Precise Ephemeris Research Articles

Applications of IGS products in GPS time comparisons

When the GPS CV (common view) technique is used for the high-precision time comparisons, the effects of the error of the satellite orbit and the ionospheric delay of the received signal are not negligible and have to be IGS precise ephemeris and CODE global total electron content maps in the GPS time transfers, and it is shown that by using these IGS products the accuracies of the single-station time determination and long-distance time transfer can be effectively improved.

Evaluation of Time Transfer Precision with GPS All in View

International time comparison network has been performed by GPS common view to calculate TAI/UTC at BIPM. Recently, since IGS provides precise ephemerides and clock offsets for the GPS satellites, we can calculate time difference between two stations via GPS Time instead of a common GPS satellite. This method is called all in view as contrasted with common view. We compared time transfer precision between all in view and common view with UTC(NPL) and UTC(TL). The results show that all in view better than common view about 1.8 times.

Preliminary Tests of GPS/HAPS Positioning System Using a Pseudolite on Helicopter

Japan Aerospace Exploration Agency (JAXA) has proposed a new positioning/navigation service using pseudolites installed on the High altitude Platforms Systems (HAPS). If pseudolites were mounted on the HAPS, their GPS-like signals would be stable augmentations that would improve the performance of GPS. In order to establish the fundamental technologies, the augmentation tests using a GPS pseudolite installed on a helicopter were conducted. The position of the pseudolite antenna underneath the helicopter, which is analogous to the GPS ‘precise ephemeris’, was precisely determined by the inverted GPS method. Also, the results demonstrate the efficacy of integrating the pseudolite signal with GPS.

A velocity field estimation of the Brazilian portion of the SOAM plate

With the proposition for the adoption of Geocentric Reference System for the Americas (SIRGAS) as a terrestrial reference frame for South America, the need for temporal monitoring of station coordinates used in its materialization has become apparent. This would provide a dynamic characterization of the frame. The Brazilian Network for Continuous Monitoring of GPS (RBMC) has collected high accuracy GPS measurements since 1996. The Brazilian Institute of Geography and Statistics (IBGE) maintains this network in collaboration with several universities and organizations. Most of the stations are also part of the SIRGAS network. The RBMC also contributes data to the International Terrestrial Reference System (ITRS) to densify the global frame. Two of the RBMC stations are also part of the International GPS Service (IGS). This paper reports initial results from these stations. To estimate the velocity field defined by these stations, ten IGS stations located on the border of the South American plate and in adjacent plates, along with nine RBMC stations, were used. Observations covering five groups of 15 days each were used. These groups of observations were at epochs 1997.3, 1997.9, 1998.3, 1998.9 and 1999.2. Seven IGS stations were chosen to have their coordinates constrained to those epochs. IGS products (precise ephemeris and clocks) were used to process the daily solutions, which were carried out with Bernese software. Carrier phase double differences were formed using the ionospheric-delay free observable. The troposphere was modeled using a combination of the Saastamoinen model and the Niell mapping function. A tropospheric parameter was estimated every two hours. The results of the daily baseline solutions were combined using the summation of normal equations technique, in which the final coordinates and velocities were estimated. The results were compared with various models, such as the NNR-NUVEL1 and the APKIM8.80. Velocity vectors estimated for the RBMC stations show good agreement with those two models, with rates approximately equal to 2 cm/year.

Precise Ephemeris Reconstruction Using the Clohessy-Wiltshire Frame and Multiple Sequential Compressions

A precise onboard ephemeris propagation method that uses a multiple sequential compression is developed. A least-square regression is used to dee ne the reference orbit in the form of polynomial functions. Then the position and velocity residuals in terms of rectangular coordinates are generated by subtracting the reference orbit from a numerically generated preciseorbit and expressed in the Clohessy ‐Wiltshire frame. The residuals are compressed using various kinds of basis functions to ree ect secular and periodic variations. Precise position and velocity reconstruction is achieved by adding the compressed residuals into the reference orbit. Numerical simulation experiments and performance comparisons with previous results support the validity of the proposed method.

