US Global Positioning System Research Articles

Bounding higher‐order ionosphere errors for the dual‐frequency GPS user

Civil signals at L2 and L5 frequencies herald a new phase of Global Positioning System (GPS) performance. Dual‐frequency users typically assume a first‐order approximation of the ionosphere index of refraction, combining the GPS observables to eliminate most of the ranging delay, on the order of meters, introduced into the pseudoranges. This paper estimates the higher‐order group and phase errors that occur from assuming the ordinary first‐order dual‐frequency ionosphere model using data from the Federal Aviation Administration's Wide Area Augmentation System (WAAS) network on a solar maximum quiet day and an extremely stormy day postsolar maximum. We find that during active periods, when ionospheric storms may introduce slant range delays at L1 as high as 100 m, the higher‐order group errors in the L1–L2 or L1–L5 dual‐frequency combination can be tens of centimeters. The group and phase errors are no longer equal and opposite, so these errors accumulate in carrier smoothing of the dual‐frequency code observable. We show the errors in the carrier‐smoothed code are due to higher‐order group errors and, to a lesser extent, to higher‐order phase rate errors. For many applications, this residual error is sufficiently small as to be neglected. However, such errors can impact geodetic applications as well as the error budgets of GPS Augmentation Systems providing Category III precision approach.

On Real-Time High Precision Velocity Determination for Standalone GPS Users

The NAVSTAR Global Positioning System (GPS) is a satellite-based radionavigation system designed for positioning, velocity determination and timing. The relative movement between a GPS receiver and a GPS satellite causes the received signal frequency to differ from the transmitted frequency due to the Doppler effect. This frequency difference, an observable in GPS measurements, is referred to as the Doppler shift. However, the Doppler shift is biased by the inherent errors in the signal propagation. Since velocities of GPS satellites are known, the velocity of a user can be determined through observing four or more satellites, similar to GPS positioning.When the GPS selective availability (SA) was activated, errors imposed on the ephemeris and satellite clocks significantly affected the accuracy of both GPS positioning and velocity determination. The removal of SA has enabled a significant improvement on the accuracy of both GPS satellite orbits and clocks. As a result, velocity accuracy at sub-centimetre per second level is achievable for standalone GPS users if all the errors associated with the Doppler observables are corrected for properly.This paper overviews the developments in precise velocity determination and investigates all error sources associated with the Doppler measurement. The properties of errors in the Doppler shift are analysed and the methods to eliminate or mitigate these errors are discussed. The relativistic errors such as the orbit eccentricity and the Sagnac effect are derived and formulated in easy forms for correction. Algorithms for modelling the atmosphere delay rates are also presented. It is concluded that real-time velocities in millimetres per second level are achievable if the error correction schemes provided are used.

Space Weather and the Global Positioning System

Space Weather and the Global Positioning System

Ionospheric scintillation effects on single frequency GPS

Ionospheric scintillation of Global Positioning System (GPS) signals threatens navigation and military operations by degrading performance or making GPS unavailable. Scintillation is particularly active within, although not limited to, a belt encircling the Earth within 20 degrees of the geomagnetic equator. As GPS applications and users increase, so does the potential for degraded precision and availability from scintillation. We examined amplitude scintillation data spanning 7 years from Ascension Island, U.K.; Ancon, Peru; and Antofagasta, Chile in the Atlantic/American longitudinal sector as well as data from Parepare, Indonesia; Marak Parak, Malaysia; Pontianak, Indonesia; Guam; and Diego Garcia, U.K. in the Pacific longitudinal sector. From these data, we calculate percent probability of occurrence of scintillation at various intensities described by the S4 index. Additionally, we determine Dilution of Precision at 1 min resolution. We examine diurnal, seasonal, and solar cycle characteristics and make spatial comparisons. In general, activity was greatest during the equinoxes and solar maximum, although scintillation at Antofagasta, Chile was higher during 1998 rather than at solar maximum.

