Correction of satellite radar altimeter data on ice-covered surfaces in Antarctica using an integrated Geographical Information System

The problem of correcting satellite altimeter measurements for errors introduced by topographic surfaces was chosen to test the capabilities of a prototype integrated geographical information system. A system to calculate and apply corrections to altimeter data has been implemented and tested using Geosat data from the Wilkins Ice Shelf, to the west of Alexander Island, West Antarctica. The method used is presented here, with particular discussion of the suitability of the GIS as a platform for performing such corrections.     

The computation of ocean wave heights from GEOS-3 satellite radar altimeter data

The GEOS-3 satellite, carrying a short pulse radar altimeter, was launched into orbit round the earth in April 1975. The altimeter was designed to provide an accurate measurement of the distance of the satellite above the earth, and also to record the shape of the radar return pulse as a measure of the mean roughness of the earth's surface. The satellite is intended to operate over the oceans, where surface height changes show variations in the earth's gravitational field, and roughness changes are due to waves.This paper is concerned with methods of determining waveheights from the shape of the radar return pulse and the corrections that have to be taken into account. The effects of timing variations on the shape of the average return pulse shape are discussed in detail. Accurate calibration of the sampling gates that measure this shape is found to be particularly critical.The waveheights deduced are compared with ground truth derived from ship reports on waveheights in the N.E. Pacific Ocean and routine measurements made at Ocean Weather Station PAPA. It is found that with suitable calibration and adjustments, the satellite measurements agree with surface observations to about 0.5 meters in $\text{H}{}_{\mathrm{<mtext>1</mtext><mtext>3</mtext>}}$.     

A model of satellite radar altimeter return from ice sheets

Abstract

Satellite-borne radar altimetry offers a unique opportunity for measuring the form and mass balance of the polar ice sheets. Changes in ice-sheet mass balance are intimately linked to climatic change and variations in the global mean sea level. However, previous altimeter measurements of ice-sheet topography have been made without the use of a well-validated model of the altimeter return. Here, we present a theoretical model of the return which, supported by both observational and experimental evidence, suggests that over vast areas of the higher altitude regions of the ice-sheets, significant radar penetration of the firn occurs at frequencies commonly used for space altimetry. This implies the need for a previously-neglected correction to height measurements which can be as much as 3-3 m, depending on the retracking method and location. Since the degree of radar penetration may exhibit variability over a range of time-scales, failure to account for the effect could lead to erroneous estimates of surface elevation change. The detection of variability in the degree of penetration is of considerable interest from the point of view of monitoring the processes of accumulation and ablation of snow over the ice-sheet surface, as the return is particularly sensitive to conditions within a few centimetres of the surface. The model has wider applications as it may be used in modified form to simulate altimeter return from all smooth surfaces which exhibit a combination of surface and volume scattering, including deserts and the surfaces of the terrestrial planets and their satellites.     

Validation of surface height from shuttle radar topography mission using shuttle laser altimeter

Spaceborne Interferometric SAR (InSAR) technology used in the Shuttle Radar Topography Mission (SRTM) and spaceborne lidar such as Shuttle Laser Altimeter-02 (SLA-02) are two promising technologies for providing global scale digital elevation models (DEMs). Each type of these systems has limitations that affect the accuracy or extent of coverage. These systems are complementary in developing DEM data. In this study, surface height measured independently by SRTM and SLA-02 was cross-validated. SLA data was first verified by field observations, and examinations of individual lidar waveforms. The geolocation accuracy of the SLA height data sets was examined by checking the correlation between the SLA surface height with SRTM height at 90 m resolution, while shifting the SLA ground track within its specified horizontal errors. It was found that the heights from the two instruments were highly correlated along the SLA ground track, and shifting the positions did not improve the correlation significantly. Absolute surface heights from SRTM and SLA referenced to the same horizontal and vertical datum (World Geodetic System (WGS) 84 Ellipsoid) were compared. The effects of forest cover and surface slope on the height difference were also examined. After removing the forest effect on SRTM height, the mean height difference with SLA-02 was near zero. It can be further inferred from the standard deviation of the height differences that the absolute accuracy of SRTM height at low vegetation area is better than the SRTM mission specifications (16 m). The SRTM height bias caused by forest cover needs to be further examined using future spaceborne lidar (e.g. GLAS) data.     

Synthesis of passive microwave and radar altimeter data for estimating accumulation rates of dry polar snow

In this paper, we compare dry-snow extinction coefficients derived from satellite radar altimeter data with brightness temperature data from passive microwave measurements over a portion of the East Antarctic plateau. The comparison between the extinction coefficients and the brightness temperatures shows a strong negative correlation, where the correlation coefficients ranged from –0·87 to –0·95. The large-scale trend shows that the extinction coefficient of the dry polar snow decreases with increasing surface elevation, while the average brightness temperature increases with surface elevation. Our analysis shows that the observed trends are related to geographical variations in scattering coefficient of snow, which, in turn, are controlled by variations in surface temperature and snow accumulation rate. By combining informa.tion present in the extinction coefficient and brightness temperature datasets, we develop a simple semi-empirical model that can be used to obtain accumulation rate estimates of dry polar snow.     

