The AUSGeoid98 geoid model of Australia: data treatment, computations and comparisons with GPS-levelling data

 The AUSGeoid98 gravimetric geoid model of Australia has been computed using data from the EGM96 global geopotential model, the 1996 release of the Australian gravity database, a nationwide digital elevation model, and satellite altimeter-derived marine gravity anomalies. The geoid heights are on a 2 by 2 arc-minute grid with respect to the GRS80 ellipsoid, and residual geoid heights were computed using the 1-D fast Fourier transform technique. This has been adapted to include a deterministically modified kernel over a spherical cap of limited spatial extent in the generalised Stokes scheme. Comparisons of AUSGeoid98 with GPS and Australian Height Datum (AHD) heights across the continent give an RMS agreement of ±0.364 m, although this apparently large value is attributed partly to distortions in the AHD. Received: 10 March 2000 / Accepted: 21 February 2001     

On the spectral consistency of the altimetric ocean and geoid surface: a one-dimensional example

Geoid models from the new generation of satellite gravity missions, such as GRACE and GOCE, in combination with sea surface from satellite altimetry allow to obtain absolute dynamic ocean topography with rather high spatial resolution and accuracy. However, this implies combination of data with fundamentally different characteristics and different spatial resolutions. Spectral consistency would imply the removal of the short-scale features of the altimetric sea surface height by filtering, to provide altimetric data consistent with the resolution of the geoid field. The goal must be to lose as little as possible from the high precision of the altimetric signal. Using a one-dimensional example we show how the spectrum is changing when a function defined only on a limited domain (ocean in the real case) is extended or not as to cover the complete domain (the whole sphere in the real case). The results depend on the spectral characteristics of the altimetric signal and of the applied filter. Referring to the periodicity condition, as it is requested in the case of Fourier analysis, the action of the two classical filters (Ideal Low Pass and Gauss filter) and of two alternative procedures (wavelets and Slepian) is studied.     

Two new methods for solving large scale least squares in geodetic surveying computations

This paper considers the solution of linear least squares problems arising in space geodesy, with a special application to multi-station adjustment by a short arc method based on Doppler observations. We recall briefly the widely used second-order regression algorithm due to Brown for reducing the normal equations system. Then we propose two algorithms which avoid the use of the normal equations. The first one is a direct method that applies orthogonal transformations to the observation matrix directly, in order to reduce it to upper triangular form. The solution is then obtained by backsubstitution. The second method is iterative and uses a preconditioned conjugate gradient technique. A comparison of the three procedures is provided on data of the second European Doppler Observation Campaign (EDOC-2).     

蚁群智能及其在大区域基础设施选址中的应用

提出了基于蚁群智能的空间选址模型,通过蚁群智能和GIS的结合来解决复杂的空间优化配置问题.这种启发式的智能搜索方法大大提高了空间搜索能力.为符合选址问题的求解,从信息素更新方式和禁忌表调整策略两方面对基本蚁群算法进行改进.同时,为了使得模型能实用于大区域的基础设施选址,提出了"分步逼近"的策略,取得了较好的效果.将所提出的模型应用于广州市公共没施的空间优化选址.实验结果表明,该方法比简单搜索方法和遗传算法更有优势.     

Analysis of some systematic errors affecting altimeter-derived sea surface gradient with application to geoid determination over Taiwan

This paper analyzes several systematic errors affecting sea surface gradients derived from Seasat, Geosat/ERM, Geosat/GM, ERS-1/35d, ERS-1/GM and TOPEX/POSEIDON altimetry. Considering the data noises, the conclusion is: (1) only Seasat needs to correct for the non-geocentricity induced error, (2) only Seasat and Geosat/GM need to correct for the one cycle per revolution error, (3) only Seasat, ERS-1/GM and Geosat/GM need to correct for the tide model error; over shallow waters it is suggested to use a local tide model not solely from altimetry. The effects of the sea surface topography on gravity and geoid computations from altimetry are significant over areas with major oceanographic phenomena. In conclusion, sea surface gradient is a better data type than sea surface height. Sea surface gradients from altimetry, land gravity anomalies, ship gravity anomalies and elevation data were then used to calculate the geoid over Taiwan by least-squares collocation. The inclusion of sea surface gradients improves the geoid prediction by 27% when comparing the GPS-derived and the predicted geoidal heights, and by 30% when comparing the observed and the geoid-derived deflections of the vertical. The predicted geoid along coastal areas is accurate to 2 cm and can help GPS to do the third-order leveling. Received 22 January 1996; Accepted 13 September 1996     

