Antenna Phase Center Offsets Research Articles

The Influence of Zenith Tropospheric Delay on PPP-RTK

Abstract The PPP technique is more and more used in different GPS precise application. The PPP precision is related to many factors and the most important are: the number of satellites, the ambiguity resolution, the ability to model the ionospheric and tropospheric delays, solid earth and ocean tides, relativistic effects and antenna phase-center offsets and variations. The article is studying the effect of the zenith tropospheric delay (ZTD) using the GPT2 model and then computing in static mode the ZTD, which is than applied on PPP-RTK method. An analysis on dual frequency ionospheric–free phase combination (LC) and dual frequency ionospheric–free range combination (PC) was made in the first stage and then comparing the two solutions relative to the nominal position. The results revealed the fact that by using the ZTD determined in static mode in the detriment of ZTD from GPT2 model, the results are improving on North, East and Up position components. Also lower RMS was obtain when we used the ZTD from static mode comparative to the GPT2 model.

Absolute IGS antenna phase center model igs08.atx: status and potential improvements

On 17 April 2011, all analysis centers (ACs) of the International GNSS Service (IGS) adopted the reference frame realization IGS08 and the corresponding absolute antenna phase center model igs08.atx for their routine analyses. The latter consists of an updated set of receiver and satellite antenna phase center offsets and variations (PCOs and PCVs). An update of the model was necessary due to the difference of about 1 ppb in the terrestrial scale between two consecutive realizations of the International Terrestrial Reference Frame (ITRF2008 vs. ITRF2005), as that parameter is highly correlated with the GNSS satellite antenna PCO components in the radial direction. For the receiver antennas, more individual calibrations could be considered and GLONASS-specific correction values were added. For the satellite antennas, all correction values except for the GPS PCVs were newly estimated considering more data than for the former model. Satellite-specific PCOs for all GPS satellites active since 1994 could be derived from reprocessed solutions of five ACs generated within the scope of the first IGS reprocessing campaign. Two ACs separately derived a full set of corrections for all GLONASS satellites active since 2003. Ignoring scale-related biases, the accuracy of the satellite antenna PCOs is on the level of a few cm. With the new phase center model, orbit discontinuities at day boundaries can be reduced, and the consistency between GPS and GLONASS results is improved. To support the analysis of low Earth orbiter (LEO) data, igs08.atx was extended with LEO-derived PCV estimates for big nadir angles in June 2013.

GNSS satellite geometry and attitude models

This article discusses the attitude modes employed by present Global (and Regional) Navigation Satellite Systems (GNSSs) and the models used to describe them along with definitions of the constellation-specific spacecraft body frames. A uniform convention for the labeling of the principal spacecraft axes is proposed by the International GNSS Service (IGS), which results in a common formulation of the nominal attitude of all GNSS satellites in yaw-steering mode irrespective of their specific orbit and constellation. The conventions defined within this document provide the basis for the specification of antenna phase center offsets and variations in a multi-GNSS version of the IGS absolute phase center model in the ANTEX (antenna exchange) format. To facilitate the joint analysis of GNSS observations and satellite laser ranging measurements, laser retroreflector array coordinates consistent with the IGS-specific spacecraft frame conventions are provided in addition to representative antenna offset values for all GNSS constellations.

Influence of GPS antenna phase center variation on precise positioning

The GPS antenna is the connecting element between the GPS satellites and the GPS receiver. It receives the incoming satellite signal and then converts its energy into an electric current, which can be managed by the GPS receiver. The accurate antenna phase center offsets’ values and phase center variation factors are critical issues in GPS precise positioning. Some GPS users simply apply the manufacturer’s recommended offset values which may not match the precise values determined by calibration process. Other users may ignore the phase center correction factors during GPS data processing. In both cases, the resulted coordinates will have errors especially the height component.In this study, some static and kinematic field experiments have been carried out to evaluate the effect of using the manufacturer’s recommended antenna phase offset and ignoring its variation on precise positioning. The GPS data have been post-processed by two commercial software. The results showed that, a significant error may occur in case of disregarding the calibrated values and applying the manufacturer’s recommended ones. Investigation is also made on the effects of mixing different types of antennas. Significant variations are observed on the height components than the associated horizontal component due to phase center variation. The maximum variations are reached about 8 and 4cm in height and northing components respectively.

Evaluation of the ITRF2008 GPS vertical velocities using satellite antenna z-offsets

We develop a method to evaluate the terrestrial reference frame (TRF) scale rate error using Global Positioning System (GPS) satellite antenna phase center offset (APCO) parameters and apply it to ITRF2008. We search for the TRF in which z-APCO parameters have the smallest drift. In order to provide realistic error bars for the z-APCO drifts, we pay attention to model periodic variations and auto-correlated noise processes in the z-APCO time series. We will show that the GPS scale rate with respect to a frame is, as a first approximation, proportional to the estimated mean z-APCO trend if that frame is used to constrain station positions. Thus, an ITRF2008 scale rate error between ?0.27 and ?0.06 mm/yr depending on the GPS analysis center can be estimated, which demonstrates the high quality of the newly constructed ITRF2008. We will also demonstrate that the traditional estimates of the GPS scale rate from 7-parameter similarity transformations are consistent with our newly derived GPS scale rates with respect to ITRF2008 within two sigmas. We find using International GNSS Service (IGS) products that the traditional approach is relevant for scale rate determination even if some of the z-APCO values supplied by the IGS were not simultaneously calibrated. As the scale rate is related to the accuracy of vertical velocities, our estimates supply a conservative evaluation that can be used for error budget computation.

