Phase Center Variation Models Research Articles

Impact of Pseudo-Stochastic Pulse and Phase Center Variation on Precision Orbit Determination of Haiyang-2A from Experimental HY2 Receiver GPS Data

Haiyang-2A (HY-2A) is the first marine dynamic environment satellite established by China, which has made significant contributions to the marine scientific research field. It carries the satellite-based GPS receiver named HY2, which was independently developed by China. It is an experimental satellite-borne GPS receiver for low earth orbit satellites, and during its operational period in orbit, the satellite-borne GPS data are not made accessible to the public. Therefore, this paper assesses the quality of HY-2A satellite-borne GPS data based on indicators such as satellite visibility, multipath effect, and ionospheric delay. The results indicate that the data acquired by the HY2 receiver are of high quality. The precise orbit determination (POD) uses the reduced-dynamic (RD) method. The adjustment effects of the pseudo-stochastic pulse time interval and a priori sigma on POD are analyzed, and the antenna phase center variation (PCV) is estimated using the direct method and residual method. Furthermore, this paper investigates the impact of PCV models with different resolutions (10° × 10° and 5° × 5°) on satellite orbit determination. To evaluate the orbit precision, three methods are used for validation, including carrier phase residual analysis, external precise science orbit (PSO) validation, and SLR three-dimensional (3D) validation, respectively. The results indicate that the highest orbit precision is achieved when the pseudo-stochastic pulse time interval is configured to 15 min, with the a priori sigma of 1 × 10−8 m/s2. The orbit carrier phase residuals reach the millimeter level. The 10° × 10°PCV model was estimated using the direct method and residual method, respectively; the root mean square of the external orbit validation for both methods show a millimeter-level improvement. The results obtained from the direct method and residual method are comparable. The resolution is increased from 10° to 5°, and the improvement in orbital precision is relatively small. The results obtained from the SLR 3D validation are consistent with those from the external PSO validation. The experimental results contribute valuable information for the POD of the HY2 series satellites.

The international DORIS service contribution to ITRF2020

For the realization of the 2020 International Terrestrial Reference Frame (ITRF2020), the International DORIS Service delivered to the International Earth Rotation and Reference Systems Service (IERS) a set of 1456 weekly solution files from 1993.0 to 2021.0 including station coordinates and Earth orientation parameters (EOPs). The data come from fourteen DORIS satellites: TOPEX/Poseidon, SPOT-2, SPOT-3, SPOT-4, SPOT-5, Envisat, Jason-1, Jason-2, Cryosat-2, Saral, HY-2A, Jason-3, Sentinel-3A and Sentinel-3B. In their processing, the four analysis centers which contributed to the DORIS combined solution used the latest time variable gravity models, the new mean pole and diurnal-subdiurnal tidal EOP models recommended by IERS. In addition, all the analysis centers included in their processing precise SPOT-5 solar panel angle values and quaternions for, at least, the Jason satellites. Furthermore, a new Alcatel phase center variation model was implemented for the ITRF2020 processing. The main objective of this study is to present the combination process and to analyze the impact of the new modeling on the performance of the new combined solution. Comparisons with the IDS contribution to ITRF2014 show that i) the application of the new phase center variations for the Alcatel DORIS ground antennas in the data processing combined with the gradual replacement over time of the Alcatel by Starec antennas implies a scale drift from 1993.0 to 2002.5 and ii) thanks to a better modeling of the surface forces on the satellites, the new combined solution shows smaller annual and 118-day signals in the geocenter. A new DORIS terrestrial reference frame was computed to evaluate the intrinsic quality of the new combined solution. That evaluation shows that over almost the full time span the intrinsic IDS scale values lie in a range of ±5 mm. After mid-2008, the new DORIS reference frame has an internal position consistency in North-East-Up better than 7.5 mm.