Ionospheric Delay Estimation for Improving the Global Positioning System Position Accuracy

The ionospheric delay is one of the largest sources of error that affects the Global Positioning System (GPS) positioning accuracy. By combining simultaneous measurements of signals at two different frequencies the effect can be minimized due to the dispersive nature of the ionosphere. But it remains a problem for single-frequency GPS receiver users. GPS signal propagation speed in the ionosphere is characterized by its refractive index. The phase and group refractive indexes are derived to calculate the ionospheric effect on the GPS signal measurement. Several electron density models such as Bent and IRI are available for the study of electron density profiles at different altitudes of the ionosphere. Klobuchar model is the only model available for estimating the ionospheric time delay for single frequency GPS users. The Klobuchar model is a simple-computational model with an ideal description for the ionosphere’s average behaviour. In dual frequency receivers, ionospheric delay can be estimated precisely by taking the advantage of the dispersive nature of the ionosphere. The delay is estimated by measuring the difference in arrival times of the two GPS frequencies. In this paper Total Electron Content (TEC) values are presented for a typical day using phase and code measurements of a dual frequency GPS receiver located at Hyderabad, India and using the Klobuchar model also. The results are analyzed and presented.

Wayfinding with a GPS-based mobile navigation system: A comparison with maps and direct experience

This study examined the effectiveness of a Global Positioning System (GPS)-based mobile navigation system in comparison to paper maps and direct experience of routes, by focusing on the user's wayfinding behavior and acquired spatial knowledge. Based on information received from one of these three media, participants walked six routes finding the way to goals. Results showed that GPS users traveled longer distances and made more stops during the walk than map users and direct-experience participants. Also, GPS users traveled more slowly, made larger direction errors, drew sketch maps with poorer topological accuracy, and rated wayfinding tasks as more difficult than direct-experience participants. Characteristics of navigation with these three learning media and possible reasons for the ineffectiveness of the GPS-based navigation system are discussed.

Methods to Convert Local Sampling Coordinates into Geographic Information System/Global Positioning Systems (GIS/GPS)–Compatible Coordinate Systems

Abstract Laying out a sampling transect in the field is a common task when researching natural systems and resources. With widespread availability of global navigation satellite systems (GNSS), such as the US global positioning system (GPS), it is becoming more common to resurvey legacy transects to establish them in globally referenced coordinate systems such as geodetic latitude/longitude or planimetric systems such as the Universal Transverse Mercator (UTM) or the State Plane Coordinate System (SPCS). Transforming local coordinates into a globally referenced coordinate system allows (1) disparate legacy surveys to be combined into a common geographic information system (GIS) database, (2) new GPS measurements to be incorporated into that same database, and (3) GPS-based navigation to be used for plot establishment and resampling. This article presents the mathematics necessary to determine the globally referenced planimetric coordinates of established linear, rectangular, or nominally rectangular transects (such as a rhombus) using formulas that are easily implemented on a spreadsheet. In addition, methods are given to determine the planimetric coordinates of new transects.

In-Flight GPS-Signal-Reception Anomalies of Helicopters

I T IS difficult to find appropriate installation points for global positioning system (GPS) antennas on helicopters, because structures such as rotors and engines that are located above the fuselage may degrade GPS positioning accuracy. Even for fixedwing airplanes that have no such obstacles on the fuselage, an inflight GPS antenna performance study has revealed that large bias errors can be caused by distortions in the GPS antenna pattern due to other antennas nearby [1]. The effects of such problems are not so great when GPS usage is limited to en-route navigation. However, they will be more significant in approach and landing with vertical guidance (APV) or precision approaches using a GPS satellite/ ground-based augmentation system (SBAS/GBAS), such as the Federal Aviation Administration’s GPS wide/local area augmentation system (WAAS/LAAS). Although the minimum operational performance standards (MOPS) for WAAS/LAAS airborne equipment [2,3] specify an airborne accuracy model that includes an airframemultipath based on flight test data from large commercial jet transports [4], there is little quantitative flight test data for helicopter GPS antenna performance, and it is thought to be difficult for helicopters to meet the MOPS specifications. This paper presents in-flight GPS-signal-reception anomalies, such as GPS signal masking and multipath that degrade positioning accuracy, observed using the Japan Aerospace Exploration Agency’s (JAXA) research helicopter.

