Orbit Selection for the WATER HM Mission
R. S. Nerem
CCAR, CIRES, University of Colorado
D. P. Chambers
Center for Space Research, University ofTexas at Austin
(with input from Gregg Jacobs, RichardRay, and John Ries)
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Colorado Center for Astrodynamics Research
Nerem 10/29/07
General Constraints on Search
lNo sun-synchronous orbits
lAltitudes lower than 1000 km to avoid radiation exposure
lInclinations greater than 75° in order to sample mostrivers in Siberia
lRepeat period ~ 20-days in order to meet groundtrackspacing requests of mesoscale oceanography andhydrology (~10 days was also considered to meet arequest for higher temporal resolution studies)
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Colorado Center for Astrodynamics Research
Nerem 10/29/07
Sun Synchronous Orbits
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Nerem 10/29/07
Why not Sun-Synchronous Orbits?
lPrevents Serious Tidal Studies
lTides poleward of 66°
lTides in shallow water and estuaries
lWavelengths shorter than 300 km
lMaps Diurnal Errors to Undesirable Periods
lInto long-period “climate-like” signals
lInto seasonal variations (annual, semi-annual)
lInto the mean sea surface
The University of Colorado
Colorado Center for Astrodynamics Research
Nerem 10/29/07
Calculating Tidal Aliasing
• P is satellite repeat period
• T is tide period
• p is tide alias period
The University of Colorado
Colorado Center for Astrodynamics Research
Nerem 10/29/07
Aliasing Considerations
lFor applications in oceans and estuaries, we desire asatellite orbit that aliases all tides to frequencies greaterthan 2 cycles-per year
lThe alias period is shorter than the semi-annual (half-year) signal
lCan observe multiple cycles of the alias period within a few years
Another important factor to consider is the separationbetween alias frequencies and between the aliases andannual/semi-annual (f)
lDetermines time needed to separate two tides in altimeterobservations
Time to separate ~ 1/f
lT/P did not have optimal separation for someconstituents
lK1 aliased to within 0.11 cpy of semi-annual
l9-years to separate
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Colorado Center for Astrodynamics Research
Nerem 10/29/07
Tidal Aliasing ~10 Day Repeat
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Colorado Center for Astrodynamics Research
Nerem 10/29/07
Tidal Aliasing ~20 Day Repeat
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Colorado Center for Astrodynamics Research
Nerem 10/29/07
Tidal Aliasing ~20 Day Repeat
The University of Colorado
Colorado Center for Astrodynamics Research
Nerem 10/29/07
Orbit 1
Altitude = 845.111 km
Inclination = 78°
Repeat Period = 20.86521 days
Orbit 2
Altitude = 845.859 km
Inclination = 78°
Repeat Period = 21.85882 days
Except for K1, these two orbits have very good aliasingproperties for 9 major constituents
The University of Colorado
Colorado Center for Astrodynamics Research
Nerem 10/29/07
General Observations
lOnly orbits with an inclination < 66° alias all majorconstituents to frequencies > 2 cpy
lRetrograde orbits have poor aliasing of solar tides
lIf we do not require that the K1 alias > 2 cpy
lGood orbits do exist up to 80° inclination
lThis covers more of the Arctic Ocean, all of the Antarcticboundary, and most rivers in Siberia
The University of Colorado
Colorado Center for Astrodynamics Research
Nerem 10/29/07
Shallow Water Tides
lIn shallow water (including estuaries) tides are nonlinearand have much smaller wavelength than in the deepocean
lIn 40 m, tide wavelength is 10x smaller than in 4000m
lShallow water tides are poorly observed by currentaltimeter systems
lExamine aliasing of select nonlinear tides for potentialorbits
The University of Colorado
Colorado Center for Astrodynamics Research
Nerem 10/29/07
Orbit 1
Altitude = 845.111 km
Inclination = 78°
Repeat Period = 20.86521 days
Orbit 2
Altitude = 845.859 km
Inclination = 78°
Repeat Period = 21.85882 days
Some close aliases between nonlinear and primary tides
Aliasing of Nonlinear Tides
The University of Colorado
Colorado Center for Astrodynamics Research
Nerem 10/29/07
Comments on the Orbit
“….. many coastal locations at mid to high latitudeswould be sampled twice in each 10-day orbit, for arepeat of about 5 days. Those interested in coastalapplications prefer that this repeat time not beincreased. Five days will still not resolve the rapidcoastal ocean response to synoptic forcing (periods of1-5 days), relying on models to assimilate the SSH dataand provide the higher temporal resolution needed.” T.Strub
“The basic recommendation on orbits for mesoscale isfor full coverage. So I would lean toward the longerrepeat (~ 20 days) if the swath were 60 km to each side.Tidal aliasing should be considered for mesoscale.” G.Jacobs
“Inclination greater than 74°, temporal sampling notcritical, but more frequent overpasses desirable.”, D.Alsdorf
The University of Colorado
Colorado Center for Astrodynamics Research
Nerem 10/29/07
Orbit Repeat Period
11-Day Repeat
15-Day Repeat
21-Day Repeat
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WaTER HM Swath
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HM sampling (9.95 days)
Nadir sampling (9.95 days)
Sampling with 200 km Swath
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Colorado Center for Astrodynamics Research
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Groundtrack Separation
140 km
200 km
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Colorado Center for Astrodynamics Research
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Precision Orbit Determination
lAvailability of GRACE gravity models has significantlyreduced gravity-driven orbit error.
lGPS has made precise continuous tracking routinely available.
lFor orbit altitudes in the 800-900 km range, 2 cm orbitaccuracy should be easily obtainable.
lExtended antennas and solar arrays may complicate the orbitdetermination.
lThought should be given to monitoring motion of antennaphase centers with respect to s/c center-of-mass, attitudedetermination, GPS multipath issues.
The University of Colorado
Colorado Center for Astrodynamics Research
Nerem 10/29/07
Radial Orbit Errors due to Gravity
Predicted using difference between GGM02C-GFZ04C
Proposed Wide-Swath Orbit
Jason Orbit
Mean Portion
RMS about Mean
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Colorado Center for Astrodynamics Research
Nerem 10/29/07
Conclusions
lFor a 120 km swath, a ~20 day repeat is desired to obtain near globalcoverage.
lA variety of orbits exist with altitudes near 800 km, inclinations of ~78°, and repeat periods near 21 days that have good tidal aliasingproperties for deep ocean tides and meet other mission requirements
lCare must be given to the precision orbit determination, but 2 cmradial accuracy should be possible.
lPreliminary Recommendation (Altitude = 845.111 km, Inclination =78°, Repeat Period = 20.86521 days)
lWill be able to additionally observe the M4 tide as well as 2MN2, 2MS6,and 2MS2, will not be able to separate MS4 and M6 tides
lMore effort might find an orbit that can sample both MS4 and M4 well
The University of Colorado
Colorado Center for Astrodynamics Research
Nerem 10/29/07
Shallow Water Tides