On the use of the dual-frequency ENVISAT altimeter to determine snowpack properties of the Antarctic ice sheet

The primary purpose of ice-sheet altimetry is to monitor the changes in ice-sheet topography which may impact on global sea-level. However, the altimetric signal is sensitive to different properties of the snowpack, and therefore can also be used to determine these properties. The radar altimeter onboard the European Space Agency's ENVISAT satellite provides a dual-frequency dataset at Ku (13.6 GHz) and S band (3.2 GHz). In this paper, these signals are studied over the Antarctic ice-sheet during the 4 first years of the mission (2002-2006), in order to retrieve snowpack properties.The altimeter signal can be described by 4 classical waveform parameters. The 4 year time-series of all these parameters are decomposed into a linear and a seasonal time component. The linear component is almost constant. The distribution of the mean parameters over the Antarctic ice-sheet shows that the altimeter signal is sensitive to small-scale (mm) surface roughness.For the first time, the amplitudes and phases of the seasonal variations are characterized. The S band amplitudes are greater than the Ku band, and the phase varies over the entire ice-sheet. Previous studies suggested that the seasonal variations of the altitude from the altimeter are created by a decrease of the snowpack height through compaction. The dual-frequency observations shown here suggest that this hypothesis is too simple. Instead, the altitude variations observed in the altimetric signal are not created by the snowpack height change, but are more likely caused by the seasonal change of the snow properties, which cause a different response between the S and Ku bands. Therefore, both the linear and the seasonal variations of the altimetric signal can be used to retrieve snowpack properties.Here, we compare the dual-frequency ENVISAT signal with a model of the altimetric echo over the Antarctic ice-sheet. The model combines a surface model with a sub-surface model, for both the S and Ku bands. The Brown model [Brown G. S. (1977). The average impulse response of a rough surface and its applications. IEEE Transactions on Antennas and Propagation, 25, 1.] is used to describe the interaction of the radar wave with the snow surface. The backscatter coefficient of the surface is derived using the IEM method [Fung, A. K. (1994). Microwave scattering and emission models and their applications, Boston, MA: Artech House.]. The sub-surface signal takes into account both the layering effects and the scattering caused by the homogeneous media which is composed of small snow grains. The model is tested in two areas of the Antarctic plateau which present very different waveform parameters. The sensitivity of the radar signal to the different snowpack properties is investigated. The analysis of the waveform behaviours shows that the sub-surface signal can be completely masked by the small-scale surface roughness signal.Finally, the temperature and surface density effects are investigated in order to explain the seasonal variations of the altimetric signal. Both the temperature and the compaction rate of the snow change seasonally. Temperature is shown to impact on the Ku band signal. Furthermore, the compaction rate of the snow surface can explain all of the seasonal variation characteristics observed at both the S and Ku bands. The seasonal change of compaction rate in the snow creates a change in the waveform shape that can bias the altitude. In particular, the snow compaction can induce a bias in the retrieved altimetric altitude of more than 80 cm for the Ku band and 1.5 m for the S band. This work underlines that the altitude time-series needs to be corrected for the shape of the altimetric echo over ice-sheets.     

Dual-frequency altimeter signal from Envisat on the Amery ice-shelf

In Antarctica, radar altimeter measurements are sensitive to dielectric and penetration properties of the sensed medium (snow) such that the spacecraft's altitude can be biased. Since 2002, relatively low frequency radar measurements over the Amery Ice Shelf, east Antarctica, have been acquired using the Envisat dual frequency altimeter at S (3.2 GHz) and Ku (13.6 GHz) bands, which penetrate a few meters into the firn.The altimeter signal is however modified in summer by the presence of snowfilled crevasses. Indeed, the specularity of the snow surfaces in summer makes the altimetric signal sensitive mostly to nadir echoes, that increases the ratio between the crevasse signal and the surrounding ice-shelf signal at nadir. Crevasses are distinguished by differences in backscattering behavior compared with the surrounding ice-shelf signal. Crevasses are characterized by a strong backscatter coefficient at Ku band and anomalies in the S band altitude estimation. These two characteristics make snowfilled crevasses detectable by the dual frequency altimeter of Envisat.We first retrieve the geometric properties of the crevasses using a hyperbolic shape function, created by strong crevasse backscatter in the Ku waveform measurements. From this retrieved crevasse signal and further waveform analysis, we assess the properties of the snow surface and its sub-surface. The crevasse, due to its small size compared to the altimeter footprint, is found to be an excellent target to study snow properties of the ice-shelf.The anomalies in the S band altitude measurements over crevasses can then be explained by the presence of a double echo in the S band waveforms. This echo is attributed to a reflection at the base of the snowbridge, where we see evidence of sub-surface echos in the individual altimeter waveforms. Based on this observation, a methodology is developed to estimate the thickness of the snowbridge. We calculate the penetration depths in the summer snow surface of the Amery at Ku band, that is found to be around 6 m.     

