Seasat Radar Altimeter Research Articles

David Edgar Cartwright. 21 October 1926 — 2 December 2015

David Cartwright was one of the world's leading authorities on the tides. However, when reflecting on his life, Cartwright made the point that his early scientific career was not a success. Indeed in 1953, at the age of 27, he had virtually despaired of any creative scientific future. At the time he was being pressurized to stop his work on the statistics of ship motions but his prospects rapidly changed when he was invited to apply for a post at the new National Institute of Oceanography (NIO) being set up by George Deacon. At NIO he soon made important contributions to the study of ocean waves, especially the calculation of directional spectrum and wave climate. His earlier involvement with ship motions also culminated in a successful joint study with Louis Rydill on the response of ships to the spectrum of waves. Following this, his use of computer methods for time-series analysis led to an invitation to the Scripps Institution of Oceanography where, with Walter Munk, he developed the response method of analysing tides making use of the very long tidal records collected from Hawaii and Newlyn. He was also made aware of the significant lack of good tidal data from the deep ocean. Returning to the UK, he continued these interests, studying the deep-ocean tides of the Atlantic and leading an international collaboration that measured deep-ocean tides. He also investigated the effect of tides on storm surges around the UK. He became assistant director in charge of the Institute of Oceanographic Sciences (IOS) Bidston laboratory, where he continued these activities and started research on estimating the tides using data from the Seasat radar altimeter. After retirement he successfully extended this work with Richard Ray at the Goddard Space Flight Center. Using Geosat altimeter data they generated accurate global maps of the tides in a set of papers that Cartwright considered to be his best work. He wrote a successful book titled ‘ Tides: a scientific history ’, and later published further work with Ray on the internal tides of the ocean.

Elevation change measurement of the East Antarctic Ice Sheet, 1978 to 1988, from satellite radar altimetry

Seasat and Geosat satellite radar altimeter measurements over the East Antarctic Ice Sheet (EAIS) are analyzed to determine surface elevation change over the period from 1978 to 1988. The results show that the average elevation change over 1.7 /spl times/10/sup 6/ km/sup 2/ of the EAIS is -0.4/spl plusmn/1.8 cm/yr and is therefore not significantly different than zero. This result is in very good agreement with ERS-1/2 radar altimeter estimates of EAIS elevation change from 1992 to 1996. The combined results suggest that the EAIS has been close to a state of balance for two decades. This further reduces the likelihood that recent changes in snow accumulation have masked a long-term mass imbalance in the EAIS.

Intensity of Satellite Radar-Altimeter Return Power Over Continental Ice: A Potential Measurement of Katabatic Wind Intensity

AbstractWe analyse, above continental ice, the various factors which affect the power return of the Seasat radar altimeter as measured by its Automatic Gain Control (AGC). Corrections of effects due to the AGC loop control are first applied. AGC is then normalized by positioning the half-power point at the middle of the instrument receiving window. This operation is valid for both surface and volume scattering. Over a part of Antarctica between long. 90° and 150°E., the remaining variations of AGC are of the order of 15 dB. Most of these variations occur on a large scale (>100km) and are correlated with the katabatic wind intensity. This indicates that AGC measures either surface roughness of the ice, which is related to wind intensity, or grain-size which could also be dependent on the wind.In-situmeasurements support the evidence that the radar altimeter is more sensitive to surface scattering. These data could therefore provide a measurement of the intensity of katabatic winds over the continental ice.

Intensity of Satellite Radar-Altimeter Return Power Over Continental Ice: A Potential Measurement of Katabatic Wind Intensity

AbstractWe analyse, above continental ice, the various factors which affect the power return of the Seasat radar altimeter as measured by its Automatic Gain Control (AGC). Corrections of effects due to the AGC loop control are first applied. AGC is then normalized by positioning the half-power point at the middle of the instrument receiving window. This operation is valid for both surface and volume scattering. Over a part of Antarctica between long. 90° and 150°E., the remaining variations of AGC are of the order of 15 dB. Most of these variations occur on a large scale (>100km) and are correlated with the katabatic wind intensity. This indicates that AGC measures either surface roughness of the ice, which is related to wind intensity, or grain-size which could also be dependent on the wind. In-situ measurements support the evidence that the radar altimeter is more sensitive to surface scattering. These data could therefore provide a measurement of the intensity of katabatic winds over the continental ice.

