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Surface Water ESSP Mission Radar Options

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Title: Surface Water ESSP Mission Radar Options


1
Surface Water ESSP Mission Radar Options
  • E. Rodriguez, D. Moller, P.S. Callahan, Y. Kim
  • Jet Propulsion Laboratory
  • California Institute of Technology

2
Introduction
  • Science Requirements
  • Mission Design Process
  • Measurement Concepts
  • Space/Time Coverage Study
  • Instrument Concepts and Measurement Accuracy
  • Summary and Recommendations

3
ESSP and Science Requirements
  • ESSPs need to have compelling, clear Science
    Goals and Objectives
  • Hornerberger Report Science Questions
  • What are the underlying causes of variation in
    the water cycle on both global and regional
    scales, and to what extent is this variation
    induced by human activity?
  • To what extent are variations in the global and
    regional water cycle predictable?
  • How will variability and changes in the cycling
    of water though terrestrial and freshwater
    ecosystems be linked to variability and changes
    in the cycling of carbon, nitrogen and other
    nutrients at regional and global scales?
  • Measurement requirements directly related to
    science objectives
  • May have levels at which baseline science is
    accomplished and goals for improved science
  • Tradeoffs, interplay between science goals and
    achievable mission, measurement

4
Science Requirements
  • 5-10 cm height accuracy (need height change for
    storage change, not absolute height)
  • River discharge, wetland/lake storage change
  • Map rivers gt 100m width
  • Would like to go to smaller rivers
  • River slope measurement accuracy 10 mrad
    (1cm/1km for Amazon)
  • River discharge
  • Revisit time
  • Ideal 3 days in the Arctic, 7 days in the
    tropics
  • Acceptable 7 days in the arctic, 21 days in the
    tropics
  • Imager with resolution better than 100 m
  • River width, wetland/lake extent
  • Should distinguish vegetated/non-vegetated
  • Global coverage, sampling all major contributors
    to surface water, is not affected by clouds
  • Wetlands, rivers, lakes in tropics, Arctic thaw

5
Mission Design Process
  • Select orbit to optimize temporal/spatial
    coverage
  • Select frequency to reduce errors and cost
  • Optimize instrument design
  • Derive instrument performance
  • Compare instruments
  • Spacecraft accommodation
  • Launch vehicle selection
  • End to End data flow
  • Mission Operations
  • Cost mission
  • The presentation below will follow the blue steps
    in order (and briefly mention the last step)

6
JPL Experience with ESSP
  • JPL has participated in 5 winning ESSP missions
    (3 radar missions)
  • GRACE
  • Cloudsat Cloud profiling radar
  • OCO Orbiting Carbon Observatory
  • Aquarius Ocean salinity radar/radiometer
  • Hydros Soil moisture radar/radiometer
  • The ESSP process requires a complex trade-off
    between science requirements, mission design,
    instrument design and performance analysis,
    partnering, and cost management

Cloudsat
GRACE
Aquarius
Hydros
OCO
7
JPL Experience with Radar Topography
  • TOPEX/Poseidon Jason Altimeters sea surface
    topography (1992-, 2000-)
  • Science applications, algorithms, retracking,
    ground system, mission management, calibration
    and validation
  • Shuttle Radar Topography Mission land topography
    (2000)
  • Instrument design and manufacture, processing
    algorithms, ground system, mission management,
    calibration and validation
  • Ocean Surface Topography Mission Wide-Swath
    Ocean Altimeter (2007)
  • Science applications, instrument design and
    manufacture, processing algorithms, ground
    system, mission management, calibration and
    validation

8
Instrument Options
  • Tomographic real aperture altimeter optical
    imager (Topex/Jason heritage)
  • Use imager to model returns and generate
    space/time templates for parameter fitting
  • Lowest cost
  • Synthetic Aperture Altimeter optical imager
    (Delay-Doppler Altimeter SAR heritage)
  • Uses synthetic aperture altimetry to improve
    along-track resolution
  • No determination of river/wetland extent without
    an additional imager
  • Medium cost
  • Near-Nadir Synthetic Aperture Interferometer
    (SRTM WSOA heritage)
  • Combines height determination and imaging in a
    single instrument
  • Height determination and imaging over a large
    swath
  • Higher cost

