Comparison of Absolute and Relative Antenna Phase Center Variations

Three major GPS antenna calibration methods are available toda: the relative field calibrations using the GPS data collected on short baselines, the absolute field calibrations, where the GPS antenna is rotated and tilted by a robot, and calibration measurements in an anechoic chamber. Mean antenna offsets and the elevation-dependent phase center variations of GPS antennas determined by all three techniques are compared to assess their accuracy. The analysis of global GPS data with these sets of calibration values reveals that the offsets and variations of the satellite antenna phase centers have to be considered, too, to obtain a consistent picture. ? 2001 John Wiley & Sons, Inc.     

Influence of different GPS receiver antenna calibration models on geodetic positioning

To better understand how receiver antenna calibration models contribute to GPS positioning error budget, we compare station positions estimated with different calibration models: igs05.atx, igs08.atx and individual antenna calibrations. First, the impact of switching from the igs05.atx antenna calibration model to the igs08.atx antenna calibration model is investigated using the EUREF Permanent Network historical data set from 1996 until April 2011. It is confirmed that these position offsets can be effectively represented by the igs05.atx to igs08.atx latitude-dependent model. Then, we demonstrate that the position offsets resulting from the use of individual calibrations instead of type mean igs08.atx calibrations can reach up to 1 cm in the up component, while in the horizontal, the offsets generally stay below 4 mm. Finally, using six antennas individually calibrated by a robot as well as in an anechoic chamber, we observe a position agreement of 2 mm in the horizontal component and a bias of 5 mm in the up component. Larger position offsets, dependent on the antenna/radome type, are, however, found when these individual calibrations are compared to type mean calibrations of two tested antennas.     

Tests of phase center variations of various GPS antennas, and some results

Phase variations of GPS receiving antennas are a significant error component in precise GPS applications. A calibration procedure has been developed by Geo++ and the Institut für Erdmessung, which directly determines absolute phase center variations (PCVs) without any multipath influence by field measurements. The precision and resolution of the procedure allows the determination of reliable azimuthal variations. PCV may affect long-term static GPS differently than real-time GPS, depending on the applications. At the same time, different antenna types are involved. Less investigations have been done on absolute PCV of rover antennas than on geodetic antennas which, however, becomes more important due to the mixed antenna situation in GPS reference networks and RTK networks. The concepts of the absolute PCV field calibration are summarized and emphasis is placed on a variety of absolute PCV patterns of geodetic and rover antennas. Electronic Publication     

Phase center modeling for LEO GPS receiver antennas and its impact on precise orbit determination

Most satellites in a low-Earth orbit (LEO) with demanding requirements on precise orbit determination (POD) are equipped with on-board receivers to collect the observations from Global Navigation Satellite systems (GNSS), such as the Global Positioning System (GPS). Limiting factors for LEO POD are nowadays mainly encountered with the modeling of the carrier phase observations, where a precise knowledge of the phase center location of the GNSS antennas is a prerequisite for high-precision orbit analyses. Since 5 November 2006 (GPS week 1400), absolute instead of relative values for the phase center location of GNSS receiver and transmitter antennas are adopted in the processing standards of the International GNSS Service (IGS). The absolute phase center modeling is based on robot calibrations for a number of terrestrial receiver antennas, whereas compatible antenna models were subsequently derived for the remaining terrestrial receiver antennas by conversion (from relative corrections), and for the GNSS transmitter antennas by estimation. However, consistent receiver antenna models for space missions such as GRACE and TerraSAR-X, which are equipped with non-geodetic receiver antennas, are only available since a short time from robot calibrations. We use GPS data of the aforementioned LEOs of the year 2007 together with the absolute antenna modeling to assess the presently achieved accuracy from state-of-the-art reduced-dynamic LEO POD strategies for absolute and relative navigation. Near-field multipath and cross-talk with active GPS occultation antennas turn out to be important and significant sources for systematic carrier phase measurement errors that are encountered in the actual spacecraft environments. We assess different methodologies for the in-flight determination of empirical phase pattern corrections for LEO receiver antennas and discuss their impact on POD. By means of independent K-band measurements, we show that zero-difference GRACE orbits can be significantly improved from about 10 to 6 mm K-band standard deviation when taking empirical phase corrections into account, and assess the impact of the corrections on precise baseline estimates and further applications such as gravity field recovery from kinematic LEO positions.     

