Per as long as mances study of interferometric radar altimeters: from the instrument to t

Per as long as mances study of interferometric radar altimeters: from the instrument to t

Per as long as mances study of interferometric radar altimeters: from the instrument to t

Brennan, Georgeanne, Free Lance Writer has reference to this Academic Journal, PHwiki organized this Journal Per as long as mances study of interferometric radar altimeters: from the instrument to the global mission definition Vivien Enjolras(1,5), Patrick Vincent(2), Jean-Claude Souyris(1), Ernesto Rodriguez(3), Laurent Phalippou(4), Bruno Cugny(1), Anny Cazenave(1,5) (1) CNES, 18 av. Edouard Belin, 31055 Toulouse Cedex 4, France (2) IFREMER, 155 rue Jean Jacques Rousseau, 92138 Issy les Moulineaux, France (3) JPL, 4800 Oak Grove Drive, Pasadena, CA 91109, USA (4) ALCATEL ALENIA SPACE, 28, av. Jean François Champollion, 31000 Toulouse, France (5) LEGOS, 14 av. Edouard Belin, 31400 Toulouse, France Presentation Plan Recall of the context of the study Geometry of the measurement Physics of the measurement Impact of external errors in the swaths: plat as long as m attitude (yaw in addition to roll angles), propagation media (ionosphere in addition to wet troposphere) Data simulation over a real phenomenon: December 2004 tsunami Proposition of an operating point as long as such an instrument Context of the study WSOA about to be embarked on Jason 2 mission as a demonstrator Main scientific objectives: 2D topography of the oceans, in addition to catching of meso scale phenomena thanks to its spatial resolution In this context, studies have been per as long as med on such an instrument in addition to its adaptation on a plat as long as m as the Jason 2 one to get a global error budget Un as long as tunately, WSOA was cancelled in 2005 The outcomes have led to the proposition of a new operating point

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Geometry of the measurement (1/2) Off-nadir bi-polar observation by two passive antennae Low look angles: compromise between a wide swath in addition to good power return on water Both swaths hits mechanically done with the feed’s position Geometry of the measurement (2/2) Knowledge of 4 main parameters required: altitude H (Doris, GPS), range r1 (onboard clock), interferometric phase (onboard calculator) in addition to roll angle (a priori ACS) Need to move to a useful reference frame, adapted to oceanography Physics of the measurement (1/2) Channels’ signals coherence characterizes the quality of the interferometric phase measurement 3 main sources of signals decorrelation : Speckle: onboard correction Thermal Noise: Main one Range Migration: negligible at low angles Need to have a very accurate knowledge of the link budget Height error directly related to the interferometric phase error Single Look Height Error non adapted to oceanographic applications

Physics of the measurement (2/2) Necessity to have a multilooking process (onboard to lower the telemetry budget): study of different scenarios Application on the nominal case WSOA on Jason 2 Impact of external errors : Attitude (1/6) Need of a yaw control on repetitive orbit like the Jason one (around 80 % of mission lifetime) Direct impact on the geometry of the swath in addition to related sampling Loss of coverage, getting worse when the Sun is far from the orbital plane (angle from 0° to 66° + 23,15° = 89.15°) 6 period of 12 days every year are optimal with a null yaw in addition to 84 km swath Impact of external errors : Attitude (2/6) Yaw period slightly different from orbit period: yaw doesn’t remain the same over an area cycle after cycle (crossovers in addition to overlap impact) Only a Sun Synchronous orbit can cope with this effect, almost erasing the plat as long as m yaw motion Going Sun Synchronous also offers a gain about 70 % in the dragging surface, lowering the rhythm of maneuvers, in addition to enabling some possible altitude reduction New Operating Point : Sun Synchronous Orbit Altitude Reduction

Impact of external errors : Attitude (3/6) Impact of the misknowledge of the roll angle in the swath Estimation process at crossovers, assuming a linear evolution on a short time Crossovers selection (time delay less than 5 days to deal with the ocean decorrelation) Study of the impact of different factors (orbit, swath sampling, nadir altimeter data) on the quality of the estimation Impact of external errors : Roll angle (4/6) The shorter the orbit cycle, the better the estimation Nadir measurements are not indispensable, but still improves the estimation slightly A thinner sampling gives more measurements, but noisier; it still improves the estimation Impact of external errors : Ionosphere (5/6) Nadir dual-frequency altimeter estimation of the ionospheric delay No delay estimation through the swath: added residual errors to consider Ionospheric wavelengths of hundreds of kilometers: mean residual errors negligible Only interests in the worst cases, around the tropics at certain periods Use of daily worldwide GPS data as long as the study

Impact of external errors : Troposphere (6/6) Identical problem with the wet tropospheric delay Use of SSMI data Non negligible zonal mean residual errors, especially in the tropics Maximum errors can reach 4 cm in the worst case Through a large perturbation, more than 50 % of the errors in the Far Range are greater than 1 cm, in addition to around 20 % are greater than 1.5 cm by the Middle Range Possible need to use external data in the ground processing to improve the media budget Overall Budget: WSOA Nominal Case Overall Per as long as mances study first nominally applied on WSOA on Jason 2 Best in addition to Worst Cases refer to attitude in addition to propagation media errors Post-processing pixels of 1614 km are considered Overall Budget between 5 in addition to 9 cm: contribution of the instrument error budget about 55 % Possibility to look at a new operating point, more adapted Data Simulation: December 2004 Tsunami Show the interest of a 2D topography mapping No proof of a warning system Simulation of a radar altimeter interferometer onboard T/P in addition to Jason when they flew over the Indian Ocean (nominal case 1614 km pixel) Yaw configuration tested In nominal mode, the 2D in as long as mation results in a good knowledge of the direction in addition to the amplitude of the wave In attitude worst cases, measurements come close to nadir ones

New Operating Point Research (1/3) Scientific Objectives: identical to WSOA ones, plus the constraint of a global coverage Orbit: Move to a sun-synchronous orbit (decrease the altitude to improve the link budget) TWTA (tube amplifier) in addition to antennae kept 10 meters mast: no more Jason 2 constraint Frequency: Ku b in addition to to keep the same feeds in addition to antennae as Jason 2 Nadir Altimeter: Kept especially as long as the ionospheric correction (dual-frequency) in addition to intercalibration PRF per antenna: half the nadir altimeter PRF (around 1000 Hz sufficient) B in addition to width: as WSOA, sufficient as long as the range resolution required (drawback as long as the link budget if increased) Pulse length: TWTA enables an increasing, assuming more telemetry (low cost) New Operating Point Research (2/3) 14 days sun synchronous orbit at 815 km with 84 km swaths covers more than 99 % of the world between -81 º in addition to 81º More than 42 % or areas between -60 in addition to 60 hit more than 3 times per cycle Only Drawback: tidal aliasing! New Operating Point Research (3/3)

Conclusion Few changes in the WSOA operating point can improve the overall budget Strong need to move to a sun-synchronous orbit Global coverage possible in 14 days 4 to 5 cm height RMS obtained on 124 km pixels Possible use of external data (GPS in addition to SSMI) Even if errors stronger than nadir altimeter errors in addition to partly correlated in the swath, we have access to a 2D topography (see tsunami) Error budget can be improved by integrating overlapping over whole mission lifetime These instruments should be part of the future of altimetry Paper just published in Journal SENSORS Special Issue on Altimetry

Brennan, Georgeanne Brennan, Georgeanne Free Lance Writer

Brennan, Georgeanne Free Lance Writer

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