High precision applications of the global positioning system

The Global Positioning System (GPS) is a constellation of U.S. defense navigation satellites which can be used for military and civilian positioning applications. A wide variety of GPS scientific applications were identified and precise positioning capabilities with GPS were already demonstrated with data available from the present partial satellite constellation. Expected applications include: measurements of Earth crustal motion, particularly in seismically active regions; measurements of the Earth's rotation rate and pole orientation; high-precision Earth orbiter tracking; surveying; measurements of media propagation delays for calibration of deep space radiometric data in support of NASA planetary missions; determination of precise ground station coordinates; and precise time transfer worldwide

Reduction of initial convergence period in GPS PPP data processing

Precise Point Positioning (PPP) has become a popular technique to process data from GPS receivers by applying precise satellite orbit and clock information, along with other minor corrections to produce cm to dm-level positioning. Although PPP presents definite advantages such as operational flexibility and cost effectiveness for users, it requires 15-25 minutes initialization period as carrier-phase ambiguities converge to constant values and the solution reaches its optimal precision. Pseudorange multipath and noise are the largest remaining unmanaged errors source in PPP. It is proposed that by reducing these effects carrier-phase ambiguities will reach the correct steady state at an earlier time, thus reducing the convergence period of PPP. Given this problem, this study seeks to improve management of these pseudorange errors. The well-known multipath linear combination was used in two distinct ways: 1) to directly correct the raw pseudorange observables, and 2) to stochastically de-weight the pseudorange observables. Corrections to the observables were made in real-time using data from the day before, and post-processed using data from the same day. Post-processing has shown 4 7% improvement in the rate of convergence, as the pseudorange multipath and noise were effectively mitigated. A 36% improvement in the rate of convergence was noted when the pseudorange measurements were stochastically de-weighting using the multipath observable. The strength of this model is that it allows for real-time compensation of the effects of the pseudorange multipath and noise in the stochastic model

Signal multipath in high precision GPS surveys

Cilj je ovoga rada pokazati da višestazni GPS signal može značajno utjecati na točnost rezultata GPS mjerenja. GPS prijemnik Ashtech Z Max, najnovije rješenje za mjerenje iz Magellana, rabljeno je u tu svrhu u ometanom području kako bi se ispitalo kolika je točnost i ponavljanje rezultata kod iste konfiguracije satelita i pod istim uvjetima u okolišu premosnika. Z satelitsko praćenje i Advanced Multipath Mitigation Technologies (Enhanced Strobe Correlator (ESC)) i Ashtech Max-Trac GPS Antenna) prijemnika Ashtech Z Max daju najveću točnost u centimetarskoj rezoluciji čak i u uvjetima slabog signala. Sva su mjerenja izvršena u okolišu mosta tijekom dva uzastopna dana. Analizom su uspoređene koordinate točke, locirane u blizini mosta, dobivene istim rješenjima višeznačnosti tijekom dva uzastopna dana. S druge strane, rezultati testiranja GPS prijemnika Ashtech Z Max usporedili su se s rezultatima mjerenja s Total Station kao dodatna provjera kvalitete. Rezultati pokazuju da je Ashtech Z Max tehnologija sigurnija za horizontalne i vertikalne koordinate. Točnost pozicioniranja u centimetarskoj rezoluciji (1 ÷ 2 cm) može se rutinski postići kad se prati dovoljan broj satelita u uvjetima višestaznog prostora.The aim of this paper is to show that GPS signal multipath can significantly influence the accuracy of the results of a GPS survey. For this purpose, Ashtech Z Max GPS receiver, which is the next generation survey solution from Magellan, is used in the obstructed area and investigates the achievable accuracy and repeatability under the same satellite configuration and site condition near a bridge environment. Z tracking and Advanced Multipath Mitigation Technologies (Enhanced Strobe Correlator (ESC)) and Ashtech Max-Trac GPS Antenna) of the Ashtech Z Max ensure the strongest centimetre level position even under weak signal conditions. All of the measurements were performed near a bridge in the two consecutive days. In the analysis, the coordinates of the point, which is located near a bridge, obtained from the same solutions of the ambiguities in the two consecutive days were compared with each other. On the other hand, the results of Ashtech Z Max GPS receiver testing were compared against results from Total Station surveying as a further quality check. The results show that Ashtech Z Max Technology is more stable for the horizontal and vertical coordinates. Positioning accuracy on the centimetre level (1 ÷ 2 cm) can be routinely achieved when observing sufficient number of satellites in the multipath environment

