Browsing by Subject "Altimeter"
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Item Altimeter methods in satellite geodesy(1982) Shum, Che-Kwan; Schutz, Bob E.The application of satellite-borne radar altimeters to the field of satellite geodesy and satellite orbital motion has been investigated. The altimeter measurement, the height of the satellite above the mean ocean surface, can be used directly or by differencing the measurements at the points where the orbit ground tracks intersect. The latter case is referred to as a "crossover measurement." The methods of utilizing these measurements in the estimation of orbit and geodetic parameters have been applied to data obtained by the oceanographic satellite, Seasat, during the 18-day period from 28 July to 14 August, 1978. Because of the near 17-day repeat cycle of the orbit ground track, this data set constitutes a reasonably uniform global distribution over the ocean surface. Supplementing the altimeter data with laser range data to provide overland coverage, a single continuous ephemeris spanning the 18-day interval resulted in a laser range residual RMS of 1.06 m and a crossover residual RMS of 1.04 m using the Goddard Space Flight Center PGS-S4 gravity field. This ephemeris was used as a reference to evaluate the time tag accuracy of the altimeter measurements, as well as to estimate geodetic parameters including the spherical harmonic coefficients which represent the gravity field of the earth. Using laser range, altimeter and crossover measurements, a batch estimation procedure based on a square root formulation of the least squares problem using Givens rotations was used to estimate two sets of spherical harmonic coefficients of degree and order 14 and 26. The 18-day orbit computed with the solved degree and order 26 gravity field has an ephemeris accuracy in terms of laser range residual RMS of 0.89 m, altimeter residual RMS of 1.36 m, and altimeter crossover residual RMS of 0.48 m, respectively. The crossover residuals reflect the influence of remaining radial orbit errors and temporal sea surface topography effects, whereas the altimeter residuals reflect the remaining geoid errors and short wave length ocean topography effects which cannot be accommodated in the estimated gravity field. The estimated orbit and geodetic parameters have been used to compute a mean ocean surface model which is compared to other similarly determined surfaces. Also obtained is the earth's mean equatorial radius and its flatteningItem The mapping of tectonic features in the ocean basins from satellite altimetry data(1988-05) Gahagan, Lisa Marie, 1963-; Not availableSatellite altimetry data provide information on the height variations of the sea surface. The angle between a line perpendicular to the sea surface and a vertical line between the satellite and the sea surface is referred to as the deflection of the vertical and is equal to the first derivative of the sea surface. This study examines two theoretical models describing the relationship between the deflection of the vertical data and the bathymetry 1) across a fracture zone in a large age-offset, fast-spreading regime and 2) across a fracture zone in a small age-offset, slow-spreading regime. The models are respectively compared to the observed relationship 1) across the Mendocino Fracture Zone which is in a large age-offset, fast-spreading regime and 2) across the DuToit Fracture Zone which is in a medium age-offset, slow-spreading regime. The strong agreement between the theoretical models and the observed relationships suggests that the models can be used with the deflection of the vertical data to locate fracture zones in known regimes. The angle between the trend of a feature and the trend of the satellite track affects the deflection of the vertical signal. As the angle becomes smaller, the amplitude of the deflection of the vertical signal, which varies with the sine of this angle, decreases and the wavelength of the signal increases. Once the feature is parallel to the track, there is no deflection of the vertical signal. The deflection of the vertical signal is also affected by the direction the satellite travels. If the feature trends between the ascending and descending tracks of the satellite, then the satellite will cross the feature from opposite directions and the ascending and descending signals will be opposite to each other. If the feature does not trend between the ascending and descending tracks, then the satellite will cross the feature from the same side and the deflection of the vertical signal will be similar for both the ascending and descending data sets. A third factor affecting the deflection of the vertical signal is the latitude at which the feature is located. The trend of the satellite track varies as a function of latitude, ranging from 18° at 0° latitude to 64.6° at 70° latitude. Because the trend of the satellite track varies, not only does the