High-Precision, Kinematic Positioning with a Single GPS Receiver

ABSTRACT: The goal of the research described in this paper is the design of a GPS data processing technique capable of producing high-precision positioning results, regardless of platform dynamics, utilizing only a single, high-quality receiver. This goal is accomplished by combining two processing philosophies: point positioning, which makes use of precise GPS constellation ephemeris and clock offset information to estimate a single receiver's state; and carrier-phase–filtered pseudorange processing, in which pseudorange-based positioning is supplemented with carrier-based position-change information. Results derived with the developed software indicate that near-decimeter-level positioning accuracy is attainable for a variety of platforms, ranging from static, terrestrial reference stations, to aircraft, to satellites. A number of modeling improvements can be applied to the existing software, and testing in the real-time environment is planned.

Mini-ASTROD: Mission Concept

Advances in laser physics and its applications triggered the proposition and development of Laser Astrodynamics. Mini-ASTROD is a down-scaled version of ASTROD (Astro-dynamical Space Test of Relativity using Optical Devices). This mission concept has one spacecraft carrying a payload of a telescope, six lasers, and a clock together with ground stations (ODSN: Optical Deep Space Network) to test the optical scheme and yet give important scientific results. These scientific results include a better measurement of the relativistic parameters (γ to 1 ppm, β to a few ppm and others with improvement), a better sensitivity (several times better) in using the optical Doppler tracking method for detecting gravitational waves, a potential of measuring the solar angular momentum via the Lense-Thirring effect and measurement of many solar system parameters more precisely. These enable us to build a more precise ephemeris and astrodynamics. The weight of this spacecraft is estimated to be about 300–350 kg with a payload of about 100–120 kg. The spacecraft goes into an inner solar orbit with several options. One option is with period 304 days as for the inner spacecraft of the standard two-spacecraft ASTROD mission concept and it takes about 900 days to reach the other side of the Sum relative to the Earth. Another option is to launch with initial period about 290 days and to pass by Venus twice to receive gravity-assistance for achieving shorter periods. For a launch on November 15, 2008, after two encounters with Venus, the orbital period can be shortened to 165 days. After about 400 days from launch, the spacecraft will arrive at the other side of the Sun and the relativistic parameter γ can be determined to 1 ppm. We discuss the payload configuration and outlook for technological developments to reach the mission goals, and summarize the conclusions and recommendations of the first and second organizational meeting for the Mini-ASTROD study.

The quality of precise glonass ephemerides

To set up a global observation network, and to get experience in modelling GLONASS satellite orbits have been identified as key issues of the International GLONASS Experiment (IGEX), proposed in the late 1997. Thus in October 1998 the official IGEX field campaign was launched as a common effort of more than 60 institutions contributing to the station network as well as offering Data- and Analysis-Center services. In the process, precise satellite orbits, consistent at the 30cm level, were (and still are) calculated by 6 analysis groups on a regular basis. These precise ephemeris are combined and evaluated in order to generate robust, reliable and complete GLONASS orbits. This paper deals with the increasing quality of the calculated GLONASS orbits, even several months after the end of the basic IGEX field campaign, and discusses differences in individual orbit solutions with respect to the orbital geometry. Finally some ideas on a possible continuation of the IGEX effort are given, even under the restriction of a currently disintegrating satellite system.

Solution of regional GPS network using Precise Point Positioning with undifferenced data

Precise Point Positioning (PPP) is the precise point positioning of a single station by carrier phase and pseudo-range data, while both the precise ephemeris and clock parameters are fixed. Because no parameters common to different stations are estimated here, there is no correlation among the stations, and so the computation is greatly reduced. We use the GPS analysis software GIPSY of Jet Propulsion Laboratory (JPL), USA, for an analysis of the APSG (Asian-Pacific Space Geodynamics Program) GPS campaigns. The results show that the PPP is comparable in repeatability to the double-difference (DD) algorithm, widely used in China. The non-fiducial algorithm is used for maintaining better the station configuration. The results after a Helmert transformation show that the PPP and the DD are comparable as regards external consistency, while the computation time for a same amount of data is 3.5 hours for the PPP and 18 ∼ 20 hours for the DD. Therefore, the PPP can be expected to contribute usefully to the data reduction of the forthcoming Chinese giant science-engineering project—earthquake monitoring GPS network with up to 2000 receivers, by reducing the computer time while maintaining the same precision.