Latest Developments in Network RTK Modeling to support GNSS Modernization

Global Navigation Satellite Systems like the US Global Positioning System GPS and the Russian GLONASS system are currently going through a number of modernization steps. The first satellites of the type GPS-IIR-M with L2C support were launched and from now on all new GPS satellites will transmit this new civil L2 signal. The first launch of a GPS-IIF satellite with L5 support is announced for spring 2008. Russia has started to launch GLONASS-M satellites with an extended lifetime and a civil L2 signal and has announced to build up a full 18 satellite system by 2007 and a 24 satellite system by 2009. Independently of that the European Union together with the European Space Agency and other partnering countries are going to launch the new European satellite system Galileo, which will also provide worldwide satellite navigation service at some time after 2011. As a consequence we can expect to have very heterogeneous receiver hardware in these reference station networks for a transition period which could last until 2015. Network server software computing network corrections will have to deal with an increased number of signals, satellites and heterogeneity of the available data. The complexity but also the CPU load for this server software will increase dramatically. With the increasing number of signals and satellites the demands for the network server software is growing rapidly. The progress on the satellite system side is going hand in hand with the tendency of the customers to operate growing numbers of reference station receivers resulting in higher demands for CPU power. The paper presents a new approach, which allows us to process data from a large number of reference stations and multiple signals via a new federated Kalman filter approach. With the newest improvements in the GLONASS satellite system, more and more Network RTK service providers have started to use GLONASS capable receivers in their networks. Today, practically all service providers, who are using GLONASS, are applying the Virtual Reference Station (VRS) technique to deliver optimized correction streams to the users in the field. Different satellite systems and generations require different weighting in network server processing and receiver positioning. The network correction quality depends very much on the satellite and signal type. New message types have been recently developed providing individualized statistical information for each rover on unmodeled residual geometric and ionospheric errors for GPS and GLONASS satellites. The use of this information leads to RTK performance improvements, which is demonstrated in practical examples.

Pedestrian navigation with high sensitivity GPS receivers and MEMS

Following a brief introduction to pedestrian navigation and dead-reckoning methods, the characteristics of the current global navigation satellite system (GNSS), namely the US global positioning system (GPS), and its suitability for pedestrian navigation are reviewed. Error sources affecting system performance under both line-of-sight and attenuated signal conditions are described. Attenuated signals can now be acquired and tracked under certain indoor environments, but accuracy and other performance measures are degraded. Pedestrian navigation examples under the forestry canopy and indoor are used to show not only performance but also performance variability as a function of the environment. The use of micro-electro-mechanical systems (MEMS) to aid GPS is introduced. Their advantages and limitations are described through the use of an urban canyon test in a city core, a very harsh RF environment due to signal multipath. It is found that although MEMS aiding substantially improve performance, test conditions have a major impact on actual performance. As a consequence, enough test results are not available at this time to fully characterize performance as a function of the environment due to the high variability of the latter. Accuracy, for instance, varied between 15 and 150 m in the few tests deported herein. Finally, a few predictions are made regarding potential future improvements.

Roundabout way of profiling Earth's atmosphere

The COSMIC (Constellation Observing Systems for Meteorology, Ionospheric and Climate) mission was initiated to take measurements of temperature and humidity in the atmosphere by means of a technique called radio occultation. A collaboration between the US and the Taiwanese National Space Organization, the mission involves the launching of six microsatellites aboard a single US Air Force Minotaur rocket. Each microsatellite is equipped with a radio receiver that will pick up signals from the US Global Positioning System. The GPS radio occultation technique used by the COSMIC system is expected to deliver precise measurements since the receivers measure changes in the phase and amplitude of radio signals transmitted by GPS satellites, and not changes in radiant energy. The COSMIC system is able to provide 2500 soundings, distributed uniformly around the globe per day.