Improving radiometry of imaging spectrometers by using programmable spectral regions of interest

Programmable imaging spectrometers can be adjusted to fit specific application requirements that differ from the instrument initial spectral design goals. Sensor spectral characteristics and its signal-to-noise ratio (SNR) can be changed by applying customized online binning patterns.We present a software utility that generates application driven spectral binning patterns by using an SNR dependent sensor model. The utility, named BinGO (BInning patterN Generator and Optimiser), is used to produce predefined binning patterns that either (a) allow an existing imaging spectrometer to optimize its spectral characteristics for a specific application, (b) allow an existing imaging spectrometer to spectral and/or spatially emulate another instrument, or (c) design new multispectral or imaging spectrometer missions (i.e. spaceborne, airborne, terrestrial). We present a variety of BinGO case studies, including the simulation of airborne (APEX) [Itten, K.I. et al., 2008. APEX — The hyperspectral ESA Airborne Prism Experiment. Sensors 8(1), 1–25], spaceborne (SENTINEL III) [Nieke, J., Frerick, J., Stroede, J., Mavrocordatos, C., Berruti, B., 2008. Status of the optical payload and processor development of ESA’s Sentinel 3 mission. In: Proceedings of the Geoscience and Remote Sensing Symposium IGARSS 2008, pp. 427–430], as well as scientific and performance optimized approaches. We conclude that the presented approach can successfully be used to increase the efficiency of spectral information retrieval by using imaging spectroscopy data and to simulate various missions and requirements, finally supporting proper trade-off decisions to be made between performance optimization and scientific requirements. In addition, if specific sensor parameters are known, BinGO can also model other imaging spectrometers.     

Use of potential coefficient models for geoid undulation determinations using a spherical harmonic representation of the height anomaly/geoid undulation difference

 This paper suggests that potential coefficient models of the Earth's gravitational potential be used to calculate height anomalies which are then reduced to geoid undulations where such quantities are needed for orthometric height determination and vertical datum definition through a potential coefficient realization of the geoid. The process of the conversion of the height anomaly into a geoid undulation is represented by a height anomaly gradient term and the usual N–ζ term that is dependent on elevation and the Bouguer anomaly. Using a degree 360 expansion of 30′ elevations and the OSU91A potential coefficient model, a degree 360 representation of the correction terms was computed. The magnitude of N–ζ reached –3.4 m in the Himalaya Mountains with smaller, but still significant, magnitudes in other mountainous regions. Received: 6 May 1996; Accepted: 30 October 1996     

Methods for the computation of geoid undulations from potential coefficients

The undulations of the geoid may be computed from spherical harmonic potential coefficients of the earth’s gravitational field. This paper examines three procedures that reflect various points of view on how this computation should be carried out. One method requires only the flattening of a reference ellipsoid to be defined while the other two methods require a complete definition of the parameters of the ellipsoid. It was found that the various methods give essentially the same undulations provided that correct parameters are chosen for the reference ellipsoid. A discussion is given on how these parameters are chosen and numerical results are reported using recent potential coefficient determinations.     

Large dust absorption of infrared radiation over Afro-Asian regions: evidence for anthropogenic impact

There is a large uncertainty in the estimation of dust radiative forcing due to the lack of adequate data about complex spatial and temporal pattern of the radiative properties of dust. Here, we examine the temporal and spatial variability of dust absorption in the thermal infrared over the Afro-Asian regions using satellite data. Large dust absorption (nearly double compared to that of pure dust) was observed in regions with large anthropogenic influence, possibly due to deposition of black carbon on dust particles. While most of the recent estimates of global mean dust radiative forcing predicted net cooling, our studies indicate that there could be large heating due to dust over vast Afro-Asian regions. It appears that large dust heating is due to its interaction with anthropogenic black carbon.     

