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。     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

Wave studies with the radar altimeter

Abstract

In this paper we review the physical understanding and the algorithms necessary to extract wave information from the radar altimeter return from the sea surface. This allows us to assess the progress made since the launch of Seasat, to identify areas where further work is necessary in order to develop our understanding of the basic mechanisms of radar interaction with the sea surface, and to suggest how new wave parameters might be obtained from the radar return. Following on from this were view the wave studies that have been carried out using Seasat and Geosat altimeter data and present some new results, on the joint probability distribution of significant waveheight and wind speed, which we have obtained recently from Seasat data.     

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 $\text{H}{}_{\mathrm{<mtext>1</mtext><mtext>3</mtext>}}$.     

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.     

Height errors introduced into the analysis of radar altimetry data by use of the simplified Brown model

Abstract

This paper presents an expression for the average return power spectrum of the full-deramped signal in a short-pulse radar altimeter and points out the relation between the response of a full-deramped signal and that of a short pulse. Specifically, this paper compares the full-deramp model with a model devised by Brown and points out the effects of using the Brown model on data from Seasat. In order to show the effect of the pulse shape on the average return response, a formula to calculate the response of a rectangular pulse is also given.     

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.     

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.     

基于MATLAB/SIMULINK的海面回波模拟器仿真与分析

星载雷达高度计是一种重要的对海洋进行精确测量的主动式微波遥感器。因为高度计能够有效地测量卫星到海面的高度、海面有效波高和海面后向散射系数,它在海洋大地水准面研究和海洋应用研究的诸多方面得到了广泛的应用。海面回波模拟器能够接收和处理高度计信号,产生海面回波波谱来调制高度计信号,最后精确延时后发射回高度计,是在地面对高度计进行测试和定标的一种有效手段。由于功能的全面性,回波模拟器是一个复杂度不亚于高度计本身的系统,因此,对回波模拟器的分析和仿真对于指导它的设计、确定它的精度以及定标的效果有着重要意义。介绍了回波模拟器的结构,分析了海面回波波谱的产生原理并设计了回波模拟器的SIMULINK仿真模型,给出并分析了仿真结果。     

Dependence of mean square slope on wave state and its application in altimeter wind speed retrieval

Mean square slope (MSS) of sea surface is a parameter describing the sea surface roughness and plays a key role in understanding the sea surface dynamics. Although MSS is influenced by many factors such as wind speed and sea surface waves, it has been traditionally parameterized by wind speed only. In this study, the surface wave impact on MSS is investigated using a collocated data set of altimeter and buoy measurements. It is found that the MSS detected by Ku-band altimeter is closely related to the wind wave components and increases with the degree of wind wave development; however, it is almost independent of the swell. The wave effect on MSS is mainly ascribed to the contribution of longer waves rather than shorter waves. An analytical spectral MSS model is proposed, and it is shown that the MSS calculated from the model agrees well with the altimeter-measured MSS when the pseudo-wave age is adjusted to the wave age corresponding to wind waves. This model is applied to derive wind speeds from altimeter data including the normalized radar cross section and the significant wave height of sea surface waves. With the collocated data set, it is shown that this new algorithm performs better than previous empirical algorithms.