Satellite-Altimeter Measurements of Surface Height in Sea-Ice Areas (Abstract)

The Seasat radar altimeter was designed to operate over the open ocean and encountered problems over sea ice. In particular, the on-board measurements of surface height were noisy and unreliable. As a consequence, published mean sea-surface and geoid maps based on the Seasat on-board height estimates either omit sea-ice-covered areas or include suspect data. We have identified and investigated the problems encountered by the Seasat altimeter over sea ice and have developed a technique for extracting accurate surface-height values from the sea-ice echo wave-form data. The retracking method is based upon fitting real wave forms to a library of model returns.

Radar altimeter data quality flagging

The Seasat radar altimeter was designed to provide precise and accurate surface height measurements over the ocean. These data have been used in studies of the ocean geoid, tides and currents. Several factors can affect the quality of the height measurements produced by the on-board processor. In particular, radar returns with time profiles which depart from a standard form can introduce significant errors. These arise over very calm seas, over rain cells or when sea ice or land are present within the altimeter footprint. Blunder point algorithms are used in the ground processing to flag outlying data points, but these fail to identify some of the anomalies that we observe. We have developed an alternative method for flagging poor quality data using a simple algorithm based on pulse shape. We demonstrate that the algorithm provides a sensitive means of editing altimeter ocean data. Furthermore changes in surface type, such as transitions between open water and sea ice can be accurately located.

Satellite-Altimeter Measurements of Surface Height in Sea-Ice Areas (Abstract)

The Seasat radar altimeter was designed to operate over the open ocean and encountered problems over sea ice. In particular, the on-board measurements of surface height were noisy and unreliable. As a consequence, published mean sea-surface and geoid maps based on the Seasat on-board height estimates either omit sea-ice-covered areas or include suspect data. We have identified and investigated the problems encountered by the Seasat altimeter over sea ice and have developed a technique for extracting accurate surface-height values from the sea-ice echo wave-form data. The retracking method is based upon fitting real wave forms to a library of model returns.

Wind-wave relationship from SEASAT radar altimeter data

We present a nonlinear relationship between ocean surface wind at 10 m height (U 10 ) and significant wave height of wind-generated gravity waves, (H 1/3)gw, over the open oceans using SEASAT radar altimeter data. The data represent a variety of fetches, durations and strength of winds. Concurrent measurement of significant wave height, (H 1/3 ), which may contain a measure of swell and U 10 obtained from the processed geophysical data record (GDR) of the SEASAT radar altimeter were used in the analysis. The total wave energy, E alt, characterised by altimeter H 1/3 measurements was compared with the energy of a fully developed sea, E fd derived from U 10 measurements using the Pierson-Moskowitz model. The criteria E alt ≤ E fd was used in data selection to minimise the influence of swell. (H 1/3)gw thus obtained was used in a regression in terms of U 10 in a second-degree polynomial. Verification with independent radar altimeter data confirmed the validity of the proposed wind-wave model, which could be used for operational wave forecasting.

Seasat Range Measurements Verified on a 3-D Ice Sheet

The Seasat radar altimeter observations of a 100 km2 area in South Greenland are compared to a detailed, ground-based survey, using “geoceivers” and pressure altimeters. The comparison shows the Seasat measurement of distance between satellite and earth to be accurate to the level of the geoceiver determined surface (±2 m). Due to the great distance between satellite and surface, finer details of surface topography are not revealed in the satellite measurements. As the satellite tends to lock onto hills in the vicinity of the sub-satellite track, the satellite tends to overestimate the true surface elevation. However, a similar altimeter would make a similar overestimate, allowing accurate differential measurement of volume changes between the two surveys.

Satellite altimeter measurements of the geoid in sea ice zones

The Seasat radar altimeter provided surface height measurements to a precision better than 10cm over the open ocean, The data have been used to produce maps of the ocean geoid which reveal details of sub-surface topography such as sea mounts, ocean trenches and mid-ocean ridges /1/. In areas of the ocean covered by sea ice, however, the quasi-specular ice returns which occurred were incorrectly handled by the on-board processor. This resulted in a significant decrease in the precision of the surface height estimates. Consequently, researchers have generally eliminated data from regions where sea ice is suspected to have been present, including large areas of the Antarctic ocean. We have developed a technique for significantly improving the height measurements over such areas permitting the mapping of the geoid in such regions. The short wavelength RMS deviation of elevation measurements from collinear passes over such areas has been reduced from 1.032m to 0.632m. The application of the technique to ERS-1 altimeter data will be particularly important, since coverage of a substantial area of the Arctic ocean (up to 82° latitude) will be possible for the first time

Seasat Range Measurements Verified on a 3-D Ice Sheet

The Seasat radar altimeter observations of a 100 km2 area in South Greenland are compared to a detailed, ground-based survey, using “geoceivers” and pressure altimeters. The comparison shows the Seasat measurement of distance between satellite and earth to be accurate to the level of the geoceiver determined surface (±2 m). Due to the great distance between satellite and surface, finer details of surface topography are not revealed in the satellite measurements. As the satellite tends to lock onto hills in the vicinity of the sub-satellite track, the satellite tends to overestimate the true surface elevation. However, a similar altimeter would make a similar overestimate, allowing accurate differential measurement of volume changes between the two surveys.

