Diapositive 1

1
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,
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
2
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

3
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

4
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
  feeds position

5
Geometry of the measurement (2/2)

• Knowledge of 4 main parameters required altitude
  H (Doris, GPS), range r1 (onboard clock),
  interferometric phase F (onboard calculator) and
  roll angle a (a priori ACS)
• Need to move to a useful reference frame,
  adapted to oceanography

6
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

7
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

8
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

9
Impact of external errors Attitude (2/6)

• Yaw period slightly different from orbit period
  yaw doesnt 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

10
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

11
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

12
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

13
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

14
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 1614 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

15
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 1614 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

16
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)

17
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!

18
New Operating Point Research (3/3)
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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 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