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END-TO-END PERFORMANCE ANALYSIS OF THE PARIS IN-ORBIT DEMONSTRATOR OCEAN ALTIMETER

This study analyzes the end-to-end performance of the PARIS In-Orbit Demonstrator Ocean Altimeter, focusing on factors affecting altimetry performance such as incidence angle, interferometric processing, waveform retracking, and ionospheric correction. The results suggest that the PARIS altimeter is well-suited for mesoscale ocean altimetry with a potential accuracy of 5 cm.

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END-TO-END PERFORMANCE ANALYSIS OF THE PARIS IN-ORBIT DEMONSTRATOR OCEAN ALTIMETER

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  1. END-TO-END PERFORMANCE ANALYSISOF THE PARIS IN-ORBIT DEMONSTRATOR OCEAN ALTIMETER Salvatore D’Addio,Manuel Martin-Neira, Chris Buck Acknowledgment: Nicolas Floury, Roberto Pietro Cerdeira TEC-ETP, Electrical Engineering Department European Space Agency

  2. The PARIS Concept PARIS = Passive Reflectometry and Interferometry System

  3. L5 L2 L1 40 MHz 40 MHz 40 MHz PARIS IOD Key Features • Direct Cross-correlation (DxR) • High gain beams for direct signals (D) • High gain beams for reflected signals (R) • Implicit use of full GNSS bandwidth (3x40 MHz) • Dual or Trial Frequency for precise estimation of ionospheric delay • Precise on-board delay calibration • Precise on-board amplitude calibration

  4. PARIS IoD L1 Power Waveform • GPS L1 Composite, Orbit Height: 500Km, Down-Looking Antenna Gain: 19dBi, Nadir Looking Geometry

  5. PARIS IoD Mission Summary Table

  6. Factors affecting altimetry performance

  7. G P Ionosphere i i s O Max Incidence Angle (1/2) The incidence angle plays an important role in the definition of critical parameters of the PARIS mission, such as:(a) Precision(b) Sampling and Coverage

  8. G P Ionosphere i i s O Max Incidence Angle (2/2) a) Altimetric performance degrades with incidence angleb) Ionospheric delay and antenna loss increase with incidence angle (a) (b) Antenna gain i The incidence angle should be restricted (<35).Coverage and sampling obtained by raising orbital height.

  9. Factors affecting altimetry performance

  10. Interferometric Processing SNR • Up/Down-looking Antenna Gain and Up/Down RX NF shall be such to ensure sufficient SNR • In PARIS IoD with interferometric processing, the final power waveform SNR is given as:

  11. Cross-Correlation Integration Time (Tc) Interferometric Processing Schematic Tc Ninc For a given target along track spatial resolution, e.g. 100Km, the coherent integration time and the incoherent averaging are inversely proportional:

  12. 1 chip delay Cross-Correlation Integration Time (Tc)

  13. Cross-Correlation Integration Time (Tc) Low Tc High Tc

  14. Cross-Correlation Integration Time (Tc) SNRSP TC

  15. Cross-Correlation Integration Time (Tc) • Analytical model for altimetry precision: • Tc shall be chosen such to have a sufficient SNR (e.g. 6dB) but cannot be increased further otherwise the altimetric precision is degraded because speckle is not averaged enough • However, Tc cannot be too low otherwise consecutive XC samples may be correlated and speckle would anyway not be averaged

  16. Factors affecting altimetry performance

  17. Waveform Retracking • The retracking has the objective of estimating the sea state parameters and the sea mean surface height by fitting a theoretical model to measured waveforms • Maximum Likelihood Estimation (MLE) or on weighted least squares estimation has been extensively used in Conventional Radar Altimetry. • The MLE method estimates the parameters by determining which values maximize the probability of obtaining the recorded waveform shape in the presence of noise of a given statistical distribution. delay SWH

  18. Factors affecting altimetry performance

  19. The impact of Ionosphere • At L-band, the ionosphere is a major contributor to the propagation delay with • Multi-frequency observations allow to remove the ionospheric effect • However this is at the expense of a severe error amplification factor

  20. Possible Ionospheric Correction Method • Starting from range equation: • Multi-frequency observations are taken: • The ionospheric delays are retrieved with the following uncertainties: • A regression is performed over N samples of ionospheric delay • The smoothed ionospheric delay is used in the range equations, resulting in an improved error amplification factor

  21. G1 G2 P Ionosphere i i s1 s2 O Ionospheric Correction: Example 1/2 • What matters in mesoscale ocean altimetry is the difference between the ionospheric delay at two specular points Worst case is between nadir (i=0) and edge of swath (i=30)

  22. +12 m Derived from RA-2 real data Vertical Delay (m) 0 m +6 m Mesoscale Delay (m) 10 m +0.15 m Residual Delay (m) 200 km averaging of mesoscale delay applied 0.15 m Ionospheric Correction: Example 2/2

  23. PARIS IoD Preliminary Error Budget

  24. CONCLUSIONS • The PARIS altimeter can provide synoptic views of the ocean being thus well suited for mesoscale ocean altimetry • The interferometric processing between direct and reflected signals maximises the altimetric performance of the system • A demonstration mission should be able to achieve 18 cm total accuracy • An operational mission could be targeted for 5 cm • A particular application of PARIS IoD is the direct observation of tsunamis: • enhance our knowledge on tsunamis and complement early warning systems

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