Ephemeris Errors Research Articles

Navigation, Interference Suppression, and Fault Monitoring in the Sea-Based Joint Precision Approach and Landing System

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> The United States Navy seeks the capability to land manned and unmanned aerial vehicles autonomously on an aircraft carrier using GPS. To deliver this capability, the Navy is developing a navigation system called the Sea-Based Joint Precision Approach and Landing System (JPALS). Because standard GPS is not sufficiently precise to land aircraft on a shortened, constantly moving runway, Sea-Based JPALS leverages dual-frequency, carrier-phase differential GPS navigation. Carrier phase measurements, derived from the sinusoidal waveforms underlying the GPS signal, are very precise but not necessarily accurate unless the user resolves the ambiguity associated with the sinusoid's periodicity. Ensuring the validity of ambiguity resolution is the central challenge for the high-integrity, safety-critical JPALS application. Based on a multi-year, multi-institution collaborative study, this paper proposes a navigation and monitoring architecture designed to meet the guidance quality challenge posed by Sea-Based JPALS. In particular, we propose a two-stage navigation algorithm that meets the aggressive integrity-risk requirement for Sea-Based JPALS by first filtering a combination of GPS observables and subsequently exploiting those observables to resolve the carrier ambiguity. Because JPALS-equipped aircraft may encounter jamming, we also discuss interference mitigation technologies, such as inertial fusion and array antennas, which, with appropriate algorithmic modifications, can ensure integrity under Radio Frequency Interference (RFI) conditions. Lastly, we recommend a fault monitoring strategy tailored to the two-stage navigation algorithm. Monitoring will detect and isolate rare anomalies such as ionosphere storms or satellite ephemeris errors which would otherwise corrupt ambiguity resolution and positioning in Sea-Based JPALS. </para>

An Adaptive Multipath Mitigation Filter for GNSS Applications

Global navigation satellite system (GNSS) is designed to serve both civilian and military applications. However, the GNSS performance suffers from several errors, such as ionosphere delay, troposphere delay, ephemeris error, and receiver noise and multipath. Among these errors, the multipath is one of the most unpredictable error sources in high-accuracy navigation. This paper applies a modified adaptive filter to reduce code and carrier multipath errors in GPS. The filter employs a tap-delay line with an Adaline network to estimate the direction and the delayed-signal parameters. Then, the multipath effect is mitigated by subtracting the estimated multipath effects from the processed correlation function. The hardware complexity of the method is also compared with other existing methods. Simulation results show that the proposed method using field data has a significant reduction in multipath error especially in short-delay multipath scenarios.

An Extended Propagation Ephemeris for GNSS

ABSTRACT: A Global Navigation Satellite Systems (GNSS) ephemeris is defined, providing satellite positions accurate to 7.7 and 48.7 meters, over 24 and 72 hours, respectively. An empirical model is created, based upon the Fourier analysis of one year of precision orbit data, using argument of latitude as the independent variable. This model was then fit to smaller batches of orbit data. Dominant frequencies in the residuals, using time as the independent variable, defined a second empirical model for corrections in an orbit-based reference frame. The ephemeris was tested using one month of data, fitting segments between 24 and 168 hours. Random error was then simulated, using standard deviations between 0.1 and 5 m, to study the sensitivity to orbit propagation accuracy. Propagation errors of 5 meters increased the 24-hour ephemeris error to 12.2 meters, but had an insignificant effect on the longer ephemerides.

TEMPO2, a new pulsar timing package - II. The timing model and precision estimates

Tempo2 is a new software package for the analysis of pulsar pulse times of arrival. In this paper we describe in detail the timing model used by tempo2, and discuss limitations on the attainable precision. In addition to the intrinsic slow-down behaviour of the pulsar, tempo2 accounts for the effects of a binary orbital motion, the secular motion of the pulsar or binary system, interstellar, Solar system and ionospheric dispersion, observatory motion (including Earth rotation, precession, nutation, polar motion and orbital motion), tropospheric propagation delay, and gravitational time dilation due to binary companions and Solar system bodies. We believe the timing model is accurate in its description of predictable systematic timing effects to better than one nanosecond, except in the case of relativistic binary systems where further theoretical development is needed. The largest remaining sources of potential error are measurement error, interstellar scattering, Solar system ephemeris errors, atomic clock instability and gravitational waves.