Terrain Profiling Using Seasat Radar Altimeter

The ability to remotely establish elevation profiles, especially in inaccessible areas of Alaska and Antarctica, could decrease the cost and time required to produce topographic maps. One possible means is the use of Seasat radar altimeter data to determine terrain profiles. Prior research has indicated the possibility of determining spot elevations accurate to about 1 m from terrain profiles derived from Seasat radar altimeter data, even though the altimeter was designed only for over‐ocean sensing. Over‐land use of the altimeter is generally limited to flat or gently rolling terrain. Using the data reduction method developed by the Geoscience Research Corporation, data from the Aleutian Islands, Alaska; Wilkes Land, Antarctica; and the Delmarva Peninsula, Delaware‐Maryland‐Virginia were investigated to assess the achievable accuracy. Terrain profiles were generated from the Antarctic and Maryland data, however no profiles could be generated from the Alaskan data. The Antarctic profiles were compared with nearby doppler determined elevations; a 50‐ft (approximately 15‐m) discrepancy between the two was found. Where the altimeter gathered data in Maryland, a profile with a root‐mean‐square error of 12 ft (3.7 m) was determined.

Swell in the Pacific Ocean observed by SEASAT radar altimeter

Abstract The first synoptic observations of ocean swell generation, propagation, and dissipation have been obtained from SEASAT radar altimeter measurements. The sequential significant wave height and surface wind speed measurements have been deduced from altimeter observations; by assuming that the Pierson and Moskowitz fully developed wave model applies, a model for determining minimum swell is used to generate three‐day average swell maps of the Pacific Ocean for the period June to October 1978, the SEASAT lifetime. The generation of numerous swell events was observed with the altimeter in the high‐latitude regions of the South Pacific Ocean (south of 40°S latitude), which was acting as a continuous swell source. Regional differences in the northward propagation and dissipation patterns of different swell events have been analyzed. In their northward propagation the swell families generated in the Ross Sea neighborhood can impinge the coasts of North America with significantly more energy than the swel...

World ocean mean monthly waves, swell, and surface winds for July through October 1978 from SEASAT radar altimeter data

Abstract Data acquired by the SEASAT radar altimeter during the satellite's three‐month lifetime are used to obtain mean monthly maps of significant wave height, minimum swell, and surface wind speed for the world oceans. The SEASAT lifetime 26 June to 10 October 1978, corresponds to the Arctic summer and Antarctic winter. Thus, the highest sea states as derived from the radar altimeter data occur in the high latitudes of the Southern Ocean, reaching the mean monthly extreme magnitudes of 6‐m significant wave height, 5‐m minimum swell amplitude, and 12 m/s surface wind speed. Several features of the wave and wind fields agree with conventional descriptions. For example, the principal zonal wind regimes of the westerlies and trade winds and the doldrums and monsoon region are clearly evident in the monthly averages. Significant wave height and swell, whose mean fields always appear to be smoother than the mean wind speed fields, exhibit minima near the doldrums and steady increases southward and northward ...

Southern ocean mean monthly waves and surface winds for winter 1978 by Seasat radar altimeter

Data acquired by the SEASAT radar altimeter during the 3 month satellite lifetime are analyzed in a study of the sea state of the southern hemisphere oceans. The lifetime of the SEASAT satellite, July 7 to October 10, 1978, corresponds to the Antarctic winter. Mean monthly maps of wind speed, significant wave height, and swell have been generated from the altimeter measurements along the satellite tracks. These maps delineate spatial and temporal differences of these parameters in the Atlantic, Indian, and Pacific oceans. Several features of the Southern Ocean wind and wave fields agree with conventional descriptions. For example, the principle zonal wind regimes established by the Southeast Trades and Westerlies are clearly evident in the monthly averages. Significant wave height and swell also exhibit minima near the Doldrums at low latitudes with steady increases southward to the latitudes of the Westerlies. However, superimposed on these general patterns is significant variability with horizontal scales as small as 1000 km. The maps also document a gradual migration of the region of absolute maximum wind and wave from the Atlantic eastward to the Indian Ocean and finally into the Pacific.