DORIS Alcatel ground antenna: Evaluation of the phase center variation models

The DORIS ground antenna phase center variation (PCV) modeling is an important factor of the DORIS solution accuracy and affects also the DORIS solutions employed in the ITRF2020 realization. There are two different elevation dependent models available for the Alcatel antenna, called here 1.0 and 2.0. For the Starec antenna we have only one PCV model. We demonstrate the effect of the Alcatel PCV model selection, reaching a value around 15 mm in the estimated DORIS station height. The post-fit Doppler observation residual analysis proved a better performance of the residuals derived from the solution employing Alcatel PCV model 2.0, particularly in several specific elevation intervals over 50 degrees. The elevation dependent systematics in the residuals have been proven by the separation of the ascending and descending portions of the residuals. The residual mean calculated for the elevation intervals reached values with a mutually opposite sign for the ascending and descending observations. Moreover, we have compared the residuals of the Starec and Alcatel stations, whereas the Starec residual RMS was lower for elevations approximately under 50 degrees, the Alcatel residual RMS was lower for higher elevations. This result could relate to an eventual Starec PCV model imperfection at higher elevations.To check the compatibility of the Starec and Alcatel PCV models impact on the station height, we have performed the comparison of the measured vertical component of the DORIS-DORIS local ties to a value calculated from the DORIS observations. The pairs of the Alcatel-Starec stations were the object of interest. When repeating the experiment with both Alcatel models 1.0 and 2.0, we have found a similar level of agreement of the measured and observed/computed height difference, but with the opposite sign. The average difference reached 8.7 ± 2.5 mm for Alcatel PCV model 1.0 and −7.2 ± 2.4 mm for Alcatel PCV model 2.0.

Improving LEO precise orbit determination with BDS PCV calibration

As the new members of the Chinese Fengyun-3 meteorological satellites series, the Fengyun-3C (FY-3C) and Fengyun-3D (FY-3D) satellites were launched in 2013 and 2017, respectively. For the first time, both FY-3C and FY-3D satellites carry a high-precision BDS and GPS dual-system space-borne receiver to fulfill their stringent requirement of precise orbit determination (POD). The fusion of BDS and GPS onboard observations can significantly improve the spatial geometry, robustness and reliability of low earth orbit (LEO) POD. In order to achieve high-quality orbit recovery from the onboard GNSS observations, the in-flight calibration of phase center variation (PCV) is a prerequisite. In this contribution, we calibrate the BDS PCV for improving the orbit precision of both FY-3C and FY-3D satellites for the first time. The results show that with the GPS PCV corrections, the residuals of GPS carrier phase observations present a reduction of about 17.6% and 21.9% for FY-3C and FY-3D satellites, respectively, and overlap differences of the GPS-only orbit are reduced on average by 3 and 5 mm. After correcting for BDS PCV, the GPS and BDS combined (GC) POD achieves a better orbit precision for both FY-3C and FY-3D satellites. The application of BDS PCV corrections can contribute to a reduction of up to 12% in BDS phase residuals and a decrease of up to 3 mm in orbit overlap differences in 1-dimension. It indicates that the introduction of the BDS PCV model can significantly improve the orbit precision. Similar orbit precision can be achieved when using the GPS PCV model to replace the BDS PCV model in the GC POD. It demonstrates that for onboard receivers, the GPS PCV model can be used to correct PCV errors of BDS signals, which is commonly used in precise point positioning of ground stations when the BDS PCV is not available. Furthermore, we find that it is feasible to generate an integrated PCV model from both onboard BDS and GPS observations. After application of the integrated PCV model, the FY-3C and FY-3D orbits of GC POD present a better precision than the solution using the single-system PCV model and the largest precision improvement of about 1 mm can be recognized in the radial component. With such a high-quality orbit, FY-3C and FY-3D satellites can be expected to make more contributions to meteorological studies and applications.

Research on Attitude Models and Antenna Phase Center Correction for Jason-3 Satellite Orbit Determination.

We focused on the researches of two models used for Jason-3 precise orbit determination (POD)—Jason-3 attitude modes and receiver phase center variation (PCV) model. A combined attitude mode for the Jason-3 satellite is designed based on experimental analysis used in some special cases, such as in the absence of quaternions or when inconvenient to use. We researched the linking of satellite attitude with antenna phase center. Specially, to verify the validity of the combined attitude, we analyzed the effects of different attitude modes on receiver phase center offset (PCO) estimation, PCO correction and POD. Meanwhile, the difference analysis of PCO correction based on attitude modes also contains the combined attitude modeling processes. The POD results showed that the orbital accuracies with the combined attitude are slightly more stable than those with attitude event file. By introducing receiver PCVs into POD, the mean residuals root-mean-square (RMS) is reduced by 1.9 mm and orbital 3D-RMS position difference is improved by 5.7 mm. The eight schemes were designed to integratedly verify the effectiveness of different attitude modes and receiver PCVs model. The results conclude that the accuracy using the combined attitude is higher than that of event file, which also prove the feasibility of the combined attitude in integrated POD and it can be as a revision of attitude event file. Using all mentioned attitude modes, the orbital accuracy by introducing PCVs can be improved by the millimeter level. The integrated effects of attitude modes and receiver PCVs on POD are almost consistent with the effects of a single variable. The optimal results of Jason-3 POD indicate that orbital mean radial RMS is close to 1 cm, and the 3D-RMS position difference is within 3 cm.

