Radar Interferometry for River, Lake, and Wetland Measurements

1
Radar Interferometry for River, Lake, and Wetland
Measurements

• E. Rodriguez and D. Moller
• Jet Propulsion Laboratory
• California Institute of Technology

2
What does the ideal instrument look like?

• It is a high resolution imager
• River width, wetland/lake extent
• It has centimetric height accuracy
• River discharge, wetland/lake storage change
• It can measure slopes to mrads
• River discharge
• It measures surface velocity to cm/s
• River discharge
• It has global coverage, sampling all major
  contributors to surface water, is not affected by
  clouds
• Wetlands, rivers, lakes
• It has a repeat period of at most a few weeks
• It can differentiate vegetated vs non-vegetated
• Wetland mapping
• It is cheap and technologically ready
• Well 7/8 88 B

3
How Many Targets Need To Be Imaged?A Lake
Monitoring Example

• Lake size data derived from GSHHS data base
  (Wessel, P., and W. H. F. Smith, A Global
  Self-consistent, Hierarchical, High-resolution
  Shoreline Database, J. Geophys. Res., 101, B4,
  pp. 8741-8743, 1996.)
• http//www.ngdc.noaa.gov/mgg/shorelines/gshhs.html
  
• Data compiled from WVS (World Vector Shoreline)
  and CIA WDB II data bases.
• Smallest lake in data base has an area of 0.1
  km2

4
How Much of the Earth Need be Mapped?

• Mapping 99.9 of all lakes requires imaging of lt
  10,000 lakes
• The total area to be mapped lt 0.5 of the
  Earths area
• For wetlands, area lt 6 of land, or lt 2.4 of the
  Earths area
• Instrument Lesson a high resolution instrument
  for river and wetland mapping does not have to
  have high total data volume
• Low average power Low data rate --gt Reduced
  relative to global imager

5
Cross-Track Interferometry for Lake Level
Monitoring
5m
10 m
System Characteristics

• Heritage SRTM, WSOA. Rain radar IIP
• Critical Technology Items
• Active antenna
• No scanning required
• Assumption only relevant data collected to
  reduce data rate

• Orbit 800 km polar orbit
• Repeat Period 10-20 days
• Baseline 10 m Antenna 5m
• Frequency 35 GHz
• Spatial resolution 2.5 m

6
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 sin Q
7
Height Measurement Precision for Lakes and
Wetlands
8
Height Slope Measurement Precision for Rivers
1 mrad 1 cm / 10 km
9
Baseline Roll Error

• dh r sin Q d Q
• An error in the baseline roll angle tilts the
  surface by the same angle.
• As an order of magnitude, a 10mrad roll error
  produces a 2.1 m height error at the footprint
• Roll knowledge error sources
• Errors in spacecraft roll estimate
• Mechanical distortion of the baseline (can be
  made negligible if the baseline is rigid enough)
• Roll errors must be calibrated using existing
  DEMs (SRTM)

10
Calibration Using Existing DEMs

• Calibrate data by estimating roll angle which
  will reduce rms height difference with existing
  DEM (SRTM?) or set of GCPs
• Improved DEM can be constructed by merging the
  interferometer data collected from multiple
  passes
• Error reduction by N(-1/2)
• There will be an error! But the error will be
  consistent for observations taken at different
  times
• Change in storage (lakes, wetlands) or discharge
  (rivers) is the best that can be obtained from
  remote sensing (reservoir and river depths are
  not known!)
• A 1m DEM average error will result in a 5mrad
  slope error.

11
River Velocity Width Slope Measurements
Measure -Doppler Velocity
Measure Topography
Example of measurement of the radial component of
surface velocity using along-track interferometry
Measure Doppler Velocity
12
Combined Cross and Vector Along-Track
Interferometry

• By forming 3 beams/antenna it is possible to
  implement an along-track interferometer with
  the same instrument configuration as a
  cross-track interferometer
• Data rate increases by a factor of 3
• Antenna technology must be demonstrated
• Estimated velocity accuracy 2cm/s for 100m x
  100m area (improves linearly with averaging area)