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.     

Surveying co-located space-geodetic instruments for ITRF computation

A new and comprehensive method is presented that can be used for estimating eccentricity vectors between global positioning system (GPS) antennas, doppler orbitography and radiopositioning integrated by satellites (DORIS) antennas, azimuth-elevation (AZ-EL) very long baseline interferometry (VLBI) telescopes, and satellite laser ranging (SLR) and lunar laser ranging (LLR) telescopes. The problem of reference point (RP) definition for these space-geodetic instruments is addressed and computed using terrestrial triangulation and electronic distance measurement (EDM) trilateration. The practical ground operations, the surveying approach and the terrestrial data processing are briefly illustrated, and the post-processing procedure is discussed. It is a geometrically based analytical approach that allows computation of RPs along with a rigorous statistical treatment of measurements. The tight connection between the geometrical model and the surveying procedure is emphasized. The computation of the eccentricity vector and the associated variance–covariance matrix between an AZ-EL VLBI telescope (with or without intersecting axes) and a GPS choke ring antenna is concentrated upon, since these are fundamental for computing the International Terrestrial Reference Frame (ITRF). An extension to RP computation and eccentricity vectors involving DORIS, SLR and LLR techniques is also presented. Numerical examples of the quality that can be reached using the authors approach are given. Working data sets were acquired in the years 2001 and 2002 at the radioastronomical observatory of Medicina (Italy), and have been used to estimate two VLBI-GPS eccentricity vectors and the corresponding SINEX files.     

Broadband convex impedance ground planes for multi-system GNSS reference station antennas

Common choke ring ground planes are known to contribute to undesirable antenna pattern narrowing in the elevation plane which is associated with difficulties of tracking low elevation satellites. Also known is the comparatively narrow frequency bandwidth of the choke grooves structure. As an alternative, using a convex impedance ground plane has been suggested for full-spectrum GNSS applications. With such ground planes a pin structure is utilized instead of choke grooves to allow a frequency bandwidth increase. A semi-spherical shape of the ground plane is shown to provide increased antenna gain for low elevation angles. Theoretical performance estimates along with experimental test data have been provided.     

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     

Adjusting VLBI data to obtain geophysical information for mobile VLBI sites

We have used Very Long Baseline Interferometry (VLBI) data to compute the site coordinates and constant velocity components for 29 fixed antenna sites and 25 mobile sites. The three singularities which occur in the adjustment with respect to the rotation of the system have been resolved by a constraint holding the net rotation of seven fixed antennas, distributed on the stable portions of four of the geologic plates, to the net rotation for these sites as defined by the NNR-NUVEL1 no net rotation model. In order to achieve a minimally constrained adjustment of this type we have found it necessary to use a new adjustment procedure in which we solve for the coordinates of each site at the weighted mean epoch of all the observations involving that site.Using the results of the above solution we have computed the departure for each site from the NNR-NUVEL1 rigid plate model. These departures show that the transition zone in western North America from the region of rigidity to the plate boundary is at least 400 km wide, in general agreement with Ward (1988,1990).     

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.     

利用不同时间同步体制钟差评估北斗三号星载原子钟性能

全球卫星导航系统(global navigation satellite system,GNSS)星载原子钟性能的优劣直接影响GNSS观测信号质量、测距精度、钟差预报与卫星自主导航能力,从而间接影响整个导航系统的服务性能。结合北斗三号系统独特的星间链路(inter-satellite link, ISL)和星地时间双向比对(two-way time transfer,TWTT)体制以及常用的精密轨道与钟差确定(orbit determination and time synchronization,ODTS)体制所估计的精密钟差数据,分析评估了北斗三号在轨原子钟服务性能。结果表明,3种钟差确定体制评估的频率准确度和漂移率结果基本一致,所有卫星频率准确度在(-4～2)×10^-11范围以内,氢钟频率准确度优于铷钟,ISL钟差评估的频率漂移率精度略优于ODTS。在评估原子钟稳定度方面,3种钟差确定体制各有优势,短期稳定度方面,ODTS钟差评估优于ISL钟差,基于ODTS评估的3 000 s稳定度可达3×10^-14,且氢钟的短期稳定性优于铷钟;中长期...     