9
Measuring Heights from Ranging
  • For all instrument concepts to be examined, the
    position vector is given by
  • P is the platform position , r is the range, and
    l is a unit vector in the look direction
  • In two-dimensions, this becomes
  • The difference between measurement techniques is
    how r and q are measured, and the spatial
    resolution is achieved

10
Radar Spatial Resolution
  • Conventional real-aperture altimeter spatial
    resolution is determined by iso-range annuli and
    antenna beamwidth
  • Left/right/front/back ambiguity
  • Pulse limited circle gives geolocation
  • Synthetic aperture processing narrows the
    along-track (azimuth) cell size
  • Left/right ambiguity is not resolved
  • Clutter from land is reduced
  • Interferometer resolves left/right ambiguity by
    illuminating only one side of the swath

11
Conventional Altimeter Measurement Concept
?
Track Point
  • Fit for leading edge center to estimate r(ange)
  • Assume that the return comes from the
    pulse-limited circle q 0
  • This procedure works well for homogeneous
    scatterers (large open water bodies, ice)
  • An alternative concept (tomographic processing)
    better suited for rivers will be described below

12
Interferometric Measurement Concept
  • Conventional altimetry measures a single range
    and assumes the return is from the nadir point
  • For swath coverage, additional information about
    the incidence angle is required to geolocate
  • Interferometry is basically triangulation
  • Baseline B forms base (mechanically stable)
  • One side, the range, is determined by the system
    timing accuracy
  • The difference between two sides (Dr) is obtained
    from the phase difference (F) between the two
    radar channels.

F 2p D r/l 2pB sin Q/l h H - r cos Q
13
Orbit Selection (1 of 3)
  • A sun-synchronous daytime orbit is desired
  • Optical imager can only operate in sunlight
  • Sun-synchronous orbits significantly simplify
    spacecraft design no batteries required, solar
    panels only move about one axis
  • An exact repeat orbit is desired for
  • Developing time series, particularly for lakes
  • Cal/val by revisiting calibration sites and other
    specific instrumentation
  • Use of waveform templates and other
    pre-calculated features
  • Repeat period needs to meet sampling criteria
  • Rapid revisit time to resolve variations (Arctic
    thaw, floods)
  • Dense tracks to hit many lakes, all important
    river features
  • These criteria conflict as 5 day repeat has only
    about 70 orbits with gt500 km spacing while lt100
    km spacing requires 400 revs or about 30 days.

14
Orbit Selection (2 of 3)
  • Higher orbit desirable for smaller perturbations,
    easier maintenance
  • Orbit maintenance maneuvers less frequent than
    once per week (Operational) and once per repeat
    (Accuracy, Operations)
  • Smaller perturbations (gravity, drag) improve
    orbit determination
  • Reach at least 75 deg latitude to cover Arctic
    rivers
  • Use frozen orbit to maintain low eccentricity and
    argument of perigee, ease orbit maintenance

15
Orbit Selection (3 of 3)
  • TOPEX studies suggestion orbit near 800 km with
    inclination 75 deg or 98 deg (Sun sync) with
    15-30 day (93-gt186 km) repeat and 6 day
    (466 km) subcycle as a good compromise of all
    considerations
  • Need to search for detailed parameters including
    frozen orbit
  • Would need orbit maintenance maneuvers 4 - 40
    days, depending on solar cycle activity level and
    spacecraft Area/Mass

16
Space/Time Coverage Study
  • Simulate instrument coverage for two scenarios
  • Pulse limited altimeter
  • Interferometer with 120 km swath
  • Examine space/time observations of rivers and
    lakes for different orbit choices
  • Orbits examined
  • ERS-2 Orbit
  • 35 day repeat
  • 98 degree inclination
  • Sun synchronous
  • Terra Orbit
  • 16 day repeat
  • 98 degree inclination
  • Sun synchronous
  • 10-Day Repeat Orbit