Ice sheet survey over Antarctica using satellite altimetry: ERS-2, Envisat,SARAL/AltiKa,the key importance of continuous observations along the same repeat orbit

From September 2002 to October 2010, the Envisat radar altimeter surveyed Greenland and Antarctica ice sheets on a 35 day repeat orbit, providing a unique data set for ice sheet mass balance studies. Up to 85 repeat cycles are available and the whole Envisat data set may be along-track processed in order to provide height variability and trend with a good spatial resolution for the objectives of ice sheet survey.

Soon, a joint Centre National d’Etudes Spatiales/Indian Space Research Organisation mission, SARAL (Satellite with Argos and AltiKa), with the AltiKa payload on board, will be launched on exactly the same orbit (less than 1 km of the nomimal orbit in the across-track direction). This will allow an extension of previous European Remote Sensing (ERS) satellite, ERS-1 and ERS-2, and Envisat missions of the European Space Agency (ESA), in particular from the point of view of ice altimetry. However, AltiKa operates in the Ka band (36.8 GHz), a higher frequency than the classical Ku band (13.6 GHz), leading to important modifications and potential improvements in the interactions between radar wave and snow-pack.

In this paper, a synthesis is presented of all available information relevant to ice altimetry scientific purposes as derived from the Envisat mission: mean and temporal derivatives of the height ? but also of the backscatter and of the two waveform parameters ? snow-pack change corrections, across-track surface slope at 1 km scale, etc. The spatial and temporal variability of ice sheet surface elevation is investigated with the help of the high-resolution Envisat along-track observations. We show that at least 50 repeat cycles are needed to reach the required accuracy for the elevation trend. It is thus advocated as potentially highly beneficial for SARAL/AltiKa as a follow-on mission. Moreover, the novel characteristics of SARAL/AltiKa are promising in improving our understanding of snow penetration impact.     

Inundated wetland dynamics over boreal regions from remote sensing: the use of Topex‐Poseidon dual‐frequency radar altimeter observations

This study presents a first attempt to estimate the extent and seasonality of northern wetlands using radar altimeter satellite observations. The sensitivity of the Topex‐Poseidon dual‐frequency radar altimeter to detect inundation is investigated and compared with passive and active microwave satellite measurements along with a land surface climatology database. The C band backscatter altimeter signal clearly tracks passive microwave emissivity observations of wetlands and is able to detect small flooded areas. Because of the nadir incidence angle, the radar altimeter also shows more capability to detect wetlands than the C band scatterometer. Monthly flooded areas are calculated by estimating flooded pixel fractional coverage using the altimeter C band backscatter magnitude and a linear mixing model with dual‐frequency altimeter backscatter difference, C–Ku, to account for vegetation effects. Because of the Topex‐Poseidon satellite spatial coverage, the results are given only from 40° N to 66° N. This region nevertheless represents more than 30% of world's inundated surfaces during the summer. A direct validation of the inundated extent is unfortunately impossible on a large scale, due to the scarcity of quantitative observations. As a consequence, the results are evaluated by comparison with other existing estimates. Radar altimetry estimates, comprising natural wetlands and river/lakes, indicate a maximum inundated area of 1.86×10⁶ km² for July 1993 and 1994 as compared with 1.31×10⁶ km² from passive microwave technique and ～2.10×10⁶ km² from climatology dataset. The wetland seasonal variability derived from the altimeter and passive microwave techniques agrees well. These promising results could soon be applied to the ENVISAT dual‐frequency radar altimeter that will provide a better survey of global inundated surfaces thanks to its much more complete spatial coverage.     