A Method for the Ionospheric Delay Estimation and Interpolation in a Local GPS Network

To estimate ionospheric delays from the Global Positioning System (GPS) measurements, satellite and receiver equipment biases have to be modeled. This paper presents a procedure based on the least squares (LS) approach, which implicitly takes into account these equipment biases in the estimation of the ionospheric effect. The second part of this work deals with the interpolation of the ionospheric correction from a permanent GPS network to a single frequency GPS user. The results obtained show that for 10-cm position accuracy the ionospheric delay can be successfully interpolated when the GPS user is within 40 km of the GPS permanent network.

Comparison of atmospheric water vapor over Antarctica derived from CHAMP/GPS and AMSU-B data

The lack of traditional meteorological observations over Antarctica is a major challenge for the operational weather prediction as well as for the application of satellite data. It was shown that the US Global Positioning System can be used to observe the vertically integrated water vapor (IWV) with a high precision using ground-based GPS receivers, e.g. with standard deviations of about 1 kg/m 2 for differences between GPS and radiosondes. Limb sounding observations of the Earth's atmosphere by a GPS receiver on board the Low Earth Orbiter (LEO) CHAMP allow to obtain sufficiently accurate measurements of temperature or water vapor. They are now available for the validation of other spaceborne data. Here we present a comparison between CHAMP/GPS IWV and the IWV obtained from the NOAA-AMSU-B radiometers on board the Polar Operational Environmental Satellite (POES) NOAA-15 over Antarctica. CHAMP allows an adequate homogenity over the entire continent. Since the 14th of May 2001 the CHAMP radio occultation profiles are available. A comparison between the IWV derived from radiances of AMSU-B and from radio occultations of CHAMP over Antarctica shows a low mean difference (−0.08 kg/m 2) and also a low standard deviation (0.79 kg/m 2). Because both datasets are independent this shows that both datasets are valuable sources for the IWV over Antarctica and should be assimilated into numerical weather prediction (NWP) models within the near future.

Precision navigation in European skies

A global system of air traffic control based entirely on global positioning systems, an inevitable idea that has been inching toward realization for more than a decade, came closer on 6 June, with the first transmission of test signals from the European Geostationary Navigation Overlay Service (Egnos). The point of Egnos is to provide error correction to geopositioning signals, relying on dedicated equipment installed on three geostationary satellites and a network of ground reference stations, so that locations can be determined at an accuracy of close to 1 meter. For now, Egnos will error-correct signals from the US Global Positioning System (GPS) and Russia's Global Navigation Satellite System (Glonass). Eventually, though, the expectation is that it will error-correct signals from Europe's own constellation of global positioning satellites, dubbed Galileo. Galileo will rely on 30 low-earth-orbiting satellites, together with ground stations, and is expected to begin transmitting its first test signals in 2006.

Caesium atomic clocks: function, performance and applications

For more than four decades, caesium atomic clocks have been the backbone in avariety of demanding applications in science and technology. Neither satellitebased navigation systems, like the US Global Positioning System, nor thesyntonization of telecommunication networks at the presently prescribed levels,would function without them. Recent years have brought major breakthroughs inthe development, operation and mutual comparison of frequency standardsbased on the same hyperfine transition in caesium as used previously,but now incorporating the technique of laser cooling. Several cold-atomfountains have been developed. Mutual agreement to within about one part in1015has been demonstrated for two of them operated side by side, and also for twooperated simultaneously, in the US and Germany. This paper gives an overview ofcurrently available commercial caesium clocks and primary standards developed innational metrology institutes.