17
Surface Water Databases Used
  • CIA World Data Bank II
  • Rivers examined major rivers, additional major
    rivers, other rivers, double-lined rivers
  • River shape, but no discharge information
  • http//www.evl.uic.edu/pape/data/WDB/
  • University of New Hampshire Global River
    Discharge (Vörösmarty, Fekete, and Tucker)
  • Time series of river discharge at stations
  • Average discharge computed and registered with
    CIA river data base
  • http//www.rivdis.sr.unh.edu/
  • University of Hawaii Global Self-Consistent,
    Hierarchical, High-Resolution, Shoreline Database
    (Wessel and Smith)
  • Lakes and lake areas
  • http//www.soest.hawaii.edu/wessel/gshhs/gshhs.htm
    l

18
ERS-2 35 Day Repeat Coverage
Global River Coverage Histogram
Global Lake Coverage Histogram
19
ERS-2 Visits/Cycle for Major Rivers
20
ERS-2 Visits/Cycle for Major Lakes
21
Terra 16 Day Repeat Coverage
Global River Coverage Histogram
Global Lake Coverage Histogram
22
Terra Visits/Cycle for Major Rivers
23
Terra Visits/Cycle for Major Lakes
24
10 Day Repeat Coverage
Global River Coverage Histogram
Global Lake Coverage Histogram
25
10-Day Repeat Visits/Cycle for Major Rivers
26
10-Day Repeat Visits/Cycle for Major Lakes
27
Coverage Study Conclusions
  • Pulse limited coverage severely undersamples
    river and especially lake coverage
  • 16-day repeat coverage misses 30 of rivers and
    70 of lakes in the data bases
  • If one restricts the study to the largest rivers
    and lakes, coverage is much better, but still
    misses major rivers and lakes
  • 16-day repeat coverage misses 14 rivers and 9
    lakes in the top 150 (ranked by discharge and
    area, respectively)
  • The rivers which are covered can have only a few
    visits per cycle, leading to problems with slope
    calculations
  • A 120 km swath instrument misses very few lakes
    or rivers
  • 1 for 16-day repeat and 7 for 10-day repeat
  • A detailed analysis of the science impact of
    these results must await the results of the
    synthetic mission study results

28
Frequency Selection Criteria
  • Ionospheric delay correction
  • Range resolution
  • Cloud/Rain attenuation
  • Technology maturity
  • Spacecraft accommodation
  • Land water separation near nadir, vegetation
    penetration

29
Ionospheric Delays
  • The ionosphere is a dispersive medium which
    introduces height errors depending on frequency
    and total electron content (TEC)
  • I16 is the total electron content in units of
    1016m-2. It varies between 20 -gt 100. If
    uncompensated, this leads to errors between
    4cm-gt22cm at Ku-band and 0.7cm -gt 3cm at Ka-band
  • TOPEX/Jason compensate using Ku/C band altimeters
    (additional instrument cost)
  • In the future, it may be possible to compensate
    using GPS derived ionospheric models
  • Ka-band is preferable if only one frequency is
    used

30
Range Resolution and Height Accuracy
  • Increased range resolution (bandwidth) is desired
  • Reduces the size of the pulse limited circle
  • Increases height accuracy for altimeter and
    interferometers
  • Increases the spatial resolution of
    interferometric images
  • There is a data rate penalty for increased
    resolution
  • Ku-band bandwidth is limited to 320 MHz (50 cm
    range resolution)
  • Ka-band can be used with bandwidths of 500 MHz
    (30 cm range resolution)
  • Given a fixed interferometric baseline and SNR,
    Ka-band will have 2.5X better height accuracy
    than Ku-band

31
Cloud and Rain Attenuation
  • Ku and Ka-band will penetrate clouds
  • Ka-band is severely impacted by higher rain rates
  • As a worst case 5 to 10 of data might be lost
    at Ka-band due to rain (TRMM and scatterometer
    data)
  • Ku band could work for lower rain rates, but rain
    distorts the waveform and degrades accuracy.