Monitoring Global Positioning System Satellite Orbit Errors for Aircraft Landing Systems

Ground-based augmentations of the global positioning system (GPS) demand the greatest safety and reliability to support aircraft precision approach and landing navigation. One troublesome failure mode for these systems is the possibility of large orbit errors; discrepancies between the locations of GPS satellites in space and the locations derived by the ephemeris data that they broadcast. To counter this possibility, several ephemeris monitor algorithms detecting orbit errors are described. The method is based on a comparison between satellite positions given by the current satellite ephemeris [today’s ephemeris (TE)] and the ephemeris broadcast by the same satellite on its preceding pass [yesterday’s ephemeris (YE)]. Variants of this YE‐TE test are shown to provide protection against ephemeris errors and also to support minimum detectable errors as low as 1145 m, which will minimize the resulting impact on ground-based augmentation system user availability. In addition, to initialize these monitors when no earlier validated ephemerides are available, means of using raw measurements are proposed. The results show that the YE‐TE and the measurement-based methods together are adequate to meet navigation integrity and availability requirements for category 1 precision approaches.

A global solution for the Mars static and seasonal gravity, Mars orientation, Phobos and Deimos masses, and Mars ephemeris

With the collection of six years of MGS tracking data and three years of Mars Odyssey tracking data, there has been a continual improvement in the JPL Mars gravity field determination. This includes the measurement of the seasonal changes in the gravity coefficients (e.g., J ¯ 2 , J ¯ 3 , C ¯ 21 , S ¯ 21 , C ¯ 31 , S ¯ 31 ) caused by the mass exchange between the polar ice caps and atmosphere. This paper describes the latest gravity field MGS95J to degree and order 95. The improvement comes from additional tracking data and the adoption of a more complete Mars orientation model with nutation, instead of the IAU 2000 model. Free wobble of the Mars' spin axis, i.e. polar motion, has been constrained to be less than 10 mas by looking at the temporal history of C ¯ 21 and S ¯ 21 . A strong annual signature is observed in C ¯ 21 , and this is a mixture of polar motion and ice mass redistribution. The Love number solution with a subset of Odyssey tracking data is consistent with the previous liquid outer core determination from MGS tracking data [Yoder et al., 2003. Science 300, 299–303], giving a combined solution of k 2 = 0.152 ± 0.009 using MGS and Odyssey tracking data. The solutions for the masses of the Mars' moons show consistency between MGS, Odyssey, and Viking data sets; Phobos GM = ( 7.16 ± 0.005 ) × 10 −4 km 3 / s 2 and Deimos GM = ( 0.98 ± 0.07 ) × 10 −4 km 3 / s 2 . Average MGS orbit errors, determined from differences in the overlaps of orbit solutions, have been reduced to 10-cm in the radial direction and 1.5 m along the spacecraft velocity and normal to the orbit plane. Hence, the ranging to the MGS and Odyssey spacecraft has resulted in position measurements of the Mars system center-of-mass relative to the Earth to an accuracy of one meter, greatly reducing the Mars ephemeris errors by several orders of magnitude, and providing mass estimates for Asteroids 1 Ceres, 2 Pallas, 3 Juno, 4 Vesta, and 324 Bamberga.

A New Autonomous Navigation Method for Martian Rover Based on Celestial Observation

It is essential for a Martian rover to know its position in order to fulfill a task. For there are some disadvantages of the rover controlled and navigated by ground station. The autonomous navigation and control of planet rover have become an important topics. Since the celestial navigation method has become one of the main trends of mobile robots navigation development, a new Autonomous navigation method for Martian rover based on celestial observation is described in this paper. This method only uses the angles between the stars and the center of Martian moons, Phobos or Deimos, which are got from an image of such objects taken by a star camera. At a given time these angles contains the information of rover's position, we can determine the position of Martian rover by the UPF (Unscented Particle Filter) method combined with the rover motion model. A simulation result demonstrated the validity and feasibility of this new method. The main factors which affect the precision, including measurement error and ephemeris error, are also analyzed.

