Showing papers by "National Ocean Service published in 1990"

Water surface detection strategy for an airborne laser bathymeter

Abstract: An algorithm is described for estimating wave heights and correcting measured local depths to the mean water level for an airborne laser bathymetry system utilizing collinear infrared and green beams. The wave heights are referenced to a mean surface derived from laser slant ranges plus vertical accelerometer data to isolate long period swell from aircraft motion. Performance of the technique under nominal operating conditions is characterized through detailed error analyses. It is seen that filters as long as several hundred seconds can be used to determine mean water level - as might be needed, for example, in the presence of swell crests parallel to the flight line. Use of the accelerometer also permits a sufficiently accurate mean water surface to be maintained for about 6 seconds without update by local surface slant range detections. This allows depths to be calculated for pulses whose surface returns are not detected. The use of slant ranges to calibrate system alignment parameters is demonstrated. Comparisons are made with an alternative approach used in the Australian LADS system.

Definition of a Terrestrial Reference Frame using IRIS VLBI Observations; Approaching Millimeter Accuracy

Abstract: The data now available from space-based geodetic observing techniques, especially Very- Long-Baseline Interferometry and Satellite Laser ranging, presently allow us to define a terrestrial reference frame with an accuracy of better than two centimeters. Improvements to the observing systems over the next few years may allow accuracies approaching a few millimeters. These unprecedented accuracy levels raise a number of interesting technical and scientific questions concerning the accuracy of the Earth models used to reduce the data. For example, observations of the changes in the lengths of baselines from North America to Europe show 25 to 30% discrepancies from the values given by the Minster- Jordan AMO-2 plate motion model, which is now uniformly adopted for processing the new data. Also, vertical crustal motions expected from glacial rebound not presently included in the data reduction can exceed 1 cm per year at some sites. Existing VLBI observing networks lack the global coverage necessary to realize the potential of the present accuracy levels. In the next few years much effort will need to be directed toward establishing observatories in under-represented portions of the globe, especially the southern hemisphere and the central Asia-Indian ocean regions.

Applying kinematic GPS techniques at our nation's airports

Abstract: The National Geodetic Survey (NGS) began surveying US airports using a mixture of static, kinematic, antenna swap, and pseudokinematic techniques. The methodologies and the associated theoretical rationale are described. The NGS performed an online operational test of GPS (Global Positioning System) kinematic surveying at several Florida airports, and it was found to be a complete success. All successful processing variations in the Apalachicola example agreed at the 1-cm level. >

Preliminary Comparisons Of Noaa's New And Old Water Level Measurement Systems

Abstract: The National Ocean Service began implementing the Next Generation Water Level Measurement System (NGWLMS) field measurement systems into the National Water Level Observation Network (NWLON) in July of 1988. At present, there are approximately 75 NGWLMS systems installed. With the exception of Global Sea Level Network stations and special project stations, the new systems are being operated side-by-side with the old technology float-dr i ven analog-to-digital recorders (ADR' s) . The NGWLMS field units will eventually replace the ADR gauges at all stations after completion of side-by-side comparisons which provide for datum ties between the historical and new time series. The engineering analysis of the new systems performance includes evaluation of the acoustic water level sensor calibration stability, data quality parameters, and air temperature gradients along the sensor sounding tube and their potential effects on measurement uncertainty. In addition, the time series data are being independently processed relative to the same station datum. Statistics are being compiled on comparisons of routine output products. This paper describes the results of the engineering analysis and the data comparisons at several NWLON stations. Included is a discussion of the differences found between the systems. Preliminary results indicate the implementation of the NGWLMS will provide improved operational capability and higher quality data over that of the old technology system.