The GLONASS-M satellite yaw-attitude model

The proper modeling of the satellites’ yaw-attitude is a prerequisite for high-precision Global Navigation Satellite System (GNSS) positioning and poses a particular challenge during periods when the satellite orbital planes are partially eclipsed. Whereas a lot of effort has been put in to examine the yaw-attitude control of GPS satellites that are in eclipsing orbits, hardly anything is known about the yaw-attitude behavior of eclipsing GLONASS-M satellites. However, systematic variations of the carrier phase observation residuals in the vicinity of the orbit’s noon and midnight points of up to ±27 cm indicate significant attitude-related modeling issues. In order to explore the GLONASS-M attitude laws during eclipse seasons, we have studied the evolution of the horizontal satellite antenna offset estimates during orbit noon and orbit midnight using a technique that we refer to as “reverse kinematic precise point positioning”. In this approach, we keep all relevant global geodetic parameters fixed and estimate the satellite clock and antenna phase center positions epoch-by-epoch using 30-second observation and clock data from a global multi-GNSS ground station network. The estimated horizontal antenna phase center offsets implicitly provide the spacecraft’s yaw-attitude. The insights gained from studying the yaw angle behavior have led to the development of the very first yaw-attitude model for eclipsing GLONASS-M satellites. The derived yaw-attitude model proves to be much better than the nominal yaw-attitude model commonly being used by today’s GLONASS-capable GNSS software packages as it reduces the observation residuals of eclipsing satellites down to the normal level of non-eclipsing satellites and thereby prevents a multitude of measurements from being incorrectly identified as outliers. It facilitates continuous satellite clock estimation during eclipse and improves in particular the results of kinematic precise point positioning of ground-based receivers.

DORIS satellite phase center determination and consequences on the derived scale of the Terrestrial Reference Frame

We analyzed 1 year of DORIS data to estimate daily corrections to the mean locations of the satellite antenna phase centers. For each DORIS satellite, we looked for possible biases, discontinuities, trends or annual signals. All SPOT satellites show very similar patterns, which are characterized by a significant constant bias of about 20 mm in the cross-track direction as well as a clear annual signal of 5 mm amplitude. All DORIS satellites show a consistent systematic radial offset of 10–20 mm (equivalent to 1.5–3.0 ppb in the Terrestrial Reference Frame) when using ITRF2000 and less than 1 ppb using ITRF2005P. A discontinuity unexpectedly appears in DORIS radial antenna phase center offset of the ENVISAT satellite on October 12, 2004, at the time of a flight software switch. GPS phase center offsets were also computed for Jason and TOPEX/Poseidon and no significant differences were found between DORIS and GPS estimations. Finally, we applied these estimated DORIS satellite antenna phase center offsets to derive weekly time series of station coordinates in 2004. Results showed very small improvement for station coordinates and confirmed that DORIS scale could match ITRF2000 scale by modifying the vector between the center of mass of the satellite and the phase center of the on-board antenna with a proper radial offset. At this point, we do not recommend the adoption of any empirical DORIS satellite phase center offsets at this point.

Impact of GPS satellite antenna offsets on scale changes in global network solutions

In this paper, we demonstrate that biases in the GPS satellite antenna phase center offsets could lead to scale biases in global network solutions, which change along with the observed satellite constellation. To validate the IGS standard offset values, satellite‐specific offsets are estimated from GPS data and the network solutions are re‐adjusted with these estimates. Both the estimated offsets and the re‐adjusted network scales confirmed that the IGS standard offsets are significantly biased and produce scale changes of more than 1 ppb. From investigations of the offset inhomogeneities among satellites belonging to the same block type, it is strongly recommended that block‐type‐specific offsets used presently as IGS standard should be replaced by satellite‐specific ones.

Estimation of elevation-dependent satellite antenna phase center variations of GPS satellites

A method for the estimation of the phase center variations of GPS satellite antennas using global GPS data is presented. First estimations have shown an encouraging repeatability from day to day and between satellites of the same block. Thus, two different satellite antenna patterns for Block II/IIA and for Block IIR with a range of about 4 cm and an accuracy of less than 1 mm could be found. The present approach allows the creation of a consistent set of receiver and satellite antenna patterns and phase center offsets. Thereby, it is possible to switch from relative to absolute phase center variations without a scale problem in global networks. This changeover has an influence on troposphere parameters, reduces systematic effects due to uncorrect antenna modeling and should diminish the elevation dependence of GPS results.

Satellite antenna phase center offsets and scale errors in GPS solutions

ITRF2000 solutions have shown that there are parts-per-billion (ppb)-level scale differences between GPS and other techniques, as well as among various GPS analysis centers (ACs). The trends of the scale differences reach 0.2 ppb per year. It is demonstrated that the uncertainty in the satellite antenna phase center offset (z-direction) is one of the major reasons for the scale differences. The scale change caused by an error in the z_offset can be approximately expressed as: scale change (in ppb)=7.8 × error of z_offset (in meters). Changing the z_offset value of the BLOCK II and IIA satellites from 102 to 95 cm could produce a 0.5-ppb scale variation. For BLOCK IIR satellites the uncertainty of the z_offset is much larger. The number of these satellites increases each year. The accuracy of IGS products could be significantly affected if this problem is not properly solved. Besides scale, satellite antenna phase center uncertainties have perceptible effects on also the clock, zenith path delay, and other solved-for parameters.