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NASA NTRS1972

The geocentric orientation vector from limited Astro-geodetic data

Test computations using the comprehensive sets of both astro-geodetic and gravimetric data indicate that the accuracy of the geocentric orientation vector determined from incomplete representations of the earth's gravitational fields, is dependent on the overall extent of the datum. The lack of an astro-geodetic determination of the geoid due to an inadequate density of astro-geodetic stations over certain parts of the datum does not materially affect these determinations. The effect is only marginal if the station density of the area referred to does not markedly differ from the average for the region.

Mather, R. S.↗

NASA NTRS1972

Determination of mean surface position and sea state from the radar return of a short-pulse satellite altimeter

Using the specular point theory of scatter from a very rough surface, the average backscatter cross section per unit area per radar cell width is derived for a cell located at a given height above the mean sea surface. This result is then applied to predict the average radar cross section observed by a short-pulse altimeter as a function of time for two modes of operation: pulse-limited and beam-limited configurations. For a pulse-limited satellite altimeter, a family of curves is calculated showing the distortion of the leading edge of the receiver output signal as a function of sea state (i.e., wind speed). A signal processing scheme is discussed that permits an accurate determination of the mean surface position--even in high seas--and, as a by-product, the estimation of the significant seawave height (or wind speed above the surface). Comparison of these analytical results with experimental data for both pulse-limited and beam-limited operation lends credence to the model. Such a model should aid in the design of short-pulse altimeters for accurate determination of the geoid over the oceans, as well as for the use of such altimeters for orbital sea-state monitoring.

Barrick, D. E.↗

NASA NTRS1972

Error analysis of earth physics satellite systems

Error analysis of distant-satellite-to-close-satellite range-rate, satellite-to-sea altimetry, and ground station to satellite range are made by simulations in which observational variances are assumed, observation equations are formed, and normal equations incremented. The final normal equation matrix is inverted to obtain standard deviations and correlation coefficients. The natural parameters solved for are the broad variations of the gravity field, represented by harmonic coefficients; local variations of gravity, represented by point masses; and the departure of the sea level from the geoid, represented by area means. A standard case of a low (263 km) polar close satellite, three equatorial geosynchronous satellites, and eight ground tracking stations is set up.

Kaula, W. M.↗

NASA NTRS1973

A solution of the geodetic boundary value problem to order e3

A solution is obtained for the geodetic boundary value problem which defines height anomalies to + or - 5 cm, if the earth were rigid. The solution takes into account the existence of the earth's topography, together with its ellipsoidal shape and atmosphere. A relation is also established between the commonly used solution of Stokes and a development correct to order e cubed. The data requirements call for a complete definition of gravity anomalies at the surface of the earth and a knowledge of elevation characteristics at all points exterior to the geoid. In addition, spherical harmonic representations must be based on geocentric rather than geodetic latitudes.

Mather, R. S.↗

NASA NTRS1972

Geodetic satellite observations in North American (solution NA-9)

A new detailed geoidal map with claimed accuracies of plus or minus 2 meters (on land), based on gravimetric and satellite data, was presented. With the new geoid and the orthometric heights given, more reliable height constraints were calculated and applied. The basic purpose of this experiment was to compute the new solution NA9 by defining the origin of the system, from the point of view of error propagation, in the most favorable position applying inner constraints and imposing new weighted height constraints to all of the stations. The major differences with respect to formerly published adjustments are presented.

Mueller, I. I.↗

NASA NTRS1973

Simultaneous observation solutions for NASA-MOTS and SPEOPT station positions on the North American datum

Simultaneous observations of the GEOS-I and II flashing lamps by the NASA MOTS and SPEOPT cameras on the North American Datum (NAD) were analyzed using geometrical techniques to provide an adjustment of the station coordinates. Two separate adjustments were obtained. An optical data only solution was computed in which the solution scale was provided by the Rosman-Mojave distance obtained from a dynamic station solution. In a second adjustment, scaling was provided by processing simultaneous laser ranging data from Greenbelt and Wallops Island in a combined optical-laser solution. Comparisons of these results with previous GSFC dynamical solutions indicate an rms agreement on the order of 4 meters or better in each coordinate. Comparison with a detailed gravimetric geoid of North America yields agreement of 3 meters or better for mainland U.S. stations and 7 and 3 meters, respectively, for Bermuda and Puerto Rico.