An interoperable spatiotemporal weather radar data dissemination system

This paper investigates an interoperable framework to disseminate Earth Science data to different application domains. The proposed framework can manage different Earth science data products and raster snapshots over time through the use of relevant metadata information. The framework generates images to be accessed by GIS software for various Earth science and web‐based applications. The access is enabled through the compliance with OpenGeospatial Consortium's Web Map Service (WMS) for interoperability such that any WMS viewer can access the service. The framework can provide GIS users the capability to incorporate geospatial information from other WMS servers. Using the United States NEXt generation weather RADar (NEXRAD) data, we demonstrate how the proposed framework can facilitate the dissemination of Earth Science data to a broad community in a near real‐time fashion. The proposed framework can be used to manage and disseminate various types of spatiotemporal Earth science data.     

Averaging of radar altimeter pulse returns with the interpolation tracker†

Abstract

Radar altimeters have tracking systems which measure the range to the surface and produce averages of the radar pulse returns. Besides the range measurement, additional geophysical information can be extracted from the average pulse return. This paper describes and analyses a novel tracking and averaging algorithm, the interpolation tracker. It produces undistorted estimates of the mean pulse return in the presence of frequent gaps in the data stream. The theoretical analysis is confirmed by calculated first-order statistics from experimental data.     

Global structure of marine wind speed variability derived from TOPEX altimeter data

As the largest source of momentum for the ocean surface, wind affects the full range of oceanic motion—from individual surface waves to complete current systems. The marine surface wind is among the critical geophysical parameters which determine the most fundamental aspects of the ocean. Using six years (1993–1998) of TOPEX altimeter data with an unprecedented accuracy and continuity, a detailed investigation of the global structure of marine wind climatology and variability is carried out. It is found that the overall pattern of wind climatology is basically determined by solar radiation and therefore dominated by zonal features, while that of the wind variability is largely ‘event’ determined and thereby dominated by regional features. Consequently, wind climatology and wind variability show a complex relationship in their magnitude of intensity. Strong winds may be associated with high variabilities, such as in the westerlies of the North Atlantic and North Pacific; they may also be associated with low variabilities, such as in the westerlies of the Southern Ocean. Meanwhile, weak wind zones like the doldrums in the western equatorial Pacific can have a very low level of annual variability, while a very high level of interannual variability. The Asian monsoon system has a lower than average climatological mean speed, but exhibits extremely high annual variability. The phase distributions of wind variations carry combined information of climatology and variability. Effects of the Asian monsoon and marine storms are manifested on top of the basically zonal phase pattern. Statistics suggest that semiannual variability exceeds annual variability for 12.2% of the world's oceans, and interannual variability exceeds annual variability for 26.4% of the world's oceans.     

Seasonal-to-decadal modes of global sea level variability derived from merged altimeter data

Motivated by the first availability of more than 16 years (1992-2009) of merged altimeter data from up to four concurrent satellite missions with TOPEX/Poseidon, ERS-1 and 2, ENVISAT, Jason-1, and Geosat Follow-On, the global sea level is reexamined with special emphasis on its distinctive scales of variability. The global sea level variability is dominated by eleven statistically significant and geographically robust principal modes at seasonal-to-decadal timescales. The identified modes can be divided into four regimes, i.e., the seasonal regime with two principal components at 3 and 6 months, the annual regime with a single 12-month component of global dominance, the interannual regime with seven principal components at 1.55, 1.74, 1.94, 2.34, 3.07, 4.20, and 5.40 years, as well as the decadal regime with a 9.28-year subdecadal component. The frequency dispersion of the identified modes is found to be at ∼ 0% level for the annual regime, at ∼ 5% level for the seasonal and interannual regimes, and at ∼ 10% level for the decadal regime. In the space domain, the locations of principal modes are found to be geographically correlated. The seasonal regime is characterized by a small number of localized amplitude highs in regional seas. The annual regime is associated with major western boundary currents of the Gulf Stream and Kuroshio. For the interannual regime, mode active zones are observed in the western Pacific warm pool (WPWP), El Niño, and western Indian Ocean regions, among which the WPWP has a well-defined two-core structure, and the northern core coincides with the most energetic zone of the decadal regime. Another interesting finding is that east-west (zonal) and equatorial-extratropical (meridional) teleconnections are evident for the major signals in a rather symmetric and systematic pattern. Joint spatiotemporal analysis reveals that multi-modality at a given location and multi-regionality for a given mode are fundamental features in global sea level variability.     

