Performances 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, Toulouse Cedex 4, France (2) IFREMER, 155 rue Jean Jacques Rousseau, Issy les Moulineaux, France (3) JPL, 4800 Oak Grove Drive, Pasadena, CA 91109, USA (4) ALCATEL ALENIA SPACE, 28, av. Jean François Champollion, Toulouse, France (5) LEGOS, 14 av. Edouard Belin, Toulouse, France

Vivien ENJOLRAS “15 Years of Altimetry”, Venice, March /19 Presentation Plan ■Recall of the context of the study ■Geometry of the measurement ■Physics of the measurement ■Impact of external errors in the swaths: platform attitude (yaw and roll angles), propagation media (ionosphere and wet troposphere) ■Data simulation over a real phenomenon: December 2004 tsunami ■Proposition of an operating point for such an instrument

Vivien ENJOLRAS “15 Years of Altimetry”, Venice, March /19 Context of the study ■WSOA about to be embarked on Jason 2 mission as a demonstrator ■Main scientific objectives: 2D topography of the oceans, and catching of meso scale phenomena thanks to its spatial resolution ■In this context, studies have been performed on such an instrument and its adaptation on a platform as the Jason 2 one to get a global error budget ■Unfortunately, WSOA was cancelled in 2005 ■The outcomes have led to the proposition of a new operating point

Vivien ENJOLRAS “15 Years of Altimetry”, Venice, March /19 Geometry of the measurement (1/2) ■Off-nadir bi-polar observation by two passive antennae ■Low look angles: compromise between a wide swath and good power return on water ■Both swaths hits mechanically done with the feed’s position

Vivien ENJOLRAS “15 Years of Altimetry”, Venice, March /19 Geometry of the measurement (2/2) ■Knowledge of 4 main parameters required: altitude H (Doris, GPS), range r1 (onboard clock), interferometric phase Φ (onboard calculator) and roll angle α (a priori ACS) ■Need to move to a useful reference frame, adapted to oceanography

Vivien ENJOLRAS “15 Years of Altimetry”, Venice, March /19 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

Vivien ENJOLRAS “15 Years of Altimetry”, Venice, March /19 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

Vivien ENJOLRAS “15 Years of Altimetry”, Venice, March /19 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 and 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 and 84 km swath

Vivien ENJOLRAS “15 Years of Altimetry”, Venice, March /19 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 and overlap impact) ■Only a Sun Synchronous orbit can cope with this effect, almost erasing the platform yaw motion ■Going Sun Synchronous also offers a gain about 70 % in the dragging surface, lowering the rhythm of maneuvers, and enabling some possible altitude reduction New Operating Point :  Sun Synchronous Orbit  Altitude Reduction

Vivien ENJOLRAS “15 Years of Altimetry”, Venice, March /19 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 YAYA YDYD XAXA XDXD

Vivien ENJOLRAS “15 Years of Altimetry”, Venice, March /19 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

Vivien ENJOLRAS “15 Years of Altimetry”, Venice, March /19 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 for the study

Vivien ENJOLRAS “15 Years of Altimetry”, Venice, March /19 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, and 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

Vivien ENJOLRAS “15 Years of Altimetry”, Venice, March /19 Overall Budget: WSOA Nominal Case ■Overall Performances study first nominally applied on WSOA on Jason 2 ■Best and Worst Cases refer to attitude and propagation media errors ■Post-processing pixels of 16*14 km are considered ■Overall Budget between 5 and 9 cm: contribution of the instrument error budget about 55 % ■Possibility to look at a new operating point, more adapted

Vivien ENJOLRAS “15 Years of Altimetry”, Venice, March /19 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 and Jason when they flew over the Indian Ocean (nominal case 16*14 km pixel) ■Yaw configuration tested ■In nominal mode, the 2D information results in a good knowledge of the direction and the amplitude of the wave ■In attitude worst cases, measurements come close to nadir ones

Vivien ENJOLRAS “15 Years of Altimetry”, Venice, March /19 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) and antennae kept ■10 meters mast: no more Jason 2 constraint ■Frequency: Ku band to keep the same feeds and antennae as Jason 2 ■Nadir Altimeter: Kept especially for the ionospheric correction (dual- frequency) and intercalibration ■PRF per antenna: half the nadir altimeter PRF (around 1000 Hz sufficient) ■Bandwidth: as WSOA, sufficient for the range resolution required (drawback for the link budget if increased) ■Pulse length: TWTA enables an increasing, assuming more telemetry (low cost)

Vivien ENJOLRAS “15 Years of Altimetry”, Venice, March /19 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 and 81º ■More than 42 % or areas between -60 and 60 hit more than 3 times per cycle ■Only Drawback: tidal aliasing!

Vivien ENJOLRAS “15 Years of Altimetry”, Venice, March /19 New Operating Point Research (3/3)

Vivien ENJOLRAS “15 Years of Altimetry”, Venice, March /19 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 12*4 km pixels ■Possible use of external data (GPS and SSMI) ■Even if errors stronger than nadir altimeter errors and 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