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.     

Rapid change of snow surface properties at Vostok, East Antarctica, revealed by altimetry and radiometry

We present results of snow surface properties using the ENVISAT dual frequency altimeter at S (3.2 GHz) and Ku (13.6 GHz) bands and the AMSR-E microwave radiometer at frequencies ranging between 6 and 36 GHz in the Vostok region, East Antarctica. The altimetric time series observed between 2002 and 2008 show variations at 3 different time scales (daily, seasonal and inter-annual), that correlate directly with variations in the snow surface properties. In this study we focus on the analysis of the rapid daily event, occurring on February 14th 2005, that created a jump of the backscatter coefficient of up to 5.3 dB at the S band and 2.5 dB at the Ku band. The ratio of V/H-polarization brightness temperature slowly decreased in December and January 2005, and suddenly increased on February 14th 2005.The origin of this rapid event is investigated using AWS data from Vostok station, altimetric and radiometric data simultaneously. Both snow surface density and roughness are found to vary during this event. This event is shown to be synchronous with strong wind occuring during a period of anomalous wind direction, and the presence of surface hoar. These particular conditions certainly modified the snow surface roughness and thus impacted the altimetric signal. We finally investigate the impact of this event on the calculation of the regional ice-sheet mass-balance using different corrections of height with echo shape variations. It is shown to be negligible only if the full echo shape correction (Legresy et al., 2006) is used.     

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.     

A ground-based radar backscatter investigation in the percolation zone of the Greenland ice sheet

Satellite radar altimeters and scatterometers deployed over ice sheets experience backscatter from the surface and from within the snowpack, termed surface and volume backscatter respectively. In order to assess the errors in satellite altimeter measurements it is vital to know where the return is originating from in the snowpack. This return can vary spatially and temporally. Seasonal variations in the volume backscatter can be a major complicating factor in the radar return from the percolation zone. Ground-based step-frequency radar was deployed in the percolation zone of the Greenland Ice Sheet at ∼ 1945 m elevation (69 51N, 47 15W). Previous measurements in this area made by scientists from the Byrd Polar Research Centre and the University of Kansas, undertaken prior to summer melt events, have shown the strongest backscatter from ice features at around 1 m depth buried beneath the previous end-of-summer surface. In autumn 2004, radar measurements in the Ku band with bandwidths of 1 and 8 GHz were made alongside detailed stratigraphic observations within a 1 km² site. The radar results revealed no continuous reflecting horizons in the upper 3.5 m of the firn. Shallow cores and snowpits also indicated that there were no spatially continuous stratigraphic horizons across the study site. An average electromagnetic wave velocity of 2.11 ± 0.05 × 10⁸ m s^− 1 was determined for the upper metre of the firn. Surface and volume backscatter at vertical incidence were calculated using a standard model. The contribution of the surface backscatter to the total backscatter was on average 6 dB higher than that of the volume backscatter. However, at the higher 8 GHz bandwidth the strongest return frequently originated not from the surface but from within the upper 30 cm of the snowpack, most probably from thin ice layers. At 1 GHz bandwidth these ice layers were not always resolved; their return merged with the surface return, causing it to broaden, with the peak and leading edge moving down. Modelling using density and thickness measurements from shallow cores and snowpits showed that the backscatter from these shallow, thin ice layers could be stronger than the surface return owing to constructive interference from the top and base of the layers.     