Space Rockets, Polar Bears, and Tundra Buggies – And a GPS Link to the Seventeenth Century

The use of the Global Positioning System (GPS) for position and attitude determination has been exploited for many years. In these systems, it was required that multiple antennas maintained phase lock to common view satellites for long periods of time in order to determine integer ambiguities. The challenge to overcome with the ballistic space probe is the high rotational spin of this particular rocket. The antennas, mounted on the outside skin of the rocket, are in full view of each satellite only for a fraction of a second before the rocket core eclipses them as it rotates. The antennas rarely see common satellites. This article describes as system that overcomes these obstacles and provides position, acceleration, attitude, and rotation rate. The approach uses a two-antenna system. One antenna is mounted either side of the rocket facing out. A hybrid tracking channel combines signals from both antennas to overcome the signal blockage due to the rocket core as well as to defeat the effect on the tracking channel due to the spin. The amplitude and phase of the spin rotation signature from each satellite is extracted from the dual antenna data and used to compute the attitude and spin rate of the vehicle. ? 1999 John Wiley & Sons, Inc.     

Assessing antenna field of view and receiver clocks of COSMIC and GRACE satellites: lessons for COSMIC-2

We provide suggestions for the approved COSMIC-2 satellite mission regarding the field of view (FOV) and the clock stability of its future GNSS receiver based on numerical analyses using COSMIC GPS data. While the GRACE GPS receiver is mounted on the zenith direction, the precise orbit determination (POD) antennas of COSMIC are not. The COSMIC antenna design results in a narrow FOV and a reduction in the number of GPS observations. To strengthen the GPS geometry, GPS data from two POD antennas of COSMIC are used to estimate its orbits. The phase residuals of COSMIC are at the centimeter level, compared to the millimeter level of GRACE. The receiver clock corrections of COSMIC and GRACE are at the microsecond and nanosecond levels, respectively. The clock spectra of COSMIC at the frequencies of 0–0.005 Hz contain significant powers, indicating potential systematic errors in its clock corrections. The clock stability, expressed by the Allan deviation, of COSMIC ranges from 10^?9 to 10^?11 over 1 to 10⁴ s, compared to 10^?12 to 10^?14 for GRACE. Compared to USO-based clock of GRACE, the clock of COSMIC is degraded in its stability and is linked to the reduction of GPS data quality. Lessons for improvement of COSMIC-2 over COSMIC in FOV and receiver clock stability are given.     

LEO GPS attitude determination algorithm for a micro-satellite using boom-arm deployed antennas