Refined analysis of radar altimetry data applied to the region of the subglacial Lake Vostok/Antarctica

Satellite altimetry is a powerful tool to map the ice sheet elevation as well as a number of other parameters linked to geometrical and geophysical properties of ice sheets. Irrespective of new instrumental developments like the laser altimeter on ICESat (Ice, Cloud and land Elevation Satellite) the well-established radar altimeter (RA) missions ERS-1,2 (European Remote Sensing satellites) and ENVISAT (ENVIronment SATellite) are unique in their temporal coverage over more than a decade and in their temporal and spatial sampling. Therefore, the full exploitation of these RA data by improved methods is imperative. Here we develop improved techniques to correct for topographically induced errors by a refined consideration of the relevant topographic conditions. Furthermore we improve the gridding procedure by adapting it to local conditions and thus preserving smaller-scale features. We apply our methods to the region of the subglacial Lake Vostok/Antarctica and derive digital elevation models (DEMs) for this region with the aim of improving/resolving smaller scales. The effect of our improvements is demonstrated in detail by comparing our DEMs and previously published DEMs to the ICESat laser measurements which are taken as a reference here. Due to our improvements, the standard deviation of the difference between RA-based DEMs of the Lake Vostok region and ICESat data decreases from more than 1.1 m to 0.5 m. This remaining error is chiefly the error inherent in the RA observations. Our RA-ICESat comparisons, supported by Fourier analyses, also reveal the presence and importance of small-scale features that can be detected by laser but not by the RA measurements.     

地形对雷达高度计卫星自主导航精度影响分析

以中低轨道卫星为应用背景,对基于星载雷达高度计的卫星自主导航系统展开研究,着重分析了雷达高度计卫星自主导航中的关键因素,即地形起伏对导航精度的影响。根据实际地形的统计特性,并结合导航精度仿真分析的需要,建立了模拟地形的数学模型;对不同地形条件下雷达高度计卫星自主导航系统的导航精度进行了仿真研究,分析了不同的地形条件对雷达高度计卫星自主导航精度的影响。研究的结论对雷达高度计自主导航技术的实际应用将具有重要的参考价值。     

Global structure of extreme wind and wave climate derived from TOPEX altimeter data

Sea surface wind speed and significant wave height (SWH) are two basic parameters, in addition to sea surface height, which can be inferred from satellite altimeter measurements. Traditionally, altimeter-derived wind and wave data are less extensively used compared to sea surface height, as they are sometimes considered as by-products of satellite altimetry (in contrast to, for example, the dedicated scatterometer missions for marine wind observations). However, it is clear that altimeter-based wind and wave data have the unique advantage of being concurrent and collocated with each other. Using eight years (1993–2000) of TOPEX altimeter data with unprecedented accuracy and continuity, the 10-, 50- and 100-year return values of global wind speed and SWH are derived, their characteristics are discussed in relation to wind climatology and wind variability. Validations against in situ observations indicate that the uncertainties of altimeter-derived extreme winds and waves are at the levels of 10% and 5%, respectively. These results suggest that satellite altimeter data, with present quality and duration, can be very useful in many aspects of coastal engineering and marine technology such as design of offshore facilities, ship routing, and preparation of other sea-going activities.     

Subglacial water presence classification from polar radar data

Ground and airborne radar depth-sounding of the Greenland and Antarctic ice sheets have been used for many years to remotely determine characteristics such as ice thickness, subglacial topography, and mass balance of large bodies of ice. Ice coring efforts have supported these radar data to provide ground truth for validation of the state (wet or frozen) of the interface between the bottom of the ice sheet and the underlying bedrock. Subglacial state governs the friction, flow speed, transport of material, and overall change of the ice sheet. In this paper, we utilize machine learning and classifier combination to model water presence from airborne polar radar data acquired on Greenland in 1999 and 2007. The underlying method results in radar independence, allowing model transfer from 1999 to 2007 radar data to produce water presence maps of the Greenland ice sheet with differing radars. We focus on how to construct a successful set of classifiers capable of high classification accuracy. Utilizing multiple machine learning algorithms is shown to be successful for this classification problem, achieving 86% classification accuracy in the best case. Several heuristics are presented for constructing teams of multiple classifiers for predicting subglacial water presence. The presented methodology could also be applied to radar data acquired over the Antarctic ice sheet.     