Generation of three-dimensional deformation maps from InSAR data using spectral diversity techniques

The capability of DInSAR (Differential Interferometric SAR) for precise large-scale deformation analysis has been shown in various case studies. Generally, DInSAR possesses a high potential for monitoring deformation, but only the velocity component parallel to the line-of-sight direction can be measured. An alternative approach, capable to retrieve the deformation velocity in both range and azimuth direction, is the so-called spectral diversity technique. Spectral diversity is based on a phase comparison between different sub-aperture interferograms of the scene and can generally be regarded as a high-performance technique for estimating the mis-registration between complex SAR images.In this paper, the following questions will be discussed: how to implement the spectral diversity technique for achieving the most accurate results; how to extract the full 3D deformation vector from a combination of ascending/descending passes and how to extract a surface deformation map if the data sets are not perfectly coherent. Finally, a statistical analysis of every individual processing step and an error propagation analysis is undertaken. In order to make a quantitative analysis of the technique, ENVISAT data sets of the Bam earthquake in Iran are used.     

Theory of oceanic levelling for improving the geoid from satellite altimetry

The problem of improving the geoid from satellite altimetry is formulated and studied within the scope of geophysical fluid dynamics. The oceanic levelling is defined by analogy to the astrogeodetic levelling and it is used to determine the sea surface topography as a function of current velocity, atmospheric pressure and viscosity. Simulating strong currents like the Gulf Stream or the Kuroshio the numerical treatment of the oceanic levelling shows that the sea surface topography can come up to an order of magnitude of1–2 m, whereby the results depend on latitude and slightly on the actual pressure conditions.     

On the solvability of the Stokes pseudo-boundary-value problem for geoid determination

A new form of boundary condition of the Stokes problem for geoid determination is derived. It has an unusual form, because it contains the unknown disturbing potential referred to both the Earth's surface and the geoid coupled by the topographical height. This is a consequence of the fact that the boundary condition utilizes the surface gravity data that has not been continued from the Earth's surface to the geoid. To emphasize the `two-boundary' character, this boundary-value problem is called the Stokes pseudo-boundary-value problem. The numerical analysis of this problem has revealed that the solution cannot be guaranteed for all wavelengths. We demonstrate that geoidal wavelengths shorter than some critical finite value must be excluded from the solution in order to ensure its existence and stability. This critical wavelength is, for instance, about 1 arcmin for the highest regions of the Earth's surface. Furthermore, we discuss various approaches frequently used in geodesy to convert the `two-boundary' condition to a `one-boundary' condition only, relating to the Earth's surface or the geoid. We show that, whereas the solution of the Stokes pseudo-boundary-value problem need not exist for geoidal wavelengths shorter than a critical wavelength of finite length, the solutions of approximately transformed boundary-value problems exist over a larger range of geoidal wavelengths. Hence, such regularizations change the nature of the original problem; namely, they define geoidal heights even for the wavelengths for which the original Stokes pseudo-boundary-value problem need not be solvable. Received 11 September 1995; Accepted 2 September 1996     

The strategy for the determination of a new local geoid in China

The strategy for the determination of a new local geoid in China is: (1) This task should be fulfilled in a short time as possible; (2) The new local geoid can be with different resolution and accuracy, which depend on the needs of the social and economical development in different regions; (3) The newly established height anomaly control network, i.e. a gridded GPS levelling network should combine as possible with the existing astro-gravimetric levelling network; (4) Global geopotential model, DTM and surface gravity data are used to densify the resolution of the local geoid with remove-restore technique. The accuracy and resolution possibly obtained nowadays within the capability of China are discussed. The required accuracies for GPS positioning, levelling and coordinate transformation are suggested.     

