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Wide-Swath Altimeter Science Goals. Wide Swath Altimetry targeted at measurements of: Oceanic mesoscale eddies, fronts, and boundary currents, which are the most energetic elements of the oceanic general circulation.

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wide swath altimeter science goals
Wide-Swath Altimeter Science Goals
  • Wide Swath Altimetry targeted at measurements of:
      • Oceanic mesoscale eddies, fronts, and boundary currents, which are the most energetic elements of the oceanic general circulation.
      • Eddy mean-flow interactions, eddy transports, and the role of eddies in climate.
      • Physical-biological interactions and the role of eddies in the carbon cycle.
      • Coastal tides and open ocean internal tides.
  • The required resolution is determined by the Rossby radius of deformation
    • Current designs are targeted at 15 km resolution
    • This resolution is not a hard limit to the technique
interferometric measurement concept
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

wsoa science rationale
WSOA Science Rationale

Topex Orbit, 2 Altimeters:

150km equatorial spacing, 10 day repeat

Topex Orbit, 1 satellite, fixed yaw coverage

Wide-Swath altimeter, 10 day repeat

High resolution ocean topography measurements requires several coordinated nadir altimeters. A better coverage from a single platform can be obtained using an instrument which can image a swath instantaneously.

wsoa intrinsic resolution and coastal contamination scenarios
WSOA Intrinsic Resolution and Coastal Contamination Scenarios

WSOA Swath

• The WSOA down-linked data resolution is ~12km in the along-track direction and < 500m in the cross-track direction.

• The data are are sampled every ~650m in the along-track direction

Land

200 km

12 km

~650m

< 500m

Best case land contamination: only the range cells over land are affected, but range cells over the ocean are unaffected.

Worst case land contamination: every resolution cell is affected by its fraction over the land.

wsoa on ostm mission status
WSOA on OSTM Mission Status
  • WSOA to be demonstrated as experimental mission on OSTM (Jason 2), contingent on:
    • Successful preliminary design review (Jan 03)
    • Obtaining a launch vehicle through the DOD Space Technology Program (STP)
    • Fitting in the Alcatel Proteus Bus without any performance impact to OSTM core mission
  • Final decision on WSOA inclusion in OSTM will probably occur in the second quarter of ‘03.
  • The WSOA demonstration mission on OSTM is not optimal for measuring geoid slopes
    • Yaw steering effects
    • However, the data are likely to be collected
    • We would like to explore geodetic applications for which the WSOA on OSTM data would be useful
ostm payload concept
OSTM Payload Concept

GPSP antennas & front-end electronics (not shown)

bus electronics include portions of GPSP and WSOA, and all of SIES

WSOA deployable antenna and feed structure

AMR antenna & electronics

y

WSOA two-sided deployable mast

WSOA tip-mounted electronics

x

LRA mounted on nadir side of spacecraft

z

wsoa on ostm error budget
WSOA on OSTM Error Budget

Single Observation Error Budget

Long wavelength (>100km)

not relevant for geodesy

Error budget based on 15 km height postings. Interferometric precision degrades by the area ratio for higher postings.

wsoa precision vs averaging time
WSOA Precision vs Averaging Time
  • Since WSOA is in a repeat orbit, temporal averaging can be used to improve measurement precision
    • - Not all observation might be useful due to yaw steering

Assuming all observations can be used

Assuming 1/4 of all observations can be used

fixed yaw coverage
Fixed Yaw Coverage

The number of observations in a 10 day period for WSOA on OSTM in fixed yaw mode is shown below.

Fixed yaw coverage occurs for about ~22 day periods every ~60 days

yaw coverage examples
Yaw Coverage Examples

Optimal Coverage

Cycle 25

Intermediate Coverage

Cycle 23

Poor Coverage

Cycle 32

raw measurement geometry
Raw Measurement Geometry

Along track direction

Along track direction

No Yaw

30 Degree Yaw

(Figures not to scale in the range direction. Number of along-track looks also underrepresented)

Although the along-track resolution is ~12 km, significant measurement overlap exists (~10 measurements/footprint)

It is possible to attempt improving the resolution through inverse or deconvolution methods

a simple inversion example
A Simple Inversion Example
  • Assume 2 years worth of data collection
  • Use eigenvector filtering expansion for deconvolution (aka, EOF expansion)
  • This example assumes only data collecting in the ascending direction is used.
  • The solution of the optimal inversion using all data is still unexplored.
comparison of wide swath and conventional altimetry
Comparison of Wide-Swath and Conventional Altimetry

Wide-Swath Altimetry can be improved by using synthetic aperture techniques ( a la K. Raney, delayed Doppler altimeter).

SAR WSOA scenario based on sea ice instrument proposed to NASA IIP in ‘02