1. NASA, CGMS-41, July 2013 Coordination Group for Meteorological Satellites - CGMS An Optimization Analysis of the GCOM-C1 and Sentinel-3A Missions for Improved Ocean Color Imaging Coverage Presented to CGMS-41 Working Group II, WGII/9 in response to CGMS Action 40.17 Brian D. Killough NASA, Langley Research Center

2. NASA, CGMS-41, July 2013 Coordination Group for Meteorological Satellites - CGMS  The CEOS Systems Engineering Office (SEO) was asked to examine the coverage capabilities of a virtual satellite constellation using GCOM-C1 and Sentinel-3A.  The goal is to improve the daily coverage available for ocean color imaging by suggesting minor adjustment to their planned orbits.  The SEO utilized its COVE tool (next chart) to conduct the analysis.  Conclusion: With minor adjustments to the planned orbit of GCOM-C1 (4 km altitude, <0.1 deg inclination, 2 min LST), the constellation can maximize the daily coverage potential and remove the oscillations in relative spacing between adjacent swaths that create periods of poor coverage. Background

3. NASA, CGMS-41, July 2013 Coordination Group for Meteorological Satellites - CGMS  Browser-based tool using Google-Earth to display satellite coverage swaths and calculate coincidence scene locations.  Automated daily satellite position data from CelesTrak.  Saved bookmarks and states, Google-Earth KML and Shapefile compatibility, collaborative sessions.  Output: position, UTC time, viewing angles, solar angles, day/night, and EXCEL tables  Large mission database: 126 missions, 263 Mission- Instrument combinations Did you know.... There are 1046 Earth orbiting missions COVE is a FREE tool ! Visit: www.ceos-cove.org CEOS Visualization Environment (COVE)

4. NASA, CGMS-41, July 2013 Coordination Group for Meteorological Satellites - CGMS GCOM-C1 Mission, SGLI-VNR Instrument  Launch in mid-2014, Altitude 796km, 10:30am LTDN, 34-day orbit repeat, Swath 1150km. Sentinel-3 Mission, OLCI Instrument  Constellation of two satellites (A and B), phased 180-degrees Sentinel-3A launch in April 2014, Altitude 800 km, 10:00am LTDN, 27 day orbit repeat, Swath 1240km. Orbit Analysis  Simple J2 orbit propagator  Arbitrary launch date and propagation start time. Other Assumptions  Modifications were only considered to the GCOM-C1 orbit since Sentinel-3A and 3B are both impacted by changes and their optimized performance, as a pair, would be lost. Assumption

5. NASA, CGMS-41, July 2013 Coordination Group for Meteorological Satellites - CGMS  Due to differences in orbits, the ground swaths of the satellites drift relative to each other. The swath spacing will oscillate between extremes over a 3 month period. Analysis Results  This oscillation results in complete overlap (top figure) and non- overlapping coverage (bottom figure).  Even in the non- overlapping case, minor gaps are present between satellite passes.

6. NASA, CGMS-41, July 2013 Coordination Group for Meteorological Satellites - CGMS  With small modifications to the GCOM-C1 orbit, it is possible to remove the oscillation in the ground coverage pattern produced by the virtual constellation.  Increase the GCOM-C1 altitude by 4km and adjust the orbit inclination by <0.1-deg to achieve a 27-day orbit repeat, similar to Sentinel-3A.  Following altitude and inclination adjustments, there is a need to adjust the local crossing time to achieve the desired separation at the equator.  Moving the LTDN to 10:32am achieves the consistent daily coverage shown on the right. Additional adjustments to the LTDN can remove this small overlap, if desired. Optimization Results

7. NASA, CGMS-41, July 2013 Coordination Group for Meteorological Satellites - CGMS  The optimized coverage pattern (previous chart) requires the two spacecraft to be flying in a tight formation separated by only a few seconds. This change may require increased fuel consumption.  Significant coordination will be required by the operations teams during launch, orbit insertion, and daily operations of the constellation.  Once Sentinel-3B launches, significant overlap will exist between GCOM-C1 and Sentinel-3B (figure on right)  Unless there is a secondary benefit to redundant coverage, either GCOM-C1 or Sentinel-3B’s coverage is largely redundant.  If the 3-satellite constellation is not desired, GCOM-C1 can perform a small propulsive burn to return to its original orbit and both teams can return to independent operations. Optimized ground coverage for GCOM-C1 (RED) and Sentinel-3A (BLUE) with the addition of Sentinel-3B (GREEN). Little gaps exist in this scenario. Additional Considerations