Reece, J. S.↗

NASA NTRS1973

Earth physics, overview

Satellite applications in earth and ocean dynamic studies are considered for: earthquake hazard assessment and alleviation; prediction of general ocean circulation, surface currents, and heat transport; monitoring of transient phenomena of the ocean surface, such as sea state and wave conditions, wind-surface interactions and storm searches; and refinement of the global geoid, the gravity and magnetic fields of the earth.

Vonbun, F. O.↗

NASA NTRS1973

A gravitational field model for the earth.

Two models of the earth's gravitational field have been computed. The first, Goddard Earth Model 1 (GEM 1), has been derived from satellite tracking data. The second, Goddard Earth Model 2 (GEM 2), has been derived from a combination of satellite tracking and surface gravimetric data. The satellite data consisted primarily of optical data processed on 300 weekly orbital arcs for 25 close earth satellites. Surface gravity data were employed in the form of 5 x 5 deg mean free-air gravity anomalies providing about 70% world coverage. Station locations were obtained for 46 tracking sites by combining electronic, laser, and additional optical tracking data with the above satellite data. Analysis of the radial positions of these stations and a value of mean gravity on the geoid indicated a mean equatorial radius for the earth of about 6378.145 meters. Results of geopotential tests on satellite data not used in the solution show that better agreement was obtained with the GEM 1 and GEM 2 models than with the 1969 Smithsonian Standard Earth II model.

Smith, D. E.↗

NASA NTRS1972

Gravimetric geodesy and sea surface topography studies by means of satellite-to-satellite tracking and satellite altimetry

A satellite-to-satellite tracking experiment is planned between ATS-F and GEOS-C with a range accuracy of 2-meters and a range rate accuracy of 0.035 centimeters per second for a 10-second integration time. This experiment is planned for 1974. It is anticipated that it will improve the spatial resolution of the satellite geoid by half an order of magnitude to about 6 degrees. Longer integration times should also permit a modest increase in the acceleration resolution. Satellite altimeter data will also be obtained by means of GEOS-C. An overall accuracy of 5-meters in altitude is the goal. The altimeter, per se, is expected to have an instrumental precision of about 2 meters, and an additional capability to observe with a precision of about 0.2 meters for limited periods.

Siry, J. W.↗

NASA NTRS1974

Engineering studies related to the GEOS-C radar altimeter

Experiment requirements, technical characteristics, and GEOS-C radar altimeter related analyses are discussed along with results of a study on engineering test data requirements. Statistical analyses related to determination of wave height resolution achievable as a function of system characteristics and averaging period are described, in addition to a discussion on the desirability of using computer procedures to compensate for altitude tracker time-jitter. Data processing considerations for the GEOS-C system are examined. An extensive analysis of the spatial filter effect is given and results of a computation of geoidal power spectral density, based on Skylab altimeter data, is displayed and interpreted in terms of projected GEOS-C random errors. This information is then used in deriving minimum-mean-square filter procedures for both geoid undulation and slope data. The characteristics of mean received waveforms as a function of off-nadir angle are used to obtain tracker bias as a function of sea state and pointing angle. The angle estimation process proposed by the GEOS-C hardware contractor is also investigated from a standpoint of achievable angular resolution.

Miller, L. S.↗

NASA NTRS1974

Gulf Stream model

Surface elevation deviations in the Gulf Stream region off the eastern coast of the United States between Wallops Island, Virginia and Miami, Florida were investigated. The main causes of surface elevation deviations are geoid perturbations due to the continental shelf and the geostrophic adjustment of the density field due to the Gulf Stream. Quantitative surface elevation profiles were calculated based on geophysical measurements of gravity anomalies and hydrographic data. The results are presented graphically along with contemporaneous weather data. Comparisons are made between the profiles based on hydrographic data and a mean theoretical model. The theory of geostrophic flows including some classical Gulf Stream models is also presented briefly.

