Bistatic WITTEX: an Innovative Constellation of Radar Altimeter Satellites

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IGARSS 2002 at Toronto
Bistatic WITTEX an Innovative Constellation of
Radar Altimeter Satellites
27 June 2002 R. Keith Raney, David L. Porter,
and Francis M. Monaldo The Johns Hopkins
University Applied Physics Laboratory Laurel,
Maryland
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The Main Points
Given N co-planar spacecraft hosting nadir
(monostatic) and virtual-nadir (bistatic) radar
altimeters
Then gt N satellites gt (2N - 1) near-parallel
surface tracks
gt Two-way link eliminates inherent bistatic
timing error Patent pending
gt Heights self-calibrate at maximum latitudes
(turnovers)
gt Baseline propagation errors gt lt 1 cm height
error
gt Comparable accuracy ( cms) along all tracks
gt Cross-track differential precision better than
2 cm
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Why?
To address requirements for improved time/space
coverage Example mesoscale features
Courtesy Gregg Jacobs, NRL
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Co-PlanarAltimeter Constellation
Earth rotation spreads surface tracks
WITTEX
Nadir track 1
Nadir track 2
Nadir track 3
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Comments
Bistatic link characteristics (coplanar
constellation)
gt Bistatic antennas known constant pointing
angle gt Polarization RCP at one end, LCP at the
other gt Single frequency (Ku-band) interpolate
TEC corrections gt No WVR interpolate wet
atmosphere corrections gt All interpolations lt 50
km ltlt correlation distance gt Baseline knowledge
to 1 cm (recommend Ka-band link) gt Two-way
bistatic gt modest clock accuracy requirement
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Self Cross-Calibration
Earth rotation spreads surface tracks
Nadir track 1
Nadir track 2
Nadir track 3
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Detail of Cross-overs
Exploit dense crossovers at maximum latitudes
Each set of crossovers spans lt 5 minutes
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Footprints Overlap (Delay/Doppler)
Unity latitude / longitude aspect ratio
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Comments
Crossover characteristics at maximal latitudes
gt Constraint Crossover sets only useful when
over water gt N satellites gt
simultaneous crossovers per set gt Eliminates
systematic height errors Eg removes height
bias due to (constant) baseline error gt Minimizes
variable height errors along orbit gt New concept
cross-track comparative accuracy (? precision) gt
Analysis and modeling differential precision 2
cm
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Tropospheric Path Length Residuals
Preferred bistatic incidence
Off-nadir path length increase residuals (after
correction) are less than 0.2 cm
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Ionospheric Path Length Increase
 
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Conclusions
Bistatic WITTEX Co-planar altimeter constellation
gt Co-planar altimeters cross-calibrate at
turnovers Eliminates systematic height
errors between parallel tracks
gt Bistatic altimetry accuracy comparable to
nadir mode Two-way link relaxes clock
synchronization gt realistic solution
gt Path length errors unique to bistatic link are
small Advisable to constrain incidence to
less than 35 degrees
gt Now feasible five accurate SSH tracks from
three s/c Meets long-standing
requirements for improved coverage