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					CryoSat Workshop, 9/2/05 - 1




        SIRAL Beam-formation Verification
        using Transponders

        Mònica Roca and Mercedes Reche
        PiLDo Labs, Barcelona




                                         PiLDo Labs
CryoSat Workshop, 9/2/05 - 2                                         ToC


      1. Introduction
      2. Relevant aspects of the SIRAL on-ground processing and
         data
      3. The ESA transponder
      4. SIRAL calibration using transponder data
      5. Conclusions and way forward




                                                        PiLDo Labs
CryoSat Workshop, 9/2/05 - 4




      2. Relevant aspects of the SIRAL ground processing
         and data




   2. SIRAL ground processing and data          PiLDo Labs
CryoSat Workshop, 9/2/05 - 5            2. SIRAL ground processing and data

  •   SIRAL has 3 operating modes:
       – Low-resolution mode (LRM): Conventional, pulse-limited altimeter, used over ice
         sheet interiors and over ocean.
       – Synthetic Aperture mode (SAR): used over sea ice. One single antenna.
       – SAR-Interferometric mode (SARin): Used over the margins of the ice sheets.
         Interferometry between echoes received on each antenna.
  •   In LRM, the range compression and incoherent averaging are performed on-
      board, and the resulting averaged echo forms the level 0 data.
  •   The FBR data in the SAR and SARin modes consists of the individual,
      complex (I and Q) echoes. In SARin mode, there are two such echoes, one
      for each antenna of the interferometer.
  •   In SAR and SARin modes, the radar echoes must first be synthetic
      aperture processed (performed on-ground) before incoherent multi-looking
      (in SARin mode also phase multi-looking will also be applied). This forms the
      Level 1b data.




   2. SIRAL ground processing and data                                 PiLDo Labs
CryoSat Workshop, 9/2/05 - 6           2. SIRAL ground processing and data
                                           Instrument-corrected and
                    FBR                    geo-located bursts

   Steering: shifting the orientation
                                           Beams steered to specific surface
             of the beams


       •    FFT over elapsed time          - Range compression
                                           - 64 Doppler beams with an equal angular
       •    FFT across the burst             separation


           Slant range correction          Computed for each Doppler beam, to align
                                           them with their respective surface samples.

        Multi-look of each stack           To reduce speckle.
    (+ phase multilooking for SARin)       Echoes from beams of successive bursts,
                                           directed at the same location, are summed.

      SAR L1b             SARin L1b



   2. SIRAL ground processing and data                             PiLDo Labs
CryoSat Workshop, 9/2/05 - 7




      3. The ESA Transponder




   3. The ESA transponder      PiLDo Labs
CryoSat Workshop, 9/2/05 - 8                                   3.1. Introduction

  •   A transponder is seen by a radar as a point target (well known).
  •   1 transponder will be available for the CryoSat project (refurbished old
      ESA transponder developed for the ERS altimeter calibration).
  •   Located in the ESA Svalbard station.
  •   Initially 2 TRP’s were located 250 meters a part. This strategic location
      would have allowed the retrieval of many passes with transponder signal
      taken with different along-track and across-track separation.
  •   Finally only 1 transponder. We can still perform the analysis and
      interpretation of the data over the transponder, acting as a point target,
      for retrieval of the beam formation.
  •   We have identified several specific studies or calibrations that will make
      use of the transponder data (after reconstruction of doppler beams).




   3. The ESA transponder                                        PiLDo Labs
CryoSat Workshop, 9/2/05 - 9   3.2. Transponder location and passes

                               •   Transponder Location: SVALBARD
                                   (selected due to its high latitude)
                               •   CryoSat Orbit repeat cycle is 369 days.
                                   Not a sun-synchronous orbit.
                               •   SARin mode over the transponder
                               •   The passes can be used so long as the
                                   power decay (with respect to the peak)
                                   is not grater than about -7dB, which
                                   implies a separation from the nadir
                                   track of about about 5 kilometres.
                               •   Days for CryoSat passes over Svalbard
                                      In 6 months: 13 days
                                      In 16 months: 37 days
                                      Irregular interval,averaging 12 days




