On the problem of the near ocean surface wind speed retrieval by radar altimeter: A two-parameter algorithm
Vladimir KaraevM. B. KanevskyG. N. BalandinaP. D. CottonPeter ChallenorChristine GommengingerMeric Srokosz
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In this paper we present results from a theoretical investigation of the scattering of microwave radar signal by the ocean surface. The aim of the research is to generate an improved altimeter wind speed retrieval algorithm, and to achieve this a new theoretical approach to the problem of microwave scattering has been developed. The inadequacy of the assumption of a single valued relationship between surface wind speed and microwave radar backscatter is discussed, and a more complex approach adopted taking into account the nature of the sea surface. Two versions of a two-parameter algorithm (NA1 and NA2) for near surface wind speed retrieval are developed and presented in a tabular form. Co-located altimeter and buoy data are used to test this new algorithm. Good agreement is found between measured and retrieved values with the new algorithms: NA1- the mean difference M=-0.03 m s -1 and rms=1.47 m s -1 ; NA2- M=0.03 m s -1, rms=1.54 m s -1 . Note, that the currently used algorithm WC91 (see below) has the following characteristics: M=0.43 m s -1 and rms=1.69 m s -1 . The importance of taking into account the regional specific conditions is emphasized.Keywords:
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Backscatter (email)
Radar altimeter
Independent clocks provide time tags for the precision orbit determination (POD) equipment and the radar altimeter onboard the HY-2A satellite, and a bias between POD data' time tag and corresponding range observation's time tag from the HY-2A altimeter exists. The time tag bias contributes a bias in the sea surface height observation due to the nonzero time rate of change of the HY-2A altimeter's height. A transponder for in-orbit radar altimeter calibration provides an approach to estimate the time tag bias. The altimeter receives the responding signals from the transponder and generates ranges. Pertinent reference ranges are obtained from the POD data and the transponder's coordinate. Using the ranges from the radar altimeter and the reference ranges, the time tag bias between the POD data and the altimeter observations can be estimated. During an in situ HY-2A altimeter calibration campaign using a reconstructive transponder from August 9, 2012, to July 20, 2014, 17 estimations of the altimeter's time tag bias were obtained. The preliminary results are presented in this letter.
Radar altimeter
Transponder (aeronautics)
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This paper presents a method estimating the HY-2A altimeter ultrastable oscillator (USO) drift with a reconstructive transponder. The frequency of the USO of the in-orbit altimeter changes with age, and a bias between the actual frequency and the nominal one exists. The USO bias contributes a portion of the altimeter range drift. The HY-2A altimeter transmits signals at a fixed time interval during calibration, and the actual interval between adjacent altimeter transitions, which is controlled by the USO, is different from the nominal one due to the USO drift. The reconstructive transponder measures the arrival times of the altimeter transmitted signals accurately with the atomic clock. The drift of the USO on board the HY-2A altimeter can be estimated accurately by using the ranges from the reconstructive transponder and the HY-2A altimeter. The USO drifts of around 40 cm/year are presented. Furthermore, the multimission cross calibration provides preliminary validation of the HY-2A altimeter USO drift.
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Transponder (aeronautics)
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Haiyang-2B (HY-2B) is the second ocean dynamic environment satellite of China and was launched on launched on October 26th, 2018. Radar altimeter is one of HY-2B's main payloads and mainly used to provide sea surface height (SSH), significant wave height (SWH) and sea surface wind speed measurements. In this paper, the range noise level of HY-2B radar altimeter is estimated and compared with Jason-2 and Jason-3 altimeters. The noise level of HY-2B radar altimeter for an SWH of 2 m is estimated to be 1.38 cm, which is lower than those of Jason-2 and Jason-3 (1.60 cm). Sea level anomaly (SLA) power spectra analysis also shows that the HY-2B altimeter has low noise level with respect to Jason-2 and Jason-3. Therefore, the HY-2B radar altimeter has the potential to provide sea surface height measurements of high precision.
