Performances study of interferometric radar altimeters: from the instrument to

the global mission definitionVivien 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

Vivien ENJOLRAS “15 Years of Altimetry”, Venice, 13-16 March 2006 2/19

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, 13-16 March 2006 3/19

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, 13-16 March 2006 4/19

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, 13-16 March 2006 5/19

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, 13-16 March 2006 6/19

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

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Vivien ENJOLRAS “15 Years of Altimetry”, Venice, 13-16 March 2006 7/19

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, 13-16 March 2006 8/19

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, 13-16 March 2006 9/19

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, 13-16 March 2006 10/19

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

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Vivien ENJOLRAS “15 Years of Altimetry”, Venice, 13-16 March 2006 11/19

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, 13-16 March 2006 12/19

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 9.14.6

Best Case (beginning of August 2001)

15.17.6Intermediate Case (September 2002)

21.911Worst Case (end of

October 2001)

Maximum Ionospheric Error

100 km (mm)

Maximum Ionospheric Error

50 km (mm)Period

9.14.6Best Case (beginning

of August 2001)

15.17.6Intermediate Case (September 2002)

21.911Worst Case (end of

October 2001)

Maximum Ionospheric Error

100 km (mm)

Maximum Ionospheric Error

50 km (mm)Period

Vivien ENJOLRAS “15 Years of Altimetry”, Venice, 13-16 March 2006 13/19

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, 13-16 March 2006 14/19

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, 13-16 March 2006 15/19

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, 13-16 March 2006 16/19

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, 13-16 March 2006 17/19

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!

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Vivien ENJOLRAS “15 Years of Altimetry”, Venice, 13-16 March 2006 18/19

New Operating Point Research (3/3)

Vivien ENJOLRAS “15 Years of Altimetry”, Venice, 13-16 March 2006 19/19

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

http://www.mdpi.org/sensors