32
Technology Maturity and Spacecraft Accommodation
  • Ku-band RF maturity is higher, but Ka-band is
    rapidly catching up.
  • Power sources are available at the desired levels
    in both frequencies
  • For a given beamwidth, Ka-band antennas will be
    2.5X smaller than Ku-band antennas, or have 2.5X
    better resolution for a given antenna size
  • For a given height accuracy, the interferometric
    baseline will be 2.5X smaller at Ka-band
  • A 10 m baseline at Ka-band is equivalent to a 70
    m baseline at C-band (SRTM baseline 60 m)

33
Vegetation Penetration and Land/Water
Discrimination
  • Both Ku and Ka bands are highly attenuated by
    vegetation
  • In order to reach the water, near nadir angles
    (lt5deg) are required in either case
  • For both frequencies water is significantly
    brighter than land in the near-nadir regime
  • The water/land contrast is slightly lower at
    Ka-band

34
Frequency Selection
  • For interferometers, Ka-band is probably superior
    to Ku-band if sufficient power is available
  • Smaller baseline and antennas easier to integrate
  • For nadir altimeters, Ka-band is better due to
    the ionospheric correction, smaller instrument,
    and higher spatial and range resolution
  • CNES has advocated a Ka-band altimeter (AltiKa)
    as the next step in nadir altimeter evolution.
  • Primary drawback of Ka-band rain data loss

35
Wet Tropospheric Delays
  • Water vapor in the atmosphere causes a range
    error which is independent of frequency, for the
    frequencies examined.
  • Ocean altimetry uses a 3-frequency radiometer to
    remove wet troposphere delays.
  • This technique cannot be used over land since the
    land brightness temperature changes dominate the
    radiometer brightness temperatures.
  • Examination of wet tropospheric delays over the
    ocean and from radiosonde data over land
    indicates that the wet troposphere error will
    induce a 3cm (1s) height error.
  • This error is the same for all the instrument
    concepts.

36
Wet Troposphere Delays Over Land
Source S. Kheim, JPL
37
Why Have Conventional Altimeters Been Limited for
River/Wetland Monitoring?
  • Onboard tracker
  • Solution remove tracker dynamics
  • Assumption about river nadir location often
    incorrect
  • Solution use imager for geolocation
  • River waveforms do not fit a single return
    waveform model
  • Solution use imager to generate fitting
    templates
  • Fitting of single waveforms has insufficient
    information
  • Solution tomographic processing
  • Poor spatial resolution
  • Solution use narrower beams, imager, tomographic
    processing or SAR processing

38
Onboard Tracker
  • Onboard range tracker designed for ocean
    waveforms.
  • Tracker often is not locked over land
  • Over land, dynamic range window location changes,
    coupled with waveform averaging, often distorts
    return waveforms so that they are not useful
  • Solution preprogram range window using SRTM
    topography to remove dynamic tracker distortions
  • Penalty need to use larger range window and
    store (or command) global range window position

39
The River is Not in the Nadir Location
  • If the river is not in the nadir, a range
    measurement is insufficient for height
    determination
  • The same range results in two different heights
  • Rivers at the same height but different distances
    from nadir will also result in different ranges
  • Additional information is required to be able to
    infer height from the range
  • Need imager to resolve geolocation

r
r
Height error
40
Coupling of Geolocation and Height Errors Imager
Requirements
  • Position error (and hence optical imager spatial
    resolution) has strong impact on height accuracy
  • In order to meet the height accuracy goals, the
    imager resolution must be 10-20 m and the
    pointing accuracy lt 5 arcsec
  • The cost impact of these requirements is
    probably not small for an optical imager

41
Amazon Simulation to Illustrate Issues
Use water/land JERS SAR classification map to
simulate radar brightness (from S. Saatchi, JPL)
Use SRTM topography to simulate waveform return
times. Topographic variation 300m
42
Pass 241 Waveforms
Color indicates return power color scale varies
from purple (lowest power) to red (highest power)
Range Direction
43
Why Using the Waveform Alone is not Sufficient
For a single river encounter, waveform shape
changes significantly and the leading edge is not
a good predictor (without geolocation
information) of the height of the river.
44
Altimeter Instrument Concept
  • Use Ka-band frequency (8 mm wavelength)
  • 1.5 m reflector antenna gt 4.3 km beam limited
    footprint
  • 500 MHz bandwidth (30 cm range resolution) gt 650
    m pulse limited footprint
  • Use preset tracker based on known topography
  • Reduce number of onboard averaging to minimize
    distortion
  • Use full-deramp processing to reduce data rate
  • For SAR mode, use bursts to reduce PRF, and
    onboard SAR compression (e.g., K. Raneys
    delay-doppler)
  • Required transmit power (10W) available from
    solid-state technology
  • SAR mode requires onboard processor, higher
    complexity and digital subsystem power

