GPS meteorolojisi : İstanbul için bir uygulama
GPS meteorology : An application for Istanbul
Yazar: ÖMER GÖKDAŞ
Danışman: PROF. DR. ERGİN TARI
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Geomatik Mühendisliği Ana Bilim Dalı
Derece: Yüksek Lisans, 2014
Konu: Jeodezi ve Fotogrametri = Geodesy and Photogrammetry ; Meteoroloji = Meteorology
Dizin: GPS = Global Positioning System ; Meteoroloji = Meteorology

Özet:
Su buharının atmosferdeki dağılımı ile ilgili bilgi, hava tahmini çalışmaları ve iklim araştırmalarında büyük öneme sahiptir. Atmosferi oluşturan temel bileşenlerden en değişken özelliğe sahip olanıdır. Ayrıca, global iklim sisteminde kritik rol oynayan sera gazı (greenhouse gas) özelliğine sahiptir. Bir başka ifade ile, su buharı, atmosferin diğer bileşenleri ile karşılaştırıldığında sera etkisine en çok katkıyı yapan gazdır. 
Meteorolojistler, su buharının yatay ve düşey dağılımını belirlemek için çeşitli yöntemler geliştirmişlerdir. Bunlardan meteoroloji balonları (radiosonde) radyo sinyallerinden yararlanarak yer istasyonlarına atmosfer içindeki sıcaklık, basınç, nem ile rüzgar hız ve yönü ile ilgili bilgiler göndermektedir. Normal radiosonde aletleri ile sıcaklık ≈ 0.2 C° ve basınç ise ≈ %3.5 doğrulukta elde edilebilmektedir. Radiosonde gözlemlerinin düşey kesitte iyi bir çözünürlük sağlamasına karşın, bazı kısıtlamaları da bulunmaktadır. Örneğin, bu aletler pahalıdır ve bu nedenle kullanıldıkları istasyon sayısı ve günlük fırlatma sayısı (örneğin günde iki kez) sınırlı tutulmaktadır. Bu kısıtlamalar nedeniyle radiosonde tekniği su buharının zamansal ve uzaysal değişkenliğini belirlemede yetersiz kalmaktadır.
Yeryüzünde tesis edilmiş olan sabit GPS istasyonlarında sürekli gözlem yapılarak toplanan veriler yağışa dönüşebilir su buharı (IPWV)'nın belirlenmesinde kullanılabilmektedir. Bu yöntem ile zamansal ve mekansal çözünürlüğü yüksek değerler elde etmek mümkündür.
Bu çalışmada İstanbul'da bulunan ISTA IGS istasyonuna ait 01.10.2013 – 31.10.2013 tarihleri arasında ölçülen sıcaklık ve basınç değerleri ve GPS gözlemleri yardımıyla yoğunlaşabilir su buharı verileri hesaplanmıştır. Ayrıca yine İstanbul'da bulunan 17062 numaralı Kartal meteoroloji istasyonundan alınan verilerle doğrulaması yapılmış olup, ±1.7 mm standart sapma ile ortalama 0 mm fark hesaplanmıştır.
Ek olarak İSKİ  Uydulardan Konum Belirleme Sistemi (UKBS) ağına ait İstanbul'daki 7 istasyon (Pala, Terkos, Küçükçekmece, Silivri, Beykoz, Şile, Tuzla) için 09.10.2013 – 12.10.2013 tarihleri arasında (72 saat) 30 dakikalık dilimleri kapsayan troposferik gecikme modelleri ile hesaplanan zenit troposferik gecikmelerden yoğunlaşabilir su buharı değerleri hesaplanmıştır. Ayrıca bu değerlerle 17062 numaralı radiosonde ölçümlerinin yapıldığı Kartal meteoroloji istasyonuna mesafelerine göre ters ağırlıklandırma yöntemiyle enterpolasyon yapılarak mevcut radiosonde verileriyle karşılaştırılmıştır. Sonuçlar incelendiğinde ±1 mm standart sapma ile 1 mm ortalama fark olduğu görülmüştür.

Abstract:
Climate change and global warming have become a major challenge for the sustainable development of our Earth and its environment. Intensive research is carried out to understand atmospheric processes and their implications. In this content, water vapor plays a key role. It is an important component of the global energy balance and is involved in many chemical reactions. Water vapor takes part in the water cycle of the earth. It is condensated and precipitated in the form of rain or snow and originates again by evaporation, sublimation and transpiration. Although, only 0.001% of the total amount of water on the earth is located in the atmosphere, water vapor plays a fundamental role in atmospheric processes. It is the most variable parameter of the major constituents of the atmosphere, both, in space and time. 
Water vapor influences or causes many chemical processes in the atmosphere. It is a major greenhouse gas and is involved in the decomposition of the ozone layer. Water vapor is constantly circulating in the atmospheric system and is coupled to the formation and distribution of clouds and rainfall as well as air pollution. The distribution of water vapor plays a crucial role in the vertical stability of the atmosphere and, therefore, in the evolution of atmospheric storms. Moreover, water vapor is the carrier of latent heat and an important component of the global energy balance as well as the transportation medium of energy in the atmosphere. Therefore, it is a fundamental quantity for climatological studies over short and long periods as well as for weather forecasting. Time series of atmospheric water vapor content as well as its spatial distribution is the basis for successful research in many climatic coherences and atmospheric processes.
Water vapor plays a fundamental role in meteorological processes that act over a wide range of spatial and temporal scales. First it plays a fundamental role in the hydrological cycle. In brief, water vapour from the sea and land to the atmosphere where cloud form. From cloud, rain and snow fall back to the Earth's surface, thus supplying rivers, which flow back to the sea. Second it is the dominant greenhouse gas in the atmosphere. The greenhouse gas can lead to global warming. Then, it is both a symptom lead to a cause of the atmospheric greenhouse effect. Generally, the amount of water vapour of the atmosphere increases with temperature. The additional water vapour traps more of the heat energy from sunlight that escapes from the Earth. This trapped of the heat energy making a warming to the Earth's surface. 
