Reservoir sedimentation surveys using Global Positioning System
GPS for position information
GPS is a satellite-based global navigation system that enables users to accurately determine 3-dimensional positions (x,y,z) worldwide. While GPS is clearly the most accurate worldwide all-weather navigation system yet developed, it still can exhibit significant errors. GPS receivers calculate its position from distance measurements to the satellites that are determined by how long a radio signal takes to reach the receiver from the satellite. Position accuracy depends on the receiverís ability to accurately calculate the time it takes for each satellite signal to travel to earth. This is where the problem lies. There are primarily four sources of errors which can affect the receiverís calculation. These errors consist of (i) time, because of clock differences, (ii) ionosphere and troposphere delays on the radio signal, (iii) signal multi-path,errors caused by signal arrival by different paths usually due to signal reflecting from obstacles, and (iv) quality of GPS receiver. The combination of these errors, can limit GPS accuracy to 10 to 16 meters. These errors are eliminated through a technique known as ďDifferentialĒ.
Differential positioning, which requires at least two receivers, does provide the means for high accurate surveying. DGPS determines the position of one receiver in reference to another and is a method of increasing position accuracies by eliminating or minimising the uncertainties. Differential positioning is not concerned with the absolute position of each unit, but with the relative difference between the positions of the two units, which are simultaneously observing the same satellites. The basic principle being that errors calculated y GPS receivers in a local area will have common errors. The inherent errors are mostly concealed because the satellite transmission is essentially the same at both receivers. One GPS receiver is programmed with the known coordinates and is stationed over a known geographical benchmark. This receiver, known as the master or reference unit, remains over the known benchmark, monitors the movement of the satellites, and calculates its apparent geographical position by direct reception from the satellites. The inherent errors in the satellite position are determined relative to the masterís programmed position and necessary corrections or differences are applied to the mobile GPS receiver on the survey vessel.
There are different ways to apply DGPS collection methods to hydrographic surveying which includes real time and post processing GPS. For real time survey a constant link between the base station and rover either radio signal or other communication techniques that broadcast the differential corrections is required. The weakness with all real-time collection systems is the communication link between the master and mobile GPS receivers. Communication problems can occur with all the systems when surveying in areas with obstructions such as mountains, cliffs, vegetation etc. When these situations occur the master receiver will have to be shifted to new locations, but at times more costly and time consuming. Post-processing DGPS is therefore, used in which instead of applying the differential correction to the rover via radio contact, it is applied after the dayís survey is completed. The GPS survey information is stored in each receiverís memory and downloaded to a computer at the end of the day. Once both receiverís have been downloaded, the correction factors can be applied to the roversí data and an accurate position obtained. The limitations of a post-processed survey include uncertainty of real-time positional accuracy and the inability to accurately navigate a precise pre-planned survey route.
A minimum of four satellite observations are required to mathematically solve for the four unknown receiver parameters ( latitude, longitude, altitude and time). For hydrographic surveying the altitude, the water surface elevation parameter, is known which realistically means only three satellite observations are needed to track the survey vessel. But to obtain highest accurate positioning the survey vessel tracks all available satellites and monitors geometric accuracy of its positions.
The GPS can also be used for vertical measurement allowing not only greater accuracy in the hydrographic survey, but also more productive data collection. The antenna is mounted a fixed distance above the transducer. This provides absolute positioning for the depth measurement without the need for a water surface measurement to define a reference datum. It also eliminates the concern for a changing water surface elevation or boat heave (vertical displacement) due to wave action during the survey.
Depth Measuring Unit
The present hydrographic survey system use sonic soundings to record continuous profiles of the bottom of small and large reservoirs. The basic components are the recorder, transmitting and receiving transducer and power supply. The manually operated sounding lines and poles used in the past are virtually outdated and replaced by modern sonic sounders which have the capability of recording continuous profiles of the reservoir bottom and providing an analog bottom profile chart and digital record stored on the computer system for later processing. The echo sounding instrument consists of the recorder, the transmitting-receiving transducers mounted in the hull or sides of the survey boat, and a power source of either a battery or generator converter combination. The depth of water is recorded continuously at hundreds of soundings a minute on chart paper and the depth digital signal is recorded by the computer at a prescribed interval. Calibration of depth measurement from the echo sounder is critical in assuring high quality depth measurements by the hydrographic survey system. The largest correction results from the variability of the sound velocity in water due to density, salinity, temperature, turbidity, and depth of water. In fresh water at 60 degree F, echo sounders are generally calibrated for a sound velocity of 1463 m/s. The indicated depth by the echo sounder needs to be corrected for the known depth or water conditions. This can be accomplished by different methods with the bar-check being one of the commonly used methods. A bar check consists of lowering an acoustic reflector, such as a flat metal plate, to a known depth below the transducer and adjusting the instrument to produce and equivalent depth reading. The bar check must be conducted in fairly calm water with minimum wind conditions. Mild to strong wind will shift the sounding vessel so that the calibrating bar will be suspended at an angle from vertical causing the narrow signal beam from the transducer to miss the bar. The survey crew should make a bar check and record results on a depth chart and log sheet. Comparisons are recorded during both descend and ascend of the bar, at a pre determined intervals through out the depth range of the survey. Any adjustments to the speed of sound of the eco sounder should be noted on the chart. With careful calibration and correct collection techniques, a high degree of bottom profile accuracy can be maintained. The accuracy of the equipment is 0.1% of the water depth.