   3. The ESA transponder                                  PiLDo Labs
CryoSat Workshop, 9/2/05 - 10   3.2. Transponder location and passes




   3. The ESA transponder                            PiLDo Labs
CryoSat Workshop, 9/2/05 - 11                                                       3.3. TRP characteristics
                       Parameter                                     Type I
  Antenna diameter                                         0.61 m

  Radar cross section (s = Gant1 x Gant2 x Gsys x l2/4π)   44.5 – 104.5 dB

  Beam width                                               2.5º

  Beam co-linearity                                        +-0.1 º

  Beam pointing                                            +-0.1º

  Antenna gain                                             35.8 dB

  Inter-antenna isolation                                  105 dB

  Amplifier frequency range                                13.2 – 14.1 GHz

  Amplifier gain                                           17-77 dB

  Amplifier gain variation across band                     <+-0.5 dB

  Amplifier 1dB suppression point                          20 or 23 dBm

  Amplifier phase linearity across band                    +-7º               Transponder developed by RAL (UK) in 1987
                                                                                                            curtsey of ESA
  Internal gain monitoring precision                       +-0.05 dB

  Gain stability over temperature                          need it !

  Internal path-length monitoring precision                +-0.5 mm

  Nominal internal path length                             9.88 m

  Path-length stability over temperature                   0.1 mm/º




   3. The ESA transponder                                                                         PiLDo Labs
CryoSat Workshop, 9/2/05 - 12




      4. SIRAL calibration using Transponder data




                                                PiLDo Labs
CryoSat Workshop, 9/2/05 - 13           4. SIRAL calibration using TRP data

    Primarily used for calibration of the interferometer baseline.
    • Other possible calibrations over the transponder:
        –   Range bias: the measured range is compared with the expected one
        –   Input to orbit studies: comparison of the range measured by the
            radar altimeter with that calculated from an ephemeris
        –   Sigma-0 bias: altimeter measured received power when flying over
            the transponder is compared with the theoretical power. The
            transponder radar cross-section has to be accurately characterised
        –   Datation: the expected time of minimum range is compared with the
            actual time of minimum range seen by the altimeter over the TRP
        –   Mispointing: traditionally by measuring the slope of the echo trailing
            edge. New methods using SARin mode echoes over a TRP
    •   SIRAL Doppler beam formation consolidation through the TRP.




   4. SIRAL calibration using transponder data                     PiLDo Labs
CryoSat Workshop, 9/2/05 - 14                                  4.1. Range bias

    4.1. Range Bias:
    • Over TRP:
        – using Level 1b data (after multi-looking)
        – using stack (after beam steering, 2D FFT, but before slant-range
          correction and multi-looking)
    • Over a natural target




   4. SIRAL calibration using transponder data                 PiLDo Labs
CryoSat Workshop, 9/2/05 - 15                                   4.1. Range bias

    Over TRP using Level 1b data
    •   Principle: the range is evaluated by retracking the one single multi-
        looked echo (from the Level 1b data) at the transponder position and
        compared with the theoretical range at that same point.
    •   We need to pay particular attention to the atmospheric corrections
        (ionosphere and troposphere).
    •   We do not have in-situ measurements so we should rely on modelling.
    •   Ionospheric correction smaller at high latitudes.
    •   We need the internal TRP delay characterised up to a value that
        depends on the range calibration requirement. This is measured by an
        TRP internal calibration (about 12 ns).
    •   We can also characterise it through the RA-2 (use RA-2 well calibrated
        over the TRP to determine TRP internal delay).




   4. SIRAL calibration using transponder data                  PiLDo Labs
CryoSat Workshop, 9/2/05 - 16                           4.1. Range bias

   • EnviSat orbits
   • CryoSat orbits




       • Need to evaluate TRP distance to EnviSat ground track




   4. SIRAL calibration using transponder data          PiLDo Labs
CryoSat Workshop, 9/2/05 - 17                    4.1. Range bias

 Over TRP using a stack of
   beams
   Principle: the range is
   evaluated by modelling
   the theoretical function
   that describes the range
   distance between the
   altimeter and the TRP,
   accounting for the
   satellite trajectory and
   velocity [r(t) hyperbola].
    This range is compared
    with the measured
    range.