Troposphere is the lowest part of the atmosphere that contacts earth. It's thickness is about 8 km above poles and 18 km above equator, and changes according to the season. Compares to the other parts of the atmosphere it is the most intensive parts. Therefore, it is a quite important source of error in determining positions of points precisely. Tropospheric delay is a function of temperature, relative humidity and air pressure, and it is related with the height of the measurement point closely. Tropospheric delay is described as the effect of non-ionized atmosphere to the electro magnetic waves that are broadcast in radio frequencies. This effect causes electromagnetic wave slow down and curve.
In microwave measurements, the tropospheric refractivity causes a delay in the arrival of the signal propagating through the atmosphere. This refraction effect is one of the limiting factors in accurate GPS positioning. The tropospheric path delay can be decomposed into a dry and wet part, where the latter part is coupled with the integrated precipitable water vapor above the GPS receiver.
On the one hand, the refraction effect has to be corrected for GPS measurements, on the other hand, it is a valuable signal to determine the spatial distribution of the water vapor. This study investigates both aspects. For the first part, two basic approaches are looked into: One method is based on meteorological measurements. Thereby, the integrated amount of water vapor and its temporal variation are the prime target. The other concept makes use of long-term GPS measurements. The arrival delay of the GPS signals are used, to estimate the integrated amount of water vapor. This result can then be the basis to determine its spatial distribution and temporal behavior
Several methods have been recently investigated to obtain an accurate tropospheric refractivity correction. The types of approaches can be divided into three groups: estimation of path delays by GPS measurements, assimilation of (mostly) meteorological measurements including water vapor radiometers. 
First method determine the path delay directly with meteorological measurements:  Radiosondes measure the pressure, temperature and relative humidity along the line of the sounding, which allows to calculate the respective path delay. 
A radiosonde is a traditional measurement device for upper air observations in meteorology. A radiosonde is equipped with different sensors, which typically measure pressure, temperature, relative humidity, and wind (both speed and direction).  All radiosonde sensors together with a radio transmitter are attached to a weather balloon, which is normally launched at the most two times per day and the measured vertical profiles of all parameters are reported back to a receiving site at the nominal time epochs 0:00 and 12:00 UTC. The integration of the vertical absolute humidity (expressed in units of mm) profiles from the surface to the top of the radiosonde profiles gives the atmospheric IPWV.
Another method estimates path delays from GPS measurements. Nowadays, this method yields the most accurate path delays. However, GPS-estimated path delays as well as path delays obtained directly from meteorological measurements can only be determined in the line of sight (or zenith direction) of the respective measurement system.
Apart from its importance in geodesy, GPS signals are also a highly valuable information for atmospheric research. The integrated amount of water vapor in the zenith direction is called integrated precipitable water vapor (IPWV). It is approximately proportional to the tropospheric path delay which can be estimated from GPS measurements. This relatively new research field is commonly called GPS meteorology.
A number of studies have shown that "GPS meteorology" offers detailed coverage and continuous observations regardless of weather conditions (e.g., heavy rainfall and clouds) and is an economical tool to complement other remote-sensing techniques to measure water-vapor content.
The advantages of the GPS measurements are that they can be performed independently on the weather and have a high temporal resolution (a few minutes) on the IPWV estimates. Meanwhile, along with densification and extension of permanent GPS site networks globally and regionally, a continuously improving spatial resolution is expected. Therefore, using the GPS measurements to provide estimates of the IPWV above receivers on the ground is a promising application.
A meteorological sensor should be installed adjacent to the GPS antenna for accurate estimation. The reality is that many meteorological sensors are not located near GPS stations. Alternatively, the surface temperature and pressure data from the nearest weather station can be interpolated. Because the vertical variability of pressure and temperature is sensitive to the altitude of the sensor, the pressure and temperature at all the weather stations are adjusted to a common reference level that refers to the Mean Sea Level (MSL). Subsequently, the pressure and temperature at the MSL are interpolated to derive the pressure and temperature at the GPS stations.
We can use the meteorological data collected hourly at the closest site for Precipitable Water Vapor (IPWV) estimation, or use the interpolated measurements instead. Generally, a method is needed to interpolate the temperature and pressure at the GPS site with measurements from surrounding weather station sites. Because the vertical variability of pressure is sensitive to the altitude of the station, the pressure and temperature measurements given at different altitudes have to be converted to the common reference level, which is often Mean Sea Level. As a result, the interpolated parameters at any point refer to this reference level. These parameters must then be converted to the station level of the GPS station, in order to generate the IPWV measurements for that station.
In this study, between 01.10.2013 and 31.10.2013, using continuous GPS data and hourly temperature and air pressure data observed at ISTA IGS station, precipitable water vapor values were computed. Additionally, these values were verified by the observed radiosonde data at Kartal meteorology station which number is 17062. It was shown that ± 1.7 mm standart deviation and 0 mm mean difference.
In Addition, between 09.10.2013 and 12.10.2013 (72 hours) with 30 minutes periods, precipitable water vapor values were calculated by using zenith tropospheric delay that gets from tropospheric delay models at 7 ISKI UKBS stations (Pala, Terkos, Kuçukcekmece, Silivri, Beykoz, Sile, Tuzla) Furthermore, these values were enterpolated by using inverse distance weighted method to Kartal meteorology station which number is 17062 and verified by the observed radiosonde data. It was shown that ± 1 mm standart deviation and 1 mm mean difference.