Hildebrand 23 Mar 04

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					            Surface Water Mission Concepts


   Peter Hildebrand, Rafael Rincon, Lawrence Hilliard, David LeVine
                     NASA/Goddard Space Flight Center
                              James Mead
                       ProSensing Corp., Amherst, MA




23 March 2004                                                         1
                RadSTAR Surface Water Concept

• Project Planning Issues
     – Science measurement requirements
• RadSTAR Scatterometer Instrument
     – Scatterometer/radiometer concept
     – Project status
• Application to Surface Water Measurement needs
     – Image: SAR processing
     – Altimetry:
• Project Planning Issues
     –   Science measurement requirements
     –   Algorithm
     –   Instrument
     –   Cost cap


23 March 2004                                      2
            Science Measurement Requirements

    •   100 m spatial resolution (50 m better)
    •   10 cm height resolution (5 cm better)
    •   River slopes to ~5 cm / 5 km
    •   ~weekly update (equator)
    •   Cloud and forest canopy penetration




23 March 2004                                    3
            Science Measurement Requirements




23 March 2004                                  4
            Science Measurement Requirements




23 March 2004                                  5
            Science Measurement Requirements




23 March 2004                                  6
            Science Measurement Requirements




                  •   100 m spatial resolution (50 m better)
                  •   10 cm height resolution (5 cm better)
                  •   River slopes to ~5 cm / 5 km
                  •   ~weekly update (equator)
                  •   Cloud and forest canopy penetration
23 March 2004                                                  7
                    RadSTAR Science &
                         Technology Goals
• Science goals:
  – measure soil moisture and sea surface salinity
     • important variables in the understanding of climate
       change, including the speed of the water cycle.
  – provide coincident Earth surface backscatter and
    emission measurements
  – support Aquarius and Hydros validation studies
• Technology goals:
  – merge 1.26 GHz digital beam-forming scatterometer,
    1.4 GHz STAR radiometer, 2-D array, patch antenna.
  – provide L-band, emission and backscatter
    measurements in a compact aircraft instrument
     • compact, cross-track scanning package with no moving
       parts
  – move on path to space for an L-band scatterometer/
    radiometer.


   23 March 2004                                              8
                       RadSTAR Scatterometer

• RadSTAR Scatterometer Instrument:
     – Designed for
          • joint operation with the existing NASA Synthetic Thinned Array
            Radiometers (STAR)
          • remote sensing of soil moisture and ocean salinity.
          • employ a single broadband antenna for simultaneous radar and
            radiometric measurements
     – Airborne imaging radar
          • capable of cross-track scanning
          • synthesizing multiple beams using digital beamforming techniques




23 March 2004                                                                  9
                                 RadSTAR Scatterometer
                                                                                           real -time
                                                                                          beamformer

•     8 transmit/receive channels
•     phased-array antenna.                                       ADC             ADC                   ADC        ADC          ...
•     amplitude and phase control                 20 MHz IF output,
                                                  bandwidth=1 MHz
      capability enables generation of
      low-sidelobe beams               transmit phase/
                                      amplitude control

•     scan range of +/-50 degrees.                               T/R
                                                                module
                                                                                  T/R
                                                                                 module
                                                                                                    T/R
                                                                                                   module
                                                                                                                   T/R
                                                                                                                  module

•     received signals down-converted to
      20 MHz.
•     IF signals digitized and passed to
      an FPGA processor for
      beamforming




                                                                         Subarrays of microstrip patch elements




    23 March 2004                                                                                                          10
                          RadSTAR Scatterometer
                                     Transmitter
                  Frequency                         1.26 GHz
                    PRF                              5 KHz
                 Pulse Width                          1 s
         Number of Transmitters                         8
      Total Output Power (Nominal)                    10 W
          Beam Steering Angles                      50 degrees


                                      Receiver
                Dynamic Range                         60 dB
                  Precision                          0.1 dB
         Intermediate Frequency                      20 MHz
        Beamforming Scan Angles                     50 degrees
           Number of Channels                           8

23 March 2004                                                     11
                                 RadSTAR Antenna

•   Based on ESTAR antenna
•   corporate fed microstrip patch array
•   10 subarrays of 8 elements with a separation
    of /2
•   only the central 8 are active, the outer two
    terminated in a matched load to reduce the
    effects of uneven mutual coupling

                                     Parameter                      Description
                                   Antenna Type                Microstrip Patch Array
                                     Frequency                       1.26 GHz
                            Bandwidth (10 dB return loss)             18 MHz
                                    Polarization                     H and V
                         3 dB Beamwidth (Nadir, along track)      15+/- 1 Degrees
                                   Subarray gain                      10.5 dB
                                     Side Lobes                 Better than – 18 dB
                                 Scan Angle Range                   50 degrees
     23 March 2004                                                                      12
                         RadSTAR Scatterometer

