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River Radar Backscatter

Delwyn Moller

Ernesto Rodríguez

Daniel Esteban-Fernandez

Collaborators: Doug Alsdorf and James Hamski (OSU)

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Experimental Motivation and Objective

A primary technical risk assessed by the NRC decadal survey and ESA Earth Explorer reviews was considered to be the lack of field

measurements of the radar cross-section (0) at Ka-band. Specifically our models indicate a gradual decay off-nadir, however concerns have been raised that the decay may be more rapid and so the instrument

may not be able to deliver the required swath

Experimental Campaign Objectives

• Provide validating data to prove that the “0 roll-off” is gradual within the proposed operating range (up to 6 degrees incidence)

• Secondarily look at temporal coherence times as relevant to synthetic aperture processing

• Integrate these results into the spaceborne system design and simulation as necessary

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Altimeter Data

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Experimental Approach

The Mars Science Laboratory (MSL) program at JPL loaned their short-pulse Ka-band Doppler Radar for our experimental campaign May ‘07

• OSU obtained site (bridge) permits for sites in Ohio including the Ohio river.

• A custom mount structure was designed to be portable between sites.

- This mount supports an antenna on a precision positioner which scans in elevation as remotely controlled by a custom data-acquisition interface.

- The entire structure is rotated to extend beam and antenna over the bridge railing.

• Backscatter and Doppler data was collected at a series of angles in elevation.

• In situ wind and current data was collected

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Sites for Deployment

1. Ohio River bridge: a low-flow river visible from space

2. Home-road Bridge over a reservoir: Indicative of no-flow.

3. Dresden unused bridge: A small faster flowing river

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The sites selected spanned a spectrum of hydrological interest to represent the variability of backscatter and correlation times we might expect from space.

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Expectations

From altimeter scattering results, one sees that sometimes very bright narrow returns are observed near nadir. These are consistent with mirror-like returns (coherent specular)

Other result (e.g., Vandermark et al., 2004) show sigma0 is well behaved with wind speed (incoherent scattering)

One expects at low wind speeds, the returns to be a mixture of coherent and incoherent returns,

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Relative 0() All Sites

+20

+15

+10

+25

+20

+15

+10

+25

+20

+15

+10

+25

+20

+15

+10

+25

Saturation

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(Preliminary) Absolute Calibration

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(Preliminary) Absolute Calibration

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(Preliminary) Absolute Calibration

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Relative 0() All Sites cont.

Design point

+20

+15

+10

+25

The nominal WatER/HM design point is 10dB sigma0 with about 5dB margin

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Relative 0() Ohio

Wind ~ 2m/s Wind < 1m/s

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Speckle Statistics

Saturation

Speckle statistics results validate the fact that river returns consist of nadir coherent/incoherent return mixtures, giving way to mainly incoherent returns away from nadir.

Since coherent returns scale as ~area2, while incoherent returns scale as area, this explains the high specular peaks measured by altimeters from space

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Temporal Coherence

Ohio River “pulse-pair”powers collected for “moderately windy” (wind ~ 2-3m/s) and “low-wind” (wind < 1m/s) conditions. These

results imply SAR apertures can be formed from space.

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Plans

•Possible ocean measurements•Possible return to Dresden/Muskingam?

•Deconvolution of antenna pattern •Study of wind statistics and patterns to simulate any

possible reduced swath during extreme low wind conditions

•Incorporation of observed 0 into virtual mission