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. AcknowledgmentsThe authors thank Prof. G. Seeber (University of Hannover) and Dr. G. Wübbena (Geo++ GmbH) and their groups for their kindness in making available the absolute field calibration results derived from robot measurements.     

Generation of a consistent absolute phase-center correction model for GPS receiver and satellite antennas

The development and numerical values of the new absolute phase-center correction model for GPS receiver and satellite antennas, as adopted by the International GNSS (global navigation satellite systems) Service, are presented. Fixing absolute receiver antenna phase-center corrections to robot-based calibrations, the GeoForschungsZentrum Potsdam (GFZ) and the Technische Universität München reprocessed more than 10 years of GPS data in order to generate a consistent set of nadir-dependent phase-center variations (PCVs) and offsets in the z-direction pointing toward the Earth for all GPS satellites in orbit during that period. The agreement between the two solutions estimated by independent software packages is better than 1 mm for the PCVs and about 4 cm for the z-offsets. In addition, the long time-series facilitates the study of correlations of the satellite antenna corrections with several other parameters such as the global terrestrial scale or the orientation of the orbital planes with respect to the Sun. Finally, completely reprocessed GPS solutions using different phase-center correction models demonstrate the benefits from switching from relative to absolute antenna phase-center corrections. For example, tropospheric zenith delay biases between GPS and very long baseline interferometry (VLBI), as well as the drift of the terrestrial scale, are reduced and the GPS orbit consistency is improved.     

Improved antenna phase center models for GLONASS

Thanks to the increasing number of active GLONASS satellites and the increasing number of multi-GNSS tracking stations in the network of the International GNSS Service (IGS), the quality of the GLONASS orbits has become significantly better over the last few years. By the end of 2008, the orbit RMS error had reached a level of 3–4 cm. Nevertheless, the strategy to process GLONASS observations still has deficiencies: one simplification, as applied within the IGS today, is the use of phase center models for receiver antennas for the GLONASS observations, which were derived from GPS measurements only, by ignoring the different frequency range. Geo++ GmbH calibrates GNSS receiver antennas using a robot in the field. This procedure yields now separate corrections for the receiver antenna phase centers for each navigation satellite system, provided its constellation is sufficiently populated. With a limited set of GLONASS calibrations, it is possible to assess the impact of GNSS-specific receiver antenna corrections that are ignored within the IGS so far. The antenna phase center model for the GLONASS satellites was derived in early 2006, when the multi-GNSS tracking network of the IGS was much sparser than it is today. Furthermore, many satellites of the constellation at that time have in the meantime been replaced by the latest generation of GLONASS-M satellites. For that reason, this paper also provides an update and extension of the presently used correction tables for the GLONASS satellite antenna phase centers for the current constellation of GLONASS satellites. The updated GLONASS antenna phase center model helps to improve the orbit quality.     

Group delay variations of GPS transmitting and receiving antennas

GPS code pseudorange measurements exhibit group delay variations at the transmitting and the receiving antenna. We calibrated C1 and P2 delay variations with respect to dual-frequency carrier phase observations and obtained nadir-dependent corrections for 32 satellites of the GPS constellation in early 2015 as well as elevation-dependent corrections for 13 receiving antenna models. The combined delay variations reach up to 1.0 m (3.3 ns) in the ionosphere-free linear combination for specific pairs of satellite and receiving antennas. Applying these corrections to the code measurements improves code/carrier single-frequency precise point positioning, ambiguity fixing based on the Melbourne–Wübbena linear combination, and determination of ionospheric total electron content. It also affects fractional cycle biases and differential code biases.     