• Beamforming:
     – far-field beams are digitally generated at various scan angles
       from sampled voltages in an antenna array.
     – the technique allows steering the received beam and controlling
       its beamwidth and side-lobe.
     – several beamforming options:
          • Focused beam on transmit and receive
                – Narrow beam, low sidelobes
          • Wide-field illumination; beam forming on receive
                – Simultaneous cross-track sampling




23 March 2004                                                        13
                    RadSTAR Science &
                         Technology Goals
• Science goals:
  – measure soil moisture and sea surface salinity
     • important variables in the understanding of climate
       change, including the speed of the water cycle.
  – provide coincident Earth surface backscatter and
    emission measurements
  – support Aquarius and Hydros validation studies
• Technology goals:
  – merge 1.26 GHz digital beam-forming scatterometer,
    1.4 GHz STAR radiometer, 2-D array, patch antenna.
  – provide L-band, emission and backscatter
    measurements in a compact aircraft instrument
     • compact, cross-track scanning package with no moving
       parts
  – move on path to space for an L-band scatterometer/
    radiometer.


   23 March 2004                                              14
                RadSTAR Surface Water Concept

• Application to Surface Water Measurement needs
     – SAR processing
     – Altimetry
• Project Planning Issues
     –   Science priorities
     –   Science measurement requirements
     –   Algorithm
     –   Instrument
     –   Cost cap




23 March 2004                                      15
                RadSTAR Surface Water Concept
•   Use digital beam forming to
    synthesize multiple beams on
    each side of the flight track.                     Spaceborne
                                                         Radar
•   420 km swath is formed from:
     – two swaths of 210 km on each                                 V
       side of the track
                                         Synthesized
     – each 210 km consists of 7            beam
       individual beams, with a ground
       resolution of 30 km.
     – SAR processing sharpens                                      Sawth
       measurements                                                 Width




23 March 2004                                                           16
                    RadSTAR Surface Water Concept

•    To avoid Doppler aliasing and
     range ambiguities, the radar PRF
     is constrained by the high ground
     speed and the cross-track (range)
     swath width.
•    The cross-track antenna length W
     is determined by:
      W = h/(S cos2θ)
      where  = wavelength, h = orbit
        height, S = swath width,
        θ=incidence angle
•    For θ < 50 degrees, S=30 km:
      – W=5.9 m (L-band),
      – W=0.55 m (Ku-band)




    23 March 2004                                   17
                    RadSTAR Surface Water Concept

•    The minimum along-track antenna
     length is set by the need to avoid
     Doppler aliasing and range
     ambiguities:
      – L = 4 Kuh tan (θ)/(Wf cos( θ))
      – where: u=orbit velocity, f=frequency
      for h=350 km, Xr=30 km, max=50 , and
          f=1.26 GHz (13.9 GHz) L=8.19 m.


•    The SAR would therefore require an
     antenna of
      –   5.9 x 8.19 m at L-band
      –   0.55 m x 8.19 m at Ku-band


•    Could reduce the long dimension using
     a complex waveform.




    23 March 2004                                   18
                RadSTAR Surface Water Concept

                  • The L-band antenna:
                     – 49 subarrays, spaced at, λ/2
                     – subarrays consist of 68 elements
                  • This antenna could provide
                     – 30 km cross-track swath




                                                    5.9 m
                                 8.2 m

23 March 2004                                               19
                          Surface Water Concept

• Mapping provided through SAR processing
     – Wide swath width increases runoff mapping
     – Effects of aliasing, sidelobes & multiple reflecting surfaces
• Altimetry provided through interferometry
     – Wide swath reduces accuracy requirements for the same slope
       measurement precision
     – Effects of aliasing, sidelobes & multiple reflecting surfaces
• Issues:
     – Seeing through clouds, precipitation and forest cover
     – Radar system issues
          • Antenna size
          • Signal processing &/or data rate
          • Complexity
     – Also consider alternatives to radar



23 March 2004                                                          20
                 Surface Water Mission Planning

• Project Planning Issues
     – Science priorities
          • Important new science with important applications
     – Science measurement requirements
          • Clearly defined sampling
          • Meet science observational needs
     – Algorithm
          • Demonstrated capability
     – Instrument
          • Demonstrated capability – high TRL
     – Cost cap
          • Compromise
          • Find partners
     – Teaming
          • Strong leadership
          • Committed, high performance team
          • Willing to compromise in order to achieve



23 March 2004                                                   21

				
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