber antenna measurements allow for the direct phase calibration of all frequencies involved and can therefore be used to determine offset and variations for all signals. TU Darmstadt and University of Bonn developed a calibration system which allows for the efficient and quick calibration of GNSS antennas in a dedicated anechoic chamber. A series of measurements was used to calibrate GPS, GLONASS and Galileo antennas. The frequency response of all antennas and the phase pattern and offset values were determined over the frequency range of the GNSS signals in use by the different systems. In this contribution we will explain the calibration procedure and setup and give an estimation of its precision. The results of the phase pattern determinations will be given. They show a good correspondence to the recommended IGS values in use for GPS in general. The variations in phase center with frequency for the antennas tested are seen to have no clear relation to the frequency, but are significantly different for different frequencies. They are small however within e.g. the GLONASS bandwidth and probably not significant. The repeatability among different antennas of the same type has to be tested and also the overall variations have to be confirmed by more samples and further tests are planned.
G21A-0094
The multipath and SNR Quality in civil code L2
The new generation of GPS satellites, with the addition of the new L2C civil code, may provide to the users better positioning capabilities. The new code in the L2 may increase the signal robustness, improve resistance to interference, reduce tracking noise and consequently, improve accuracy and provide better positioning inside buildings and in wooded areas. The second civil frequency code will eliminate the need of using fragile semi- codeless tracking techniques currently used in connection with L2. The L2C has a different structure that allows civil and military share the same code. L2C owns two codes of different length: moderate code (CM) and long code (CL). The CM was chosen to have 10.230 chips repeated to every 20 millisecond. The CL was chosen to have 767250 chips with period of 1.5 second. The main reasons for these choices were due to excellent correlation properties. Furthermore, L2C enhances performance by having no data modulation on CL code, which improves, among others, the threshold tracking performance. Comparing the L2C acquisition with the C/A, the CM code is ten times longer than the C/A and the two components have half the total power. This is an important feature for many low-power applications. Although this signal has several advantages, some investigations about its performance are necessary, mainly about the provided accuracy under some effects, for example, multipath. Thus, this paper aims to analyze the L2C signal, as well as its quality using some parameters, such as Signal to Noise Ratio (SNR) and multipath level (MP). The experiment was realized at Sao Paulo State University UNESP in Presidente Prudente, Brazil. The data were collected by two receivers of different brands, both able to collect the L2C signal, and connected to the same antenna, thought the use of a splitter. The results showed that the MP and SNR values were better for the modernized satellites. Furthermore, the SNR values of the two receivers were similar while the MP values of one receiver showed better results than the other model. Therefore, the satellites with L2C code presented, as expected, better performance than those not modernized satellites in relation to SNR and MP.
G21A-0095
PPP versus "network" positioning: first results of the GINS CNES/GRGS software.
CNES/GRGS has implemented the capability to process GPS data in the PPP mode in its GINS software which was already capable to estimate classical network adjustment using Zero-Difference and Double-Difference observables. The PPP and "network" strategies are compared in the framework of two different classes of scientific positioning objectives: - hourly estimation of 3D crustal deformations due to sub-diurnal solid earth tides and ocean tide loading effects - daily/weekly computation of seasonal and secular vertical displacements induced by atmospheric and hydrologic loading phenomena. In addition different PPP strategies are evaluated using cross-combinations of different orbit and clock products as well as antenna phase centre corrections. The sources of discrepancies between the different sets of solutions are discussed.
G21A-0096
A study on the standardization of GPS quality control through simulation tests
GPS quality control is to determinate the quality of observations in terms of various error factors such as atmospheric effect, multipath, etc. It could be used to predict the accuracy of positions in field surveys. Because errors in observations deteriorate the accuracy of positioning, it is an important process to predict how much errors are contained in observations and determine if the accuracy goal of the survey is achievable. In this study, we present the standards of quality control based on theories and various simulation tests. The simulated data are constructed from GPSSIM module of Bernese GPS software Version 5.0. We created simulated data with various levels of error factors in two baselines lengths of 80km and 30km. Then, the simulated data are processed by TGO ver1.5, LGO ver2.0, and GPSurvey ver2.35. Through this procedure, we determine the relations between the quality of the observations in terms of each error sources and achievable positioning accuracy. It is expected that this study increase the efficiency of the general field survey by predicting the positional accuracies before the post processing.
G21A-0097
A Critical Assessment of the Current EGNOS Performance
The main purpose of this presentation is to evaluate the current performance of the European Geostationary Navigation Overlay Service (EGNOS) in comparison to commercial, local DGPS services. In full operational capability (FOC) EGNOS provides orbit and clock-corrections of all GPS satellites as well as the ionospheric delay for real-time users within the service area by means of three geostationary satellites. In addition EGNOS should disseminate integrity information in the near future. The analysis is mainly based on the comparison of the trajectories of a slowly moving vehicle obtained by two real-time correction techniques – EGNOS and WEP (Wienstrom Positioning Service Provider). The tests are carried out in medium obstructed environment which slightly harms the visibility of the EGNOS satellites. In order to set up a comparable test environment, the GPS-signals were received by one single antenna, divided in two data streams by an antenna-splitter and subsequently forwarded to both a GPS/EGNOS receiver and a GPS- receiver with RTCM-input capability. During the trial session also raw data of the rover receivers as well as the reference station will be logged. This allows to verify a posteriori the calculated real-time positions and to test different positioning algorithms using EGNOS correction data by means of the program SISSIM (SISNet Simulation). Additionally an evaluation of the EGNOS ionospheric model will be presented.