45
What is the Precision of Altimeter Heights?
  • The range resolution for all radar altimeters is
    much coarser than the estimated height precision
  • Example Topex range resolution 50 cm. Height
    precision 2 cm
  • The reason this can be achieved is that a priori
    information can be used to constrain the height
    inversion
  • Example for ocean altimetry, the Brown waveform
    model reduces the estimation to fitting 5
    parameters
  • For rivers, there is no universal parametric
    model which can be used for inversion
  • The solution use imager water delineation and
    land topography to create templates for waveform
    fitting
  • The accuracy of the estimated height must be
    assessed for each crossing geometry

46
Large River Crossing Waveforms
47
Smaller River Crossing Waveforms
48
Tomographic Height Estimation
  • Use entire range/time power history instead of
    single waveforms
  • Use imager to obtain water mask and geolocation
  • Generate simulated waveform templates and
    optimize fit with data by varying river height
    and reflectivity
  • The problem can be recast as a Maximum Likelihood
    or MAP estimation problem for a limited set of
    model parameters
  • Formal estimates of measurement errors can be
    obtained by error propagation

49
Tomographic Estimation Height Error
  • If template is close enough to the true
    waveform, the difference can be written as
  • The parameters can be ML estimated by minimizing
  • The estimation error follows by error
    propagation. In the case of a single parameter
    (optimistic estimate)

50
Large River Waveform Height Derivatives
SAR Altimeter Waveform Height Derivatives
Nadir Track
Range Direction
Interferometer River Image
RAR Altimeter Waveforms Height Derivatives
51
Smaller River Waveform Height Derivatives
SAR Altimeter Waveform Height Derivatives
Nadir Track
Range Direction
Interferometer River Image
RAR Altimeter Waveform Height Derivatives
52
Predicted Height Precision
  • Predicted precision optimistic because only 1
    parameter is estimated (results indicative)
  • Accuracy will be dominated by modeling errors
    (water mask, brightness, and location) and
    tropospheric delays
  • Both real aperture and synthetic aperture
    altimeters will meet the precision goals.

53
Surface Water Interferometer Concept
  • Ka-band SAR interferometric system with 2 swaths,
    50 km each
  • Produces heights and co-registered all-weather
    imagery
  • 200 MHz bandwidth (0.75 cm range resolution)
  • Use near-nadir returns for SAR altimeter/angle of
    arrival mode (e.g. Cryosat SIRAL mode) to fill
    swath
  • No data compression onboard data downlinked to
    NOAA Ka-band ground stations

54
Simulated Interferometer Return
The interferometer return signal contains both
radar brightness (for water boundary delineation)
and phase (color) for height estimation Image
geolocation accuracy given by timing accuracy,
not platform attitude, unlike optical imager
55
Interferometric Error Budget Dominant Contributors
Orbit Error
Media Delay Error (Iono, Tropo)
Phase Error
Baseline Roll Error
Other error sources (e.g., baseline length, yaw
errors) can be controlled so that errors are
smaller by an order of magnitude, or more.
56
Interferometric Phase Error
dh l r tan Q/(2 p B) dF
  • For incidence angles considered, a roll error
    and a phase error are indistinguishable
  • Sources of phase noise
  • Thermal noise in radar signal (random)
  • Decorrelation of the two returns due to speckle
    decorrelation of scattered fields (random)
  • Phase imbalance between the two interferometric
    channels
  • Temperature driven (slow change)
  • Can be calibrated using calibration loop.