Detection of seamount signatures in SEASAT altimeter data using matched filters

Matched‐filter techniques are described for the detection of characteristic geoid undulation signatures of seamounts in SEASAT radar altimeter data. The technique requires models of the expected undulation signatures and a statistical description of the background (seamount‐free) data. Examples are given of the detection of 11 known seamounts on four distinct tracks. Further extensions and applications of the matched‐filter methods are discussed.

Mapping Ice-Sheet Margins from Radar Altimetry Data

The Seasat radar altimeter, which was designed to measure ranges to the sea surface, has also provided the most accurate available maps of ice-sheet elevation. Seasat operated for three months during the austral winter of 1978, when Antarctica was girdled by sea ice. As the satellite approached the continent from the ocean, the altimeter obtained very strong reflections from the sea ice, and it continued to measure ranges to the sea ice even for a short time after passing over the ice front. The measured ranges are oblique distances to the nearest portion of sea ice, and the sequence of oblique ranges gives the position of the sea ice along the ice front. After the satellite crossed the ice front travelling seaward, oblique ranges were measured to the nearest portion of ice shelf. Examples are given from Seasat orbits crossing the Amery and Fimbul ice shelves. The entire Seasat data set provides an opportunity for mapping most of the East Antarctic coastline to an absolute horizontal accuracy of ±(0.1 → 1) km. The resulting map would effectively be an instantaneous view of the ice margin, requiring no adjustment for ice movement during the survey period.

Mapping Ice-Sheet Margins from Radar Altimetry Data

The Seasat radar altimeter, which was designed to measure ranges to the sea surface, has also provided the most accurate available maps of ice-sheet elevation. Seasat operated for three months during the austral winter of 1978, when Antarctica was girdled by sea ice. As the satellite approached the continent from the ocean, the altimeter obtained very strong reflections from the sea ice, and it continued to measure ranges to the sea ice even for a short time after passing over the ice front. The measured ranges are oblique distances to the nearest portion of sea ice, and the sequence of oblique ranges gives the position of the sea ice along the ice front. After the satellite crossed the ice front travelling seaward, oblique ranges were measured to the nearest portion of ice shelf. Examples are given from Seasat orbits crossing the Amery and Fimbul ice shelves. The entire Seasat data set provides an opportunity for mapping most of the East Antarctic coastline to an absolute horizontal accuracy of ±(0.1 → 1) km. The resulting map would effectively be an instantaneous view of the ice margin, requiring no adjustment for ice movement during the survey period.

Sea‐state‐related altitude errors in the SEASAT radar altimeter

For the work reported here, a six‐parameter model waveform is least squares fitted to 10‐s averages of the data from the 63 waveform sampling gates in the SEASAT radar altimeter. One of the fitted parameters is a time‐origin parameter that provides a measure of the position of the actual mean radar waveform relative to the sampling gate set and thus provides a correction to the instrument's altitude measurement. For the data set analyzed this correction appears to vary quadratically as a function of the significant waveheight (SWH). The result to date is that the correction ΔG, in centimeters, which should be added to the surface elevation measurements on the SEASAT geophysical data record (GDR), is given by ΔG = 0.593 SG + 0.476 SG2 where SG is the GDR value for SWH, in meters. The discussion in this paper reviews waveform assumptions built into the SEASAT onboard altitude tracker and subsequent altitude data processing and lists several additional important effects that were not included in that processing but which are incorporated into the SEASAT waveform analysis reported here.

Waveheight and wind speed measurements from the SEASAT radar altimeter

Comparisons between the SEASAT radar altimeter inferred estimates of significant waveheight (SWH) and wind speed with buoy measurements for near overflights are presented. Details of the ‘calibration’ of SEASAT's measurement of σ0(0°) against those made by the GEOS‐3 radar altimeter are presented. The ‘onboard’ estimate of SWH is shown to be biased high by 0.5 m for SWH > 2 m. Comparisons of buoy measurements of SWH with altimeter waveform data processed by using the Fedor algorithm for 51 cases show a mean difference of 0.07 m with a standard deviation of 0.29 m over the range of 0.5–5 m SWH. The SEASAT inferred estimates of σ0(0°) are biased high by 1.6 dB relative to GEOS‐3 data. For 87 corrected comparisons with buoy measurements of wind speed, the mean difference is −0.25 m/s, and the standard deviation of the difference is 1.6 m/s over the range of 1–10 m/s wind speed. The numbers are very similar to GEOS‐3 data comparisons and thus provide a second corroboration of the waveheight and wind speed measurement capability of a short pulse spaceborne radar altimeter.