Radar altimeter
Significant wave height
Sea-surface height
Space-based radar
Wave height
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This paper investigates the spatial and temporal variability of significant wave height (Hs) and wind speed (U10) using altimeter data from the Australian Ocean Data Network (AODN) and buoy data from the National Data Buoy Center (NDBC). The main goal is to evaluate spatial and temporal criteria for collocating altimeter data to fixed-point positions and to provide practical guidance on altimeter collocation in deep waters. The results show that a temporal criterion of 30 min and a spatial criterion between 25 km and 50 km produce the best results for altimeter collocation, in close agreement with buoy data. Applying a 25 km criterion leads to slightly better error metrics but at the cost of fewer matchups, whereas using 50 km augments the resulting collocated dataset while keeping the differences to buoy measurements very low. Furthermore, the study demonstrates that using the single closest altimeter record to the buoy position leads to worse results compared to the collocation method based on temporal and spatial averaging. The final validation of altimeter data against buoy observations shows an RMSD of 0.21 m, scatter index of 0.09, and correlation coefficient of 0.98 for Hs, confirming the optimal choice of temporal and spatial criteria employed and the high quality of the calibrated AODN altimeter dataset.
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Collocation (remote sensing)
Significant wave height
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HY-2A, the China's first satellite for oceanic dynamic environment measurement, was launched on August 16th, 2011. As one of main payloads of HY-2A satellite, HY-2A radar altimeter is a dual-frequency (Ku and C band) altimeter with a conventional pulse-limited method. The altimeter can measure sea surface height, significant wave height, and sea surface wind speed. The accuracy of the altimeter is about 2cm (@4m SWH) or 4cm (@20m SWH). The system techniques and capabilities of the HY-2A altimeter are discussed in this paper.
Radar altimeter
Significant wave height
Ku band
Space-based radar
Wave radar
Sea-surface height
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SIRAL (SAR Interferometer Radar Altimeter) is the new spaceborne altimeter designed for CryoSat mission. The instrument is currently in the development phase, in ALCATEL SPACE, which encompasses an Engineering Model and a Flight Model. This ESA mission, planned for 2004, will be used to estimate - on a global scale - the fluctuations in mass of sea-ice and land-ice. This paper discusses the flexibility of the instrument and in particular its capability to operate in various modes (conventional altimeter, SAR and interferometer modes), and gives main results from the pre-developments and breadboard activities.
Radar altimeter
Breadboard
Space-based radar
Earth observation
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ALCATEL ESCAPE, France, has designed, developed and manufactured a radar altimeter for CNES, the French Space Agency. This altimeter was integrated onto the TOPEX POSEIDON satellite as part of a cooperative program between CNES and NASA. TOPEX POSEIDON was successfully launched on 10 August 1992 by an ARIANE-4 rocket and the CNES/ALCATEL ESPACE altimeter turned on 21 August for the first time. Since then, the altimeter has been operated 15% of the time, the remaining 85% being dedicated to the NASA supplied APL altimeter (both altimeters share the same antenna and operate at the same frequency). Analysis of collected data have demonstrated exceptional performances of ALCATEL ESPACE altimeter. This paper includes the presentation of the altimeter objectives, the altimeter basic characteristics, and the in-flight measured performances. The low weight, low power, low TM rate and low cost altimeter was specifically designed to be flown on any satellite platform, including small satellites. A project of a small satellite using the ALCATEL ESPACE altimeter is also presented.
Radar altimeter
Doris (gastropod)
Rocket (weapon)
Orbit Determination
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Abstract Satellite altimetry is regularly used to map the global distribution of mode‐1 internal tides. Validating altimeter measurements of internal tides with in situ data is difficult as the mooring observations typically are too short compared with the altimetry. Here taking advantage of the long‐term (since 1999) Tropical Atmosphere‐Ocean/Triangle Trans‐Ocean Buoy Network (TAO/TRITON) buoy observations at 2°S–156°E located at a TOPEX/Poseidon crossover in a region of strong internal tides, direct comparisons are made between altimeter and in situ measurements over a common 15 year period. The hourly buoy data are decimated every 9.9 day to emulate altimeter measurements. The altimeter and decimated buoy data are highly coherent at the aliased semidiurnal period (~60 day), and they agree well for the stationary semidiurnal internal tide. For the nonstationary internal tide, which consists of a significant annual cycle, the demodulated amplitude from the altimeter appears to be contaminated by nontidal signals. The demodulated amplitude from the decimated buoy data, on the other hand, is capable of resolving the temporal variability. This suggests the potential of mapping nonstationary internal tides from satellites.
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Internal tide
Mooring
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