Evaluating the differences and accuracies between GNSS applications using PPP

Global Navigation Satellite Systems (GNSS) are satellite systems with global coverage. There are currently several GNSS systems in operation today including the United States NAVSTAR Global Positioning System, Russian GLONASS, Chinese Beidou and the European Union’s Galileo system. The Galileo and Beidou systems are currently undergoing upgrading in order to achieve more sustainable and comprehensive worldwide exposure, ultimately providing users with a broader option of systems and wider more reliable coverage. In recent years, in addition to the GPS constellation, the ability to utilise extra satellites made available through the GLONASS and Beidou systems has enhanced the capabilities and possible applications of the precise point positioning (PPP) method. Precise Point Positioning has been used for the last decade as a cost-effective alternative to conventional DGPS-Differential GPS with an estimated precision adequate for many applications. PPP requires handling different types of errors using proper models. PPP precision varies with the use of observations from different satellite systems (GPS, GLONASS and mixed GPS/GLONASS/Beidou) and the duration of observations. However, the fundamental differences between GPS, GLONASS, Beidou and Galileo and the lack of a fully tested global tracking network of multi-Global Navigation Satellite Systems necessitate the evaluation of their combined use. More studies are required in order to confirm the reliability and accuracy of the results obtained by the various methods of PPP. This is outside the scope of this paper. This research paper will evaluate and analyse the accuracy and reliability between different GNSS systems using the Precise Point Positioning technique with emphasis on the function and performance of single systems compared with combined GNSS systems. A methodology was designed to ensure accurate and reliable results have been achieved. Solutions generated from identical data will be compared for bias, accuracy and reliability between single standalone GPS and combined GNSS systems. This study focused on the performance of these systems over a twenty four hour observation period, decimated into 1, 2, 6, 12 and 24 hours. The study found that the reliability and performance of GNSS systems over standalone GPS was insignificant over a twenty four hour period. In fact, where satellite availability and constellation are at a premium, standalone GPS systems can produce equivalent quality results compared with combined GNSS. Having said this, the combined GNSS systems achieved quicker convergence times than standalone systems. With limited access and availability to resources, in particular GNSS receivers, the results can be seen as preliminary testing enhancing the knowledge of GNSS users. Nonetheless, this dissertation covers a wide range of topics and field testing providing relevant reliable data on the accuracy, precision and performance of both standalone and combined Global Navigation Satellite Systems

Factors That Affect the Global Positioning System and Global Navigation Satellite System in an Urban and Forested Environment.

The purpose of this study was to evaluate the accuracy in real time measurements acquired from GPS and GLONASS satellite observations using RTK techniques in an urban and forested environment. To determine this accuracy, 2 data sets of 3-dimensional coordinates were created and compared at 14 stations situated at East Tennessee State University. One data set included coordinates determined by conventional land survey methods; the second was solved by RTK GPS/GLONASS. Once the magnitude of any deviation in the coordinate positions was determined, the contributions to the accuracies from cycle slips, multipath, satellite availability, PDOP, and fixed or float solutions were evaluated. Three points in the urban environment varied from the conventional data set. Multipath was assumed to be the major bias in these points. Seven points in the forested environment varied from the conventional data set. The use of float solutions and high PDOP may have caused this bias

GPS for Marine Navigation and Hydrography

Current marine navigation and shipbome surveying accuracy requirements are reviewed. The technical characteristics of GPS are summarized and its single point positioning performance is given and compared with the above requirements. A detailed description and analysis of the three types of observables possible with GPS, namely code, carrier and Doppler frequency measurements, are presented. The following error sources are discussed: cycle slips, Selective Availability, ionospheric and tropospheric effects and multipath. A description of the various receiver measuring techniques currently available, namely C/A code LI, L2 squaring, L2 codeless, P codeless and P code, is given, together with advantages and disadvantages for marine positioning. The single and double differenced observables used in differential GPS (DGPS) mode are analysed in terms of real time versus postmission suitability. The latest techniques for quasi-instantaneous ambiguity resolution such as wide and extra wide-laning are discussed in terms of receiver requirements and operational procedures. An attempt is made at providing DGPS kinematic accuracy estimates for various cases with and without Selective Availability. Trends and prospects are forecast in the following five areas: system enhancements, user equipment, observable types and modelling, marine applications and GPS-related services

