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Preliminary SWOT Orbit Design Study

Preliminary SWOT Orbit Design Study. R. Steven Nerem, Ryan Woolley, George Born, James Choe Colorado Center for Astrodynamics Research, University of Colorado Richard Ray NASA/Goddard Space Flight Center Ernesto Rodriguez Jet Propulsion Laboratory. Orbit Design Considerations.

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Preliminary SWOT Orbit Design Study

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  1. Preliminary SWOT Orbit Design Study R. Steven Nerem, Ryan Woolley, George Born, James Choe Colorado Center for Astrodynamics Research, University of Colorado Richard Ray NASA/Goddard Space Flight Center Ernesto Rodriguez Jet Propulsion Laboratory

  2. Orbit Design Considerations • Latitudinal coverage (orbit inclination) • Temporal Sampling • Spatial Sampling • Tidal Aliasing • Starting Point: • 15-25 day repeat • 800-1000 km altitude • Near 78° inclination • Other Considerations: • Calibration/Validation • Multiple Orbit/Mission Phases • Orbit Maintenance • Final Orbit Design Derived from Science Requirements

  3. Sensor Swath Pattern ~3.5° ~3.5° ~0.6° 800-1000 km ~60 km ~60 km ~10 km

  4. 15-Day Orbit Coverage Gaps 3° N 0° 400 km 60 km 3° S

  5. 22-Day Orbit Coverage 2° N 0° 2° S

  6. Repeat Period vs Equatorial Spacing

  7. Repeat Period vs Coverage (i = 78°) ~130 km total swath width

  8. Day 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Base Interval ~25° or ~3000 km 15-Day Repeat, 1-Day Subcycle

  9. Day 3 6 9 1 4 7 2 13 5 12 10 8 11 14 Base Interval ~25° or ~3000 km 14-Day Repeat, 3-Day Subcycle

  10. 2 1 Base Interval ~25° or ~3000 km 22-Day Repeat, 3-Day Subcycle Day 3 6 9 12 15 18 21 5 8 11 14 17 20 4 7 10 13 16 19 22

  11. 2 1 Base Interval ~25° or ~3000 km 3-Day Repeat Day 3 6 9 12 15 18 21 5 8 11 14 17 20 4 7 10 13 16 19 22

  12. 1-Day Repeating Ground Track

  13. 3-Day Repeating Groundtrack

  14. 4-Day Repeating Groundtrack

  15. 22 Day Repeat – 3 Day Subcycle

  16. 22 Day Repeat – 3 Day Subcycle

  17. 22 Day Repeat – 3 Day Subcycle

  18. Possible Orbit Altitudes: i = 78° Repeat Length (days) + Repeat Orbit at Subcycle

  19. 16 21 24 19 18 23 17 21 25 23 19 15 16 19 22 25 23 20 17 3-5 Day Subcycles Repeat Length (days)

  20. Properties of Repeat Track Orbits • Complete exactly N orbits in C days • N is an integer, C is not (except for SS orbits) • Altitude precisely determined by i, N, and C • Ground track forms a grid on Earth’s surface, one point fixes the whole grid • Grid “denser” for increasing C • Sub-cycle length is a complex function of N and C

  21. Candidate Orbits Repeat Length Equatorial Spacing # of Orbits to Repeat

  22. Tidal Aliasing • This initial analysis does not consider possible benefits of swath coverage (tidal solutions using swath “crossover” measurements) • Tidal aliasing frequencies completely determined by orbit repeat period (function of altitude and inclination) • Desirable characteristics: • Good separation of major tide constituents aliasing frequencies • Alias frequencies should not be close to one cycle per year • Tides should not alias to very long periods (<< 1 year)

  23. Aliasing Near Diurnal Solar Tides The precession rate of the satellite orbit plane determines which frequency is aliased to zero. To avoid unfavorable aliasing generally requires a precession rate ≤ –2°/d (cf. Topex), which limits satellite inclination. We must trade off inclination and aliasing. Four main solar diurnal tides are separated in frequency by 1 cpy.

  24. Tidal Alias Frequencies: i = 75°

  25. Tidal Alias Frequencies: i = 77°

  26. Tidal Alias Frequencies: i = 80°

  27. Tidal Alias Frequencies: i = 85°

  28. Average Tidal Frequency Separation

  29. Average Tidal Frequency Separation

  30. X X X X X X X X X X X X X X X X X X Tidal Aliasing: i = 78° X X X

  31. Candidate Orbits Minimal Tidal Aliasing

  32. 16 21 24 19 18 23 17 21 25 23 19 15 16 19 22 25 23 20 17 3-5 Day Subcycles

  33. How Does This Analysis Change for SWOT? • Many measurement locations have 2 or more ascending/descending passes. • Most measurement locations are “cross over” points.

  34. Example Sampling of Tides by SWOT 818 d 21.8635-day repeat latitude 32.0° 68 d 111 d 160 d 285 d Case 1: One ascending arc per repeat cycle 48 d 89 d 80 d 143 d Nominal alias period

  35. Example Sampling of Tides by SWOT 21.8635-day repeat latitude 32.0° 818 d 68 d 111 d 160 d 285 d Case 2: Two ascending arcs per repeat cycle 48 d 89 d 80 d Added sampling helps lunar tides, but not solar. 143 d Nominal alias period

  36. Example Sampling of Tides by SWOT 21.8635-day repeat latitude 32.0° 818 d 68 d 111 d 160 d 285 d Case 3: Two ascending arcs + two descending arcs per repeat cycle 48 d 89 d 80 d Added sampling helps solar diurnal tides, but not solar semidiurnals. 143 d Nominal alias period

  37. Example Sampling of Tides by SWOT 21.8635-day repeat latitude 60.0° 818 d 68 d 111 d 160 d 285 d Case 3b: Two ascending arcs + two descending arcs per repeat cycle 48 d 89 d 80 d Added sampling helps solar tides, depending on latitude. 143 d Nominal alias period

  38. Nadir vs Swath Sampling of the Tides • Additional sampling within a repeat period generally solves aliasing issues of lunar tides. • At most latitudes, additional sampling of solar tides does not help resolve semidiurnal tides. • For some sea level studies, additional sampling will help mitigate solar tide-model errors, depending on data processing strategies. • For tide model improvement studies, swath altimetry provides only marginal improvement for the solar tides over what is offered from conventional nadir altimetry. • Therefore, Nadir-type aliasing studies generally apply to SWOT - for solar tides. Most lunar tides will not alias to long periods, so we can neglect them during orbit design (but it’s easy to check M2, O1, etc.).

  39. Coverage Analysis • 3 Cases studied to get representative coverage for different latitude bands: • Mid-latitude to high-latitude: Aghulas current region (Gulf Stream is similar) • Equatorial: Amazon River • High-latitude: Lena River • Plots of number of visits within a cycle, for 10 day and 4 day sampling periods • Histograms of temporal revisits within a cycle (i.e., no revisits between cycles considered)

  40. 22-Day Repeat, Aghulas

  41. 10 days of 22-Day Repeat, Aghulas

  42. 4-Days of 22-Day Repeat, Aghulas

  43. 22-Day Repeat, Aghulas

  44. 22-Day Repeat, Amazon

  45. 10 Days of 22-Day Repeat, Amazon

  46. 4-Days of 22-Day Repeat, Amazon

  47. 22-Day Repeat, Amazon

  48. 22-Day Repeat, Lena

  49. 10 Days of 22-Day Repeat, Lena

  50. 4 Days of 22-Day Repeat, Lena

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