DORIS data processing in the INASAN Analysis Center and the contribution to ITRF2014

We present results of the DORIS data processing for the ITRF2014 carried out in the Institute of Astronomy, Russian Academy of Sciences (short INA) for the time period 1993.0–2015.0. The evaluated SINEX free-network (inawd08) weekly solutions for station positions and polar motion have been submitted to the IDS Combination Center. We applied the upgraded models of the gravity field, troposphere, and used corrected data for SPOT-5 satellite which permitted us to improve the result precision for Helmert transformation parameters and EOP. The agreement with the IERS C04 solution turns out better than 0.1 mas with a dispersion of 1 mas. The evaluated amplitudes of annual geocenter variations derived from inawd08 weekly solutions are 3.4±0.5mm, 4.4±0.5mm, and 3.3±1.0mm for X, Y, and Z components, respectively. The recovered amplitudes and phases are in a good agreement with the geophysical models. The results of two single satellite campaigns related to the scale increase in 2012 and 1994 for IDS combination are studied. Unlike other Analysis Centers, we did not apply a phase center law for ground antennas. It was shown that the correction for the phase center variation (PCV) of ground DORIS antennas results in the shift of the mean scale factor by 1.24ppb (i.e., ∼8mm) with respect to the case without correction. The PCV model and the satellites HY-2A and SARAL are included into the INA latest solutions (inawd10) after the submission for the ITRF2014. The future work is discussed.

Field tests of L1 phase centre variation models of surveying-grade GPS antennas

GNSS antenna electrical phase center variability is a source of errors in precise geodetic measurements, particularly in reference frame maintenance, satellite precise levelling, deformation monitoring, the establishment of geodetic control networks, geodynamic research, etc. It has a considerable influence on the precision and accuracy of the resulting coordinates. Previous research shows that changing the antenna on the monitored point often results in a considerable bias in the derived height coordinate component. In theory, if electrical phase center is sufficiently modeled, antenna changes should have negligible effect on coordinates. We present the analysis of the influence of the GPS antenna models on the resulting marker position. Numerical tests are based on field measurements. The GPS data collected at the test baseline were processed using Bernese GPS Software Version 5.0. The research shows that the IGS and NGS phase center variation models for some types of the surveying (rover) antennas are still imperfect and are contaminated by constant errors, which may exceed even ± 5 mm for horizontal coordinates components and ± 25 mm for vertical ones.

Effects and disturbances on GPS-derived zenith tropospheric delay during the CONT08 campaign

The zenith total delay (ZTD) can be retrieved from space geodetic techniques, e.g., Global Positioning System (GPS) and Very Long Baseline Interferometry (VLBI), which plays a key role in climatological and atmospheric sciences. However, ZTD estimates still have lots of effects and uncertainties, particularly in GPS model errors. The continuous VLBI observations provide an opportunity to assess GPS ZTD estimates during the Continuous VLBI Campaign 2008 (CONT08) at 11 co-located stations from August 12 to 26, 2008. In this paper, the effects on GPS ZTD estimate and its disturbances are investigated using different mapping function models (GMF, NMF, and VMF1), Phase Center Variation (PCV) models (AZEL and ELEV) and Ocean Tide Loading (OTL) models (FES2004, CSR4.0 and GOT00). It has shown that the ZTDs from VLBI and GPS have an agreement in –3.88–3.74mm with correlation coefficients of higher than 0.87. For mapping function models, there are no obvious differences, while the PCV model of ELEV is always a little better than AZEL for large scale network with mixed antenna types. For stations near to the coastlines, ocean loading effects must be corrected. While for short period, the effects with OTL models of FFES2004, CSR4.0 and GOT00 are always at the same level. In addition, significant diurnal cycles S1 (24h period) and semidiurnal cycles S2 (12h period) of GPS ZTD are found with amplitudes between 0.82 and 13.84mm and 0.30 and 5.23mm, respectively, which are closer to VLBI ZTD estimates. The correlation coefficients between VLBI and GPS ZTD are 0.85 and 0.95 in S1 and S2, respectively.

