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ПОВЫШЕНИЕ ТОЧНОСТИ DGPS и RTK ДЛЯ GPS-СЕТИ
Фавзи Зарзура
Сибирская государственная геодезическая академия, 630108, Россия, г. Новосибирск,
ул. Плахотного, 10, аспирант кафедры высшей геодезии, тел. (383)343-29-11, e-mail: fawzyhamed2011 @yahoo. com
Борис Тимофеевич Мазуров
Сибирская государственная геодезическая академия, 630108, Россия, г. Новосибирск,
ул. Плахотного, 10, кафедра высшей геодезии, профессор, д.т.н., тел. 3432911, e-mail: btmazurov@mail. ru
Быстрое и точное относительное позиционирование для базовых линий становится возможным при использовании двухчастотной GPS-аппаратуры. Несмотря на то, что для повышения точности путем уменьшения ошибок используется технология DGPS, она не может исключить орбитальные ошибки, а также ошибки, вызванные ионосферой и тропосферой [1]. В данной статье описано применение метода точного относительного позиционирования для обработки данных GPS (преимущества и недостатки) и выполнено сравнение с результатами измерений тех же самых точек сети постоянных базовых станций в Мекке.
Приведены выводы о повышении точности измерений DGPS при использовании точных эфемерид международной службы IGS сети с использованием модели ионосферы Клобучара и модели тропосферы Хопфилда или Саастамойнена.
Ключевые слова: GPS-сеть, точное позиционирование, уменьшение влияния ошибок.
ACCURACY ENHANCEMENT OF DGPS AND RTK FOR GPS NETWORK
Fawzy Zarzoura
Siberian State Academy of Geodesy (SSGA), 630108, Russia, Novosibirsk, 10 Plakhotnogo Ul., aspirant, department of high geodesy, tel. 3432911, e-mail: fawzyhamed2011@yahoo.com
Boris T. Mazurov
Siberian State Academy of Geodesy (SSGA), 630108, Russia, Novosibirsk, 10 Plakhotnogo Ul., professor, department of high geodesy, tel. 3432911, e-mail: btmazurov@mail.ru
This paper outlines the use of accurate relative positioning for processing GPS data and compares the results with relative positioning in the same point of Mecca permanent GPS observation network.
Key words: DGPS, RTK, Klobucher model, Hopfield, Saastimoinen and IGS.
Our research is focused on improving the accuracy of differential GPS and Real Time Kinematic (RTK) observations using wide area GPS systems. Differential GPS (DGPS) and Real Time Kinematic (RTK) are observation techniques that can be used to remove or reduce the ionosphere effects arising in ordinary GPS [2] .In order to obtain precise coordinates of points from GPS data, a number of nuisance parameters first needs to be removed from the data. These may be classified as satellite errors, atmospheric errors, and receiver errors [8].
Observations Methodology
This procedure involves four observables for each of the visible satellites in each epoch. The two pseudo range and carrier phase observables can be linearly combined, thus reducing the effects of the ionosphere refraction. The use of a troposphere model, together with parameterization techniques, can reduce the troposphere refraction effects [11]. It is possible to obtain precision of a few millimeters and a few centimeters in the horizontal and vertical components, respectively. Such levels of accuracy can be obtained for static point position, using a period of 24 hours of data. Once the coordinates for all stations are daily estimated, a solution for a specific epoch/can be obtained, as there is no correlation between the coordinates of different stations, such a solution may be obtained independently for each station [7].
Observation Sites and used Instruments
The location for the proposed GPS network is shown in Figure (1). A pilot network has been established over the Holly Mecca. The system involves permanently running GPS reference stations, at spacing up to 30 km, then feeding GPS data to a central processing computer. Five LEICA GPS SR530 dual frequency receivers collected the GPS data on 12 th February 2006, where point (G182) was a reference point of the whole DGPS work. At first, the static observations with rate in legal two seconds are performed. Four receiver of the same LEICA type is setup at the other points for more than 24 hours. The Reference Stations are designed to support high-precision positioning over a wide area.