Absolute phase center corrections of satellite and receiver antennas

Results of the estimation of azimuth-dependent phase center variations (PCVs) of GPS satellite antennas using global GPS data are presented. Significant variations of up to ±3–4 mm that are demonstrated show excellent repeatability over eight years. The application of the azimuthal PCVs besides the nadir-dependent ones will lead to a further reduction in systematic antenna effects. In addition, the paper focuses on the benefit of a possible transition from relative to absolute PCVs. Apart from systematic changes in the global station coordinates, one can expect the GPS results to be less dependent on the elevation cut-off angle. This, together with the significant reduction of tropospheric zenith delay biases between GPS and VLBI, stands for an important step toward more consistency between different space geodetic techniques.     

GNSS接收机天线相位中心变化相对检测方法

在精密定位中,GNSS接收机天线相位中心变化是必须进行改正的影响因素。目前成熟的微波暗室法和自动机器人法,对于一般用户而言,不具备相关实验条件,而野外相对法相对简单、易操作。为此,本文利用相对检测法,对GNSS接收机天线相位中心变化进行检测。实例表明,此方法可获得精度优于±3 mm的检测结果,因此可利用此方法对其他类型天线PCV值进行检测,也可借鉴此方法对北斗接收机天线相位中心变化进行检测。同时论文分析了影响检测精度,提出了有益改进建议。      

Evaluating pseudorange multipath effects at stations in the National CORS Network

A preliminary study was conducted to evaluate the amount of pseudorange multipath at 390+ sites in the National Continuously Operating Reference Station (CORS) Network. The National CORS Network is a cooperative effort involving over 110 different agencies, universities, and private companies who seek to make GPS data from dual-frequency receivers located throughout the United States and its territories available to the general public. For CORS users, pseudorange multipath can seriously degrade the accuracy of any application that relies on precise measurements of the pseudorange observable over a short period of time, including differential pseudorange navigation, kinematic and rapid-static surveying, and ionospheric monitoring. The main objectives of this study were to identify the most affected and least affected sites in the network, to closely investigate problematic sites, and to compare various receiver/antenna combinations. Dual-frequency carrier phase and pseudorange measurements were used to estimate the amount of L1 and L2 pseudorange multipath at each site over a one-year period. Some of the most severely affected sites were maritime Differential GPS and Nationwide Differential GPS (DGPS/NDGPS) sites. Photographs obtained for these sites verified the presence of transmission towers and other reflectors in close proximity to the GPS antennas. Plotting the variations of the L1 and L2 pseudorange multipath with respect to azimuth and elevation further verified that even above a 60° elevation angle there was still as much as five meters of pseudorange multipath at some sites. The least affected sites were the state networks installed in Ohio and Michigan; these sites used excellent antenna mounts, choke ring antennas, and new receiver technology. A comparison of the 12 most commonly used receiver/antenna combinations in the CORS Network indicated that newer receivers such as the Ashtech UZ-12, Leica RS-500, and Trimble 5700 help to significantly mitigate pseudorange multipath, while the receivers/antennas at some DGPS/NDGPS sites, and the antennas formerly used at the Wide Area Augmentation System (WAAS) sites, are among those most affected by pseudorange multipath. The receiver/antenna comparison did not take into account the potential presence of reflectors at the sites (i.e., it is possible that a well-performing receiver/antenna combination could have been consistently placed at very poor site locations, and vice-versa).Product Disclaimer: Mention of a commercial company or product does not constitute an endorsement by the National Oceanic and Atmospheric Administration. Use for publicity or advertisement purposes of information from this paper concerning proprietary products or the comparison of such products is not authorized.     