57
Interferometer Height and Slope Precision
Height and slope estimates are made by using
radar image to isolate water body and fitting a
best fit linear height change over the
swath. Precision depends on water brightness and
the length and width of the imaged water body
58
Assumed Water Sigma0
Sigma0 derived from Geometric Optics prediction
and Fresnel reflection coefficient
59
Histogram of River Lengths in Swath
60
Baseline Roll Error
  • dh r sin Q d Q??? C d Q
  • An error in the baseline roll angle tilts the
    surface by the same angle.
  • As an order of magnitude, a 0.1arcsec roll error
    results in a 2.2cm height error at 50km from the
    nadir point
  • Roll knowledge error sources
  • Errors in spacecraft roll estimate
  • Mechanical distortion of the baseline (can be
    made negligible if the baseline is rigid enough)

C
61
Removing Roll Error by Using SRTM
  • Roll errors produce long-wavelength across-track
    tilts
  • It is not yet feasible to have star trackers with
    sufficient accuracy to guarantee cm level
    accuracy
  • Adjust cross-track tilts to match reference
    topographic map (e.g., SRTM) for long wavelength
    land heights and slopes
  • Removes long-wavelength errors to cm accuracy
  • Loses absolute height information tracks storage
    changes, rather than absolute height (consistent
    with science goals)
  • Removes long wavelength tropospheric errors
  • Potential issue seasonal variations of height
    due to vegetation at higher latitudes
  • Make adjustments based on non-vegetated regions
  • Make cross-over adjustments using data near in
    time ocean calibration (SRTM algorithms)
  • Derive height correction based on mean height
    changes between vegetated and non-vegetated
    regions

62
Interferometer Instrument Heritage
  • SRTM--First spaceborne radar interferometry
    mission (2000)
  • Absolute height accuracy 6m (90) at 30m
    spatial posting

Wide-Swath Ocean Altimeter
  • Wide-Swath Ocean Altimeter (WSOA)
  • Experimental demonstration as part of the OSTM
    mission
  • WSOA has successfully passed a Preliminary Design
    Review
  • Technology maturity and feasibility demonstrated
  • Ku-band (2.5 cm) real aperture radar
    interferometer. Applicable Technologies
    Ultra-stable mast (SRTM heritage) reflectarray
    antennas RF receive chain design
  • Required Ka-band tube exists, but is not space
    qualified

63
Instrument Options Summary
  • The 3 designs considered are technologically
    mature and ready for an ESSP proposal
  • The interferometer option gives significantly
    better performance and coverage than either of
    the altimeter options
  • The altimeter options have poor space/time
    coverage of rivers and lakes slope measurement
    problematic
  • Height estimation using the real or synthetic
    aperture altimeters needs to be coupled to an
    optical imager in order to remove geolocation
    errors
  • The cost of acquiring an optical instrument with
    the desired accuracy remains to be studied
  • The performance of the altimeter options is hard
    to quantify when the water body does not cover
    the footprint performance depends on river
    geometry and imager capabilities and needs to be
    studied further
  • The RAR and SAR instruments may be combined into
    a single instrument and the interferometer acts
    as an altimeter

64
Mission Cost Considerations
  • Assume that the ESSP cap slightly higher than
    last ESSP (175M)
  • The altimeter options (without imager) will
    probably fit under the cost cap without any
    partners
  • However, imager cost is an unknown, at this point
  • The interferometer option will probably need a
    partner to fit under the cost cap
  • Potential collaboration ESA explorer coordinated
    proposal, other space agencies
  • The interferometer option has the potential of
    meeting additional science objectives
  • Sea ice thickness
  • Ocean mesoscale variability
  • Ocean coastal coverage
  • Next generation topography maps

65
Recommendations
  • Since interferometer option offers significant
    advantages over nadir altimeters, examine
    partnering options in the near future
  • Nadir altimeter options have significant
    questions which must be answered prior to
    proposal
  • Space/time coverage impact
  • Imager requirements
  • Refinement of height error budget
  • Two tasks are ongoing to resolve these issues
  • NASA synthetic hydrology mission study
  • JPL approved internal funding for surface water
    radar instrument trade-offs
  • In order to select the instrument properly, we
    should complete these studies and the
    investigation of foreign collaboration
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