Comparison of Using Relative and Absolute PCV Corrections in Short Baseline GNSS Observation Processing

GNSS antenna phase center variations (PCV) are defined as shifts in positions depending on the observed elevation angle and azimuth to the satellite. When identical antennae are used in relative measurement the phase center variations will cancel out, particularly over short baselines. When different antennae are used, even on short baselines, ignoring these phase center variations can lead to serious (up to 10 cm) vertical errors. The only way to avoid these errors, when mixing different antenna types, is by applying antenna phase center variation models in processing. Till the 6th November 2006, the International GNSS Service used relative phase center models for GNSS antenna receivers. Then absolute calibration models, developed by the company Geo++, started to be used. These models involved significant differences on the scale of GNSS networks compared to the VLBI and SLR measurements. The differences were due to the lack of the GNSS satellite antenna calibration models. When this problem was sufficiently resolved, the IGS decided to switch from relative to absolute models for both satellites and receivers. This decision caused significant variations to the results of the GNSS network solutions. The aim of this paper is to study the height differences in short baseline GNSS observations processing when different calibration models are used. The analysis was done using GNSS data collected at short baselines moved with different receiver antennas. The results of calculations show, that switching from relative to absolute receiver antenna PCV models has a significant effect on GNSS network solutions, particularly in high accuracy applications.

Tsukuba GPS Dense Net Campaign Observation: Improvement in GPS Analysis of Slant Path Delay by Stacking One-way Postfit Phase Residuals

In order to study small-scale water vapor variations over distances from a few km to 20 km, two campaign observations with a dense GPS network were carried out for 2.5 months in total at Tsukuba, Japan. For the observations 79 GPS antennas were installed at 75 sites within a 20 km by 20 km square area, at 1 to 3 km intervals. The PCV models provided by the US National Oceanic and Atmospheric Administration (NOAA) were applied to remove unmodeled phase center variation (PCV) specific to GPS antenna type. In addition, new PCV maps (MPS map) were constructed for all the antennas by stacking one-way postfit residuals over both campaign periods, to remove not only azimuth dependent PCV, but also the errors due to multipath effects. After MPS maps were introduced into the analysis, strong elevation dependence as well as azimuth dependence of postfit phase residuals, almost disappeared for all the antennas. In addition, the time variations in postfit residuals which were common to all the GPS sites, were subtracted to remove satellite orbits and/or clock errors. This led to the accurate estimate of slant path delay (SPD), which enabled the SPD to be applied to tomography analyses of water vapor (Seko et al. 2003). The horizontal scale of SPD was estimated using correlation distributions. As a result, the horizontal scale of the zenith total delay, the gradient component, and the postfit residual may be roughly considered as 644±120 km, 62±23 km, and 2-3 km, respectively. Improvement of the postfit residuals following the application of MPS maps also showed a positive impact on PWV estimation. Systematic biases of GPS derived PWV between different antenna types (Trimble and Ashtech) were reduced, resulting in a better agreement of GPS PWV, with RMS errors of 2.0 mm or less relative to PWV by rawinsonde or water vapor radiometer observations. The distribution of time-averaged PWV estimated at the 75 GPS sites showed a systematic pattern which has a negative correlation with the antenna height of each site.

Anatomy of apparent seasonal variations from GPS‐derived site position time series

Apparent seasonal site position variations are derived from 4.5 years of global continuous GPS time series and are explored through the “peering” approach. Peering is a way to depict the contributions of the comparatively well‐known seasonal sources to garner insight into the relatively poorly known contributors. Contributions from pole tide effects, ocean tide loading, atmospheric loading, nontidal oceanic mass, and groundwater loading are evaluated. Our results show that ∼40% of the power of the observed annual vertical variations in site positions can be explained by the joint contribution of these seasonal surface mass redistributions. After removing these seasonal effects from the observations the potential contributions from unmodeled wet troposphere effects, bedrock thermal expansion, errors in phase center variation models, and errors in orbital modeling are also investigated. A scaled sensitivity matrix analysis is proposed to assess the contributions from highly correlated parameters. The effects of employing different analysis strategies are investigated by comparing the solutions from different GPS data analysis centers. Comparison results indicate that current solutions of several analysis centers are able to detect the seasonal signals but that the differences among these solutions are the main cause for residual seasonal effects. Potential implications for modeling seasonal variations in global site positions are explored, in particular, as a way to improve the stability of the terrestrial reference frame on seasonal timescales.