一种顾及现势指向的上行天线阵相位中心精确标校方法

针对上行天线阵相位中心标校技术粗糙、精度不高等问题,引入精密工程测量技术测定天线实际状态,提出了一种顾及现势指向的相位中心精确标校方法。首先,利用工业摄影测量系统获取天线各姿态的型面数据,并通过最小二乘法拟合求解出现势性强的机械轴;然后,用矩阵法解算各姿态下机械轴的交点作为旋转中心;最后,基于反角度加权插值法推估得到天线在任意姿态下的机械轴,进而从投影中心沿机械轴延伸既定长度获得可靠的相位中心。以3台φ3 m上行阵天线为试验对象,通过工程控制网统一摄影测量坐标系,并按照本文方法标校天线相位中心。电信号合成效果表明,本文方法能有效克服天线自重变形、机械安装等因素的影响,实现了相位中心的精确标校,增强了上行天线阵合成信号的幅度。     

Precise orbit determination for the FORMOSAT-3/COSMIC satellite mission using GPS

The joint Taiwan–US mission FORMOSAT-3/ COSMIC (COSMIC) was launched on April 17, 2006. Each of the six satellites is equipped with two POD antennas. The orbits of the six satellites are determined from GPS data using zero-difference carrier-phase measurements by the reduced dynamic and kinematic methods. The effects of satellite center of mass (COM) variation, satellite attitude, GPS antenna phase center variation (PCV), and cable delay difference on the COSMIC orbit determination are studied. Nominal attitudes estimated from satellite state vectors deliver a better orbit accuracy when compared to observed attitude. Numerical tests show that the COSMIC COM must be precisely calibrated in order not to corrupt orbit determination. Based on the analyses of the 5 and 6-h orbit overlaps of two 30-h arcs, orbit accuracies from the reduced dynamic and kinematic solutions are nearly identical and are at the 2–3 cm level. The mean RMS difference between the orbits from this paper and those from UCAR (near real-time) and WHU (post-processed) is about 10 cm, which is largely due to different uses of GPS ephemerides, high-rate GPS clocks and force models. The kinematic orbits of COSMIC are expected to be used for recovery of temporal variations in the gravity field.     

Absolute Calibration of an Ashtech Z12-T GPS Receiver

Dual-frequency carrier phase and code measurements from geodetic type receivers are a promising tool for frequency and time transfer. In order to use them for clock comparisons, all instrumental delays should be calibrated. We have carried out the calibration of one such receiver, an Ashtech Z12-T type, by two different methods: first, by absolute calibration using a GPS simulator; second, by differential calibration with respect to a time transfer receiver that had previously been calibrated. We present the experimental set-ups and the results of the two experiments and estimate the uncertainty budget. An ultimate uncertainty of order 1 ns in the absolute calibration seems to be attainable. ? 2001 John Wiley & Sons, Inc.     

New global positioning system reference station in Brazil

Co-located very long baseline interferometry (VLBI) and global positioning system (GPS) reference stations were installed near Fortaleza, Brazil, in 1993. Both have been important in the realization and maintenance of the International Terrestrial Reference Frame. A new-generation GPS system was installed in 2005 to replace the original station. Experience gained in the prior 12 years was used to improve the design of the GPS antenna mount. Preliminary indications are greatly improved data quality from the new station. Simultaneous observations from the nearly half-year of overlapping operation have been used to determine the local tie between the new and old GPS reference points to about 1 mm accuracy. This can be used to update the 1993 survey tie between the original GPS and the VLBI points, although there are questions about the accuracy of that measurement based on a comparison with space geodetic data. A test of removing the conical radome over the old GPS antenna indicates that it has biased the station height by about 16 mm downward, which probably accounts for most of the previous survey discrepancy.     

