مقايسه ميزان كارآيي داده هاي هواشناسي راديوسوند و ERA-Interim جهت تصحيح تروپوسفري در تعيين موقعيت مطلق دقيق با بكارگيري روش رديابي اشعه سه بعدي

Comparing the Efficiency of Radiosonde and ERA-Interim Meteorological Data in Precise Point Positioning Tropospheric Delay Correction Using Three Dimensional Ray Tracing Method

تعيين موقعيت دقيق نقاط در تمام شرايط آب و هوايي همواره از مهمترين اهداف سيستم­هاي ماهواره­اي ناوبري جهاني (GNSS) بوده است. انكسار امواج ارسالي از ماهواره­ها بدليل عبور از لايه­ هاي مختلف جو به ويژه پايين­ترين لايه آن كه تروپوسفر نام دارد، زمينه­ ساز بروز يكي از مهمترين انواع خطاهاي موجود در تعيين موقعيت مي­باشد. در اين مقاله به مقايسه كارآيي داده ­هاي هواشناسي ERA-Interim و داده ­هاي راديوسوند در افزايش دقت تعيين موقعيت مطلق نقاط با اعمال تصحيحات مربوط به اثرات اين لايه بر مشاهدات GPS پرداخته شده است. بدين منظور با بكارگيري روش رديابي اشعه سه بعدي و انتخاب دو ايستگاه بندرعباس و بيرجند و آماده ­سازي داده ­هاي مذكور در اين مناطق، تصحيحات تروپوسفري با هر دو نوع داده هواشناسي فوق براي سه تاريخ متفاوت محاسبه گرديد. ابتدا با استفاده از نرم ­افزار Bernese موقعيت دو ايستگاه بندرعباس و بيرجند يكبار با مجهول در نظر گرفتن تاخير مربوط به لايه تروپوسفر و محاسبه آن و بار نيز با صرف نظر از آن تعيين گرديد. در ادامه تصحيحات تروپوسفري بدست آمده از روش رديابي اشعه با استفاده از مشاهدات راديوسوند و داده­ هاي هواشناسي ERA-Interim بر فايل مشاهدات GPS اعمال گرديد و تعيين موقعيت در اين دو حالت نيز انجام گرفت. نتايج بدست آمده علاوه بر نشان دادن اهميت خطاي تروپوسفري در محاسبات تعيين موقعيت مطلق، بيانگر آن است كه موقعيت بدست آمده با استفاده از تصحيحات راديوسوند در ايستگاه بندرعباس 0.0162 متر دقيق­ تر از موقعيت بدست آمده با استفاده از تصحيحات داده ­هاي هواشناسي ERA-Interim مي­باشد. در حالي كه نتايج دو نوع داده تفاوت چنداني در ايستگاه بيرجند ندارند. اين موضوع را مي­ توان به تغييرات كم بخار آب و ساير شاخص­ هاي جوي در راستاهاي ارتفاعي و مسطحاتي در ايستگاه بيرجند در مقايسه با ايستگاه بندرعباس نسبت داد.

The Earth’s atmosphere can be described by a model of layers. Although there are also horizontal gradients of the meteorological parameters, it is sufficient for a general model to divide the atmosphere into a vertical layer structure since the vertical gradients of the meteorological parameters are significantly larger than the horizontal ones. Furthermore due to the influence of the gravity the regional horizontal differences become smoothed out towards higher altitudes. The atmosphere can be divided into the troposphere, the stratosphere, the mesosphere, the thermosphere and the exosphere. The tropospheric path delay is one the errors in GNSS observations and reduces the accuracy of GNSS positioning. Accurate estimation of tropospheric path delay in GNSS signals is necessary for meteorological applications. The tropospheric delay is divided into the dry and wet parts. The dry tropospheric delay depends on the pressure variations between satellite and Earth’s surface and can be determined accurately using the Saastamoinen and Hopfield models. The wet delay can be determined by subtracting the dry delay from the total GPS derived delayIn this paper the effect of radiosonde and ERA-Interim data in increasing the accuracy of positioning is compared. European center for medium range weather forecasting (ECMWF) is currently publishing ERA-I, a global reanalysis of the data. This reanalysis provides values of several meteorological parameters on a global ∼75 km. The vertical stratification is described on 37 pressure levels. The piecewise-linear (PWL) is a simple and powerful 2D ray tracing technique which is fast and accurate in processing. The refined piecewise-linear (RPWL) technique is another 2D ray tracing technique. The 3D ray tracing technique based on Eikonal equation is the strongest and newest ray tracing method. These equations are solved in order to get the ray path and the optical path length. The Eikonal equation itself is the solution of the so-called Helmholtz equation with respect to electro-magnetic waves. In this method the ray paths are not limited to a certain azimuthally fixed vertical plane. Tropospheric corrections were calculated using both types of data using 3-D ray tracing method in Bandar Abbas and Birjand stations. The station coordinates were determined using two methods: 1-the tropospheric error was considered as unknown, 2-this error was not considered. Then tropospheric corrections obtained from ray tracing method was applied to the GPS observations and positioning was done. The Bernese GPS software version 5.2 has been used to process the GPS data. It performs ionosphere-free linear combination equation of dual frequency GPS observations from each site within the regional network. The ZTD was calculated using this software including raw data from the GPS observation network and the ZHD was calculated according to the Saastamoinen model. The results indicate the importance of tropospheric correction in precise positioning. In addition, these results indicate that the results obtained using radiosonde corrections in Bandar Abbas are more accurate than the results obtained using ERA-Interim data. Although the results of two types of data in Birjand station do not have much difference. This can be attributed to small variations of water vapor and other atmospheric parameters in Birjand station.