Sea Level Slides

1
Orbit Selection for the WATER HM Mission
R. S. Nerem CCAR, CIRES, University of
Colorado D. P. Chambers Center for Space
Research, University of Texas at Austin (with
input from Gregg Jacobs, Richard Ray, and John
Ries)
2
General Constraints on Search

• No sun-synchronous orbits
• Altitudes lower than 1000 km to avoid radiation
  exposure
• Inclinations greater than 75 in order to sample
  most rivers in Siberia
• Repeat period 20-days in order to meet
  groundtrack spacing requests of mesoscale
  oceanography and hydrology (10 days was also
  considered to meet a request for higher temporal
  resolution studies)

3
Sun Synchronous Orbits
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Why not Sun-Synchronous Orbits?

• Prevents Serious Tidal Studies
• Tides poleward of 66
• Tides in shallow water and estuaries
• Wavelengths shorter than 300 km
• Maps Diurnal Errors to Undesirable Periods
• Into long-period climate-like signals
• Into seasonal variations (annual, semi-annual)
• Into the mean sea surface

5
Calculating Tidal Aliasing

• P is satellite repeat period
• T is tide period
• ?p is tide alias period

6
Aliasing Considerations

• For applications in oceans and estuaries, we
  desire a satellite orbit that aliases all tides
  to frequencies greater than 2 cycles-per year
• The alias period is shorter than the semi-annual
  (half-year) signal
• Can observe multiple cycles of the alias period
  within a few years
• Another important factor to consider is the
  separation between alias frequencies and between
  the aliases and annual/semi-annual (Df)
• Determines time needed to separate two tides in
  altimeter observations
• Time to separate 1/Df
• T/P did not have optimal separation for some
  constituents
• K1 aliased to within 0.11 cpy of semi-annual
• 9-years to separate

7
Tidal Aliasing 10 Day Repeat
8
Tidal Aliasing 20 Day Repeat
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Tidal Aliasing 20 Day Repeat
10
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
aliasing properties for 9 major constituents
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General Observations

• Only orbits with an inclination lt 66 alias all
  major constituents to frequencies gt 2 cpy
• Retrograde orbits have poor aliasing of solar
  tides
• If we do not require that the K1 alias gt 2 cpy
• Good orbits do exist up to 80 inclination
• This covers more of the Arctic Ocean, all of the
  Antarctic boundary, and most rivers in Siberia

12
Shallow Water Tides

• In shallow water (including estuaries) tides are
  nonlinear and have much smaller wavelength than
  in the deep ocean
• In 40 m, tide wavelength is 10x smaller than in
  4000m
• Shallow water tides are poorly observed by
  current altimeter systems
• Examine aliasing of select nonlinear tides for
  potential orbits

13
Aliasing of Nonlinear Tides
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
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Comments on the Orbit

• .. many coastal locations at mid to high
  latitudes would be sampled twice in each 10-day
  orbit, for a repeat of about 5 days. Those
  interested in coastal applications prefer that
  this repeat time not be increased. Five days will
  still not resolve the rapid coastal ocean
  response to synoptic forcing (periods of 1-5
  days), relying on models to assimilate the SSH
  data and provide the higher temporal resolution
  needed. T. Strub
• The basic recommendation on orbits for mesoscale
  is for full coverage. So I would lean toward the
  longer repeat ( 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
  not critical, but more frequent overpasses
  desirable., D. Alsdorf

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Orbit Repeat Period
11-Day Repeat
15-Day Repeat
21-Day Repeat
16
WaTER HM Swath
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Sampling with 200 km Swath
Nadir sampling (9.95 days)
HM sampling (9.95 days)
18
Groundtrack Separation
200 km
140 km
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Precision Orbit Determination

• Availability of GRACE gravity models has
  significantly reduced gravity-driven orbit error.
• GPS has made precise continuous tracking
  routinely available.
• For orbit altitudes in the 800-900 km range, 2 cm
  orbit accuracy should be easily obtainable.
• Extended antennas and solar arrays may complicate
  the orbit determination.
• Thought should be given to monitoring motion of
  antenna phase centers with respect to s/c
  center-of-mass, attitude determination, GPS
  multipath issues.

20
Radial Orbit Errors due to Gravity
Proposed Wide-Swath Orbit
Jason Orbit
Mean Portion
RMS about Mean
Predicted using difference between GGM02C-GFZ04C
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Conclusions

• For a 120 km swath, a 20 day repeat is desired
  to obtain near global coverage.
• A variety of orbits exist with altitudes near 800
  km, inclinations of 78, and repeat periods
  near 21 days that have good tidal aliasing
  properties for deep ocean tides and meet other
  mission requirements
• Care must be given to the precision orbit
  determination, but 2 cm radial accuracy should be
  possible.
• Preliminary Recommendation (Altitude 845.111
  km, Inclination 78, Repeat Period 20.86521
  days)
• Will 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
• More effort might find an orbit that can sample
  both MS4 and M4 well

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Shallow Water Tides