GPS天线的绝对校正方法

介绍了由德国Geo＋＋研究的直接在野外进行的绝对校准方法,该方法可以消除多路径效应,测得和方位角相关的PCV．经过进一步的研究,该校正过程高效准确且可以实时提供校正结果。简单列举了AOAD／M—T类型天线的相对和绝对PCV的结果,对PCV的改正精度要求做了简单介绍。     

GPS Antenna Calibration at the National Geodetic Survey

The precise point whose position is being measured when a GPS baseline is determined is generally assumed to be the phase center of the GPS antenna. However, the phase center of a GPS antenna is neither a physical point nor a stable point. For any given GPS antenna, the phase center will change with the changing direction of the signal from a satellite. Ideally, most of this phase center variation depends on satellite elevation. Azimuthal effects are only introduced by the local environment around each individual antenna site. These phase center variations affect the antenna offsets that are needed to connect GPS measurements to physical monuments. Ignoring these phase center variations can lead to serious (up to 10 cm) vertical errors. This article will describe the procedure by which the National Geodetic Survey is calibrating GPS antennas and how this information may be obtained and used to avoid problems from these antenna variations. ? 1999 John Wiley & Sons, Inc.     

Antenna phase center calibration for precise positioning of LEO satellites

Phase center variations of the receiver and transmitter antenna constitute a remaining uncertainty in the high precision orbit determination (POD) of low Earth orbit (LEO) satellites using GPS measurements. Triggered by the adoption of absolute phase patterns in the IGS processing standards, a calibration of the Sensor Systems S67-1575-14 antenna with GFZ choke ring has been conducted that serves as POD antenna on various geodetic satellites such as CHAMP, GRACE and TerraSAR-X. Nominal phase patterns have been obtained with a robotic measurement system in a field campaign and the results were used to assess the impact of receiver antenna phase patterns on the achievable positioning accuracy. Along with this, phase center distortions in the actual spacecraft environment were characterized based on POD carrier phase residuals for the GRACE and TerraSAR-X missions. It is shown that the combined ground and in-flight calibration can improve the carrier phase modeling accuracy to a level of 4 mm which is close to the pure receiver noise. A 3.5 cm (3D rms) consistency of kinematic and reduced dynamic orbit determination solutions is achieved for TerraSAR-X, which presumably reflects the limitations of presently available GPS ephemeris products. The reduced dynamic solutions themselves match the observations of high grade satellite laser ranging stations to 1.5 cm but are potentially affected by cross-track biases at the cm-level. With respect to the GPS based relative navigation of TerraSAR-X/TanDEM-X formation, the in-flight calibration of the antenna phase patterns is considered essential for an accurate modeling of differential carrier phase measurements and a mm level baseline reconstruction.

使用参照物的射电望远镜中心坐标测量方法

为满足新建射电望远镜在单站或多站联合进行的深空探测或射电天文观测对中心坐标精确测定的要求,提出一种利用已知精确中心坐标的望远镜作为参照物,测量地平式射电望远镜中心点坐标的方法和测量数据处理方法。这一方法对场地和设备的要求较低,能够得到毫米级或亚毫米级的位置精度。尤其适合对天线阵列的中心位置进行测量。对国家天文台密云观测站的40 m射电望远镜进行了中心坐标测量,位置均方根误差为2.312 mm,满足了后续的观测工作对其位置的需求。     

Accuracy assessment of the GPS-based slant total electron content

The main scope of this research is to assess the ultimate accuracy that can be achieved for the slant total electron content (sTEC) estimated from dual-frequency global positioning system (GPS) observations which depends, primarily, on the calibration of the inter-frequency biases (IFB). Two different calibration approaches are analyzed: the so-called satellite-by-satellite one, which involves levelling the carrier-phase to the code-delay GPS observations and then the IFB estimation; and the so-called arc-by-arc one, which avoids the use of code-delay observations but requires the estimation of arc-dependent biases. Two strategies are used for the analysis: the first one compares calibrated sTEC from two co-located GPS receivers that serve to assess the levelling errors; and the second one, assesses the model error using synthetic data free of calibration error, produced with a specially developed technique. The results show that the arc-by-arc calibration technique performs better than the satellite-by-satellite one for mid-latitudes, while the opposite happens for low-latitudes.