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