Comparison of Envisat radar and airborne laser altimeter measurements over Arctic sea ice

Sea ice thickness is a crucial, but very undersampled cryospheric parameter of fundamental importance for climate modeling. Advances in satellite altimetry have enabled the measurement of sea ice freeboard using satellite microwave altimeters. Unfortunately, validation of these new techniques has suffered from a lack of ground truth measurements. Therefore, an airborne campaign was carried out in March 2006 using laser altimetry and photo imagery to validate sea ice elevation measurements derived from the Envisat/RA-2 microwave altimeter.We present a comparative analysis of Envisat/RA-2 sea ice elevation processing with collocated airborne measurements collected north of the Canadian Archipelago. Consistent overall relationships between block-averaged airborne laser and Envisat elevations are found, over both leads and floes, along the full 1300 km aircraft track. The fine resolution of the airborne laser altimeter data is exploited to evaluate elevation variability within the RA-2 ground footprint. Our analysis shows good agreement between RA-2 derived sea ice elevations and those measured by airborne laser altimetry, particularly over refrozen leads where the overall mean difference is about 1 cm. Notwithstanding this small 1 cm mean difference, we identify a larger elevation uncertainty (of order 10 cm) associated with the uncertain location of dominant radar targets within the particular RA-2 footprint. Sources of measurement uncertainty or ambiguity are identified, and include snow accumulation, tracking noise, and the limited coverage of airborne measurements.     

Multi-look processing of synthetic aperture radar data from dynamic ocean surfaces

The basic theory is described of multi-look processing of synthetic aperture radar (SAR) data from dynamic ocean surfaces. The principal effects inherent to multi-look SAR on the impulse response function are first illustrated using a moving point target model. The theory is then extended to the multi-look imagery of diffusely scattering ocean surface waves. Finally, the use of a sliding synthetic aperture is suggested to investigate the short life-times of small scale surface waves that are considered to be the predominant scatterers on the ocean surface.     

An explanation of enhanced radar backscattering from flooded forests

Abstract

A simpie structural backscatter model for a forest stand, suitable for use with L-band HH polarized radar imagery, is used to explain the increased level of backscattering observed from flooded forests. Measurements made of relative levels of backscatter from SIR-B image data of a flooded Australian forest are consistent with an interpretation based upon scattering mechanisms involving both the tree components and the understorey or forest floor. The change in Fresnel power reflection coefficient of the ground with flooding is advanced as the cause of the enhancement in backscattered power levels.     

An AVHRR mosaic image of Antarctica

Abstract

This project was started in October 1985 with the objectives of (1) producing a 1 km-resolution database of Antarctica and (2) producing a 1:5 000000 scale image map. So far, 28 three-band AVHRR scenes have been corrected for variable Sun reflectance and transformed to a polar stereographic projection using a digitized coastline for control. Relatively cloud-free segments from 18 scenes have been incorporated in a provisional mosaic and further segments are to be included to improve the picture.     

On the utilization of ERS-1 altimeter data for offshore oil exploration

In this paper, methods for the generation of residual and prospecting geoids and free-air gravity anomaly using satellite altimeter data are developed. Maps of residual geoid, gravity anomaly, and their spectral components generated over Indian offshore using ERS-1 altimeter data are presented and interpreted. A number of known megastructures over the study area, for example, Bombay High, Saurastra platform, 90 east ridge, etc., are interpreted successfully from these maps. Some of the anomalous zones (the basins near Mangalore and off Bombay High in the western offshore and Palar basin off Madras coast in the eastern offshore) have been identified as potential sites for occurrences of hydrocarbon-bearing structures, for which detailed ship-borne geophysical surveys have been commissioned by the Oil & Natural Gas Corporation (ONGC), Dehradun, India.     

Simulation of radar with a knife-like antenna beam using precipitation radar data

The concept of radar using a knife-like antenna beam is discussed and compared with precipitation radar (PR). An example of PR data processing is given. Two approaches are used to retrieve variance of slope: point-to-point algorithms and a linear angular algorithm. The algorithm for the PR data transformation into data of radar with a knife-like antenna beam has been developed. Transformation of real satellite data into data of radar with a knife-like antenna beam enables one for the first time to simulate measurements of future space radar qualitatively. Transformed data are used to validate the retrieval algorithms developed for radar with a knife-like antenna beam. Numerical simulation is also used to test the retrieval algorithms. The noise effect on the precision of slope variance retrieval for the PR and radar with a knife-like antenna beam is considered. Data processing confirms that the precision of radar with a knife-like antenna beam is significantly higher than the precision of PR. Numerical simulation is also used to explore the possibility of sea-surface slope imaging. The obtained results confirm that only radar with a knife-like antenna beam can be employed to retrieve real images of the processes occurring on sea-surface slopes.     

Preprocessing of side-looking airborne radar data†

Abstract

Studies on microwave surface scattering in The Netherlands have indicated the need for accurate radar systems for applications in remote sensing. An SLAR system with digital recording was developed and is now being used for several programmes. This system was designed with special attention to speckle reduction and system accuracy. The digital data are processed in a computer with algorithms for geometric and radiometric corrections. In the future aircraft position and attitude measurements will also be used in these correction algorithms. Examples of the results are shown.