First use of IGEX precise ephemerides for intercontinental GLONASS P-code time transfer

Until recently, the Global Positioning System (GPS) was the only operational means of distributing time to an arbitrary number of users and of synchronizing clocks over large distances with a high degree of precision and accuracy. Over the last few years it has been shown that similar performance can be achieved using the Russian Global Navigation Satellite System (GLONASS). GLONASS time transfer between continents was initially hampered by the lack of post-processed precise ephemerides. Results from the International GLONASS Experiment (IGEX) campaign are now available, however, and this paper reports on the first use of IGEX precise ephemerides for GLONASS P-code intercontinental time links. The results of GLONASS P-code and GPS C/A-code time transfer are compared under similar conditions.

The international GLONASS experiment: products, progress and prospects

In October 1998 the IGEX field campaign, the first coordinated international effort to monitor GLONASS satellites on global basis, was started. Currently about 40 institutions worldwide support this effort either by providing GLONASS tracking data or in operating related data and analysis centers. The increasing quality and consistency of the calculated GLONASS orbits (about 25 cm early in 2000), even after the end of the official IGEX field campaign, are shown. Particular attention is drawn to the combination of precise ephemerides in order to generate a robust, reliable and complete IGEX orbits product. Some problems in modeling the effect of solar radiation pressure on GLONASS satellites are demonstrated. Finally, the expected benefits and prospects of the upcoming International GLOnass Service-Pilot Project (IGLOS-PP) of the International GPS Service (IGS) are discussed in more detail.

First Results for the GPS Survey of South Africa Tide Gauges

Tide gauge measurements are used for a variety of scientific purposes, not least of which are the definition of vertical data and the detection of long-term variations in mean sea level. GPS measurements at tide gauge sites provide a means of separating local verticl motions from sea level rise, and a means of unifying vertical data in a single reference system. This paper describes a GPS survey to determine the positions and heights of reference stations at South African tide gauge sites. The data were processed in baseline mode using a commercial software package. The heights of the tide gauge stations relative to the fixed ITRF reference station HRAO were determined at a precision of around 3 cm – better than 0.1 ppm. Analysis of the error sources showes that use of the precise ephemeris contributed to a substantial improvement in accuracy, as did the use of ionosphere-free fixed integer baseline solutions. Variations in the antenna phase centers also contributed significant changes in height. © 2001 John Wiley & Sons, Inc.

Precision ERS-2 orbit determination combining multiple tracking techniques

Both of the oceanographic research by using altimetry and the application of InSAR of ERS-2 satellite need the precise ephemerides of the satellite. In this note, the ERS-2 orbit is precisely determined by using SLR and PRARE observations to take advantage of the high accuracy of SLR and the widespread data coverage of PRARE. Based on the software system SODS-VCE established by us, the ERS-2 orbits in 1997 are calculated. The evaluation from tracking data fits, endpoint consistency of orbital arcs and comparison with independent orbit implies an orbit accuracy of 5–6 cm in radial component which can satisfy the requirement of being better than 10 cm in oceanographic research by altimetry, being less than 20 cm in cross-track component which can meet the requirement of InSAR. In orbit computation, the method of variance component estimation is employed to weight the SLR, PRARE range and Doppler measurements properly. The results indicate that the application of the method can improve the computation accuracy efficiently.

Analytical theories of satellite motion constructed with a new package of formula manipulation and compared with numerical integration

Specialized to the Lie series based perturbation method of Kirchgraber and Stiefel (1978) a new computer algebra package called ANALYTOS has been developed for constructing analytical orbital theories either in noncanonical or canonical form. We present results on the (extended) Main Problem of orbital theory of artificial earth satellites and related issues. The order of the solutions achieved is generally one order higher than those known from literature. Moreover, the analytical orbits have been checked succesfully against precise numerical ephemerides.

Crustal deformation state shown by fault measurement and GPS

In Capital area of China, many faultdeformational surveying sites were set up sequentiallyfrom 1967. With short leveling, short base line andshort ranging net across fault, a lot of data havebeen accumulated. In 1992, GPS (Global PositioningSystem) measurements were started in China. Two timesof GPS measurements have been made in 1992 and 1995respectively. The International Terrestrial ReferenceFrame 93 and the Precision Ephemeris provided byInternational GPS Geodynamics Service (IGS) fromGlobal GPS tracing network solution are used. In GPSanalysis and data processing, GAMIT software and GLOBKsoftware are used. The results from these two kinds ofdata are discussed. For earthquake risk area judgment,it is of interest to know where is compressional areaand where is tensional area, and generally thecompressional areas are considered as risk area andcalled abnormal area. All the data from shortleveling, short ranging (measured bylaser-range-finder), short base line (measured by baseline tape) and GPS methods can give us the descriptionabout the deformational state of the crust –compressional state or tensional state. The comparisonshows that, some of the two kinds of data areconsistent, a few of them are contradictory, and thenthe results are discussed.