基于有源定标器的卫星雷达高度计近海测高基准重建方法研究

利用卫星雷达高度计开展高精度近海高度测量面临诸多问题,重构受污染的近海回波波形,建立近海测高基准是实现高精度近海高度测量的重要基础。介绍一种利用有源定标器重建卫星雷达高度计近海测高基准的方法,以HY-2卫星雷达高度计为例,分别分析了离岸距离大于20km、离岸距离小于2km及离岸距离处于2~20km之间处近海地区雷达高度计回波波形受影响情况,并从理论上推导了该方法建立近海测高基准的精度。在2012年7月5日唐山近海地区HY-2高度计在轨定标试验中,首次观测到近海地区有源定标器转发高度计信号的回波波形,结果表明:利用有源定标器重建卫星雷达高度计近海测高基准的方法可行,该方法将有助于提高近海测高精度。     

Observations of Lake Baikal ice from satellite altimetry and radiometry

We demonstrate the potential of combining satellite altimetry and radiometry for lake ice studies using the example of Lake Baikal in Siberia. We show the synergy using active and passive microwave observations available from the recent satellite altimetry missions (TOPEX/Poseidon, Jason-1, ENVISAT and Geosat Follow-On), complemented by the SSM/I passive data. We assess the applicability of altimetric and radiometric data for ice/water discrimination, and discuss the drawbacks and benefits of each type of sensor. An ice discrimination method, based on the combined use of the data from the four altimetric missions and SSM/I, is proposed and validated using available in situ observations and MODIS imagery. The method is applied to the entire satellite data set and used to define specific dates of ice events (first appearance of ice, formation of stable ice cover, first appearance of open water, complete disappearance of ice) and associated uncertainties. Using these satellite-derived estimates, we can extend the existing time series of ice events in the Southern Baikal up to 2004 and provide new information on the Middle and Northern Baikal, regions where no recent in situ ice cover observations are available. Our data show that over the last 10-15 years, trends towards earlier ice formation and later ice break-up result in a tendency for longer fast ice duration over the whole Lake Baikal. The methods proposed here have been tested for Lake Baikal, but they are applicable for other lakes and water bodies with seasonal ice cover.     

Preliminary results of ENVISAT RA-2-derived water levels validation over the Amazon basin

Since the launch of the ENVISAT satellite in 2002, the Radar Altimetry Mission provides systematic observations of the Earth topography. Among the different goals of the ENVISAT Mission, one directly concerns land hydrology: the monitoring of the water levels of lakes, wetlands, and rivers. The ENVISAT Geophysical Data Records products contain, over different type of surfaces, altimeter ranges derived from four specialized algorithms or retrackers. However, none of the retrackers are intended to the processing of the radar echoes over continental waters. A validation study is necessary to assess the performances of the different ENVISAT-derived water levels to monitor inland waters. We have selected four test-zones over the Amazon basin to achieve this validation study. We compare first the performances of these retracking algorithms to deliver reliable water levels for land hydrology. Comparisons with in-situ gauge stations showed that Ice-1 algorithm, based on the Offset Centre of Gravity technique, provides the more accurate water stages. Second, we examine the potentiality to combine water levels derived from different sensors (Topex/Poseidon, ERS-1 and -2, GFO).     

Soil moisture mapping based on ASAR/ENVISAT radar data over a Sahelian region

The analysis of feedbacks between continental surfaces and the atmosphere is one of the key factors to understanding African Monsoon dynamics. For this reason, the monitoring of surface parameters, in particular soil moisture, is very important. Satellite remote sensing appears to be the most suitable means of obtaining data relevant to such parameters. The present paper presents a methodology applied to the mapping and monitoring of surface soil moisture over the Kori Dantiandou region in Niger, using data provided by the ASAR/ENVISAT radar instrument. The study is based on 15 sets of ASAR/ENVISAT C‐band radar data, acquired during the 2004 and 2005 rainy seasons. Simultaneously with radar acquisitions, ground soil moisture measurements were carried out in a large number of test fields. Soil moisture was estimated only for fields with bare soil or low‐density vegetation, using low‐incidence‐angle radar data (IS1 configuration). A mask was developed, using SPOT/HRV data and DTM, for use over areas characterized by high‐density vegetation cover, pools, and areas with high slopes. Soil moisture estimations are based on horizontal‐ and vertical‐polarization radar data. In order to double the temporal frequency of soil moisture estimations, IS2 data were used with IS1 data, with all data normalized to a single incidence angle. A high correlation is observed between in situ measurements and processed radar data. An empirical inversion technique is proposed, to estimate surface soil moisture from dual‐polarization data with a spatial resolution of approximately 1 km. Surface soil moisture maps are presented for all the studied sites, at various dates in 2004 and 2005. Of particular interest, these maps reveal convective precipitation scales associated with strong spatial variations in surface soil moisture.     

