Wyniki wyszukiwania - BazTech

1

Single-epoch precise positioning using Modified Ambiguity Function Approach

Cellmer S.

Technical Sciences / University of Warmia and Mazury in Olsztyn

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2013

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nr 16(4)

265--280

EN

Single-epoch positioning is a great challenge in recent research related to GNSS data processing. The Modified Ambiguity Function Approach (MAFA) method can be applied to perform this task. This method does not contain a stage of ambiguity resolution. However the final results take into account their integer nature. The functional model of the adjustment problem contains the conditions ensuring the integer nature of the ambiguities. A prerequisite for obtaining the correct solution is a mechanism ensuring appropriate convergence of the computational process. One of such mechanisms is a cascade adjustment, applying the linear combinations of the L1 and L2 signals with the integer coefficients and various wavelengths. Another method of increasing the efficiency of the MAFA method is based on the application of the integer de-correlation matrix to transform observation equations into equivalent, but better conditioned, observation equations. The next technique of improving the MAFA method is search procedure. This technique together with the de-correlation procedure allows to reduce the number of stages of the cascade adjustment and to obtain correct solution even in the case when a priori position is a few meters away from the actual position. This paper presents some problems related to search procedure. The results of single-epoch positioning using improved MAFA method are presented.

2

On-the-fly ambiguity resolution using an estimator of the modified ambiguity covariance matrix for the GNSS positioning model based on phase data

Cellmer S.

Artificial Satellites : Journal of Planetary Geodesy

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2012

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Vol. 47, No. 3

81--90

EN

On-the-fly ambiguity resolution (OTF AR) is based on a small data set, obtained from a very short observation session or even from a single epoch observation. In these cases, a classical approach to ambiguity resolution (e.g. the Lambda method) can meet some numerical problems. The basis of the Lambda method is an integer decorrelation of the positive definite ambiguity covariance matrix (ACM). The necessary condition for the proper performing of this procedure is a positive definiteness of ACM. However, this condition is not satisfied in cases of very short observation sessions or single epoch positioning if phaseonly observations are used. The subject of this contribution is such a case where phase-only observations are used in the final part of the computational process. The modification of ACM is proposed in order to ensure its positive definiteness. An estimator of modified ACM is a good ACM approximation for the purpose of performing the LAMBDA method. Another problem of short sessions (or a single epoch) positioning is the poor quality of the float solution. In this paper, a cascade adjustment with wide-lane combinations of signals L1 and L2 as a method of solving this problem is presented.

3

Using the integer decorrelation procedure to increase of the efficiency of the MAFA method

Cellmer S.

Artificial Satellites : Journal of Planetary Geodesy

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2011

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Vol. 46, No. 3

103--110

EN

The Modified Ambiguity Function Approach (MAFA) is a method of GNSS carrier phase processing. In this method, the functional model of the adjustment problem contains the conditions ensuring the "integerness" of the ambiguities. These conditions are expressed in the form of differentiable function. A prerequisite for obtaining the correct solution is a mechanism ensuring not only the "integerness" of the ambiguity but also appropriate localization of the search space in the place where the ambiguities have correct values. One of such mechanisms is cascade adjustment, applying the linear combinations of the signals L1 and L2 with the integer coefficients and various wavelengths. This paper presents another, independent from the previous, approach to increase the efficiency of the MAFA method. It is based on the application of the integer decorrelation matrix to transform observation equations into equivalent, but better conditioned, observation equations. The transformation matrix is obtained in the well-known ambiguity variance-covariance matrix integer decorrelation process.

4

Comparison of using relative and absolute PCV corrections in short baseline GNSS observation processing

Dawidowicz K.

Artificial Satellites : Journal of Planetary Geodesy

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2011

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Vol. 46, No. 1

19--31

EN

GNSS antenna phase center variations (PCV) are defined as shifts in positions depending on the observed elevation angle and azimuth to the satellite. When identical antennae are used in relative measurement the phase center variations will cancel out, particularly over short baselines. When different antennae are used, even on short baselines, ignoring these phase center variations can lead to serious (up to 10 cm) vertical errors. The only way to avoid these errors, when mixing different antenna types, is by applying antenna phase center variation models in processing. Till the 6th November 2006, the International GNSS Service used relative phase center models for GNSS antenna receivers. Then absolute calibration models, developed by the company “Geo++”, started to be used. These models involved significant differences on the scale of GNSS networks compared to the VLBI and SLR measurements. The differences were due to the lack of the GNSS satellite antenna calibration models. When this problem was sufficiently resolved, the IGS decided to switch from relative to absolute models for both satellites and receivers. This decision caused significant variations to the results of the GNSS network solutions. The aim of this paper is to study the height differences in short baseline GNSS observations processing when different calibration models are used. The analysis was done using GNSS data collected at short baselines moved with different receiver antennas. The results of calculations show, that switching from relative to absolute receiver antenna PCV models has a significant effect on GNSS network solutions, particularly in high accuracy applications.

5

Problem of antenna phase centr variations in satellite levelling

Dawidowicz K.

