GLOBAL POSITIONING SYSTEMThe positioning system used for all the major services offered ``will be GPS (Global Positioning System), which is an all weather, radio based, satellite navigation system that enable users to accurately determine three-dimensional position. The Global Positioning System’s primary mission is to provide passive global positioning and navigation for land, air, and sea based strategic and tactical forces. The GPS receiver measures distances between satellites and itself to determine the receiver’s position from intersections of the multiple range vectors. Distances are determined by accurately measuring the time, which a signal pulse takes to travel from satellite to the receiver.
The GPS receivers use the satellites as reference points for triangulation their position on earth. The position is calculated from distance measurements to the satellites that are determined by how long a radio signal takes to reach the receiver from the satellite. To calculate the receiver’s position on earth, the satellite distance and the satellite’s position in space are needed. The satellites transmit signals to the GPS receivers for distance measurements along with the data messages about their exact orbital location and operational status. The satellites transmit two “L” band frequencies for the distance measurement signals called L1 and L2. Minimums of four satellite observations are required to mathematically solve the four unknown receiver parameters (latitude, longitude, altitude and time).
At a known geographical benchmark, one GPS receiver is programmed with the known coordinates and stationed over the geographical benchmark. The receiver, known as the master or reference unit, remains over the known benchmark, monitors the movement of the rover (secondary mobile GPS receiver), and calculates its apparent geographical position. The inherent errors are determined relative to the master receiver's programmed position and the necessary corrections or differences are transmitted to the rover.
TOPOGRAPHY
The RTK GPS502 will be used for establishing a calibration station, correlating benchmarks, and undertaking any required topographic survey in an open area.
The RTK system can be operated in either dynamic mode, fixing every second or at a user specified time interval or distance, for topographic survey. Alternatively the system can be used in fast static mode, for detail surveys, occupation times are in the order of a few seconds, or in static mode, geodetic baselines can be observed yielding accuracies of + 5mm + 1ppm. The system is either deployed by pedestrian in a backpack, with the GPS antenna on a plumbing pole, or is mounted on our eight wheel drive tracked amphibious All Terrain Vehicle (ATV). In less rugged terrain the unit is mounted on a conventional four wheel drive vehicle.
Detail surveys to locate man made features carried out by ‘stop and go’ pedestrian operations and by total station observations. Total Station will be employed to survey any areas where satellite coverage is restricted due to obstructions. The equipment will consist of a Leica Total Station and Electronic Notebook observations being recorded on the memory card.
BATHYMETRY
Bathymetric data and simultaneously side scan sonar data shall be acquired using a high precision multi frequency Geo Swath multi beam echo sounder.
The mobile DGPS Receiver will interface to the GEO SWATH multi beam echo sounder. The Geo Swath processor receives the depth data and time, synchronized with the GPS position. Thus position and depth data along with date, time will log on the hard disk at one-second intervals. The Geo Swath Sonar transducer array shall be mounted on a pole mount over the bow of the vessel. The DGPS antenna shall be mounted on top of the transducer pole, directly above the Geo Swath ‘V’ plate, to reduce lever arm offsets. Geo Swath 250kHz transducers, the MRU, the Precision Altimeter and the Mini SVS is mounted on the ‘V’ Plate. The SG Brown Gyro compass will be secured aligned with the vessel centerline and SVP dips shall be taken as required
The Geo Swath software allows coverage to be monitored in real time, and several data displays are available for the surveyor to maintain a high level of data quality control.
The Geo Swath is supplied with a complete set of software for data collection; calibration, filtering, generation of digital elevation models (DEMs) and side scan mosaics. Data quality control and data visualization tools are an important part of the package. Raw swath data was filtered to remove outliers, sound velocity corrections, tidal corrections were applied and navigation data was checked and edited using the Geo Swath Swath32 software. The calibration parameters is applied to the filtered swath data, giving geo referenced, calibrated data files on a line-by-line basis.
The speed of sound is measured in the beginning and ends of the work by deploying a CTD valeprot 600.