   4. SIRAL calibration using transponder data   PiLDo Labs
CryoSat Workshop, 9/2/05 - 18                                    4.1. Range bias

    •   We can use SARin mode, although we only need one antenna received
        echo.
    •   In order to retrieve the range measured by the altimeter, the
        individual I&Q echoes shall be processed by performing the 2D-FFT
        (Doppler beams and range compression), and retracking.
    •   We will compare the above with the computed theoretical range and
        retrieve the range bias.
    •   Same considerations about the characterisation of the internal delay of
        the transponder apply.
    •   Same considerations about the atmospheric corrections apply.




   4. SIRAL calibration using transponder data                   PiLDo Labs
CryoSat Workshop, 9/2/05 - 19                                 4.1. Range bias

 Over natural targets: the salar de
   Uyuni, Bolivia (PI- H.A. Fricker)
 •    measurement of the surface height
      retrieved by the altimeter is compared
      with independent measurement (GPS
      survey)                                      ICESat
                                                              360
                                                                    085
                                                    EnviSat
 •    we will use SAR In                           TOPEX
 •    we will reproduce the processing
      performed for RA-2: retrack the
      waveforms with a gaussian retracker
 •    we can cross-calibrate RA-2 and SIRAL
      SAR In
 •    we can also use the salar to calibrate
      the interferometric baseline




     4. SIRAL calibration using transponder data              PiLDo Labs
CryoSat Workshop, 9/2/05 - 20                                     4.2. Sigma-0 bias

   4.2. Sigma-0 bias
   •   The TRP can be used to calibrate the radar measurements of the
       surface backscatter
   •   2 ways of approaching the problem
        – using Level 1b data (after multi-looking)
        – using stack (after beam steering, 2D FFT, but before slant-range correction
          and multi-looking)
   •   The altimeter measures the power of a target, the transponder, which
       has a well known radar cross section (RCS).
   •   Therefore the RCS shall be extremely well characterised, depending on
       sigma-0 calibration requirements (aprox. ±0.1 dB).
   •   By applying the radar equation it is possible to compute the theoretical
       power the altimeter is supposed to measure.




   4. SIRAL calibration using transponder data                        PiLDo Labs
CryoSat Workshop, 9/2/05 - 21                                                            4.2. Sigma-0 bias
 •    Over a TRP, the altimeter is looking at a ground fixed point target and therefore the
      altimeter is not measuring at nadir but at a certain angle given by the geometry. At
      the same time, the TRP is not seen by the altimeter from zenith.
 •    Since the radar cross section of the transponder depends on the transponder antenna
      gain, the angle in which the transponder is seen by the altimeter also becomes
      important.



                                                                      R                    
                                   CryoSat              t   sin1       sin   t 
                                                                                  180
                                                                     R  h                 

                                                               rt           or         rd
                                     

                                                    A ,t    T ,  t  Gelec  l4
                                                                           2           T          2
               TRP                                  G                G
                                             P P                                                         [W]
                                             
                                              r    t
                                                                    4 
                                                                               4
                                                                                    r t   L
                                                                                     4




     4. SIRAL calibration using transponder data                                              PiLDo Labs
CryoSat Workshop, 9/2/05 - 22                    4.2. Sigma-0 bias

   • Power received by
     the altimeter over
     the TRP




   4. SIRAL calibration using transponder data     PiLDo Labs
CryoSat Workshop, 9/2/05 - 23                              4.2. Sigma-0 bias

   Processing (conceptually similar to range bias):
   •   We can use SARin mode, although we only need one antenna received
       echo.
   •   If using the stack beams: in order to retrieve the power measured by
       the altimeter, the individual I&Q echoes shall be processed by
       performing: the 2D-FFT (Doppler beams and range compression), and a
       retracking (gaussian fitting and integration).
   •   If using the Level 1b: the multi-looked echo shall be retracked.
   •   We will compare the above with the computed theoretical power and
       retrieve the power bias.
   •   We need the TRP RCS characterised up to a value that depends on the
       sigma-0 calibration requirement (about ± 0.1 dB).
   •   Otherwise we will characterise it through the RA-2 (use RA-2 well
       calibrated over the TRP to determine TRP internal delay).