This paper describes a low earth orbiter micro-satellite attitude determination algorithm using GPS phase and pseudorange data as the only observables. It is designed to run in real-time, at a rate of 10 Hz, on-board the spacecraft, using minimal chip and memory resources. The spacecraft design includes four GPS antennas deployed on boom arms to improve the antenna separations. The boom arms feature smart sensors, from which time-varying deformation data are used to calculate changes in the body-fixed system (BFS) co-ordinates of the attitude antennas. These data are used as input to the attitude algorithm to improve the accuracy of the output. The conventional double-difference phase observation equations have been re-arranged so that the only unknown parameters in the functions (once the ambiguities have been determined) are the spacecraft Euler angles. This greatly increases the redundancy in the mathematical model, and is exploited to enhance the algorithm's ability to trap observations contaminated by unmodelled multipath. This approach has been shown to be successful in identifying phase outliers at the 5–10 mm level. Speed of execution of the program is improved by utilising numerical differentiation of the model equations in the linearisation process. Furthermore, as the number of solve-for parameters is reduced to three by the chosen mathematical model, matrix inversion requirements are minimised. A novel approach to ambiguity resolution and determination of initial estimates of the attitude parameters has been developed utilising a heuristic technique and the known, and time varying, BFS co-ordinates of the antenna array. Algorithm testing is based on a simulation of the micro-satellite trajectory combined with variations in attitude derived from spin-stabilisation and periodic roll and pitch parameters. The trajectory of the spacecraft centre of mass was calculated by numerical integration of a force model using Earth gravity field parameters, third body effects due to the Sun and the Moon, dynamic Earth tide effects (solar and lunar), and a solar radiation pressure model. Frame transformations between J2000 and ITRF97 used the IERS conventions. A similar approach was used to calculate the trajectories of all available GPS satellites during the same period, using initial conditions of position and velocity from IGS precise orbits. RMS differences between the published precise orbit and the integrated satellite positions were at the 5-mm level. Phase observables are derived from these trajectories, biased by simulation of receiver and satellite clock errors, cycle slips, random or systematic noise and initial integer ambiguities. In the actual simulation of the attitude determination process in orbit, GPS satellite positions are calculated using broadcast ephemerides. The results show that the aim of 0.05° (two sigma) attitude precision can be met provided that the phase noise can be reduced to the level of 1–2 mm. Attitude precision was found to vary strongly with constellation geometry, which can change quite rapidly depending on the variations in spacecraft attitude. The redundancy in the mathematical model was found to be very effective in trapping and isolating cycle slips to the double difference observations that are contaminated. This allows for the possibility of correcting for cycle slips without full recourse to the ambiguity resolution algorithm. Electronic Publication     

气象参数对对流层折射影响的相关研究

对流层折射是卫星导航测量的重要误差源之一。针对卫星导航仿真系统高精度和强实时性的要求,本文利用Hopfield模型和Saastamoinen模型分析了不同气象环境下气象参数对计算对流层天顶折射的影响。研究表明天顶折射量是各气象参数的增函数,在相同气象环境下,对各气象参数的敏感度各不相同。当气象环境改变时,敏感度的变化也不相同:气温的敏感度变化幅度最大,相对湿度次之,而大气压的敏感度保持不变。在此基础上利用距离平方反比插值方法栅格化气象站资料建立全国范围的气象环境。栅格数据的应用可将因气象参数的不准确而导致的对流层天顶折射量误差减小一个量级,对于提高卫星导航仿真系统的精度具有重要意义。     

Effects of Resonances in Corrugated Horn Antennas for a 22-GHz Balancing Radiometer

The Stratospheric WAter vapor RAdiometer (SWARA) is a microwave radiometer designed for ground-based measurements of water vapor (H₂O) in the middle atmosphere (20 to 80 km), including the stratosphere and mesosphere. The instrument is operating in a noncryogenic balancing calibration mode. Since its deployment, features have been observed in the spectrum which can be attributed to resonant variations of the antenna pattern of the corrugated horn. This paper presents copolar and crosspolar antenna pattern measurements of two sister antennas of the SWARA horn, as well as water vapor measurements from both antennas on the ground-based microwave radiometer MI ddle Atmospheric WAter vapor RA diometer. We show that small irregularities in the frequency spectrum at the -20-dB level are visible in the copolar pattern, which, due to the balancing operation scheme used for the radiometer, lead to features in the spectrum that have the same or even higher brightness temperature as the line of interest.     