G164
Figure (1): The shape of Mecca network
The weighted average position of points obtained from the solution of points in code- phase solution with Hopfield troposphere model, Klobucher Ionosphere model, precise orbit and mask angle 15°. This value is to be adopted as the position to be used as a reference to test the accuracy and precision in all subsequent investigations [9].
Table (1): The two reference solutions, namely Code solution and Code-Phase solution
point Code solution Code (ac -phase solution opted value) The difference
East (m) North (m) Ht (m) East (m) North (m) Ht. (m) AE (m) AN (m) AH (m)
G079 604847.109 2358799.827 344.232 604847.605 2358799.313 344.1557 -0.4975 0.5137 0.0758
G097 581954.337 2379143.209 265.318 581954.305 2379142.648 264.7447 0.0319 0.5618 0.5729
G164 584332.702 2353111.952 222.583 584333.051 2353112.241 222.2416 -0.3488 -0.2893 0.3416
G182 reference 603577.634 2377008.347 418.155 603577.634 2377008.347 418.1548 0.0000 0.0000 0.0000
M305 584002.934 2366022.109 273.465 584003.240 2366022.323 273.1244 -0.3053 -0.2142 0.341
Observations Analysis
Leica Geostationary Office programme ( LGO ) is used for analysis the data . The software is particularly well suited for the rapid processing of small size single and dual frequency surveys, permanent network processing, ambiguity resolution on long baselines, ionosphere and troposphere modeling, clock estimation and time transfer, combination of different receiver types, simulation studies, orbit determination and estimation of Earth rotation parameters and the generation of so called free network solutions [6]. The results and analysis of observations will be introduced into three steps as following:
Orbital errors
To study the effect of satellite position on the solution, a process of Mecca Code-Phase observations have been done twice. Every run utilized the same processing parameters except that the first run used the broadcast ephemeris, and the second run used the precise ephemeris as produced by International GPS Service “IGS”. The differences between the resulted coordinates [10].
As it is demonstrated in table (2), the horizontal Position coordinates varies in a wide range from 0.53 mm to 0.605 mm. The range for height is varies from 0.83 mm to 1.94 mm. As a closing remark for this section, on can easy detect the contribution of how can precise ephemeris improve the solution against the broadcast solution.
Table (2): The differences between the default code- phase solution and the code-phase solution by replacing the orbit model to precise model at Mecca network
Point
AE(mm) AN(mm) AH(mm)
G-079 0.23 0.51 1.4
G-097 0.13 0.45 0.83
G-164 0.52 0.54 0.98
G-182 0.00 0.00 0.00
M-305 0.71 0.69 1.94
Ionosphere errors
To study the effect of ionosphere error on the solution, a process of Mecca Code-Phase observations have been done several times, in this observation using of klobucher ionosphere model with the adopted values [S]. Every run utilized the same processing parameters except that the first run utilized an ionosphere model from the following models [5]:
• Computed Model
• Standard
• Global/Regional
Table (3): The differences between the default solution and the other models solution by changing the ionosphere model at Mecca network
point AE(mm) AN(mm) AH(mm)
The differences between the G-079 0.00 0.00 0.01
default solution and the G-097 0. 01 0.00 -0. 03
solution by replacing the G-164 0.00 0.00 0.00
ionosphere model to computed G-1S2 0.00 0.00 0.00
model M-305 0. 11 -0. 11 0. 04
The differences between the G-079 0.00 0.00 -0. 01
default solution and the G-097 0.00 0.00 0. 01
solution by replacing the G-164 0.01 -0. 03 -0. 1
ionosphere model to standard G-1S2 0.00 0.00 0.00
model M-305 -0. 22 0. 04 -0. 11
The differences between the G-079 0.00 0.00 0.00
default solution and the G-097 0.00 0.00 0. 01
solution by replacing the G-164 0.00 0.00 0.00
ionosphere model to global G-1S2 0.00 0.00 0.00
model M-305 0. 34 0. 14 0. 13
As it is indicated in table The coordinates vary in a clear range from sub millimeter with respect to all types of Ionosphere models but for Then the use of Klobucher model rather than other Ionospheres models as the most common ionosphere model used.