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NASA NTRS1972

Interaction of marine geodesy, satellite technology and ocean physics

The possible applications of satellite technology in marine geodesy and geodetic related ocean physics were investigated. Four major problems were identified in the areas of geodesy and ocean physics: (1) geodetic positioning and control establishment; (2) sea surface topography and geoid determination; (3) geodetic applications to ocean physics; and (4) ground truth establishment. It was found that satellite technology can play a major role in their solution. For solution of the first problem, the use of satellite geodetic techniques, such as Doppler and C-band radar ranging, is demonstrated to fix the three-dimensional coordinates of marine geodetic control if multi-satellite passes are used. The second problem is shown to require the use of satellite altimetry, along with accurate knowledge of ocean-dynamics parameters such as sea state, ocean tides, and mean sea level. The use of both conventional and advanced satellite techniques appeared to be necessary to solve the third and fourth problems.

Mourad, A. G.↗

NASA NTRS1975

Earth and ocean physics

A procedure for obtaining a parameterization of the marine geoid for suitable orthogonality properties in altimetry data is discussed. The application of the technique to the Puerto Rico trench is explained and a map of the data is developed. The Goddard Earth Model (GEM-6) is described to show the method for determining the earth gravity field using data obtained from satellite tracking stations. The derivation of a global ocean tide model from satellite data is explained. The influence of solid earth and ocean tides on the inclination of GEOS-1 is plotted. The delineation of the geographical fracture pattern and boundary system of the tectonic plates using ERTS satellite is shown.

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NASA NTRS1974

Scientific and economic potential of the SEASAT Program

The SEASAT satellite system is planned as a user-oriented system for timely monitoring of global ocean dynamics and mapping the global ocean geoid. The satellite instrumentation and modular concept are discussed. Operational data capabilities will include oceanographic data services, direct satellite read-out to users, and conversational retrieval and analysis of stored data. A case-study technique, generalized through physical and econometric modeling, indicates potential economic benefit from SEASAT to users in the following areas: ship routing, iceberg reconnaissance, arctic operations, Alaska pipeline ship link, and off-shore oil production.

Mccandless, S. W.↗

NASA NTRS1974

Skylab radar altimeter - Short-wavelength perturbations detected in ocean surface profiles

Short-wavelength anomalies in sea surface topography, caused by the gravitational effects of major ocean bottom topographic features, have been detected by the radar altimeter aboard Skylab. Some features, such as deep ocean trenches, seamounts, and escarpments, displace the ocean surface by as much as 15 meters over 100-kilometer wavelengths. This experiment demonstrates the potential of satellite altimetry for determining the ocean geoid and for mapping major features of the ocean bottom.

Leitao, C. D.↗

NASA NTRS1974

International Symposium on Applications of Marine Geodesy, Columbus, Ohio, June 3-5, 1974, Proceedings

Requirements for marine geodesy are examined, taking into account accuracy requirements for certain marine operations, boundary and positioning problems in offshore Norway, navigation requirements for nodule exploration and mining, and the determination of marine boundaries at sea. Subjects related to marine geodesy and positioning/navigation are discussed along with topics concerned with marine geodesy and ocean physics. Satellite altimetry and modern geoids method are considered and attention is given to marine gravity anomalies and geodesy. Individual items are announced in this issue.

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NASA NTRS1974

Requirements and applications of marine geodesy and satellite technology to operations in the oceans

Marine geodesy is concerned with the determination of marine geographic positions, geodetic controls, and the geoid. The activities of marine geodesy are related to the utilization of satellite technology, electronic distance measurement, geodetic astronomy, gravimetric geodesy, and the potentially usable very long baseline interferometry. Assessments are made of the different accuracy and/or precision requirements involved in the various approaches. It is shown that marine geodesy is highly relevant to man's various practical and scientific operations at sea, including those on the continental shelves.

Mourad, A. G.↗

NASA NTRS1974

SEASAT-A - A user oriented systems design

The SEASAT-A spacecraft is tentatively scheduled for launch in 1978 into a nearly circular orbit with an altitude of approximately 800 km. The spacecraft is to demonstrate a capability for the global monitoring of wave height and spectra, surface winds, ocean temperature, and current patterns. Other objectives include the charting of ice fields and a mapping of the global geoid. The sensors to be carried by the spacecraft are discussed and a description is given of the satellite system and the subsystems.

Mccandless, S. W., Jr.↗