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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 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 • 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
Sensor Swath Pattern ~3.5° ~3.5° ~0.6° 800-1000 km ~60 km ~60 km ~10 km
15-Day Orbit Coverage Gaps 3° N 0° 400 km 60 km 3° S
22-Day Orbit Coverage 2° N 0° 2° S
Repeat Period vs Coverage (i = 78°) ~130 km total swath width
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
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
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
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
Possible Orbit Altitudes: i = 78° Repeat Length (days) + Repeat Orbit at Subcycle
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)
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
Candidate Orbits Repeat Length Equatorial Spacing # of Orbits to Repeat
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)
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.
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
Candidate Orbits Minimal Tidal Aliasing
16 21 24 19 18 23 17 21 25 23 19 15 16 19 22 25 23 20 17 3-5 Day Subcycles
How Does This Analysis Change for SWOT? • Many measurement locations have 2 or more ascending/descending passes. • Most measurement locations are “cross over” points.
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
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
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
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
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.).
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)