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.     

Sea surface wind speed and sea state retrievals from dual-frequency altimeter and its preliminary application in global view of wind-sea and swell distributions

Altimeter radar backscatter intensity, in terms of the normalized radar cross section (NRCS), is known to be modulated by surface wind forcing and the state of wind-sea development. Based on a data set of collocated altimeters (including Topex/Poseidon, Jason-1 and Jason-2) and in situ measurements, different responses to various wind speeds and wave ages (i.e. the state of wind-sea development) were illustrated for altimeter dual-frequency NRCSs (K_u-band at 13.6 Hz and C band at 5.4 Hz), which can facilitate the retrieval of wind speed and wave age parameters. A statistical parametric algorithm was developed to retrieve the two dynamic parameters from the altimeter dual-frequency NRCSs using the neutral network method. The wind-sea significant wave height (SWH) was estimated from wind speed and wave age parameters, which partitions the swell SWH from the altimeter SWH measurement. All newly derived parameters were well validated by comparison against in situ buoy measurements. A preliminary application of the method in examining the swell or wind-sea contributions to global waves was performed; it was found the swell dominance in an open ocean, and the wind-sea dominance in marginal and semi-enclosed seas. The methods would benefit other applications such as studies of air–sea interactions, validation of wave model, determination of swell decay rate and studies of wave climate.     

Observations of internal waves with high sampling data of radar altimetry and MODIS images

ABSTRACT

The 20 Hz high sampling radar altimetry data and quasi-synchronous satellite images are applied to investigate internal waves in the South China Sea (SCS) and the Sulu Sea. The response characteristics and band sensitivities of radar altimetry data to the modulation of internal waves were analysed. With quasi-synchronous observations, the responses of Sigma0 and significant wave height (SWH) in the Ku band and the C band to the modulation were discussed, respectively. The results demonstrate that for Sigma0, the Ku band is more sensitive, whereas the C band is more sensitive for SWH. The propagation speeds of internal waves in the SCS and the Sulu Sea were calculated with quasi-synchronous observation of radar altimetry high sampling data and MODIS images. The calculated speed of the internal waves is close to the phase speed obtained from the KdV equation. This work indicates that the quantitative parameter of internal waves could be extracted from radar altimetry high sampling data. The altimetry data could serve as an additional data source for the investigation of internal waves.     

An atlas of Antarctica north of 72.1degreesS from GEOSAT radar altimeter data

Although the importance of Antarctica in the global system has long been recognized and discussed in the literature, data as basic as topographic maps of a resolution amenable to geophysical analysis are still lacking for large parts of the continent. Mapping, surveying, and monitoring of the large expanses in remote areas are facilitated by remote-sensing technology. The best source for topographic mapping is satellite altimetry. It is often argued that satellite radar altimeter data over ice cannot be used to map ice surfaces with a slope exceeding 0.65degree. In the work presented in this paper, we extend the limits of altimeter data evaluation using ( a ) geostatistical methods, and ( b ) an atlas approach to mapping all of Antarctica north of 72.1degreesS at 3-km resolution. Ordinary Kriging is applied to altimeter data from the GEOSAT Geodetic Mission, selected for its denser coverage as compared to the Exact Repeat Mission. The resultant maps yield a wealth of new information, in particular along the margin of the East Antarctic Ice Sheet including location and topography of drainage systems of small glaciers and location of some of the ice shelves. The atlas sheets Riiser-Larsen Peninsula, Prince Olav Coast, Mawson Coast West (Kemp Coast), Lambert Glacier, Ingrid Christensen Coast, Pennell Coast, Napier Mountains, Knox Coast, Sabrina Coast, and Antarctic Peninsula (Graham Land) are presented and analyzed. Repeated mapping facilitates the monitoring of changes in surface elevation may indicate dynamically and climatically induced mass changes of the East Antarctic Ice Sheet.     

