Вестник Каспия №4/2008 Caspian Sea Bulletin От Редакции журнала «Вестник Каспия»
GPS data collection at land and at sea
жүктеу/скачать 5.89 Mb.
|
- Бұл бет үшін навигация:
- Weather Conditions
- Preliminary results from data analysis
- Plans for installing a permanent cal/val station
- Acknowledgements
GPS data collection at land and at seaWe noted that the main pier is not in perfect conditions. We decided that this pier would not be safe for GPS measurements. Annual visits to check levelling and benchmarks would be necessary. The GPS base station (S/N T224969) was installed on the roof of the guardian white house. A first session was done on Monday, 9 June at 30 sec rate. When the buoy was deployed on Tuesday, 10 June the GPS base station was set to 1 sec rate. A bench mark has been installed on the floor, on the right corner of the guardian house. A first order levelling between the antenna (S/N 3085567) of this benchmark has been made. The distance between the upper edge of the antenna metal ring and the top of benchmark is 3234 +/- 3 mm. Then the distance between Antenna Reference Point (ARP) and the bench mark is: (3234 – 27.5 – 2.5) = 3204 +/- 3 mm We were not able to rent an optical level system and we used a rudimental level pole for levelling. For the next missions, we will plan the delivery or local renting of an accurate levelling system. The GPS buoy was deployed on the same pier as the pressure gauge few tens of meters away from the gauge. It was difficult to find an appropriate location for the buoy. We were not able to deploy the buoy close to the bottom pressure. We had to find a compromise between sky mask and flat water. This part of the Caspian Sea seems to be very windy. The simple system of a lifeguard GPS buoy should be improved for stability. Due to the rough sea surface conditions the buoy has been weighted with a heavy rock. The plastic surface of the buoy was not perfectly horizontal because of the rock’s moment. The distance from plastic surface to the sea surface has been calibrated at 4 points of the buoy: 60 , 65, 75, 85 mm. This leads to an approximate slope between 2 and 3 degrees. Without taking into account the slope of the buoy the distance between the Antenna Reference Point (ARP) and the sea surface is 171,6 mm. Taking into account this slope the distance is 171,5 mm. We used the Buoy Antenna marked S/N 3085273. We were able to collect around 45 hours of measurements. Weather ConditionsWe observed that the area of Absheron Port is extremely windy. On June 9, we were able to observe strong wind coming from the open sea. Local inhabitants confirmed that wind is present in Baku year around. We noted that winds and pressure data are important to interpret sea level measurements. We visited Pirallahi Island (5 km offshore from the terminal) which is connected to land by an artificial bridge built in 1948 (Fig.3). At Pirallahi Island there are two meteorological stations. The first one is located along the road between the island and oil fields (not accessible at the time of visit) and handled by Amir Alyev who is responsible for the hydro-meteorological observations along the coasts. The second one is handled by another department. Amir Alyev confirmed that measurements (wind amplitude and direction, water and air temperature, air pressure, height and length of waves) are collected every 6 hours. There are different methodologies of data acquisition: (1) observation during daytime and (2) graphs during night time. Wind speeds are averages of 10 minutes at sampling time, while wind directions are averages of 2 minutes at sampling time. A 5 year-long historical record is available in electronic format. Amir Alyev will install a new air pressure sensor at Pirallahi Island collecting hourly data. Preliminary results from data analysisWe made a preliminary analysis on the basis of the data collected. Fig. 9 shows the bottom pressure variability recorded at port Absheron during the first days of the mission. We can clearly detect the presence of a high frequency oscillation phenomena. The spectra of the time series (Fig. 10) shows that the different periods of this oscillation are in the range between 1.5 and 3 hours. This oscillation could be the different mode of a seiche caused by the closure of the end of the Island by a dam or a product of the Caspian Sea metocean dynamics itself. We plan to investigate this phenomena using a barotropic modelling. 
Fig. 9. Time series of bottom pressure variation during the period of data collection. Рис. 9. Показания датчика давления с 10 по 12 июня 2008 г. |