Acta Scientiarum Polonorum. Geodesia et Descriptio Terrarum

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2011

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Vol. 10, nr 2

17-30

EN

Satellite leveling is the procedure used to determination orthometric (normal) heights, on the base of ellipsoidal heights derived with GNSS techniques and additional information which make possible geoid (quasi-geoid) undulation determination. Geoid to ellipsoid separations can be get from geoid models, which accuracy in last years has significantly grown. However, for accurate determination of orthometric (normal) heights it is also important exact determination of ellipsoidal heights from GNSS measurements, which accuracy degrades a number of factors. One of the most important in heights determining, is antenna phase center variations problem. It is well known that magnitude of antenna phase center variations (PCV) can reach several centimeters. Unfortunately part of so-called commercial GNSS post-processing software does not include corrections to the antenna PCV. The paper presents results of solutions this problem with help of a subroutine which introduces PCV corrections to code and phase observations. This approach has been tested using GPS data at four measurement points. Three different types of antenna were used in observations. Processing GPS observations ware done with Ashtech Solutions and Topcon Tools software. The heights derived with satellite leveling were compared to heights got from geometrical precise leveling. The results of studies on one hand confirmed significant influence of antenna PCV onto exactitude of heights determination as well as usefulness of proposed procedure to introducing correction to GNSS observations.

PL

Niwelacja satelitarna jest procedurą wykorzystywaną do wyznaczania wysokości ortometrycznych (normalnych), na podstawie wysokości elipsoidalnych uzyskanych z pomiarów GNSS oraz dodatkowych informacji, które umożliwiają wyznaczenie przebiegu geoidy (quasi-geoidy). Odstęp między geoidą a elipsoidą może być określony z modeli geoidy, których dokładność w ostatnich latach znacząco wzrosła. Jednak dla dokładnego wyznaczenia wysokości ortometrycznych (normalnych) istotne jest również właściwe wyznaczenie wysokości elipsoidalnych z pomiarów GNSS, których dokładność degraduje szereg czynników. Jednym z istotniejszych przy wyznaczaniu wysokości jest problem zmienności położenia centrum fazowego anteny. Powszechnie wiadomo, że wartość zmian położenia centrum fazowego anteny (Phase Center Variations – PCV) może osiągać kilka centymetrów. Niestety, część tzw. programów firmowych nie zawiera modeli służących do korekty PCV anten. W pracy zaprezentowano wyniki rozwiązania tego problemu z pomocą autorskiego programu, który wprowadza poprawki PCV do obserwacji kodowych i fazowych. Podejście to zostało sprawdzone przy wykorzystaniu obserwacji GPS wykonanych na czterech punktach. Trzy różne typy anten zostały użyte w trakcie pomiarów. Opracowania obserwacji dokonano z użyciem programów: Ashtech Solutions i Topcon Tools. Wysokości uzyskane z niwelacji satelitarnej zostały porównane z wysokościami uzyskanymi z niwelacji precyzyjnej. Wyniki analiz z jednej strony potwierdzają istotny wpływ PCV anteny na dokładność wyznaczenia wysokości oraz z drugiej – przydatność zaproponowanej procedury do wprowadzania poprawek ze względu na PCV do obserwacji GNSS.

6

Antenna phase center variations corrections in processing of GPS observations with use of commercial software

Dawidowicz K.

Technical Sciences / University of Warmia and Mazury in Olsztyn

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2010

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nr 13

120-132

EN

It is well known that magnitude of antenna phase center variations (PCV) can reach several centimeters. Unfortunately part of commercial GPS post-processing software does not include corrections related to antenna PCV. This paper presents a proposal of a subroutine which introduces PCV corrections to code and phase observations basing on ANTEX file and RINEX observations and navigation files. This approach has been tested using GPS data collected at four measurement points. Three different types of antenna were used in observations. Observation processing was done with Ashtech Solutions and TopconTools software. The results of calculations show, that the proposed subroutine can be successfully used in commercial software which does not include such correction algorithms. The subroutine can also serve to verify antenna PCV corrections algorithms in commercial software where there is often no possibility of looking into applied algorithms.

PL

Powszechnie wiadomo, że zmiany położenia centrum fazowego anteny (phase center variations - PCV) mogą osiągać wartość kilkunastu centymetrów. Niestety, część programów komercyjnych do opracowania obserwacji GPS nie zawiera poprawek związanych z PCV. W artykule przedstawiono propozycję podprogramu, który na podstawie pliku ANTEX oraz obserwacyjnych i nawigacyjnych plików RINEX wprowadza poprawki PCV do obserwacji kodowych i fazowych. Podejście zostało przetestowane na podstawie danych GPS zebranych w czterech punktach pomiarowych. W pomiarach wykorzystano trzy różne typy anten. Obserwacje opracowano z wykorzystaniem programu Ashtech Solutions i TopconTools. Wyniki obliczeń dowodzą, że proponowany podprogram może być skutecznie wykorzystany jako uzupełnienie programu firmowego nie mającego takich poprawek. Podprogram może również służyć do weryfikacji zastosowanych poprawek PCV w programach komercyjnych, w których często brakuje wglądu w zastosowane algorytmy.