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Salton Sea 1995 Hydrographic GPS Survey

Hydrographic Survey Method and Equipment

The hydrographic survey equipment was mounted in the cabin of a 24-foot tri-hull aluminum vessel equipped with twin inboard motors. The hydrographic system contained on the survey vessel consisted of a GPS (global positioning system) receiver with a built-in radio and an omnidirectional antenna, a dual frequency depth sounder, a helmsman display for navigation, a plotter, a computer, and hydrographic system software for collecting the underwater data. Power to the equipment was supplied by an on-board generator.

The shore equipment included a second GPS receiver with a built-in radio and an omnidirectional antenna. The GPS receiver and antenna were mounted on a survey tripod over a known datum point. A radio booster was used for this survey because of the massive area of the Sea. The power for the shore units was provided by two 12-volt batteries. Depending on conditions, the radio data link between the two GPS units ranged from 10 to 15 miles. To obtain the maximum radio transmission range, known datum points near the Sea and high above the water surface were selected.

GPS Technology and Equipment

The positioning system used at the Salton Sea was NAVSTAR (NAVigation Satellite Timing and Ranging) GPS, an all weather, radio based, satellite navigation system that enables users to accurately determine three-dimensional position. The NAVSTAR system's primary mission is to provide passive global positioning and navigation for land, air, and sea based strategic and tactical forces and is operated and maintained by the DOD (Department of Defense). The GPS receiver measures distances between satellites and itself and determines the receiver's position from intersections of the multiple range vectors. Distances are determined by accurately measuring the time a signal pulse takes to travel from the satellite to the receiver.

The NAVSTAR system consists of three segments:

The GPS receivers use the satellites as reference points for triangulating 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. A minimum of four satellite observations are required to mathematically solve for the four unknown receiver parameters (latitude, longitude, altitude, and time). The time unknown is caused by the clock error between the expensive satellite atomic clocks and the imperfect clocks in the GPS receivers. For hydrographic surveying the altitude, the Salton Sea water surface elevation parameter was known, which realistically meant only three satellite observations were needed to track the survey vessel. During the Salton Sea survey, a minimum of five satellites were used for position calculations, but the majority of the time, the best six available satellites were used.

The GPS receiver's absolute position is not as accurate as it appears in theory because of the function of range measurement precision and geometric position of the satellites. Precision is affected by several factorsótime, because of the clock differences, and atmospheric delays caused by the effect on the radio signal by the ionosphere. GDOP (geometric dilution of precision) describes the geometrical uncertainty and is a function of the relative geometry of the satellites and the user. Generally, the closer together in angle two satellites are from the receiver, the greater the GDOP. GDOP is broken into components: PDOP is position dilution of precision (x,y,z), and HDOP is horizontal dilution of precision (x,y). The components are based only on the geometry of the satellites. The PDOP and HDOP were monitored during the Salton Sea Survey, and for the majority of the time, they were less than 3, which is well within the acceptable limits of horizontal accuracy for Class 1 and 2 level surveys.

An additional and larger error source of GPS collection is caused by false signal projection, called S/A (selective availability). The DOD implements S/A to discourage the use of the satellite system as a guidance tool by hostile forces. Positions determined by a single receiver when S/A is active can have errors of up to 100 meters.

A method to resolve or cancel GPS errors (satellite position or S/A, clock differences, atmospheric delay, etc.) is called DGPS (differential GPS). DGPS was used during this survey to determine positions of the moving survey vessel in real time. DGPS determines the relative position of one receiver to another and can increase position accuracies by eliminating or minimizing 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. Inherent errors are mostly canceled because satellite transmission is essentially the same at both receivers.

At a known geographical benchmark, one GPS receiver is programmed with the known coordinates and stationed over the 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 master receiver's programmed position and the necessary corrections or differences are transmitted to the mobile GPS receiver on the survey vessel. For the Salton Sea Survey, position corrections were determined by the master receiver and transmitted via a UHF radio link every 3 seconds to the survey vessel mobile receiver. The survey vessel's GPS receiver used the corrections along with the satellite information it received to determine the vessel's differential location. Using DGPS resulted in positional accuracies of 1 to 2 meters for the moving vessel compared to positional accuracies of 100 meters with a single receiver.