Apparent clock variations of the Block IIF-1 (SVN62) GPS satellite

The Block IIF satellites feature a new generation of high-quality rubidium clocks for time and frequency keeping and are the first GPS satellites transmitting operational navigation signals on three distinct frequencies. We investigate apparent clock offset variations for the Block IIF-1 (SVN62) spacecraft that have been identified in L1/L2 clock solutions as well as the L1/L5-minus-L1/L2 clock difference. With peak-to-peak amplitudes of 10?C40?cm, these variations are of relevance for future precision point positioning applications and ionospheric analyses. A proper characterization and understanding is required to fully benefit from the quality of the new signals and clocks. The analysis covers a period of 8?months following the routine payload activation and is based on GPS orbit and clock products generated by the CODE analysis center of the International GNSS Service (IGS) as well as triple-frequency observations collected with the CONGO network. Based on a harmonic analysis, empirical models are presented that describe the sub-daily variation of the clock offset and the inter-frequency clock difference. These contribute to a better clock predictability at timescales of several hours and enable a consistent use of L1/L2 clock products in L1/L5-based positioning.     

IPWV Estimation from Different GNSS Antenna

Global Navigation Satellite System (GNSS) is widely used nowadays in variety of applications. The observation file for the near real time estimation of Integrated Precipitable Water Vapour (IPWV) received at the ground-based receiver is mixed with ambiguities. Multi-path effects affect the positional accuracy as well as range from satellite to ground based receiver of the system. The designing of the antenna suppress the effect of multi-path, cycle slips, number of observations, and signal strength and data gaps within the data streams. This paper presents the preliminary data quality control findings of the Patch antenna (LeicaX1202), 3D Choke ring antenna (LeicaAR25 GNSS) and Trimble Zephyr antenna (TRM 39105.00). The results shows that choke ring antenna have least gaps in the data, cycle slips and multi-path effects along with improvement in IPWV. The signal strength and the number of observations are more in case 3D choke ring antenna.     

Modeling thermal deformation of VLBI antennas with a new temperature model

Temperature variations at very long baseline interferometry (VLBI) sites cause thermal deformations of the VLBI antennas and corresponding displacements of the VLBI reference points. The thermal deformation effects typically contain seasonal and daily signatures. The amplitudes of the annual vertical motion of the antenna reference point can reach several millimeters, depending on the design of the antenna structure, on the material, and on the environmental effects such as global station position, station height and climatology effects. Simple methods to correct this effect use the difference of the environmental temperature with respect to a defined reference temperature, the antenna dimensions, the elevation of the antenna, the material of antenna structure. Applying these simple models for thermal deformation in the VLBI data analysis improves the baseline length repeatability by 3.5%. A comparison of these simple models with local thermal deformation measurements at the antennas in Onsala and Wettzell show that the local measurements and the modeled corrections agree well when the temperature of the antenna structure is used, but agree less good when the surrounding air temperatures are used. To overcome this problem we present a method to model temperature penetration into the antenna structures, that allows to model thermal deformation effects that agree with the observed vertical deformation of the Onsala and Wettzell radio telescopes with a root mean square deviation of 0.07 and 0.13 mm, respectively. Possible implementations in the VLBI analysis are presented, and the definition of an adequate reference temperature is discussed.     

Examining the C1-P1 Pseudorange Bias

Use of GPS tracking data from different dual-frequency receiver types (cross-correlating vs. codeless) has revealed satellite-dependent biases in pseudorange observables P1 (Y-code) and C1 (C/A, Clear Acquisition code). These biases can have a direct effect on clock estimates, carrier phase bias fixing, and other parameters estimated in GPS data processing. A set of satellite-specific compensatory pseudorange offsets is calculated, and each is applied to a wee of daily global network analyses in which satlellite, receiver, atmospheric, and Earth rotation parameters are estimated. Results from these analyses are then compared to those from corresponding baseline cases in which no biases were applied. There is also some evidence that suggests that the pseudorange biases differ even among codeless receiver models. Hence, a second set of offsets is computed on a different basis, and compared with the baseline model in a similar manner. A preliminary examination of C1-P1 variations over time is presented. Finally, recommendations are made for the use of the calculated offsets, and consideration is given to a future dissemination of updates to these values as necessary. ? 2001 John Wiley & Sons, Inc.     

