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    FR4.TO5.5.ppt FR4.TO5.5.ppt Presentation Transcript

    • END-TO-END PERFORMANCE ANALYSIS OF 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
    • The PARIS Concept
      • PARIS =
      • Passive Reflectometry and Interferometry System
    • 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
      L1 L5 L2 40 MHz 40 MHz 40 MHz
    • PARIS IoD L1 Power Waveform
      • GPS L1 Composite, Orbit Height: 500Km, Down-Looking Antenna Gain: 19dBi, Nadir Looking Geometry
    • PARIS IoD Mission Summary Table Main passenger Piggy back Launcher Configuration PROTEUS-like PROBA-like Platform < 5-7.5 cm < 17-20 cm Total Altimetry Accuracy (1  ) All 3 bands of GPS, GLONASS, GALILEO, BEIDOU GPS L1+L5 Galileo E1+E5 Frequencies 16 4 Number of Beams 2.4 m / 30 dB 0.9 m / 19 dB Antenna Diameter / Gain < 100 km 100 km Spatial Resolution 2 days 3 days Revisit Time 1500 km 900 km Swath 1500 km 800 km Orbital Height Polar Sun Synchronous Polar Sun Synchronous Orbit Operational Mission In-orbit Demonstrator Tsunami detection Particular Application Ionospheric total electron content Wind speed and wind direction over ocean Significant Wave Height Mean Square Slope of ocean surface Sea Ice Extent River and lake water level Scientific by-products Polar Ice Thickness Secondary Scientific Product Mesoscale Ocean Altimetry Main Scientific Product PARIS Altimeter Instrument
    • Factors affecting altimetry performance On-board Delay/Amplitude Calibration needed Residual instrument errors after calibration Uncertainties on Geometry/Orbit
      • Use of 2 or 3 Frequencies
      • Estimated or measured
      • Frequency Dependent
      • Fraction of EM Bias
      • Uncertainties on Propagation:
      • Ionosphere
      • Troposphere
      • EM Bias
      • Skewness Bias
      Accuracy Optimum retracker to be defined Waveform Retracking Performance
      • Improve SNR
      • Optimized for maximum precision
      • Instrument Characteristics:
      • Antenna Gain, Receiver NF
      • XC Coherent integration time
      • Higher incidence angle reduces the height precision, but increases swath
      • Scanning reduces antenna gain
      Allowed incidence angle of specular point Precision Comment Factors affecting altimetry performance
    • 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 P G O s Ionosphere i i
    • Max Incidence Angle (2/2) a) Altimetric performance degrades with incidence angle b) Ionospheric delay and antenna loss increase with incidence angle The incidence angle should be restricted (<35  ). Coverage and sampling obtained by raising orbital height. (a ) (b) i Antenna gain P G O s Ionosphere i i
    • Factors affecting altimetry performance On-board Delay/Amplitude Calibration needed Residual instrument errors after calibration Uncertainties on Geometry/Orbit
      • Use of 2 or 3 Frequencies
      • Estimated or measured
      • Frequency Dependent
      • Fraction of EM Bias
      • Uncertainties on Propagation:
      • Ionosphere
      • Troposphere
      • EM Bias
      • Skewness Bias
      Accuracy Optimum retracker to be defined Waveform Retracking Performance
      • Improve SNR
      • Optimized for maximum precision
      • Instrument Characteristics:
      • Antenna Gain, Receiver NF
      • XC Coherent integration time
      • Higher incidence angle reduces the height precision, but increases swath
      • Scanning reduces antenna gain
      Allowed incidence angle of specular point Precision Comment Factors affecting altimetry performance
    • 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:
    • Cross-Correlation Integration Time (Tc) Interferometric Processing Schematic For a given target along track spatial resolution, e.g. 100Km , the coherent integration time and the incoherent averaging are inversely proportional : Tc Ninc
    • Cross-Correlation Integration Time (Tc) 1 chip delay
    • Cross-Correlation Integration Time (Tc) Low Tc High Tc
    • Cross-Correlation Integration Time (Tc) SNR SP T C
    • 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
    • Factors affecting altimetry performance On-board Delay/Amplitude Calibration needed Residual instrument errors after calibration Uncertainties on Geometry/Orbit
      • Use of 2 or 3 Frequencies
      • Estimated or measured
      • Frequency Dependent
      • Fraction of EM Bias
      • Uncertainties on Propagation:
      • Ionosphere
      • Troposphere
      • EM Bias
      • Skewness Bias
      Accuracy Optimum retracker to be defined Waveform Retracking Performance
      • Improve SNR
      • Optimized for maximum precision
      • Instrument Characteristics:
      • Antenna Gain, Receiver NF
      • XC Coherent integration time
      • Higher incidence angle reduces the height precision, but increases swath
      • Scanning reduce antenna gain
      Allowed incidence angle of specular point Precision Comment Factors affecting altimetry performance
    • 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.
      SWH delay
    • Factors affecting altimetry performance On-board Delay/Amplitude Calibration needed Residual instrument errors after calibration Uncertainties on Geometry/Orbit
      • Use of 2 or 3 Frequencies
      • Estimated or measured
      • Frequency Dependent
      • Fraction of EM Bias
      • Uncertainties on Propagation:
      • Ionosphere
      • Troposphere
      • EM Bias
      • Skewness Bias
      Accuracy Optimum retracker to be defined Waveform Retracking Performance
      • Improve SNR
      • Optimized for maximum precision
      • Instrument Characteristics:
      • Antenna Gain, Receiver NF
      • XC Coherent integration time
      • Higher incidence angle reduces the height precision, but increases swath
      • Scanning reduce antenna gain
      Allowed incidence angle of specular point Precision Comment Factors affecting altimetry performance
    • 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
    • Possible Ionospheric Correction Method
      • Multi-frequency observations are taken:
      • 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
      • The ionospheric delays are retrieved with the following uncertainties:
      • Starting from range equation:
    • 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  )
      P G 2 O s 2 Ionosphere i i G 1 s 1
    • Ionospheric Correction: Example 2/2 Vertical Delay (m) Mesoscale Delay (m) Residual Delay (m) 200 km averaging of mesoscale delay applied  0.15 m +0.15 m +6 m  10 m +12 m 0 m Derived from RA-2 real data
    • PARIS IoD Preliminary Error Budget 5 cm Troposphere (Wet and Dry) 2 cm EM Bias 1 cm Skewness Bias 5 cm Orbit / Geometry 2 cm Instrument error residuals Ionosphere Residual Total RMS Height Accuracy Instrument Noise and Speckle Ionosphere Averaging Noise 18 cm at Edge of Swath 5 cm 9.5 cm (2 frequencies, N=3) 12.5 cm IOD Height Total Accuracy on 100Km, G=20dBi, H=800Km Parameter
    • 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