The TSC (Technical Service Center) mobile and reference GPS units are identical in construction and consist of a 6-channel L1 C/A code continuous parallel tracking receiver, an internal modem, and a UHF radio transceiver. The differential corrections from the reference station to the mobile station are transmitted using the industry standard RTCM (Radio Technical Commission for Maritime Services) message protocol via the UHF radio link. The programming to the mobile or reference GPS unit is accomplished by entering necessary information via a notebook computer. The TSC's GPS system has the capability of establishing or confirming the land base control points by using notebook computers for logging data and post-processing software. The GPS collection system has the capability of collecting the data in 1927 or 1983 NAD (North American Datums) in the surveyed area's state plane coordinate system's zone, which for the Salton Sea was California Zone 6.

Survey Method and Equipment

The Salton Sea hydrographic survey collection took a total of 22 days, starting on November 4, 1994, and concluding on February 3, 1995. During this time the water surface elevations of the Sea ranged from 227.8 to 227.2 feet below sea level. The bathymetric survey was run using sonic depth recording equipment interfaced with a DGPS capable of determining sounding locations within the Sea. The survey system software was capable of recording depths and horizontal coordinates on 1-second increments as the survey boat moved along the predetermined gridlines or transects covering the Salton Sea. Because of the constant sloping underwater terrain of the Salton Sea, the data were recorded every 2 to 3 seconds; the average width between the transects was 2000 feet. The majority of the transects were run in a mostly east-west direction. Data were also collected along the shore as the boat traversed to the next transect. Transects were also run in a mostly north-south direction to provide additional data for complete contour development. The survey vessel's guidance system gave directions to the boat operator to assist in maintaining course along these predetermined gridlines. During each run, the depth and position data were recorded on the notebook computer hard drive for subsequent processing by TSC personnel. The underwater data set includes about 133,400 data points. A graph plotter was used in the field to track the boat and ensure adequate coverage during the collection process. Water surface elevations recorded by the USGS gage (near Westmorland, California) during the time of collection were used to convert the sonic depth measurements to true lake bottom elevations. Little to no wind occurred during the majority of the underwater collection, and the Sea's water surface was very calm.

The hydrographic survey crew used benchmarks as control points for shore station sites that were previously established and verified by other Federal, State, and county agencies. The hydrographic survey crew obtained additional verification by performing a static survey using the GPS receivers. Because of the size of the Sea, four master shore station locations were used for relaying correction information to the survey vessel (DGPS). Shore unit locations were Desert Shores, the Navy Base, and Travertine Rock on the west shore of the Sea and at Red Island on the south shore of the Sea. These points were selected because they had known coordinates, were accessible, were located near the Sea, and were high above the water surface. These locations allowed for good radio transmission range from the known reference survey points to the mobile survey vessel. For this survey, range varied from 10 to 15 miles between the reference and mobile GPS units. At times, the signal between the reference and mobile receivers was broken; thus, the mobile GPS receiver did not receive position corrections (DGPS). Trouble shooting determined the problem was a faulty antenna at the master GPS unit. During post processing of the collected data, all points without differential correction were removed.

The TSC's depth sounder is a dual frequency sounder with 41- and 208-kilohertz transducers available. The depth sounder determines the bottom by measuring the elapsed time between the transmission of the sound pulse from the transducer to the waterway bottom and the reception of its echo back to the transducer. The dual frequencies can be operated alone or simultaneously. The high frequency reflects off the first bottom surface and the low frequency penetrates and perhaps traces the harder sub-bottom information. After consulting with the manufacturer, it was determined that a 24-kilohertz low frequency transducer would have the best success in penetrating and tracing the harder sub-bottom. Because this information was of interest to the study team, a lease of the 24-kilohertz equipment was obtained. The collection with the 24-kilohertz equipment was conducted in areas of concern in the southern and northern portions of the Sea where soft bottom conditions were expected. Results of this collection yielded little evidence of soft bottom conditions, but no general conclusions can be made from this information because this was the first extensive use of this equipment by the TSC operator.

Periodically, the depth sounder was calibrated by lowering a deflector plate below the boat by cables with known depths marked by beads. The depth sounder was calibrated by adjusting the speed of sound, which can vary with density, salinity, temperature, turbidity, and other conditions. The accuracy of an instantaneous reading from the depth finder is estimated to be ±0.5 feet, but errors are minimized over the entire survey. The estimated accuracy takes into consideration calibration error and the collection of depth data when the boat is moving. The collected data were digitally transmitted to the computer collection system via an RS-232 port. The TSC collection system only allows one of the frequencies at a time to be stored by the computer. The high frequency data were recorded for the Salton Sea Survey. The depth sounder also produces an analog hard copy chart of the measured depths. These graphed analog charts were printed for all survey lines as the data were collected and recorded by the computer. The charts were analyzed during post processing, and when the analog charted depths indicated a difference from the recorded computer bottom depths, the computer data files were modified.

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