地灾监测的大数据信息平台研究及应用

重点对地灾监测方面的方案研究进行了阐述,在构建大数据信息平台的设计方案时,以实现高度自动化的全方位监测功能为导向,依托导航卫星数据接收机和多源传感器,集成构建大数据信息平台的系统解决方案,主要包括扼流圈卫星天线、GNSS数据接收机、MEMS传感器、环境测量单元、防雷区域单元等功能元件的组成、多传感器的系统集成,以及全方位监测的功能实现。按照地灾监测大数据信息平台建设、构建大数据平台关键技术突破及检测数据解算软件功能实现的顺序,完成了GNSS实时监测预警应用网络系统HCmonitor的研发。综合应用多种手段实现地灾监测的功能升级,为解决重大灾害预警提供了新的思路和解决方案,该项成果在甘肃舟曲的灾后重建工程得到应用与推广。     

Monte Carlo simulations of the impact of troposphere,clock and measurement errors on the repeatability of VLBI positions

Within the International VLBI Service for Geodesy and Astrometry (IVS) Monte Carlo simulations have been carried out to design the next generation VLBI system (“VLBI2010”). Simulated VLBI observables were generated taking into account the three most important stochastic error sources in VLBI, i.e. wet troposphere delay, station clock, and measurement error. Based on realistic physical properties of the troposphere and clocks we ran simulations to investigate the influence of the troposphere on VLBI analyses, and to gain information about the role of clock performance and measurement errors of the receiving system in the process of reaching VLBI2010’s goal of mm position accuracy on a global scale. Our simulations confirm that the wet troposphere delay is the most important of these three error sources. We did not observe significant improvement of geodetic parameters if the clocks were simulated with an Allan standard deviation better than 1 × 10⁻¹⁴ at 50 min and found the impact of measurement errors to be relatively small compared with the impact of the troposphere. Along with simulations to test different network sizes, scheduling strategies, and antenna slew rates these studies were used as a basis for the definition and specification of VLBI2010 antennas and recording system and might also be an example for other space geodetic techniques.     

An investigation of GNSS atomic clock behavior at short time intervals

A technique for obtaining clock measurements from individual GNSS satellites at short time intervals is presented. The methodology developed in this study allows for accurate satellite clock stability analysis without an ultra-stable clock at the ground receiver. Variations in the carrier phase caused by the satellite clock are isolated using a combination of common GNSS carrier-phase processing techniques. Furthermore, the white phase variations caused by the thermal noise of the collection and processing equipment are statistically modeled and removed, allowing for analysis of clock performance at subsecond intervals. Allan deviation analyses of signals collected from GPS and GLONASS satellites reveal distinct intervals of clock noise for timescales less than 100 s. The clock data collected from GPS Block IIA, IIR, IIR-M, and GLONASS satellites reveal similar stability performance at time periods greater than 20 s. The GLONASS clock stability in the 0.6–10 s range, however, is significantly worse than GPS. Applications that rely on ultra-stable clock behavior from the GLONASS satellites at these timescales may therefore require high-rate corrections to estimate and remove oscillator-based errors in the carrier phase.     

Spin rate estimation of sounding rockets using GPS wind-up

Carrier phase wind-up is a well-known effect that arises from the relative rotation between a transmitting and receiving antenna. In GPS measurements at L1 frequency, this effect translates into an error of 19.029 cm per full relative rotation of antennas. Since this effect is independent of the satellite elevation for pure rotation about the antenna boresight axis, it is usually absorbed by the clock estimation in navigation algorithms. Therefore, the impact of wind-up is usually neglected for applications that do not require accuracies to the cm level like RTK. However, in receiving platforms with high rotation rate, the accumulated wind-up value can be important and actually be larger than receiver noise or even ionospheric variations. Therefore, in such scenarios, the wind-up contribution can be isolated and used as a source of information to compute the spin rate of such platforms using an appropriate combination of GPS observables. This work shows some results of a coarse, yet simple, approach to monitor the rotation angle and spin-rate of spin stabilized sounding rockets flown by DLR.