Performance Analysis of Korean WADGPS Algorithms with NDGPS Data

To provide more accurate and reliable positioning and timing services to Korean nationwide users, the Ministry of Maritime Affairs and Fisheries of Korea is implementing Korean NDGPS (Nationwide DGPS), which is operational partly. And it also has a plan to construct WADGPS (Wide Area Differential GPS) system using sites and equipments of the NDGPS reference stations. For that, Seoul National University GNSS Laboratory is implementing and testing prototypes of WRS (Wide-area Reference Station) and WMS (Wide-area Master Station). Until now, because there are not enough installed WRSs to be used for computing wide area correction information, we cannot test algorithms of WMS with the data processed actually in WRSs. Therefore to evaluate the performance of the algorithms, we made a MATLAB program which can process RINEX (Receiver INdependent Exchange) format data with WADGPS algorithm. Using that program which consists of WRS, WMS and USER modules, we processed the data collected at NDGPS reference stations, which are saved in RINEX format. In WRS module, we eliminate the atmospheric delay error from the pseudorange measurement, smooth the measurement by hatch filter and calculate pseudorange corrections for each satellite. WMS module collects the processed data from each reference stations to generate the wide area correction information including estimated satellite ephemeris errors, ionospheric delays at each grid point, UDRE (User Differential Range Error), GIVE (Grid Ionosphere Vertical Error) and so on. In USER part, we use the measurements of reference stations as those of users and estimate the corrected users' positions and protection levels (HPL, VPL). With the results of estimation, we analyzed the performance of the algorithms. We assured the estimated UDRE /GIVE values and the protection levels bound the corresponding errors effectively. In this research, we can expect the possible performance of WADGPS in Korea, and the developed modules will be useful to implementation and improvement of the algorithms.

Influence of Satellite Departure and Experiment Research on Integrated Positioning With Two Geostationary Satellites and GIS

建立了双星定位和双星/GIS组合定位的计算模型.在此基础上研究了星历误差对双星定位结果的影响,卫星位置偏差对组合定位结果的影响,在组合定位中,在经、纬度方向上,分别给卫星位置加入均方差为1°的位置偏差.结果表明,给卫星位置加入均方差为1°的经、纬度方向位置偏差,并不会降低组合定位的精度.对这一重要结论,从几何上给出了解释.最后进行了双星/GIS组合定位的物理试验.试验结果表明,双星/GIS组合定位确实可以大大提高双星定位的精度.

Detecting Global Positioning Satellite Orbit Errors Using Short-Baseline Carrier-Phase Measurements

Asystem to detectorbit errorsaffecting differential global positioning system integrity isdescribed.Themethod isbased on the use of carrier-phase ranging measurements madeby two or more ground-based referencereceivers separated by short baselines. A dual-frequency, geometry-free widelane approach is used to resolve cycle ambigu- ities present in the carrier-phase measurements. The performance of the proposed integrity monitor is evaluated relative to existing integrity requirements for aircraft landing navigation applications. The results show that real- time protection is achievable against all types of ephemeris errors that can ine uence aircraft precision approach and landing navigation. LTHOUGH initially developed to support military applica- tions, the global positioning system (GPS) has quickly grown into an important and versatile civil navigation utility. The differ- ential GPS (DGPS) technique, in particular, has provided improve- ments in positioning accuracy sufe cient for many demanding ap- plications, including aircraft navigation during precision approach andlandingphasesofe ight.However,forsuchdife cultapplications, highaccuracyisgenerallynotsufe cient.Thenavigationsystemmust also provide a means to ensure its own integrity by reliably detect- ing navigation system failures or anomalies. In this work, detailed consideration is given to the real-time detection of a specie c type of navigation failure: incorrect knowledge of a GPS satellite orbit. Each GPS satellite broadcasts orbit ephemerides so users can compute satellite locations at any time of interest. The satellite lo- cations, together with ranging measurements also obtained from the satellite signals, are used to compute user position. An error in knowledge of satellite position will, therefore, cause a resulting error in the computed user position. Nominally, these errors are negligibly small for DGPS users, but integrity considerations for aircraft precision landing navigation dictate that anomalous con- ditions must be quickly detected. Furthermore, the effects of orbit anomalies differ from those of other navigation failures, for exam- ple, GPS receiver failures, in that orbit errors ultimately cause nav- igation errors that are dependent on the time-varying displacement between the aircraft and ground-based DGPS reference receiver. Therefore, the impact of undetectable orbit errors on navigation must ultimately be assessed separately by each individual aircraft within the DGPS service volume. In this regard, the Federal Avia- tion Administration (FAA), RTCA, Inc., and the International Civil Aviation Organization (ICAO) are developing specie c ephemeris integrity performance standards for DGPS-based aircraft precision landing. The FAA' s local-area augmentation system (LAAS) will serve as the baseline DGPS architecture considered in this paper. Whenever the GPSdata broadcast by the satellites do not contain the correct satellite orbit parameters, an ephemeris anomaly is said to exist. Although there may be a variety of potential causes for such anomalies, for example, unscheduled maneuvers, incorrect or- bit uploads, and faulted data decoding in the receiver, all ephemeris