Inverse Vening Meinesz formula and deflection-geoid formula: applications to the predictions of gravity and geoid over the South China Sea

Using the spherical harmonic representations of the earth's disturbing potential and its functionals, we derive the inverse Vening Meinesz formula, which converts deflection of the vertical to gravity anomaly using the gradient of the H function. The deflection-geoid formula is also derived that converts deflection to geoidal undulation using the gradient of the C function. The two formulae are implemented by the 1D FFT and the 2D FFT methods. The innermost zone effect is derived. The inverse Vening Meinesz formula is employed to compute gravity anomalies and geoidal undulations over the South China Sea using deflections from Seasat, Geosat, ERS-1 and TOPEX//POSEIDON satellite altimetry. The 1D FFT yields the best result of 9.9-mgal rms difference with the shipborne gravity anomalies. Using the simulated deflections from EGM96, the deflection-geoid formula yields a 4-cm rms difference with the EGM96-generated geoid. The predicted gravity anomalies and geoidal undulations can be used to study the tectonic structure and the ocean circulations of the South China Sea. Received: 7 April 1997 / Accepted: 7 January 1998     

Spatial analysis of landuse over Idukki district using remote sensing techniques

Use of Landsat imagery for the preparation of small scale thematic maps has proved a quicker and economic device. Visually interpreted Land use categories supported by intensive ground truth have been classified for Idukki district in Kerala State. With this quicker device, Landuse and Land Cover clasess are made, considering all the geographical conditions of the area. The classified map can certainly prove a good tool to the planners for acquiring optimal Land use of the area.     

Feasibility of the short ARC adjustment model for precise determinations of the oceanic geoid

A key element in any attempt to achieve a0.1 m precision in geoid representation via satellite altimetry is obtaining an ephemeris of comparable precision, or circumventing this requirement by adjusting the ephemeris in some way, together with the geoid. The first possibility requires extensive satellite tracking and involves an enormous number of adjustable parameters in the long arc approach. The second psssibility allows for a piece-wise treatment of short orbital arcs considered mutually independent, in which slight adjustments of the state vector parameters can compensate for an inherent modeling error. The main question to be answered when pondering the possibility of using the short arc adjustment model in SEASAT-A altimetry reductions is whether or not this method is inherently capable of representing the detailed geoid with0.1 m precision. An analysis of computer simulations provides at least a partial answer to this question by pointing out the necessary conditions in order to achieve the desired precision; under certain favorable circumstances, not described in detail, these conditions could prove to be also sufficient.     

Performance of three types of Stokes's kernel in the combined solution for the geoid

When regional gravity data are used to compute a gravimetric geoid in conjunction with a geopotential model, it is sometimes implied that the terrestrial gravity data correct any erroneous wavelengths present in the geopotential model. This assertion is investigated. The propagation of errors from the low-frequency terrestrial gravity field into the geoid is derived for the spherical Stokes integral, the spheroidal Stokes integral and the Molodensky-modified spheroidal Stokes integral. It is shown that error-free terrestrial gravity data, if used in a spherical cap of limited extent, cannot completely correct the geopotential model. Using a standard norm, it is shown that the spheroidal and Molodensky-modified integration kernels offer a preferable approach. This is because they can filter out a large amount of the low-frequency errors expected to exist in terrestrial gravity anomalies and thus rely more on the low-frequency geopotential model, which currently offers the best source of this information. Received: 11 August 1997 / Accepted: 18 August 1998     

Statistics of GPS ionospheric scintillation and irregularities over polar regions at solar minimum

A statistical study of the occurrence characteristic of GPS ionospheric scintillation and irregularity in the polar latitude is presented. These measurements were made at Ny-Alesund, Svalbard [78.9°N, 11.9°E; 75.8°N corrected geomagnetic latitude (CGMLat)] and Larsemann Hills, East Antarctica (69.4°S, 76.4°E; 74.6°S CGMLat) during 2007–2008. It is found that the GPS phase scintillation and irregularity activity mainly takes place in the months 10, 11 and 12 at Ny-Alesund, and in the months 5, 6 at Larsemann Hills. The seasonal pattern of phase scintillation with respect to the station indicates that the GPS phase scintillation occurrence is a local winter phenomenon, which shows consistent results with past studies of 250 MHz satellite beacon measurements. The occurrence rates of GPS amplitude scintillation at the two stations are below 1%. A comparison with the interplanetary magnetic field (IMF) B _y and B _z components shows that the phase scintillation occurrence level is higher during the period from later afternoon to sunset (16–19 h) at Ny-Alesund, and from sunset to pre-midnight (18–23 h) at Larsemann Hills for negative IMF components. The findings seem to indicate that the dependence of scintillation and irregularity occurrence on geomagnetic activity appears to be associated with the magnetic local time (MLT).