Troposphere errors
To study the effect of tropophere error on the solution, a process of Mecca Code-Phase observations have been done several times. Every run utilized the same processing parameters except that the first run utilized a troposphere model from the following models [3]:
• Simplified Hopfiled Model
• Saastimoinen Model
• Essen & Froome Model
• No Troposphere Model
The differences between the resulted coordinates for each used troposphere model and the original values are depicted in table 4
Table (4): The differences between the default code- phase solution and the code-phase solution by replacing the troposphere models at Mecca network
point AE(mm) AN(mm) AH(mm)
The differences between the default code- phase solution and the codephase solution by replacing the troposphere model to simplified Hopfield G-079 -1.0 0.4 9.6
G-097 0.2 0.5 20.3
G-164 0.1 1.0 26.3
G-182 0.0 0.0 0.0
M-305 0.2 0.6 19.1
The differences between the default code- phase solution and the codephase solution by replacing the troposphere model to Saastamoinen model G-079 -0.1 0.0 0.3
G-097 0.1 0.1 0.7
G-164 0.1 0.1 1.0
G-182 0.0 0.0 0.0
M-305 0.0 0.1 0.7
The differences between the default code- phase solution and the codephase solution by replacing the troposphere model to Essen and Froome model G-079 0.0 -0.2 -7.4
G-097 0.1 -0. 3 -16.2
G-164 0.1 -0. 6 -21.1
G-182 0.0 0.0 0.0
M-305 0. 2 -0. 3 -15.6
The differences between the default code- phase solution and the codephase solution by replacing the troposphere model to no troposphere model G-079 -13.6 25.9 -83.4
G-097 13.5 -5.6 -143.7
G-164 4.5 20.2 -217.8
G-182 0.0 0.0 0.0
M-305 16.7 20.0 -132.9
The differences between the Simplified Hopfield troposphere model values and the computed values are ranging between -0.1mm and -0.2mm in east component, 0.5mm to 1.0 mm in north component and ranging between 9.6 to 26.3 mm in height component.
The differences between the Saastimoinen troposphere model values and the computed values are ranging between 0.3mm and 1.0 mm in height component and 0.1mm differences in east and north component.
The differences between the Essen & Froome troposphere model values and the computed values are ranging between 0.1mm and 0.2mm in east component, -0.2 mm to -0.6 mm in north component and ranging between -0.7 to -21.1 mm in height component.
The differences between the computed values and the values, which no troposphere model used, are ranging between -13.6mm and 16.7mm in east component and -5.6 mm to 25.9 mm on north component and in height component ranging between -83.4 to -217.8 mm.
Finally, it can be easy to detect that the effect of troposphere models on the height component has the greatest effect than on the east and north component. The difference between the troposphere models was very small but it was high if no
troposphere model used, so that the use of any troposphere model is better than no model used.
Conclusions
In this research 5 stations of Holly Mecca were processed with Leica Geostationary Office software (LGO) and compare the result with those obtain in ITRF2000 with. The result shows the general agreement. Solution is better than 40mm for daily solutions, and the repeatability is about 20mm, 35mm, 45mm for N,E,H components. The difference between the coordinates and those obtain in relative mode in ITRF 2000 are due to the ambiguity resolution and combination of solution in software. The difference between baseline computed and relative mode is better than 30 mm. The use of precice ephemirace rather than broadcast ephemirace, Klobucher ionosphere model, and Hopfield or saastimoinen troposphere model would give an apperciable improvement for all baselines. Also, The troposphere models have the same effect on the all observation teqniques , the Hopfield model give the same results with the Saastimoinen model as addition of midel result values between the Simplified Hopfield model and Essen & Froome model.
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© Ф. Зарзура, Б. Т. Мазуров, 2013
