ارزیابی کارایی تعیین موقعیت مطلق دقیق به عنوان جایگزینی برای تکنیک های نسبی

در دهه اخیر استفاده از شیوه تعیین موقعیت مطلق دقیق (precise point positioning) در کاربردهای استاتیک و کینماتیک ماهواره‌ای متداول شده و چندین نرم‌افزار و سایت‌های پردازش برخط متعدد برای این منظور تولید شده‌اند. در این تکنیک از مشاهدات عاری از اثر یونسفر کد و فاز یک گیرنده دو فرکانس به همراه محصولات دقیق مدار و ساعت ماهواره که از سایت‌های مراکز مختلف igs در دسترس‌اند، استفاده می‌شود. در نتیجه اگرچه در ppp به طور مستقیم نیازی به استفاده از مشاهدات ایستگاه‌های مرجع نیست، اما همچنان وجود یک شبکه مبنایی، حتی با فواصل بلند مانند شبکه igs برای تولید محصولات دقیق مدار و ساعت ماهواره نیاز خواهد بود. همچنین استفاده از این شیوه تعیین موقعیت در کاربردهای مختلف، نیازمند داشتن دانش کافی از دقت، صحت و روش اجرای آن است. در مقاله حاضر، مشاهدات استاتیک چهار گیرنده gps دو فرکانس از شبکه ایستگاه‌های دائمی ایران و مشاهدات کینماتیک گیرنده gps مستقر روی هواپیما، با استفاده از نرم‌افزار بِرنیز (bernese) و به دو شیوه مطلق دقیق و نسبی با استفاده از ایستگاه‌ها و محصولات دقیق igs پردازش شده و نتایج آنها با هم مقایسه شده است. همچنین به منظور تعیین مدت زمان بهینه جمع آوری مشاهدات برای رسیدن به دقت بهتر از ده سانتیمتر در حالت استاتیک، مشاهدات ایستگاه‌های دائمی در هر مرحله با اضافه کردن یک ساعت به مشاهدات قبلی پردازش شدند. در نهایت مشخص شد که با داشتن مشاهدات gps به مدت زمان بیش از یک ساعت، رسیدن به دقت بهتر از ده سانتیمتر در حالت استاتیک تضمین شده و با استفاده از تکنیک ppp دقت مشاهدات کینماتیک به طور متوسط و در مقایسه با حالت نسبی بهتر از ده و بیست سانتیمتربه ترتیب در حالت دو بعدی و سه بعدی است که می‌تواند در بسیاری از کاربردهای نقشه‌برداری مانند هیدروگرافی و فتوگرامتری کافی باشد.

Performance assessment of PPP as an alternative for relative techniques

Extended Abstract Precise Point Positioning (PPP) is a technique to determine the position of a single receiver using undifferenced dualfrequency code and carrier phase observations. In this technique, the precise satellite orbit and clock products obtained from the GPS reference station network are also required. Unlike the relative positioning techniques, the network needed for PPP is not necessary to be dense, and even a sparse network with long baselines like the International GNSS Service (IGS) network can be used. The IGS collects, archives, and distributes GPS observation data sets of sufficient accuracy to satisfy the objectives of a wide range of applications and experimentation. These data sets are used by the IGS to generate the data products which are made available to interested users through the IGS website. Moreover, in contrast to the relative positioning techniques, PPP can provide a uniform accuracy throughout the world without having the reference station observations. In the last decade, PPP has been widely used for the static and kinematic applications. The use of this technique in various applications requires to know its accuracy, processing software requirements, and performing methods. The aim of this paper is to study the performance of PPP by using the static and kinematic observations in comparison with the double difference relative solutions. For this purpose, the static observations of four dualfrequency receivers within Iranian Permanent GNSS Network (IPGN), namely AHVA, SFHN, SNDJ and TORQ, and the kinematic observations of GPS receiver installed on airplane were processed in the PPP and double difference relative solutions by the Bernese GNSS software version 5.0. The Bernese GNSS Software is a scientific, highprecision, GNSS data processing software developed at the Astronomical Institute of the University of Bern (AIUB). It is, e.g., used by Center for Orbit Determination in Europe (CODE) for its international (IGS) and European activities. In the double difference relative solution, the coordinates of 10 IGS stations in ITRF2008, which have been located around Iran, have been chosen as the weighted constraints, where the accuracy of constraints for horizontal and vertical components has been taken equal to 1 mm and 2 mm, respectively. The double difference relative results are assumed as reference values for comparisons. To find the optimum time interval of PPP for obtaining the accuracy better than 10 cm in the horizontal and vertical components, the various sessions have been taken in to account. The GPS station observations of each session are separately processed by the Bernese software in the PPP mode regarding the required parameters such as solid earth tide, ocean tidal loading, windup, antenna phase center offsets and variations for satellites and receivers, and satellite Differential Code Biases (DCBs). Then, the double difference relative results as reference values are subtracted from the obtained PPP results in X, Y and Z coordinates. To show the performance of PPP in both of horizontal and vertical components, the coordinate differences from Earth Centered Earth Fixed (ECEF) reference frame are transferred to the Local Geodetic (LG) reference frame in order to provide Northing (N), Easting (E) and Up (U) coordinates. From the PPP static results, we find that the minimum required time interval of the GPS observations is one hourin order to obtain the accuracy better than 10 cm. For assessment of the PPP performance in kinematic mode, the GPS observations collected by mounted GPS receiver on airplane are processed in relative and PPP modes. The duration of these observations is about 6 hours. In the relative kinematic processing by the Bernese software, the observations of 4 GPS reference stations within IPGN and IGS precise satellite orbit and clock products are used. The outputs of this step are three coordinates of GPS antenna mounted on airplane in 30second epochs, which are considered as reference values. Like the static mode, the reference values are subtracted from the PPP kinematic results in X, Y and Z coordinates and transferred to the LG frame. The results show that the accuracy better than 10 cm and 20 cm can be obtained using the PPP kinematic technique in the horizontal and vertical components, respectively. These accuracies are enough for many applications such as hydrography, aerial photogrammetry and navigation. As a result, this study shows that the PPP technique can be an adequate alternative for the relative techniques.