   4. SIRAL calibration using transponder data                PiLDo Labs
CryoSat Workshop, 9/2/05 - 24                                   4.3. Mispointing
   4.3. Mispointing
   •   Historically, due to the nature of the past altimeters, mispointing have
       only been evaluated irrespectively of the direction (pitch and roll).
   •   We have now the opportunity of measuring mispointing as a function of
       the angle of arrival.
   •   Considerations:
        – along-track mispointing can be misinterpreted as a datation error;
        – across-track mispointing can be misinterpreted as interferometric
           baseline error.
   •   We will therefore estimate the mispointing with the traditional way:
       slope of the trailing edge of 10 minutes averaged waveform over ocean
       (LRM).




   4. SIRAL calibration using transponder data                    PiLDo Labs
CryoSat Workshop, 9/2/05 - 25                      4.4. Datation

   4.4. Datation:
   •   TRP’s can be used to
       measure the datation
       bias.
   •   The expected time
       of minimum range
       (black) is compared
       with the actual time
       of minimum range
       seen by the altimeter
       over the TRP.
   •   We can use a stack
       of echoes or single
       multi-looked echo.




   4. SIRAL calibration using transponder data   PiLDo Labs
CryoSat Workshop, 9/2/05 - 26                                      4.5. Angle of arrival

   4.5. Angle of arrival
   •   The across track angle of an incoming ray, Fmeas, can be inferred by
       determining the phase difference between the 2 received signals (one
       from each antenna).
                                                 l  phase 
                                 Fmeast   sin 
                                               1
                                                              
                                                  2B 
       where B is the distance between the 2 antenna phase centres.
   •   The theoretically computed angle of arrival, Ftheo, can be compared with
       the angle of arrival retrieved from the data.
                     
                                                 ht 
                                  Ftheo  is cos satellite height.
       where r(t) is a hyperbola and ht   the  
                                                    1

                                                 rt  
   •   If using Level 1b data we compare a single angle measured against
       theoretical angle. If using the stack we compare the 2 equations.
                            




   4. SIRAL calibration using transponder data                          PiLDo Labs
CryoSat Workshop, 9/2/05 - 27                                4.5. Angle of arrival



                                      r1
                                              h1
                                                                    l  phase 
                                                   Fmeast   sin1           
                                                                     2B 
                                 r0
                       h0

                                                                    ht 
                                            d1   Ftheo t   cos 
                                                                   1
                                                                            
     h -1       r -1                                                  rt  
                                                                    
                            d0

                                           TRP
               d-1                            




   4. SIRAL calibration using transponder data                          PiLDo Labs
CryoSat Workshop, 9/2/05 - 28




      5. Conclusions and way forward




                                       PiLDo Labs
CryoSat Workshop, 9/2/05 - 29        5. Conclusions and way forward

  • Transponders have demonstrated through the years the suitability
    of certain type of calibrations.
  • Doppler beam formation processing can be verified over the TRP.
  • We will use the transponder deployed in Svalvard for SIRAL
    calibration of angle of arrival and:
            • range bias,
            • sigma-0 bias,
            • datation.
  • A transponder RCS characterisation is recommended for Sigma-0
    bias determination.
  • We will compare the retrieved biases using the stack beams
    (before multi-looked) and using the single multi-looked echo.




   5. Conclusions and way forward                     PiLDo Labs
CryoSat Workshop, 9/2/05 - 30




            PiLDo
               Labs

             C. Llacuna, 162
             Barcelona 08018
             T +34 93 401 97 82
             F +34 93 401 97 83
             PiLDo@pildo.com



             Monica.Roca@pildo.com
                            Mercedes.Reche@pildo.com
             tel.: +34 93 401 9707       tel.: +34 93 401 9755




                                                                 PiLDo Labs

				
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