angle between the trend of a feature and the trend of the satellite track vary with latitude, but the amplitude of the deflection of the vertical signal varies with latitude as well.Item A new approach to the determination of a mean sea surface model using multi-satellite altimeter data(2015-05) Kim, Hyo-Jin; Tapley, Byron D.Models for the mean sea surface (MSS) are created by combining and interpolating on a specified spatial grid inhomogenous data sets from different satellites with different ground track coverage. There are various approaches in which the sea surface height (SSH) data from different satellites can be combined to create an accurate reference surface. The orbit errors (especially from the early missions) need to be reduced, and systematic biases between different satellites can be decreased by re-processing them using the improved models and geophysical corrections. In this research, a new method for the data adjustment (or error reduction), which attempts to compensate for both long-wavelength orbit errors and systematic biases, simultaneously and efficiently. The approach is based on using an accurate sea surface profile as a reference surface for the integration process. The new data adjustment technique is based on along-track SSH gradients computed for each satellite, which are integrated along-track with initial values obtained by dual crossover computation with respect to an accurate set of sea surface heights. The accurate Jason-1 SSH data were used to determine the reference surface, and a total of 5 different satellites (Geosat ERM, ERS-2, T/P, Envisat and ERS-1 geodetic mission) data were adjusted to the Jason-1 SSH data. After editing, the new homogeneous SSH datasets were averaged into mean SSH profiles. Then, they were gridded into a 5-minute resolution mean sea surface over the global ocean within ±60º latitudes, as defined by the Jason-1 mean profile, using a 2-D spline interpolation in tension with Green’s function approach. The new gridded mean sea surface, named CSRMSS14 was validated by three comparisons. First, it was compared with two accurate altimeter data sets: 7-year Jason-1 and 8-year Envisat mean profiles. Second, two recent MSS models, DNSC08 and DTU10, were compared to investigate the accuracy of CSRMSS14. Third, a somewhat independent test is obtained by comparing a 2-year Jason-2 mean profile with the three MSS models (CSRMSS14, DTU10 and DNSC08), since Jason-2 data were not used in their construction. These three validations demonstrated that CSRMSS14 mean sea surface model obtained with this new approach is comparable in accuracy to DNSC08 and DTU10.Item Pointing angle and timing verification of the geoscience laser altimeter using a ground-based detection system(2001-12) Magruder, Lori Adrian, 1971-; Schutz, Bob E.The Ice, Cloud, and land Elevation Satellite (ICESat) will begin science operations in 2002 with an emphasis on determination of the ice sheet temporal variations in the Arctic and Antarctic regions. The ICESat bus will serve as the transport for an instrument called the Geoscience Laser Altimeter System (GLAS). GLAS will provide altimetry and lidar measurements with a high level of accuracy. To meet the scientific goals of the mission, specific accuracy requirements for the GLAS data products have been established. For example, the laser pointing angle must be known to within 1.5 arcseconds while the time tag must have an accuracy of 0.1 msec. Both of these data products contribute to the determination of the measured altitude vector from the spacecraft to the ice surface. A unique calibration technique has been developed for verification of the pointing direction and the time tag of the GLAS measurement. This calibration technique is a ground-based system comprised of electro-optical detectors distributed in a grid along the ground track of the satellite. The detectors will trigger "on" when illuminated by the 1064 nm wavelength of the laser footprint. Based on the GPS coordinates of the illuminated detectors and the time tag recorded on the arrival of the pulse, the centroid of the laser footprint can be determined to within 4.5 m, corresponding to 1.5 arcsecond pointing accuracy, and the time tag is determined to within 0.1 msec. This in situ measurement of the footprint location and time tag from the ground array will be compared to the corresponding data products provided by GLAS. The comparison will verify accuracy or will indicate the existence of any errors in the GLAS pointing knowledge or timing determinations. The detectors have been designed and tested in the laboratory. Using a laser pulse similar to what is expected from GLAS, the detectors were analyzed for energy level detectability, system stability, temperature response, and overall performance. In addition, simulations were created to determine possible error sources during the calibration implementation as well as the array sizing and the grid spacing.