An analytical method of computing the state transition matrix in orbit determination: ZHANG Qiang & LIU Lin Department of Astronomy, Nanjing University, Nanjing 210008

Because of the complexity of force models, the precise ephemeris and the state transition matrix in orbit determination are usually obtained by solving two groups of ordinary differential equations with numerical integration. In this paper, a simplified analytical method for computing the state transition matrix is presented. This method is not only very efficient for the case that the orbit arc is not too long, but also it can avoid the integration of two groups of differential equations at the same time. A practical test justifies the efficiency of the method.

Geographically correlated errors observed from a laser‐based short‐arc technique

The laser‐based short‐arc technique has been developed in order to avoid local errors which affect the dynamical orbit computation, such as those due to mismodeling in the geopotential. It is based on a geometric method and consists in fitting short arcs (about 4000 km), issued from a global orbit, with satellite laser ranging tracking measurements from a ground station network. Ninety‐two TOPEX/Poseidon (T/P) cycles of laser‐based short‐arc orbits have then been compared to JGM‐2 and JGM‐3 T/P orbits computed by the Precise Orbit Determination (POD) teams (Service d'Orbitographie Doris/Centre National d'Etudes Spatiales and Goddard Space Flight Center/NASA) over two areas: (1) the Mediterranean area and (2) a part of the Pacific (including California and Hawaii) called hereafter the U.S. area. Geographically correlated orbit errors in these areas are clearly evidenced: for example, −2.6 cm and +0.7 cm for the Mediterranean and U.S. areas, respectively, relative to JGM‐3 orbits. However, geographically correlated errors (GCE) which are commonly linked to errors in the gravity model, can also be due to systematic errors in the reference frame and/or to biases in the tracking measurements. The short‐arc technique being very sensitive to such error sources, our analysis however demonstrates that the induced geographical systematic effects are at the level of 1–2 cm on the radial orbit component. Results are also compared with those obtained with the GPS‐based reduced dynamic technique. The time‐dependent part of GCE has also been studied. Over 6 years of T/P data, coherent signals in the radial component of T/P Precise Orbit Ephemeris (POE) are clearly evidenced with a time period of about 6 months. In addition, impact of time varying‐error sources coming from the reference frame and the tracking data accuracy has been analyzed, showing a possible linear trend of about 0.5–1 mm/yr in the radial component of T/P POE.

Remote sensing precipitable water with GPS

The principle and method of remote sensing precipitable water (PW) by the Global Positioning System (GPS) are introduced. A series of tropospheric delay have been obtained from the dual frequency receiver's data of global tracking stations in East Asia and precise satellite ephemeris of IGS from July 31 to August 20, 1997. The continuous PW at the interval of 30 min have been estimated at Shanghai and Wuhan GPS sites. The results are compared with PW from conventional radiosonde during the same period, and the RMS is about 0.50 cm.

Problem of algorithm in precision orbit determination

In orbit determination, the precision ephemeris and state transition matrix are usually obtained by solving two groups of ordinary differential equations with numerical integration method due to the complexity of the force models. A kind of simplified analytical method to compute the state transition matrix is given. The method is not only very efficient for the case where the orbit arc is not too long, but also can avoid the integration of two groups of ordinary differential equations at the same time. Some practical test examples also show the efficiency of the method.

FurtherGingaObservations of PSR B0540−69

We report the pulse-timing history of PSR B0540-69, based on all applicable data from the Ginga X-ray observatory. Using these data, together with other, overlapping optical and X-ray observations, we have determined an improved value for the braking index, n=2.080, as well as a new estimate for the celestial position of PSR B0540-69. Our results are based on an analysis of local pulse frequencies, using a methodology designed to efficiently utilize the sparsely sampled observations of PSR B0540-69. This strategy rests on iteratively determining a sequence of successively more precise pulse ephemerides from successively more precise estimates of local pulse frequencies. We also present improved X-ray and optical pulse profiles and evidence for a close alignment between profiles in these two energy regimes. We may have detected a small, Crab-like glitch in the pulse frequency of this pulsar in a reanalysis of previously published optical data.