成層圏プラットフォームを用いた疑似ＧＰＳ衛星による航法測位サービスについて

Japan has been investigating the use of an airship system that will function as a stratospheric platform (SPF) for applications such as environmental monitoring, communications and broadcasting. If pseudolites were mounted on the platforms, their GPS-like signals would be stable augmentations that would improve the accuracy, availability, and integrity of GPS-based positioning systems because the airship network would cover all of Japan. The accuracy of the pseudolite positions would be a limiting factor for such a service since the pseudolite ‘ephemeris error’ is more serious than GPS due to the lower height of the platform. In this paper, a conceptual design of the SPF-based augmentation system is first introduced. Then some schemes for estimating the pseudolite position are described.

Derivation of Ranging Source Integrity Requirements for the Local Area Augmentation System (LAAS)*

: Several satellite latent fault modes can cause errors that may not be removed by pseudorange corrections or detected unless special augmentation-based integrity monitors are employed. The error classes resulting from satellite fault modes are distorted signal waveform, uncorrectable ephemeris error, excessive clock acceleration, excessive code-carrier divergence, and low signal power. This paper derives the integrity monitoring parameters for these errors through mathematical analyses and error-bounding rationales. The integrity parameters are the maximum allowable error in the pseudorange corrections and the probability of missed detection of this error. The inputs to the analyses are the integrity monitoring operational requirements contained in the Minimum Aviation System Performance Standards for the Local Area Augmentation System (LAAS). The results of the analyses are used as the basic performance requirements for ranging source integrity monitoring in the FAA's LAAS Ground Facility specification. The paper includes model derivations and an example application to the signal waveform distortion error class.

Orbit and Clock Error Estimation for ARWAAS

The ARWAAS is a Wide Area Augmentation System to be built around GPS constellation for improving the accuracy, integrity, and availability of the basic GPS signals over the ARab Countries region This system should allow UPS to be used as a primary means of navigation for en-route travel and for Category I approaches to selected airports throughout the region. This system would be extended to cover the Middle East region as well as most of the African continent. The system inherently should correct for ephemeris error (through satellite orbit estimation) and some other sources of errors.In this paper, several techniques for GPS satellite orbit estimation (Non-dynamic, Kinematic, and Dynamic) for ephemeris error calculation are considered. Moreover, the problem of clock error estimation and reduction is investigated. A Least-Square algorithms developed for predicting the system performance under an initial layout for the ARWAAS so as to evaluate the anticipated performance.

Ionosphere Effects at the Nanosecond Level Observed in Common-View Time Transfer

The single-channel “common-view” GPS time-transfer technique is frequently used to minimize the inaccuracy caused by satellite clock errors, including Selective Availability (SA). Even when carefully conducted, these measurements are subject to receiver noise, satellite ephemeris errors, multipath effects, and variable propagation delays. Many of these errors can be reduced if continuous measurements are made on all of the satellites that are in common view. This paper reports results of using this method with multichannel, modular, L1 C/A-code time receivers over a 4,000 km east-west baseline. These results show measurement noise at a level of about 1 ns for averaging times of between 100 s and 2 days without postprocessing. In addition, significant diurnal effects at the level of up to 15 ns peak-to-peak are often observed. These effects appear to be due to differences between the actual ionospheric delays and the predictions of the GPS built-in model. Techniques for mitigating these errors are discussed.