Mass-Market Receiver for Static Positioning: Tests and Statistical Analyses

Nowadays, there are several low cost GPS receivers able to provide both pseudorange and carrier phase measurements in the L1band, that allow to have good realtime performances in outdoor condition. The present paper describes a set of dedicated tests in order to evaluate the positioning accuracy in static conditions. The quality of the pseudorange and the carrier phase measurements let hope for interesting results. The use of such kind of receiver could be extended to a large number of professional applications, like engineering fields: survey, georeferencing, monitoring, cadastral mapping and cadastral road. In this work, the receivers performance is verified considering a single frequency solution trying to fix the phase ambiguity, when possible. Different solutions are defined: code, float and fix solutions. In order to solve the phase ambiguities different methods are considered. Each test performed is statistically analyzed, highlighting the effects of different factors on precision and accurac

Enhancement of the accuracy of single epoch positioning for long baselines with application to structure deformation monitoring

Phd ThesisUsing single-epoch GPS positioning has many advantages, especially when monitoring dynamic targets (e.g. structural movements). In this technique, errors occurring in previous epochs cannot affect the current epoch’s accuracy. However, careful processing is required. This research uses the GPS Ambiguity Search Program (GASP) single-epoch software. Resolving the phase ambiguities is essential in this technique. Some statistical ambiguity resolution functions have been introduced to estimate the best values of these ambiguities. The function inputs are the base station position, the approximate roving receiver position, and the shared GPS phase measurements at both receivers. This work investigates different GPS pseudorange solutions to find the optimal ambiguity function inputs. The noise level in an undifferenced pseudorange coordinate solution is less than in the double-differenced case; thus, using it in the ambiguity function improves the results. Regional correlation between the pseudorange-computed positioning errors exists; therefore, applying a regional filter reduces their effects. Multipath errors approximately repeat themselves every sidereal day in the case of static or quasi-static receivers and applying a sidereal filter mitigates their effects. The IGS ionospheric model reduces the effect of the ionosphere on the GPS phase measurements. Also, a local code-based ionospheric correction model can be generated. Applying these models improves the quality of the phase measurements, which leads to improvement of the ambiguity function outputs. A Kalman filter applied to the code-based ionospheric model further improves the corrected phase measurements. There is a correlation between the ambiguity function outputs’ quality and the phase measurement residuals’ . Applying a threshold filter reduces the probability of obtaining inaccurate results. Data for various baseline lengths, with synthetic displacements added, indicate that the improved GASP results are reliable for monitoring movements exceeding 10 cm for baselines up to 60 km.Aleppo University, Syria, Postgraduate Research Studentship

A demonstration of high precision GPS orbit determination for geodetic applications

High precision orbit determination of Global Positioning System (GPS) satellites is a key requirement for GPS-based precise geodetic measurements and precise low-earth orbiter tracking, currently under study at JPL. Different strategies for orbit determination have been explored at JPL with data from a 1985 GPS field experiment. The most successful strategy uses multi-day arcs for orbit determination and includes fine tuning of spacecraft solar pressure coefficients and station zenith tropospheric delays using the GPS data. Average rms orbit repeatability values for 5 of the GPS satellites are 1.0, 1.2, and 1.7 m in altitude, cross-track, and down-track componenets when two independent 5-day fits are compared. Orbit predictions up to 24 hours outside the multi-day arcs agree within 4 m of independent solutions obtained with well tracked satellites in the prediction interval. Baseline repeatability improves with multi-day as compared to single-day arc orbit solutions. When tropospheric delay fluctuations are modeled with process noise, significant additional improvement in baseline repeatability is achieved. For a 246-km baseline, with 6-day arc solutions for GPS orbits, baseline repeatability is 2 parts in 100 million (0.4-0.6 cm) for east, north, and length components and 8 parts in 100 million for the vertical component. For 1314 and 1509 km baselines with the same orbits, baseline repeatability is 2 parts in 100 million for the north components (2-3 cm) and 4 parts in 100 million or better for east, length, and vertical components