Elevation changes in Pine Island Glacier,Walgreen Coast,Antarctica, based on GLAS (2003) and ERS‐1 (1995) altimeter data analyses and glaciological implications

The Geoscience Laser Altimeter System (GLAS) aboard ICESat, launched in January 2003, has been designed to detect and monitor changes in the cryosphere. The first objective of this paper is to present high‐resolution ice‐surface elevation maps derived from GLAS data, using geostatistical analysis. In a regional study of Walgreen Coast and Northern Ellsworth Land, West Antarctica, differences in the representation of geographic and morphologic features in maps based on ERS‐1 radar altimeter data and on GLAS data are investigated, with the result that in particular in topographically complex coastal areas and the margin of the ice sheet the improvement in precision and accuracy of the laser altimeter is significant.

A second, applied objective is to map elevation changes in Pine Island Glacier, a glacier that plays a key role in the question of stability of the West Antarctic Ice Sheet and has been changing rapidly in recent years. Results of elevation differencing of 2003‐GLAS‐data and 1995‐ERS‐1‐radar‐altimeter‐data DEMs (1) show that thinning rates have been increasing and (2) are applied to attribute the observed changes in Pine Island Glacier to internal processes in the glacier, related to dynamic thinning. More generally, this application serves to demonstrate that GLAS data facilitate study of cryospheric change.     

Comparison of space borne radar altimetry and airborne laser altimetry over sea ice in the Fram Strait

This paper describes the first comparison of satellite radar and airborne laser altimetry over sea ice. In order to investigate the differences between measurements from the two different instruments we explore the statistical properties of the data and determine reasonable scales in space and time at which to examine them. The resulting differences between the data sets show that the laser and the radar are reflecting from different surfaces and that the magnitude of the difference decreases with increasing surface air temperature. This suggests that the penetration depth of the radar signal, into the snow, varies with temperature. The results also show the potential for computing Arctic wide snow depth maps by combining measurements from laser and radar altimeters.     

The contribution of Seasat to ice sheet glaciology

Abstract

The suite of sensors flown onboard Seasat during 1978 has provided glaciologists with valuable tools for the study of ice masses, particularly in the polar regions. Of the sensor package, the most useful instruments for glaciology have been the radar altimeter and the synthetic aperture radar. The former has demonstrated the ability to map the surface of ice sheets in considerable detail (possibly to better than 50 mm over ice shelves) and over a very short period of time. Such maps provide the first step towards evaluating the long term mass balance of these ice masses. Such studies are of central importance to global climate modelling, investigation of the ‘greenhouse effect’ and prediction of world sea levels. Radar altimeter mapping has also provided unparalleled detail on surface topography relevant to ice dynamics investigations. The small dataset of Seasat Synthetic Aperture Radar (SAR) imagery gathered over ice masses, principally in Iceland and Greenland (there was no coverage of Antarctica), has begun to reveal useful detail of surface and near-surface phenomena such as flowlines, meltwater percolation, and snow and ice facies invaluable for glaciolog-ical reconnaissance. In particular recent studies have shown the value of a multi-sensor approach with the combination of SAR and multi-spectral imagery. It is likely that X- and C-band SARs will prove better for snow and ice discrimination than the L-band system on Seasat. The Scatterometer and Scanning multi-channel microwave radiometer instruments on Seasat have yielded data over ice masses which are still in the early stages of evaluation. Nevertheless there are strong indications of the value of these data for investigation of surface melt phenomena and temperature-accumulation patterns.     

From satellite altimetry to ocean topography A survey of data processing techniques

Abstract

Satellite altimetry was introduced as a brief experiment on Skylab in 1973. Since this experiment, operational radar altimeters have been flown on GEOS-3 and Seasat and currently the Geosat altimetry misssion is in progress. For the early 1990s additional altimeters are planned for launch on the European ERS-1 and ERS-2 satellites, the U.S.-French Topex/Poseidon satellite, the U.S.NROSS satellite and the Japanese MOS-2 satellite. This paper presents a brief survey of altimetry missions and of some important aspects of altimeter data processing. In addition, the topic of orbit computation accuracy is addressed in somewhat more detail and some examples of geophysical products obtained from Seasat altimeter measurements are discussed. These examples do not necessarily represent products of the highestachievablequality, but they serve to illustrate the possibilities and limitations of present-day processing techniques.