Hughes aircraft's architectural design of the federal aviation administration wide-area augmentation system: An international system

Hughes Aircraft is currently developing under contract with the Federal Aviation Administration (FAA) a GPS-based navigation system that is intended to become the primary navigational aid for commercial aviation during all phases of flight—from enroute through Category I precision approach. This innovative system, named the Wide-Area Augmentation System (WAAS), will make use of a network of reference stations distributed throughout the U.S. National Airspace System. These reference stations will collect GPS measurements and send them to master stations. The master stations will process the data to provide correctional information for each GPS satellite. This information will include as separate components the GPS ephemeris errors, satellite clock bias and ionospheric estimation data. The corrections will be sent to the users by means of a Geosynchronous Earth Orbit (GEO) satellite using a specific signal and data format. WAAS is first expected to provide supplemental radio navigation, and eventually to become the primary system of navigation. The system will add the following features to the current GPS system: a GEO ranging function that will improve availability and reliability; differential GPS corrections that will improve accuracy; and integrity monitoring that will provide and enhance safety. To meet the requirements associated with a primary navigation system, WAAS should be able to provide fault-free position fix with a time availability of 0.999 for Category I approaches, and 0.99999 for domestic enroute, terminal and nonprecision approach phases of flight. This paper presents the improvements that this augmentation system offers to stand-alone GPS in order to become the primary navigational system in the U.S. The paper also covers the features that make this wide-area system a naturally expandable system worldwide that will allow the integration of future local-area differential systems across the world.

Description and Assessment of Real-Time Algorithms to Estimate the Ionospheric Error Bounds for WAAS*

The Federal Aviation Administration (FAA) is developing a Wide-Area Augmentation System (WAAS) to augment GPS for use during en route through Category I (CAT I) precision approach flight operations. WAAS has three components : (1) extra ranging sources using geostationary earth orbiting (GEO) communications satellites ; (2) a vector of corrections to the GPS signal-in-space, including components for ionospheric delay estimations and clock and ephemeris errors ; and (3) integrity monitoring to alert users of out-of-tolerance conditions. This paper describes real-time ionospheric algorithms that are part of the accuracy and integrity monitoring functions. A performance assessment is made with respect to the requirements specified in the WAAS procurement.

Communication and radio determination system using two geostationary satellites. II. Analysis of positioning accuracy

A new communication and radio determination system has been developed and experiments to evaluate the positioning accuracy of the system have been made. Theoretical estimates of the positioning accuracy and the results of experimental data analysis are presented. The relationship between the positioning error and the satellite ephemeris error while considering the dynamics of orbital motions are the main topics of discussion. Estimated improvements in positioning accuracy using benchmark stations are also described.

Weighted differential GPS method for reducing ephemeris error

Weighted differential GPS method for reducing ephemeris error

GPS positioning accuracy using precise real‐time ephemeris and clock correction

Global positioning system (GPS) has found applications in various areas including marine geodesy. GPS positioning accuracy, however, is greatly degraded by GPS ephemeris and clock errors, particularly errors due to Selective Availability (SA). Thus, it is crucial to use precise ephemeris and clock corrections for users who require high position accuracy. Presently, precise ephemeris and clock corrections are available only in post‐mission. This paper investigates the generation of precise real‐time ephemeris and clock corrections and the positioning accuracy using them. In this research, precise real‐time ephemeris is generated from accurate dynamic orbit prediction and clock corrections are calculated using instantaneous GPS measurements. Numerical analysis using data from an actual GPS tracking network is performed that indicates use of precise ephemeris and clock corrections can improve the positioning accuracy to the one meter level. This accuracy is attainable in real‐time as the precise real‐time ep...

Radar ranging to Ganymede and Callisto

Arecibo observations from 1992 February to March have yielded the first successful radar range measurements to the Galilean satellites. Round-up time delays were measured for Ganymede and Callisto with accuracies of 20 to 50 micrometer (3 to 7 km) and 90 micrometer (14 km), respectively. Both satellites showed round-trip delay residuals (relative to the E-3 ephemeris) of about a millisecond, most of which can be attributed to errors in the predicted along-track positions (orbital phases). Using a simple model that assumed that all of the ephemeris error was due to constant orbital phase and Jupiter range errors we estimate that Ganymede was leading its ephemeris by 122 +/- 4 km, Callisto was lagging its ephemeris by 307 +/- 14 km, and Jupiter was 11 +/- 4 km more distant than predicted by the PEP740 planetary ephemeris.