Local Height Determination Using GPS-Monitored Atmospheric Path Delays

National satellite control points, or any fixed points determined rel- ative to a known control point, can serve as monitoring stations when equipped with dual-frequency global positioning system (GPS) receivers. The available mon- itoring station coordinates are used to advantage to derive time-varying ionospheric and tropospheric path delays from the observed carrier phase data. Distinguishing atmospheric parameters from geometric positions is necessary in many cases be- cause the relative tropospheric zenith delays are strongly correlated with the inter- station height differences. In addition, GPS users may work with the single-fre- quency receivers that track satellite signals for only a few minutes at a time. The algorithm in this paper is based on the cosine functions of double-difference carrier phase measurements. After removing the first-order temporal and spatial iono- spheric effects, the relative tropospheric delays are found to have correlation co- efficients of up to 0.85, with the Saastamoinen model delays using some standard values for atmospheric pressure, temperature, and humidity. When comparing the 19 GPS-derived orthometric heights with the independently leveled heights, in a 5.5 3 4.0 km 2 area, the use of GPS-monitored atmospheric path delays led to an improved root-mean-square height error of 61.48 cm.

Base Band Data for Testing Interference Mitigation Algorithms

AbstractDigital signal processing is one of many valuable tools for suppressing unwanted signals or inter-ference. Building hardware processing engines seems to be the way to best implement some classes of interference suppression but is, unfortunately, expensive and time-consuming, especially if several miti-gation techniques need to be compared. Simulations can be useful, but are not a substitute for real data. CSIRO’s Australia Telescope National Facility has recently commenced a ‘software radio telescope’ project designed to fill the gap between dedicated hardware processors and pure simulation. In this approach, real telescope data are recorded coherently, then processed offline. This paper summarises the current contents of a freely available database of base band recorded data that can be used to experiment with signal processing solutions. It includes data from the following systems: single dish, multi-feed receiver; single dish with reference antenna; and an array of six 22 m antennas with and without a reference antenna. Astronomical sources such as OH masers, pulsars and continuum sources subject to interfering signals were recorded. The interfering signals include signals from the US Global Positioning System (GPS) and its Russian equivalent (GLONASS), television, microwave links, a low-Earth-orbit satellite, various other transmitters, and signals leaking from local telescope systems with fast clocks. The data are available on compact disk, allowing use in general purpose computers or as input to laboratory hardware prototypes.

GPS Satellite Interference in Hungary

Civil users of the NAVSTAR Global Positioning System (GPS) in Hungary occasionally experience interference at the 1575.42-MHz GPS signal frequency. As the application of the GPS technique spreads rapidly in our country, radio frequency interference (RFI) should be considered a serious threat. The new geodetic control network (OG-PSH) in Hungary is based on GPS measurements and incorporates more than 1100 sites. The paper reports the experiences gained during the establishment of the network. Interference sources were tracked to ground-based digital data transmissions for telecommunications, which operate mostly in the Western part of Hungary. Telecommunication regulations exceptionally allow such transmissions in specified countries. In order to warn potential GPS users, the interference sources are being mapped.

GPS-based precise orbit determination of the very low Earth-orbiting gravity mission GOCE

A prerequisite for the success of future gravity missions like the European Gravity field and steady-state Ocean Circulation Explorer (GOCE) is a precise orbit determination (POD). A detailed simulation study has been carried out to assess the achievable orbit accuracy based on satellite-to-satellite tracking (SST) by the US global positioning system (GPS) and in conjunction the implications for gravity field determination. An orbit accuracy at the few centimeter level seems possible, sufficient to support the GOCE gravity mission and in particular its gravity gradiometer.

Galileo or for whom the bell tolls

Satellite-based navigation rapidly evolved into an efficient tool extensively used in a wide variety of civilian applications covering numerous modes of transportation, communication, administration, geodesy, agriculture, and many others. The current systems globally available are the US Global Positioning System (GPS) and the conceptually very similar Russian Global Navigation Satellite System (GLONASS). Considering the worldwide applications, GPS clearly predominates over GLONASS. However, GPS and GLONASS are mainly under military control of single nations and, also critical, do not fulfill certain performance requirements of the civil users, especially in terms of safety-critical applications. Thus, augmentations to the current systems and even completely new systems are under investigation. These are usually summarized under the abbreviation Global Navigation Satellite Systems (GNSSs). The various types of GNSS are described where emphasis is put on the future US and European contributions to the second-generation GNSS, i.e., the modernized GPS and the definition of the new European Galileo system. These two systems may be characterized as “compatible competitors”—thus, one might ask for whom the bell tolls.